Hydroponic grass field system

Abstract

The hydroponic grass field system comprises at least a root zone layer 12), a capillary layer (10) and a water flow layer (6) which are arranged on top of each other in a water impermeable reservoir (2). By means of a water table control system, the level of the water table in the reservoir (2) can be raised and lowered. At a steady state level, the water table is situated in the water flow layer (6) and water can rise by capillary action upto the top of the root zone layer (12). Due to the fact that the maximum height of capillary rise of the capillary layer (10) is larger than 6.0 cm but smaller than the maximum height of capillary rise of the root zone layer (12), saturation of the lower portion of the root zone layer (12) with water can be avoided so that the roots of the grass can also develop well in the lower portion of the root zone layer (12). The granulometry of the granular materials of each of the successive layers meets the bridging factor so that no sheet materials, which would hamper the capillary action, need to be applied between the successive layers to avoid migration of fine particles of the upper layers into a coarser lower layers.

Description

"Hydroponic grass field system"The present invention relates to a hydroponic grass field system having a sand substrate growing medium. The hydroponic grass field system comprises at least one water impermeable reservoir which contains water forming a water table. The water contained in the water impermeable reservoir is a hydroponic nutrient solution providing the nutrients which are required for the growth of the grass. The hydroponic grass field system further comprises a plurality of successive layers of different granular materials which are arranged on top of each other in the water impermeable reservoir. These layers include at least a root zone layer, wherein said grass is rooted, and a water flow layer arranged underneath the root zone layer. The root zone layer is composed of the sand substrate growing medium. It has an upper and a lower surface. The water flow layer is configured to enable water to flow into and out of the water impermeable reservoir. The hydroponic grass field system also comprises a water table control system configured to raise and lower the water table by feeding water into and removing water out of said water impermeable reservoir via said water flow layer.Compared to traditional grass fields a hydroponic grass field system has a number of important advantages. First of all it enables to save water resources. Rain water can indeed not percolate into the subsoil layers but is retained in the water impermeable reservoir. It can thus be collected and stored in suitable water buffer reservoirs. For some plays such as football plays, the grass field is often sprayed with water before a match in order to increase for example the ball speed. This can be done with the collected rain water. Since the root zone layer is made of a sand substrate growing medium, the rain water or the water sprayed onto the grass field can quickly percolate through the sand substrate layer so that this layer retains its structural properties also in case of rainfall.Another advantage of the hydroponic grass field system is that the grass can be fed with the optimum amount of fertilizers without causing any pollution of the subsoil or of the water drained out of the soil since all the fertilizers are kept in the system and do not leach out. This is especially important for the macro-nutrients such a nitrogen, phosphorus and potassium which are an important source of pollution of ground water and surface water and which can thus be used in a hydroponic grass field system in larger, optimum amounts to achieve strongly rooted, healthy grass.A hydroponic grass field system is for example disclosed in WO 2020 / 140023. This system comprises a water impermeable first layer and on top thereof either only a root zone layer or a combination of a root zone layer and a porous second layer in between the root zone layer and the water impermeable first layer. The porous second layer may in particular be a mixture of cement and a particulate stone material to produce Capillary Concrete®. The porous second layer thus provides a foundation layer for the root zone layer and is also intended to allow for the water to flow through. Water may be transported by capillary forces from this porous second layer to the root zone layer.An important aspect of the invention disclosed in WO 2020 / 140023 is that the water table in the hydroponic grass field system is periodically raised and lowered to change the water level in the root zone layer. This is done a number of times a day in order to try to aerate the root zone.The applied aerification principle is illustrated in Figure 3 of WO 2020 / 140023. In the graph shown in this figure, the lower part of the root zone layer is saturated with water. The level upto which the root zone layer is saturated with water fluctuates between a minimum and a maximum level. In this way air would be pushed out of the root zone layer and sucked back into the root zone layer.A drawback of this system is however that a large part of the root zone layer is permanently saturated with water and is thus not suited to support root growth. The growth of the roots will thus be restricted to the upper part of the root zone layer so that the grass is not rooted strongly and deeply in the root zone layer. As a result thereof, the grass is for example not well adapted to dry periods. In case of dry weather conditions with high evaporation rates, the stomata of the grass will indeed be closed which will hamper the exchange of carbon dioxide and oxygen and thus the growth and development of the grass.An object of the invention is therefore to obviate this drawback and to provide a hydroponic grass field system which enables the roots of the grass to develop down to a larger depth into the root zone layer or even underneath the root zone layer.To this end the hydroponic grass field system of the present invention is characterised in thatthe granular materials of said successive layers each have a D15 sieve size value and a D85 sieve size value, the D15 sieve size value of each of the layers which are situated underneath another one of said successive layers being smaller than 8 times the D85 sieve size value of the layer which is situated on top of the respective layer;the granular materials of said successive layers each have a maximum height of capillary rise and the successive layers an average thickness, the average thickness of each of said successive layers, different from said water flow layer, being smaller than the maximum height of capillary rise of the granular material of the respective layer;said root zone layer has an average thickness TRZL of at least 15.0 cm, a saturated hydraulic conductivity KRZL, measured in accordance with ASTM F1815-97, of at least 20.0 cm / h and the granular material of the root zone layer has a maximum height of capillary rise hc,RZL of at least 20.0 cm and a D15 sieve size value D15,RZL, a D85 sieve size value D85,RZL and a D90 sieve size value D90,RZL;said plurality of successive layers includes a capillary layer which is situated underneath the root zone layer adjacent the lower surface thereof and which is configured to feed water by capillary rise of said water from the capillary layer into the root zone layer, which capillary layer has an average thickness TCL of between 4.0 cm and 15.0 cm and a saturated hydraulic conductivity KCL, measured in accordance with ASTM F1815-97, of at least 40 cm / h and the granular material of the capillary layer has a maximum height of capillary rise hc,CL which is larger than 6.0 cm but smaller than hc,RZL and a D15 sieve size value D15,CL, a D85 sieve size value D85,CL and a D90 sieve size value D90,CL; andsaid water flow layer has an average thickness TWFL of at least 5.0 cm, preferably of between 5.0 and 50.0 cm, and a saturated hydraulic conductivity KWFL, measured in accordance with ASTM F1815-97, of at least 500 cm / h and the granular material of the water flow layer has a maximum height of capillary rise hc,WFL of at least 1.0 cm but preferably less than 5.0 cm and a D15 sieve size value D15,WFL, a D85 sieve size value D85,WFL and a D90 sieve size value D90,WFL.The hydroponic grass field system according to the present invention comprises a number of successive layers, each composed of a granular material wherein water can rise by capillary forces. Since, apart from the water flow layer, each of the layers has an average height which is smaller than the maximum height of capillary rise of the granular material of the respective layer, water can be supplied by capillary rise through each of the successive layers.Moreover, a sufficiently low bridging factor is maintained between the different successive layers. The bridging factor between two successive layers is defined as the ratio between the D15 sieve size value of the lowermost layer and the D85 sieve size value of the layer which is situated right on top of this layer. In the hydroponic grass field system of the present invention this bridging factor is for each couple of adjacent layers smaller than eight or preferably even smaller. In this way, migration of finer sand particles of a higher layer into the sand or gravel of the layer situated underneath this higher layer can be prevented or limited without having to provide a sheet material such as a geotextile fabric in between the different layers. It has been found that such a geotextile fabric does not only hamper the capillary rise of water but it especially also hampers draining of water out of the lower portion of the uppermost layer so that the lower portion of the uppermost layer may remain saturated for a long time with water.The root zone layer has a relatively large saturated hydraulic conductivity KRZL. In this way, after rain or after having sprayed water onto the grass field, this water drains quickly out of the root zone layer. However, it also retains a sufficient amount of capillary water, which is readily available for the grass, since its maximum height of capillary rise is larger than or equal to 20.0 cm.An essential feature of the hydroponic grass field system of the present invention is that it comprises a capillary layer which is situated directly underneath the root zone layer. The granular material of this capillary layer has a maximum height of capillary rise which is larger than 6.0 cm but which is smaller than the maximum height of capillary rise of the granular material of the root zone layer. In this way, the capillary layer cannot only feed sufficient water by capillary rise into the root zone layer but it exerts a suction head underneath the root zone layer by which water is sucked out of the lower portion of the root zone layer. The suction head which can be exerted by the capillary layer onto the root zone layer depends on the thickness of the capillary layer and also on the maximum height of capillary rise. Indeed, the thicker the capillary layer and the larger the maximum height of capillary rise thereof, the larger the suction head at the upper surface of the capillary layer. By the presence of the capillary layer directly underneath the root zone layer, too high moisture contents in the root zone layer can be avoided or in case the root zone layer has been saturated with water, too high moisture contents in the root zone layer can be remediated quite quickly so that the roots of the grass can develop more deeply into the root zone layer or even through the root zone layer. A restriction of the development of grass roots into the root zone layer of a golf green is a known problem which has been described for example in the article “Layers in golf green construction” by Dr. S. W. Baker and D. J. Binn of The Sports Turf Research Institute, UK, December 1999. This articles describes the construction of golf green profiles with a 300 mm root zone layer and a 50 mm intermediate grit layer arranged on top of a gravel layer. For the gravel, 84% of the particles fell between 6.3 mm and 9.5 mm and 85% of the root zone contained sand particles between 0.25 and 1.0 mm. The D15 sieve size value of the root zone sand was equal to about 0.270 mm. The root zone sand was thus relatively coarse. The intermediate layer contained a clean 1-4 mm grit. The authors observed that in many situations root penetration was not sufficient to exploit most of the moisture retained below 150 mm. Figure 9 shows the moisture content in the root zone layer as measured after saturation with water followed by gravitational drainage for 48 hours. In the uppermost 150 mm of the profile the volumetric moisture content was equal to about 15% but then increased exponentially to about 45%, i.e. to a saturated situation, down to a depth of 300 mm. Notwithstanding the high porosity and the fact that the root zone sand was relatively coarse, water did not drain out of the lower portion of the root zone layer into the grit layer.The maximum height of capillary rise of the grit layer appears not to be large enough to withdraw the water from the lower part of the root zone layer. In the hydroponic grass field system according to the present invention, there is thus no gravel (grit) layer directly underneath the root zone layer, which gravel layer would provide a textural barrier so that the pores of the root zone layer would have to be saturated for water to move into the gravel layer.The present inventor has found that by providing a capillary layer, with a sufficiently high hc value and a sufficiently high saturated hydraulic conductivity value, underneath the root zone layer, the suction head exerted by this capillary layer onto the water in the root zone layer enables to lower the water content of the lower zone of the root zone layer more quickly to lower values. Moreover, in contrast to the described golf greens, the root zone layer of the hydroponic grass field system of the present invention is configured to be fed with water mainly by capillary action. The granular material of the water flow layer has in particular a limited maximum height of capillary rise which enables, especially in combination with the limited capillary contact of this water flow layer with the layer arranged on top thereof, to feed water at a controlled flow rate to the root zone layer. By keeping the water table in the water flow layer sufficiently low, water can thus be fed at a reduced flow rate to the root zone layer so that saturation of the lower portion of this root zone layer with water can be avoided. In case the root zone layer, in particular the lower portion thereof, would become saturated with water, for example due to rainfall, it can be desaturated quite quickly by means of the suction head which can be exerted thereon by the capillary layer.Due to its smaller hc value, the capillary layer has a smaller capillary porosity than the root zone layer and thus drains more quickly and completely than the root zone layer. The roots of the grass can thus not only grow into the lower portion of the root zone layer but even in the capillary layer, all depending on the height of the water table which is controlled by the water table control system. Initially, when sowing the grass, the water table can be quite high so that a lot of water is available at the top of the root zone layer to support the germination of the grass seed and the initial root development. The development of the roots can then be guided to deeper zones, even into the capillary layer, by lowering the water table.Preferably, said plurality of successive layers include a number of successive layers which are arranged on top of the water flow layer and each of which has an average thickness selected to allow, in case the water flow layer is completely saturated with water, rise of water by capillary action from the water flow layer upto a height which is at most 5.0 cm, preferably at most 3.0 cm, underneath the upper surface of the root zone layer.Water can thus be supplied by capillary rise from the water flow layer upto the root zone layer. It is thus not necessary to soak or saturate one or more of the higher layers completely with water to provide water into the top zone of the grass field system, which would require quite some time to drain the water back out of that water saturated layer to such an extent that the roots can develop also in the lower part of that layer. The water flow layer, on the other hand, may be saturated even completely with water since it only serves to transport at the bottom of the hydroponic grass field system. By keeping sufficient water in the root zone layer so that the grass roots have enough water in the higher layers, the grass roots will not grow down to the water flow layer, which is advantageous in view of the fact that the pores of the water flow layer should not be clogged by plant roots.In an embodiment of the hydroponic grass field system according to the present invention, the D15 sieve size value of the root zone layer D15,RZL is smaller than 230 µm, preferably smaller than 210 µm and more preferably smaller than 180 µm, with D15,RZL being preferably larger than 100 µm.The smaller the D15,RZL value, the smaller the pores of the root zone layer and thus the higher the maximum height of capillary rise and the water retention properties of the root zone layer. Notwithstanding the higher water retention properties of the root zone layer, it has been found that due to the presence of the capillary layer directly underneath the root zone layer, saturation of the lower portion of the root zone layer can be avoided so that the roots of the grass can grow into this lower portion and even entirely through the root zone layer into the capillary layer.In an embodiment of the hydroponic grass field system according to the present invention, D15,WFL is larger than 10 mm and the ratio D90,WFL / D15,WFL is smaller than or equal to 3.0, preferably smaller than or equal to 2.5 and more preferably smaller than or equal to 2.0, but larger than 1.0.An advantage of the relatively small D90,WFL / D15,WFL is that the water flow layer has a high porosity. Moreover, as a result of the relatively large D15,WFL value the water flow layer has large pores. Water can thus flow quite freely through the water flow layer so that it can be fed quickly into and out of the system. Moreover, due to the large size of the pores, they get clogged less quickly, in particular with decaying organic material.In an embodiment of the hydroponic grass field system according to the present invention, or according to the preceding embodiment, D15,WFL is smaller than 8.0 times D85,CL, preferably smaller than 7.0 times D85,CL and more preferably smaller than 6.0 times D85,CL.An advantage of this embodiment is that it is not necessary to arrange an intermediate layer in between the capillary layer and the water flow layer in order the meet the required bridging factor to avoid migration, or too much migration, of fine particles between the successive layers. In other words the profile of the grass field system may consist of only three layers, namely the root zone layer, the capillary layer and the water flow layer. Since all of the layers have to be levelled when installing the hydroponic grass field system, a system which consists of only three layers of granular material is easier to install.In an embodiment of the hydroponic grass field system according to the present invention, or according to any one of the preceding embodiments, the granular material of said capillary layer comprises a porous granular material, preferably a granular lava material, with D15,CL being larger than 1.25 mm, preferably larger than 1.75 mm and more preferably larger than 2.0 mm.An advantage of making the capillary layer of a porous granular material is that the required maximum height of capillary rise can be achieved with a coarser granular material. The pores present in the particles of the granular material contribute indeed also in providing a capillary suction so that the pores in between the particles of the granular material may be larger. As described in the master’s thesis “Extent of Capillary Rise in Sands and Silts” of Rachel Lynn Salim, 2016, the interparticle pore radius depends in particular on the D10 sieve size value, i.e. of the Hazens effective size, which may thus be larger in case the capillary rise is provided partially by the pores in the particles themselves. The D15 sieve size value is closely related to the D10 sieve size value. As a result of the larger D15 sieve size, i.e. as a result of the larger pore radius, water drains more quickly from the capillary layer and less water is retained in the interparticle pores of that layer so that more air, i.e. more open pore space, is available for the roots to grow even in this capillary layer.In an embodiment of the hydroponic grass field system according to the present invention, or according to any one of the preceding embodiments, said plurality of successive layers include an intermediate layer arranged between said capillary layer and said water flow layer, which intermediate layer has a D15 sieve size value D15,IL, a D85 sieve size value D85,IL and a D90 sieve size value D90,IL, with D15,IL being smaller than 8.0 times D85,CL, preferably smaller than 7.0 times D85,CL and more preferably even smaller than 6.0 times D85,CL, and with D15,WFL being smaller than 8.0 times D85,IL, preferably smaller than 7.0 times D85,IL and more preferably even smaller than 6.0 times D85,IL.An advantage of this embodiment is that it is not necessary to arrange a further layer in between the capillary layer and the intermediate layer in order the meet the required bridging factor to avoid migration, or too much migration, of fine particles from the capillary layer into the intermediate layer. Moreover, it is not necessary to arrange a further layer in between the intermediate layer and the water flow layer in order the meet the required bridging factor to avoid migration, or too much migration, of fine particles from the intermediate layer into the water flow layer. In other words the profile of the grass field system may consist of only four layers, namely the root zone layer, the capillary layer, the intermediate layer and the water flow layer. Since all of the layers have to be levelled when installing the hydroponic grass field system, a system which consists of only four layers of granular material is easier to install. Such a hydroponic grass field system can be made without having to use a porous granular material, such as a porous lava material, for the capillary layer.The intermediate layer preferably has an average thickness TIL of between 3.0 and 10.0 cm.Such a thickness is sufficient to prevent migration of fine particles from the capillary layer into the water flow layer. A minimum layer thickness also allows for larger thickness tolerances, which makes applying the intermediate layer on top of the water flow layer easier. Since the intermediate layer is coarser than the capillary layer, its maximum height of capillary rise is smaller so that the thickness of the intermediate layer should not be too large in order to enable the water to rise by capillary forces from the water flow layer, through the intermediate layer, into the capillary layer.The granular material of the intermediate layer preferably has a maximum height of capillary rise hc,IL of between 4.0 and 12.0 cm, and more preferably between 5.0 and 10.0 cm.The maximum height of capillary rise of the intermediate layer should be larger than the average thickness thereof but is preferably not too large. In this way, the suction head drops more quickly in the vertical direction in the intermediate layer so that the supply of water by capillary action through the intermediate layer can be better controlled by raising and lowering the water table. When the water table is in the water flow layer, the intermediate layer reduces the capillary supply of water from the water flow layer to the capillary layer. The capillary supply of water to the root zone layer can be substantially increased, by raising to water table so that it extends in the intermediate layer. The higher the water table in the intermediate layer, the larger the water supply rate. A much higher water supply rate can be achieve by further raising the water table so that it is situated in the capillary layer itself.The D15 sieve size value D15,IL of the intermediated layer is preferably larger than 1.0 mm, more preferably larger than 1.5 mm and most preferably larger than 2.0 mm, but smaller than 5.0 mm, preferably smaller than 4.0 mm and more preferably smaller than 3.0 mm.An advantage of a larger D15,IL value is that the intermediate layer has a larger pore radius so that water drains more quickly from the intermediate layer. Moreover, water can also flow more easily horizontally through the intermediate layer in case the water table is situated in the intermediate layer and water is circulated through the water impermeable reservoir. The advantage of a smaller D15,IL value is that the capillary layer may be a finer layer since the D85 sieve size value D85,CL of the capillary layer should be larger than one eighth of times the D15,IL value. The advantage of such a finer capillary layer is that it can exert a larger suction head on the root zone layer and that it can retain more water to support also the root growth in the capillary layer in case the water table is kept sufficiently low.The D85 sieve size value D85,IL of the intermediated layer is preferably smaller than 10.0 mm, more preferably smaller than 8.0 mm and most preferably smaller than 6.0 mm.For a same D15,IL sieve size value a smaller D85,IL sieve size value corresponds to a steeper particle size distribution resulting in a higher porosity of the intermediate layer. Such a higher porosity enable to provide more open porosity or in other words to retain more air in the intermediate layer when the water table is below the intermediate layer. In this respect, the D90,IL / D15,IL ratio is preferably smaller than or equal to 3.0, more preferably smaller than or equal to 2.5.Preferably, the D90,IL / D15,IL ratio is larger than 1.4, more preferably larger than 1.5. In this way, the D15 sieve size value D15,WFL of the water flow layer may be larger. Moreover, the particles of the intermediate layer fit better in between each other to provide a more compact and thus more stable layer.In an embodiment of the hydroponic grass field system according to the present invention, or according to any one of the preceding embodiments, the D90,CL / D15,CL ratio is larger than 1.5, preferably larger than 2.0 and more preferably larger than 2.5, but smaller than or equal to 5.5, preferably smaller than or equal to 4.5.A D90,CL / D15,CL ratio larger than the lower limits is advantageous since this corresponds to a higher D85,CL value so that the D15,IL value, or the D15,WFL value in case there is no intermediate layer, may be larger. An important advantage of a larger D90,CL / D15,CL ratio is also that the particles of the capillary layer fit better in between each other resulting in a more compact and thus more stable capillary layer. The D90,CL / D15,CL ratio is preferably smaller than the upper limits to increase the porosity of the capillary layer, thus enabling more open porosity and hence more air in this layer enabling the roots to grow also into the capillary layer, at least in case the water table is kept sufficiently low.In an embodiment of the hydroponic grass field system according to the present invention, or according to any one of the preceding embodiments, the D90,RZL / D15,RZL ratio is larger than 1.5 and preferably larger than 2.0, but smaller than or equal to 5.5, preferably smaller than or equal to 4.5.A D90,RZL / D15,RZL ratio larger than the lower limits is advantageous since this corresponds to a higher D85,RZL value so that the D15,CL value may be larger. An advantage of a larger D90,RZL / D15,RZL ratio is that the particles of the root zone layer fit better in between each other resulting in a more compact and thus more stable root zone layer. This is especially important for the root zone layer since the root zone layer forms the upper layer of the hydroponic grass field system, which should thus be sufficiently stable especially in case it is a sports field. The D90,RZL / D15,RZL ratio is preferably smaller than the upper limits to increase the porosity of the capillary layer, thus enabling more open porosity and hence more air in this layer for allowing to achieve a highly developed root system in the root zone layer or even more deeply in the grass field system.In an embodiment of the hydroponic grass field system according to the present invention, or according to any one of the preceding embodiments, the maximum capillary rise hc,CL of the granular mater of the capillary layer is larger than 8.0 cm, preferably larger than 10.0 cm.Such a higher capillary rise value allows the capillary layer to exert a larger capillary suction onto the root zone layer so that saturation of the lower portion of the root zone layer can be prevented or remediated more easily.In an embodiment of the hydroponic grass field system according to the present invention, or according to any one of the preceding embodiments, the average thickness TRZL of the root zone layer is smaller than 30.0 cm and preferably smaller than 25.0 cm, with TRZL being preferably larger than 18.0 cm, more preferably larger than 20.0 cm.Such root zone layer thicknesses are advantageous to enable a highly developed root system in the root zone. Since the maximum height of capillary rise does not have to be too large, the granular material of the root zone layer may have relatively large pores, compared to for example a silt or a clay layer, so that it may be a relatively coarse sand, out of which any excess water can be drained quickly and efficiently. A sand layer also provides the required stability in case the grass field is to be used as a sports field.In an embodiment of the hydroponic grass field system according to the present invention, or according to any one of the preceding embodiments, it comprises one or more moisture sensors which are arranged in the root zone layer to measure the moisture content of the root zone layer. Preferably, a first moisture sensor is arranged at a first depth in the root zone layer and a second moisture sensor at a second depth which is larger than said first depth.Water is removed from the root zone layer by evapotranspiration and is replenished in the root zone layer by capillary action. Most of the time, the moisture content of the root zone layer measured by means of the moisture sensors can be kept substantially constant by controlling the level of the water table by means of the water table control system. Usually, the water table is kept within the water flow layer so that the water rises by capillary action out of the water flow layer, through any intermediate layer and the capillary layer into the root zone layer.When the evapotranspiration is too high, the water table can be raised into the intermediate layer, if any, or even into the capillary layer.In an embodiment of the hydroponic grass field system according to the present invention, or according to any one of the preceding embodiments, the water table control system is therefore preferably configured to control capillary feeding of water into the root zone layer by adjusting the water table in the water flow layer to a steady state level wherein at least part of the water loss by evapotranspiration is compensated for by the capillary feeding of water into the root zone layer.In an embodiment of the hydroponic grass field system according to the present invention, or according to any one of the preceding embodiments, the water table control system comprises an ebb and flow system which is configured to control capillary feeding of water into the root zone layer by temporarily raising the water table to a flooding level which is situated above the water flow layer but below the lower surface of the root zone layer and by lowering the water table back to a lower level, in particular to the steady state level or to a draining level wherein the water is drained out of the water flow layer.When the water table is at the flooding level, water is fed at a higher flow rate into the root zone layer. The lower portion of the root zone layer may thus become saturated with water. In order to avoid a too high water content in the root zone layer, the flooding level is thus only maintained for a limited period of time. When the water table is lowered back to its normal level, or to the draining level, the lower portion of the root zone layer is quickly desaturated again by the suction head exerted thereon by the capillary layer and optionally also by a capillary rise of the water further upwards into the root zone layer. An equilibrium position is then preferably established again, namely a position wherein the rate of capillary rise of the water into the root zone layer is substantially equal to the evapotranspiration rate of the water out of the root zone layer.Preferably, the ebb and flow system is further configured to temporarily lower the water table to a draining level wherein the water is drained out of the water flow layer, the ebb and flow system preferably comprising drains which are recessed in the bottom of the water impermeable reservoir.In this embodiment, the water impermeable reservoir can thus be emptied substantially completely. This can be done when heavy rainfall is expected or before water is sprayed onto the grass to make it wet. In this way, any excess of water can be removed quickly. When the water has been removed substantially completely, the pores of the water flow layer are filled nearly completely with air. Due to the large particle size of the granular material of the water flow layer, the water retention thereof is indeed quite limited. Any organic matter, for example of decaying root material, can thus decompose further efficiently under aerobic conditions so that no anaerobic conditions are produced which may hamper the development of the roots.Raising the water table upto the flooding level is done in general preferably with intervals of at least one day, preferably at least two days and more preferably at least three days. Emptying the water reservoir is also done in general preferably with intervals of at least one day, preferably at least two days and more preferably at least three days.In an embodiment of the hydroponic grass field system according to the present invention, or according to any one of the preceding embodiments, the water table control system comprises at least one water buffer reservoir connected to said water impermeable reservoir and at least one pump for pumping water from said water buffer reservoir into said water impermeable reservoir and / or vice versa.Water may flow by gravity from the water impermeable reservoir into the water buffer reservoir and may be pumped back from the water buffer reservoir into the water impermeable reservoir or vice versa it may be pumped from the water impermeable reservoir into the water buffer reservoir. Alternatively, the water may be pumped from the water impermeable reservoir into the water buffer reservoir and from the water buffer reservoir back into the water impermeable reservoir.The hydroponic grass field system preferably comprises at least two water impermeable reservoirs wherein said plurality of layers are arranged, said two water impermeable reservoirs being connected to each other and / or to said water buffer reservoir.An advantage of this embodiment is that when one of the water impermeable reservoirs is drained, this water can be pumped into the other water impermeable reservoir to raise the water table therein to the flood level. In this way, less water storage capacity is needed.In an embodiment of the hydroponic grass field system according to the present invention, or according to any one of the preceding embodiments, the grass field system comprises a circulation system for circulating water via said water flow layer in at least one direction through said water impermeable reservoir, the circulation system being preferably configured to circulate the water in said one direction and subsequently in the opposite direction through the water impermeable reservoir.By circulating the water through the water impermeable reservoir, it is possible to control some predetermined properties thereof such as the pH, the EC value (Electric Conductivity indicating the content of nutrient elements), the temperature of the water, the oxygen content, etc. These properties can then be adjusted by adding an acid or a base, by adding a fertilizer solution, by heating or cooling the water or by injecting oxygen in the circulating water.The circulation system preferably comprises at least one set of first drain pipes arranged in a substantially horizontal plane in the water impermeable reservoir and at least one set of second drain pipes arranged in the same substantially horizontal plane in the water impermeable reservoir, the second drain pipes being interposed between the first drain pipes, when circulating water through the water impermeable reservoir, the second drain pipes being configured to drain water from the water impermeable reservoir when water is fed into the water impermeable reservoir via the first drain pipes and the first drain pipes being configured to drain water from the water impermeable reservoir when water is fed into the water impermeable reservoir via the second drain pipes.By reversing the flow direction between the drain pipes in the water impermeable reservoir, the properties of the water in the water impermeable reservoir, such as its oxygen content or its nutrition element content, can be kept more uniform. Moreover, when any decomposed organic matter is present in the water flow layer, this can be evacuated more easily and more completely out of the water flow layer. Clogging of the in- or outlet openings in the drain pipes can also be prevented. When a small piece of organic matter blocks an inlet opening in a drain pipe, this opening is again released when reversing the flow direction so that water is fed via this opening into the water impermeable reservoir.Additionally or alternatively, the circulation system preferably comprises an oxygenator for injecting oxygen, an oxygen containing gas or an ozone containing gas in the water circulated through the water impermeable reservoir.In the water impermeable reservoir, the oxygen or ozone is released by the water so that the granular materials of the different layers are better aerated. This may prevent anaerobic decay of any dead organic matter and can enhance the aerobic decomposition thereof. The release oxygen also has a positive effect on the development of the roots of the grass.In an embodiment of the hydroponic grass field system according to the present invention, or according to any one of the preceding embodiments, the water contained in the hydroponic grass field system is a hydroponic plant nutrient solution which comprises preferably nitrogen, potassium, phosphorous and micro-nutrients including manganese, zinc and boor and preferably also copper and molybdenum, which hydroponic grass field system preferably comprises a dosing apparatus for dosing these plant nutrients in said water.The hydroponic plant nutrient solution preferably comprises all the nutrients which are required for the growth of the grass. It is thus not necessary to spread addition fertilizers onto the surface of the hydroponic grass field system. The plant nutrition elements rise together with the water by capillary action to the surface of the root zone layer. Under dry weather conditions, they can thus accumulate to some extent at the surface of this layer. On the other hand, the plant nutrition elements can be extracted by the grass roots out of the nutrient solution. In order to achieve a more uniform distribution of the plant nutrition elements in the root zone layer, the grass can be sprayed with the plant nutrient solution, especially in case of dry weather conditions.In an embodiment of the hydroponic grass field system according to the present invention, or according to any one of the preceding embodiments, there are no sheet materials in between the different layers.Such sheet materials may indeed form capillary barriers which may prevent the capillary rise of the water, or they may hamper the capillary rise of water and the may especially also reduce the suction head exerted by the capillary layer onto the root zone layer so that the lower portion of the root zone layer may remain too wet for a too long period of time so that the roots of the grass will not develop well in this lower part of the root zone layer.Other advantages and particularities of the present invention will become apparent from the following description of some particular embodiments of the hydroponic grass field system according to the invention. This description is only given by way of example and is not intended to limit the scope of the invention. The reference numerals used in the description relate to the annexed drawings wherein:Figure 1 is a cross sectional view through the successive layers of granular materials of a first embodiment of the hydroponic grass field system according to the present invention;Figure 2 is a cross sectional view through the successive layers of granular materials of a second embodiment of the hydroponic grass field system according to the present invention;Figure 3 is a diagram showing schematically the entire layout of a particular embodiment of the hydroponic grass field system according to the present invention;Figure 4 shows the particle size distribution of a coarse gravel, which can be used for the water flow layer, a fine gravel, which can be used as an intermediate layer in the embodiment illustrated in Figure 1 and a lava which can be used as capillary layer in the embodiment illustrated in Figure 2;Figure 5 shows the particle size distribution of two capillary sands, which can be used as capillary layer in the embodiment illustrated in Figure 1, and of four different sands, which can be used for the root zone layer in the embodiments illustrated in Figures 1 and 2;Figure 6 shows the amount of precipitation on the hydroponic grass field system illustrated in Figure 1 on one specific day and the corresponding evolution of the level of the water table on that same day; andFigure 7 shows the amounts of precipitation on the test field during the test wherein the evolution of the moisture content was measured at different depths in the root zone layer.The present invention relates to a hydroponic grass field system. It comprises a sand substrate growing medium wherein the grass is rooted. The sand substrate growing medium does not have to contain organic matter to provide the required plant nutrition elements for the growth of the grass. Organic matter such as for example peat is thus normally not included in the sand substrate growing medium when making the hydroponic grass field system although some organic matter may be mixed into the sand substrate growing medium in order to increase the nutrient element retention properties thereof and, at the same time, the water retention properties. The saturated hydraulic conductivity of the sand substrate growing medium should however be high enough as defined by the present invention. Preferably, it remains higher than the defined minimum values also when after a number of years some organic matter will accumulate in the sand substrate growing medium (root zone layer) as a result of the growth and the subsequent death and decay of the roots of the grass. Preferably, the sand substrate growing medium comprises less than 10% by dry weight, more preferably less than 5% by dry weight and most preferably less than 2% or even less than 1% by dry weight of organic matter, measured in accordance with method A of ASTM F1647-11(2018).In the hydroponic grass field system the plant nutrition elements are provided to the grass via the water, which is a hydroponic plant nutrient solution. This solution comprises the macro-nutrients nitrogen, potassium and phosphorus. It preferably also comprises the micro-nutrients manganese, zinc and boron and preferably also copper and molybdenum. Further nutrients can be included in the nutrient solution. The composition of nutrient solutions which are suitable for the growth of grasses in hydroculture are known and therefore does not need to be described in detail in the present specification. As an example of a suitable nutrient solution, reference can be made to the article “The Use of Hydroponics in Abiotic Stress Tolerance Research” by Yuri Shavrukov, Yusuf Genc and Julie Hayes, March 2012 (DOI: 10.5772 / 35206). The improved nutrient solution disclosed in this publication has the following final concentration of nutrient element salts. Table 1: Nutrient solution compositionElementsSalts usedFinal concentration (mM)NNH4NO30.2K, NKNO35.0Ca, NCa(NO3)22.0Mg, SMgSO42.0P, KKH2PO40.1SiNa2SiO30.5Micro-elements (µM)FeNaFe(III)EDTA100.0BH3BO312.5MnMnCl22.0ZnZnSO43.0CuCuSO40.5MoNa2MoO30.1NiNiSO40.1ClKCl25.0 The hydroponic grass field system according to the invention comprises a plurality of successive layers of different granular materials which are arranged on top of each other. The different layers and the different granular materials have a number of properties which can be defined or measured as follows.The granular materials each have a particle size distribution which can be determined by sieving. This particle size distribution has a number of different D sieve size values. The D90 sieve size value is for example the particle size, i.e. the sieve size, at 90% by volume cumulative passing. The D85, the D50, the D15 and the D10 sieve size values are the particle sizes at 85%, 50%, 15% and 10% by volume cumulative passing. The particle size distribution can be tested according to ASTM D6913 / D6913M-17 using sieves having square openings. The volume percent of the particles can in particular be calculated by dividing the weight of the particles passing through the sieve by the average density of the material forming the particles.The average thickness of a layer is defined as the volume thereof divided by its upper surface area.The saturated hydraulic conductivity K of a layer is the conductivity of the granular material of that layer, expressed in cm / h, as determined in accordance with ASTM F1815-06.The maximum height of capillary rise of a granular material of one of the layers is determined as described under the heading “Open tube method” in the publication “Capillary Tests by Capillarimeter and by Soil filled Tubes”, by K. S. Lane, D. E. Washburn and D. P. Krynine, 1947 (https: / / onlinepubs.trb.org / Onlinepubs / hrbproceedings / 26 / 26-038.pdf). The tests are done by measuring the height of capillary rise directly in a transparent plastic tube having a diameter of about 5 cm. The tube is closed at the bottom with a perforated brass plate, screen and cloth and rests on supports in a pan of water to an approximate depth of 5 cm. The samples are compacted air-dried in to the tubes using a round hardwood rammer accompanied by tapping the sides of the tubes with a mallet. The samples of granular material are compacted to a density which is substantially equal to the density of the granular material in the layer in the hydroponic grass field system. The samples are compacted in layers. The compaction planes at the top of each layers are scarified before adding the following layer. The tube filled with the granular material is then set in a pan of tap water, with care not to trap an air pocket under the brass plate and the test is started. The test is done at room temperature in a lab under normal air humidity of for example between 50 and 60%. Readings are taken regularly until completion of the test, i.e. until the capillary rise stopped. The test is however stopped after one month in case the capillary rise of the water would continue even after one month. The granular materials applied in the different layers of the hydroponic grass field system are indeed quite coarse, in particular sands or gravels, so that the capillary rise is normally finished within one month.Figure 1 shows the plurality of successive layers of different granular materials of a first embodiment of the hydroponic grass field system according to the present invention which is intended to grow grass 50. The grass field system comprises a water impermeable foil 1, for example a rubber membrane, applied in a recess in the ground to form a water impermeable reservoir 2. The bottom of the water impermeable reservoir 2 is made level horizontally. However, trenches 3 are made in the ground which are also covered with the water impermeable membrane 1. Drains 4 are arranged in the trenches 3 so that the water impermeable reservoir 2 can be drained substantially completely.On top of the water impermeable foil 1 a decking may be arranged on supports which form a suspended floor. The decking can be produced by positioning for example trays upside down onto the water impermeable foil 1. In this way, a water buffer reservoir is formed underneath the suspended floor. This floor has not been shown in the drawings as it is only a possible option to be able to store more water in the water impermeable reservoir 2 itself.In the embodiment illustrated in Figure 1 the bottom of the water impermeable reservoir 2 and the drains 4 arranged therein are covered directly with a layer of a coarse granular material 5, in particular a coarse gravel material, forming a water flow layer 6. Optionally, this water flow layer could be arranged on top of the suspended floor. The upper surface of the water flow layer 6 is made level horizontally. Also the upper surfaces of the layers arranged on top of the water flow layer 6 are made level horizontally so that they all have a substantially uniform thickness.On top of the water flow layer 6 is a layer of a finer granular material 7, in particular a fine gravel material, forming an intermediate layer 8. This intermediate layer 8 is covered by a layer of sand 9 forming a capillary layer 10. The capillary layer 10 is covered by a layer of the sand substrate growing medium 11 forming the root zone layer 12 wherein the grass 50 is rooted. The sand of the root zone layer 12 is finer than the sand of the capillary layer 10.The water flow layer 6 is configured to enable water to flow into and out of the water impermeable reservoir 2. It is made of a coarse gravel material 5, i.e. of a coarse gravel material 5 which has in particular a D15,WFL value which is larger than 10 mm. As a result of such a large D15,WFL value, it has large pores. Moreover, the D90,WFL / D15,WFL ratio is preferably smaller than or equal to 3.0, more preferably smaller than or equal to 2.5 and most preferably smaller than or equal to 2.0. As a result of such a small D90,WFL / D15,WFL ratio the porosity of the water flow layer 6 is high. In accordance with the present invention, the water flow layer 6 should thus have a saturated hydraulic conductivity KWFL of at least 500 cm / h. Moreover, the maximum height of capillary rise hc,WFL should be at least 1.0 cm but preferably less than 5.0 cm, which is due to the large pore size. The average thickness TWFL of the water flow layer 6 should be equal to or larger than 5.0 cm. In this way, water can flow at a relatively high flow rate through the water flow layer 6. The average thickness TWFL of the water flow layer 6 is preferably larger than 8.0 cm and preferably larger than 10.0 cm, and is preferably smaller than 20.0 cm, preferably smaller than 15.0 cm. When the thickness of the water flow layer is smaller, less water needs to be drained to drain it completely.The particle size distribution of a 16 / 22 mm coarse gravel material which can be used for the water flow layer 6 is illustrated in Figure 4. This coarse gravel has a D10, a D15, a D85 and a D90 sieve size value of respectively 15.7, 16.5, 21.7 and 22.5 mm. After 8 hours, the maximum height of capillary rise was substantially achieved and was equal to about 2.0 cm. After 20 hours, the capillary rise was still equal to about 2.0 cm.Under normal conditions water, i.e. the nutrient solution, is supplied by capillary action from the water flow layer 6 upto the root zone layer 12. The water table is then situated at its steady state level in the water flow layer 6, in particular at a distance from the upper surface of the water flow layer 6 which is smaller than the maximum height of capillary rise hc,WFL. Water can thus be supplied by capillary action, including some transport of water in the vapour phase, from the water flow layer 6 into the intermediate layer 8. Due to the large pores in between the particles of the water flow layer 6, there is a reduced capillary contact between the water flow layer 6 and the intermediate layer 8. The rate of capillary rise of water from the water flow layer 6 into the intermediate layer 8 is thus limited so that the supply of water to the upper layers can be easily controlled by adjusting the steady state level of the water table in the water flow layer. The supply of water can thus be adjusted in this way depending on the rate of water loss by evapotranspiration at the upper side of the hydroponic grass field system. If a larger supply of water is required, the steady state level of the water table can be raised to a level above the water flow layer 6, for example to a level in the intermediate layer 8. Keeping the water table sufficiently low enables to feed water by capillary action into the upper layers without having to saturate in particular the root zone layer with water. However, it is also possible to raise the water table to a level in the capillary layer 10. In this way, at least the lower portion of the root zone layer 12 can be saturated with water and the water will rise by capillary action upto the upper surface of the root zone layer 12. Evaporation of water from the upper surface of the root zone layer is thus increased considerably so that the root zone layer and the grass rooted therein will cool down. This is in particular advantageous in case of very hot weather.All the layers situated above the water flow layer 6 should have a maximum height of capillary rise which is larger than the average thickness of the respective layer. The water can preferably rise by capillary action from the water flow layer, at least when this layer is completely saturated with water, into the root zone layer upto a height which is at most 5.0 cm below the upper surface of the root zone layer. This can be achieved by making the different successive layers sufficiently thin, especially the layers which are made of a granular material which has only a limited height of capillary rise, such as in particular the intermediate layer and the capillary layer.The intermediate layer 8 preferably has an average thickness TIL of between 3.0 and 10.0 cm. It is made of a granular material 7 which is finer than the granular material of the water flow layer 6. The granular material 7 may consist for example of a fine gravel. The granular material 7 of the intermediate layer 8 preferably has a maximum height of capillary rise hc,IL of between 4.0 and 12.0 cm, more preferably between 5.0 and 10.0 cm. The maximum height of capillary rise hc,IL should be larger than the average thickness TIL of the intermediate layer 8. The D15 sieve size value of the intermediate layer 8, i.e. of the granular material thereof, is comprised between 1.0 mm and 5.0 mm. Its D85 sieve size value is smaller than 10.0 mm, more preferably smaller than 8.0 mm and most preferably smaller than 6.0 mm.The particle size distribution of a 2 / 5 mm fine gravel material which can be used for the intermediate layer 8 is illustrated in Figure 4. This fine gravel has a D10, a D15, a D85 and a D90 sieve size value of respectively 2.2, 2.5, 4.75 and 5.0 mm. After 8 hours, after 20 hours and after 36 hours, the height of capillary rise was respectively equal to 5.5 cm, 6.0 cm and 7.0 cm. The maximum height of capillary rise can thus be estimated to be somewhat higher, for example about 8 cm.In between the water flow layer 6 and the intermediate layer 8 there is no sheet material, in particular no geotextile layer, which might hamper the capillary rise of the water and the drainage of water out of the intermediate layer 8. To prevent or limit migration from particles of the intermediate layer 8 into the water flow layer 6, the D15 sieve size value D15,WFL of the water flow layer 6 is smaller than 8.0 times, preferably smaller than 7.0 times and more preferably even smaller than 6.0 times the D85 sieve size value D85,IL of the intermediate layer 8. Since the D85 sieve size value D85,IL may be relatively large, the D15 sieve size value of the water flow layer may be quite large so that the water flow layer may have large pores.The intermediate layer 8 provides a bridge between the capillary layer 10 and the water flow layer 6. The presence of the intermediate layer 8 between the capillary layer 10 and the water flow layer 6 enables to use a finer material 9 for the capillary layer 9, more particularly a granular material 9 which has a maximum height of capillary rise hc,CL which is larger than 6.0 cm, and preferably larger than 8.0 cm, but smaller than the maximum height of capillary rise hc,RZL of the root zone layer 12. To prevent or limit migration from particles of the capillary layer 10 into the intermediate layer 8, the D15 sieve size value D15,IL of the intermediate layer 8 should be smaller than 8.0 times, preferably smaller than 7.0 times and more preferably even smaller than 6.0 times the D85 sieve size value D85,CL of the capillary layer 10. Since the intermediate layer 8 has a smaller D15 sieve size value than the water flow layer 6, the D85 sieve size value of the capillary layer 10 may be smaller as a result of the presence of the intermediate layer 8.The capillary layer 10 is situated underneath the root zone layer 12 adjacent, i.e. in contact with, the lower surface of this root zone layer 12. The capillary layer 10 should have a relatively large hydraulic conductivity KCL, namely a KCL value of at least 40 cm / h. In case it is saturated with water, for example upon a heavy rainfall, its water content is quickly reduced again to provide open porosity filled with air in the capillary layer 10. Moreover, it should have an average thickness TCL of between 4.0 cm and 15.0 cm, but smaller than the maximum height of capillary rise hc,CL. It has been found that in this way also the water content of the root zone layer 12 is quickly reduced again when it has been saturated with water for example as a result of a heavy rainfall so that the required amount of open porosity, filled with air, is quickly achieved again to enable the roots to grow. Due to the relatively large maximum height of capillary rise hc,CL of the capillary layer 10, it is not only able to withdraw excess water from the root zone layer 12 but when the root zone layer 12 becomes too dry, the capillary layer 10 can quickly replenish the root zone layer 12 with water, without saturating it completely with water, by raising the water table upto a level in the intermediate layer 8. Usually, such a high level of the water table is sufficient. If not, the water table can be raised upto a level in the capillary layer 10. The rate at which water is supplied by capillary action from the intermediate layer 8 into the capillary layer 10 and further into the root zone layer 12 can then be controlled quite accurately by adjusting the level of the water table in the intermediate layer 8.The granular material 9 of the capillary layer 10 consists preferably of sand. The granular material 11 of the root zone layer 12, i.e. the sand substrate growing medium 11, consists of a finer sand. The maximum height of capillary rise hc,RZL of the root zone layer 12 is thus larger than the maximum height of capillary rise hc,CL of the capillary layer 10, and is in particular larger than or equal to 20.0 cm.According to the equation of Polubarinova-Kochina (1952) the maximum height of capillary rise depends on the D10 sieve size value of the granular material and the porosity thereof. This equation is as follows:Hc = 0.45 ((1-n) / n) / D10n = porosityhc = capillary rise in cmD10 = effective grain diameter in cm.For a same porosity, the maximum height of capillary rise is thus larger the smaller the D10 sieve size value. For a same D10 sieve size value, the maximum height of capillary rise is larger when the porosity is smaller. When compacted to a same extent, the porosity depends on the D90 / D15 ratio. The larger this ratio, the smaller the porosity. Based on these parameters, the particle size distribution of the granular material, i.e. of the sand, of the root zone layer and of the granular material, i.e. of the sand, of the capillary layer can thus be selected such that the root zone layer has a higher maximum height of capillary rise than the capillary layer, whilst the maximum height of capillary rise of the capillary layer is larger than 6.0 cm.The root zone layer 12 has an average thickness TRZL of at least 15.0 cm. Its saturated hydraulic conductivity KRZL is at least 20.0 cm / h. An advantage of this relatively large hydraulic conductivity is that upon rainfall the water drains relatively quickly out of the root zone layer so that the field retains the required mechanical properties, i.e. the required strength or stability which can be measured for example with the Cleg hammer test (in particular in accordance with ASTM F1702-10), to be suited as sport field. Due to the relatively high maximum height of capillary rise, the root zone layer retains enough water to enable a healthy growth of the grass. In dryer regions, the water retention can still be increased by selecting a finer sand for the root zone layer, for example an M32 or even an M34 type sand instead of an M31 type sand.The M31, M32 and M34 sands are quartz sands, in particular silica sands of Mol, Belgium. They are commercially available. A typical particle size distribution of these sands is shown in Figure 5. The D10 sieve size values of these sands are respectively 200, 176 and 125 µm so that the M34 sand has a higher maximum height of capillary rise and also a higher water retention. This sand is thus more suitable for hot, dry regions. The D85 sieve size values of these sands are respectively 440, 320 and 208 µm. The saturated hydraulic conductivity values of these sands are respectively 62 cm / h, 49.8 cm / h and 23.6 cm / h.The D85 sieve size value of these sand should be larger than 1 / 8th, preferably 1 / 7th and more preferably 1 / 6th of the D15 sieve size value D15,CL of the capillary layer 10 to prevent or limit migration of particles from the root zone layer into the capillary layer. Also between these two layers there is indeed no sheet material, in particular no geotextile layer. Such a sheet material would indeed prevent the capillary layer from sucking water out of the lower portion of the root zone layer when this lower portion of the root zone layer would have been saturated with water.The particle size distributions of two capillary sands, namely capillary sand 1 and capillary sand 2, which can be used for the capillary layer 10 are shown in Figure 5. Capillary sand 1 has a D10, a D15, a D85 and a D90 sieve size value of respectively 225, 250, 460 and 478 µm. After 4 hours, after 20 hours and after 36 hours, the height of capillary rise was respectively equal to 10 cm, 11 cm and 12 cm. The maximum height of capillary rise can thus be estimated to be somewhat higher, for example about 14 cm. The saturated hydraulic conductivity value of this sand is equal to 59.6 cm / h.Capillary sand 2 is composed to have a similar content of fine particles but to contain more coarser particles. The porosity of this sand is therefore smaller, and was equal to 36%. This lower porosity provides a higher stability to the sand. It has a D10, a D15, a D85 and a D90 sieve size value of respectively 215, 245, 820 and 880 µm. The maximum height of capillary rise is equal to 22.2 cm. The saturated hydraulic conductivity value of this sand is equal to 79.6 cm / h.The hydroponic grass field system illustrated in Figure 1 comprises four successive layers of different granular materials namely the root zone layer 12, the capillary layer 10, the intermediated layer 8 and the water flow layer 6. It is possible to add one or more further layers, in particular one or more further intermediate layers. The bridging factors, i.e. the D85 / D15 ratios, between the layers should be selected to avoid migration of the finer particles from the finer layers into the adjacent coarser layers.The capillary layer 10 may also consist of one or more sublayers. The capillary layer 10 as a whole, i.e. the combination of the one or more sublayers, should meet the properties as defined for the capillary layer 10, in particular the average thickness TCL, the saturated hydraulic conductivity KCL and the maximum height of capillary rise hc,CL, which should be larger than the average thickness of the capillary layer. In order to measure the maximum height of capillary rise of a capillary layer which consists of two or more sublayers, the lowermost sublayer or sublayers should be applied at the bottom of the transparent tube, just above the water level in the pan, and the material of the uppermost sublayer of the capillary layer should be used to fill the transparent tube further to the top thereof.In case the uppermost sublayer meets as such the properties as defined for the capillary layer, this uppermost sublayer is not a sublayer of the capillary layer but is as such the capillary layer 10 and the lower sublayer is then also no sublayers of the capillary layer but is then a further intermediate layer.In an alternative embodiment, illustrated in Figure 2, the capillary layer 10 is composed of a porous granular material 13, in particular of a granular lava material, instead of a sand material. The lava material consists for example of a 2 to 8 mm lava material. The particle size distribution of this lava material is illustrated in Figure 4. This lava material has a D10, a D15, a D85 and a D90 sieve size value of respectively 0.5, 2.5, 7.6 and 8.0 mm. After 36 hours, the maximum height of capillary rise was substantially achieved and was equal to about 10.5 cm. Since the particles of the porous granular material 13 comprise capillary pores, they contribute as such to the maximum height of capillary rise hc,CL of the capillary layer 10. The particles of the porous granular material 13 may thus be larger since less capillary pores are required between the particles of the capillary layer. The D15 sieve size value D15,CL of this porous granular material may in particular be larger than 1.25 mm, preferably larger than 1.75 mm and more preferably larger than 2.0 mm. Also the D85 sieve size value of the porous granular material 13 may be larger, more particularly to such an extent that it is larger than 1 / 8th, preferably 1 / 7th and more preferably 1 / 6th of the D15 sieve size value D15,WFL of the water flow layer 6. In other words, the D15 sieve size value D15,WFL of the water flow layer 6 is smaller than 8.0 times the D85 sieve size value D85,CL of the capillary layer, preferably smaller than 7.0 times D85,CL and more preferably smaller than 6.0 times D85,CL. In general, the D85,CL sieve size value may be larger than 4.0 mm, preferably larger than 5.0 mm and more preferably larger than 6.0 mm in case the capillary layer is made of a porous granular material. In this way no intermediate layer 8 is required to prevent or limit migration of particles from the capillary layer 10 into the water flow layer 6 although it is possible to provide one or more intermediate layers. The hydroponic grass field system illustrated in Figure 2 however does not comprise an intermediate layer 8. It only consists of the root zone layer 12, the capillary layer 10 and the water flow layer 6. In this way, the total thickness of the different layers can be kept limited whilst each of the layers has a sufficient thickness to enable to apply the layers easily, and within the required thickness tolerances, onto each other.As can be seen in Figure 4, the D15 sieve size value of the lava material is relatively large. The D15 sieve size value of this lava material is more particularly equal to 2.5 mm. Since this lava material has to function as capillary layer 10 which is arranged directly underneath the root zone layer 12, the D85 sieve size value of this root zone layer 12 should at least be larger than 312 µm (=2500 / 8) and preferably larger than 357 µm (=2500 / 7) or even larger than 417 µm (=2500 / 6). The root zone layer 12 could thus be made of M31 sand but the finer M32 and M34 sand appear to be not or less suitable.A mixed sand composition has been made to solve this problem. The particle size distribution of a this mixed sand is illustrated in Figure 5. This mixed sand has a D10, a D15, a D85 and a D90 sieve size value of respectively 120, 135 , 370 and 410 µm. Due to the small D10 sieve size value, it has very small pores and thus a high water retention so that it is particularly suited for dry weather conditions. Due to the small pore size it has a lot of capillary pores which not only retain a lot of water but which also provide for a large maximum height of capillary rise. After 6.5 hours, after 20 hours and after 72 hours, the height of capillary rise was respectively equal to 14 cm, 16.5 cm and 29 cm. The maximum height of capillary rise is thus considerably larger than 29 cm when continuing the test for upto one month.The capillary layer 10 made of the porous granular material 13 has a saturated hydraulic conductivity KCL which is larger than the saturated hydraulic conductivity of the capillary layer which is made of sand. Water thus drains even more quickly out of this capillary layer. Moreover, due to the larger size of the particles of the capillary layer 10 there is a reduced capillary contact between the capillary layer 10 and the root zone layer 12. It has been found that in this way, the moisture content of the root zone layer 12 can be controlled, especially increased, more easily or accurately when raising the water table to a level in the capillary layer 10. The water will be supplied indeed at a lower and thus better controllable rate from the coarser capillary layer 10, made of the porous granular material, into the root zone layer 12.Different porous granular materials can be used. Preference is however given to lava material as the particles of such a lava material have a high strength, in particular a high compressive strength.Apart from the water impermeable reservoir 2 and the stack of layers contained therein, the hydroponic grass field system according to the invention also comprises a water table control system. This water table control system is configured to raise and to lower the water table in the water impermeable reservoir 2 by feeding water into an removing water out of the water impermeable reservoir 2 via the water flow layer 6. Figure 3 shows schematically the layout of a particular embodiment of the hydroponic grass field system according to the invention.This hydroponic grass field system comprises two separate grass field parts 14A and 14B, which are composed in substantially the same way and which contain in particular each a water impermeable reservoirs 2A, 2B filled with a same plurality of successive layers as described with reference to Figure 1 or 2.Different sensors are provided in both grass field parts 14A, 14B. All the sensors are connected to a programmable control unit 15. For clarity’s sake the connections to the programmable control unit 15 have not been shown in Figure 3, also not the connection of other elements which provide signals to the programmable control unit 15 or the operation of which is controlled by the control unit 15.The sensors include two water level sensors 16A, 16B which are each in fluid communication with the water in the water flow layer 6 of one of the water impermeable reservoirs 2A, 2B and which are arranged to measure the level of the water table in the respective water impermeable reservoir 2A, 2B. In the root zone layer 12 a number of temperature sensors 17 and moisture sensors 18 are embedded. The moisture sensors 18 do not measure the level of the water table but they measure the volumetric moisture content at the location of the moisture sensor 18 in the root zone layer 12. At each location, there are preferably at least two moisture sensors 18 arranged at a different level underneath the upper surface of the root zone layer 12. Moisture sensors at different levels allow for example to monitor the capillary rise of the water when the water table has been raised in order to increase the moisture content in the root zone layer. It is for example possible to lower the water table again once the uppermost moisture sensor has detected an increase of the moisture content. In this way, oversaturation of the root zone layer with water can be avoided.The hydroponic grass field system comprises a water buffer reservoir 19 which is connected over a valve 23 to a series of six interconnected tanks 24 in order to increase the amount of water that can be stored. The water buffer reservoir 19 is further connected via a common water feeding pipe 20 to both water impermeable reservoirs 2A, 2B and via separate water return pipes 21A, 21B to each of the water impermeable reservoirs 2A, 2B. The water feeding pipe 20 is provided with a pump 22 to pump water from the water buffer reservoir 19 into the water impermeable reservoirs 2A, 2B. The water can flow by gravity through the water return pipes 21A, 21B from the water impermeable reservoirs 2A, 2B to the water buffer reservoir 19. Each of the water return pipes 21A, 21B is provided with a slide valve 25A, 25B which have a blade that can be moved up to close the valve and down to open the valve. The vertical position of the blades of the valves 25A, 25B can each be controlled in order to control the level of the water table in the respective water impermeable reservoir 2A, 2B.Water is fed into and drained from the water impermeable reservoirs 2A, 2B via the drain pipes 4 provided on the bottoms of these reservoirs. These drain pipes are all arranged in a same substantially horizontal plane. In each of the reservoirs 2A, 2B the drain pipes comprise a set of first drain pipes 4A and a set of second drain pipes 4B. The first drain pipes 4A are interposed between the second drain pipes 4B. In the embodiment illustrated in Figure 3, all the drain pipes are parallel to each other, on a limited mutual distance from one another, for example on a mutual distance of between 1 and 3 m, for example 2 m.The first drain pipes 4A of each water impermeable reservoir 2A, 2B are connected to a collector pipe 26A, 27A which is connected via a valve 28A, 29A to the water feeding pipe 20 and via a valve 30A, 31A to the respective water return pipe 21A, 21B. The second drain pipes 4B of each water impermeable reservoir 2A, 2B are also connected to a collector pipe 26B, 27B which is connected via a valve 28B, 29B to the water feeding pipe 20 and via a valve 30B, 31B to the respective water return pipe 21A, 21B. All of the valves are connected to and controllable by the control unit 15.To fill the water impermeable reservoirs 2A, 2b upto the required level with water, water can be pumped by means of the pump 22 from the water buffer reservoir 19 into the water impermeable reservoirs 2A, 2B. The water level in these reservoirs can be controlled by adjusting the slide valves 25A, 25B. When the level of the water table is too high, the blades of these slide valves can be lowered.The water table control system also comprises a circulation system for circulating water through the water impermeable reservoirs 2A, 2B. This circulation system is comprised or programmed in the control unit 15. With valves 28A and 30B open and valves 28B and 30A closed, water can be circulated through the first water impermeable reservoir 2A from the first drain pipes 4A to the second drain pipes 4B. The same can be done for the second water impermeable reservoir 2B. With valves 29A and 31B open and valves 29B and 31A closed, water can be circulated through the second water impermeable reservoir 2B from the first drain pipes 4A to the second drain pipes 4B. Water can also be circulated in the opposite direction. Through the first water impermeable reservoir 2A with valves 28A and 30B closed and valves 28B and 30A open and through the second water impermeable reservoir 2B with valves 29A and 31B closed and valves 29B and 31A open.Water is preferably circulated alternately in one direction and in the opposite direction a number of times a day, with the water table being at its steady state level, in particular in the water flow layer 6. By alternately circulating the water in two opposite directions, the apertures in the drain pipes 4 are prevented from becoming clogged. Moreover, fine organic particles produced for example by the decay of dead root parts or dead micro-organisms, can be removed out of the water impermeable reservoir 2A, 2B via the circulating water. The circulation system may preferably also comprise an oxygenator 32 for injecting oxygen, an oxygen containing gas or an ozone containing gas in the water circulated through the water impermeable reservoir 2A, 2B. This oxygenator 32 may inject the gas for example in the water in the water buffer reservoir 19. However, as illustrated in Figure 3, the oxygenator 32 preferably injects the gas in the water feeding pipe 20. In this way, no gas will escape and all of the gas will arrive in the water flow layers 6 at the bottom of the grass field system. The oxygenator preferably generated nanobubbles which are more stable in the water and which are thus released more slowly, i.e. upto a larger distance from the drain pipes, into the grass field system. The oxygen contained in the water will oxygenate or aerate the water flow layer and, when released from the water, it will rise in the different layers which are situated on top of the water flow layer and it will thus also oxygenate or aerate these layer. Undesired anaerobic decomposition processes can thus be avoided. Moreover, any dead organic matter will be decomposed more quickly so that accumulation of dead organic matter, in particular in the root zone, can be avoided. The hydroponic grass field system will thus remain for a longer period of time in optimal condition.The water table control system moreover comprises an ebb and flow system which is also comprised or programmed in the control unit 15. The ebb and flow system is configured to control capillary feeding of water into the root zone layer 12 by temporarily raising the water table to a flooding level which is situated above the water flow layer 6 but below the lower surface of the root zone layer 12. The flooding level can either be situated in the capillary layer 10 or in the intermediate layer 8, if any. After a predetermined period of time, or after the moisture sensors 18 have detected a moisture level which is higher than a threshold level, the water table can be lowered again by the ebb and flow system to the steady state level or to a lower level. The ebb and flow system is indeed preferably also configured to temporarily lower the water table to a draining level wherein the water is drained out of the water flow layer 6. Since this is done by means of the drain pipes 4 which are recessed in the bottom of the water impermeable reservoir 2, the water impermeable reservoir can be completely drained. In this completely drained state, most of the pores in the water flow layer 6 and in any intermediate layer 8 will be completely open and air will have been sucked by the descending water table into the different layers so that all of the layers will be aerated. In this way, anaerobic decomposition processes will also be avoided. During the growing season, raising the water table upto the flooding level is done in general preferably with intervals of at least one day, preferably at least two days and more preferably at least three days. Emptying the water reservoir is also done in general preferably with intervals of at least one day, preferably at least two days and more preferably at least three days. During the winter period, the water reservoir may stay empty for several weeks or months.In order to reach the flooding level, a lot of water is needed. An advantage of the two (or more) grass field parts 14A, 14B, and in particular of the two water impermeable reservoirs 2A, 2B, is that water of one water impermeable reservoir 2A can be used to raise the water table upto the flooding level in the other water impermeable reservoir 2B. When pumping water from the water buffer reservoir 19 in the second water impermeable reservoir 2B upto the flooding level, water can simultaneously be drained from the first water impermeable reservoir 2A into the water buffer reservoir 19 and vice versa, when pumping water from the water buffer reservoir 19 in the first water impermeable reservoir 2A upto flooding level, water can simultaneously be drained from the second water impermeable reservoir 2B into the water buffer reservoir 19. When one of the water impermeable reservoirs 2A, 2B is filled upto the flooding level, the other water impermeable reservoir 2B, 2A can be drained completely. In this way, less water storage capacity is needed.In another embodiment not illustrated in the drawings water both water impermeable reservoirs 2A, 2B could be connected via piping to one another and water from one water impermeable reservoir 2A, 2B could be pumped directly via this piping into the other water impermeable reservoir 2B, 2A. Raising the water table in the water impermeable reservoirs 2A, 2B only with water from the water buffer reservoir 19 is however to be preferred. Any organic matter that has been sedimented in one of the water impermeable reservoirs 2A, 2B and removed therefrom via the drain pipes 4 will indeed also be able to settle at least partially in the water storage tank 19 and / or in any of the tanks 24 connected thereto. The organic matter which has been sedimented onto the bottom of the water storage tank 19 or of the tanks 24 can be removed therefrom quite easily for example by means of a suction device, similar to a pond cleaner. This is however not possible in the water impermeable reservoirs 2A, 2B so that organic material which has been sedimented in one of these reservoirs 2A, 2B is preferably not pumped directly in the other water impermeable reservoir 2B, 2A.The hydroponic grass field system illustrated in Figure 3 also includes an apparatus 33 for dosing concentrated solutions of plant nutrition elements from two tanks 34A and 34B into the water buffer reservoir 19. The concentrated solutions are so-called A and B solutions and are well known in hydroponic cultivation. The plant nutrition elements are divided over these two tanks in order to avoid precipitation reactions in the concentrated solutions. To determine whether additional nutrients have to be dosed in the water buffer reservoir 19, the EC value (Electrical Conductivity) of the nutrient solution is measured in the tank 19 is measured by means of an EC meter 35. When the EC value is below a threshold value due to the uptake of nutrient elements by the grass in the grass field, an amount of the concentrated A and B solutions is dosed by means of the dosing apparatus 33 in the water buffer reservoir 19. Also the pH value of the nutrient solution is measured in the water buffer reservoir 19 by means of a pH meter. Via an acid dosing apparatus 37 an acid solution, for example a nitric acid solution, is dosed from an acid tank 38 into the water buffer reservoir 19. Dosing of the concentrated nutrient solutions A and B and of the acid solution is controlled by the control unit 15, which is connected to the EC and pH meters 35, 36 and to the dosing apparatuses 33, 37.The water buffer reservoir 19 is preferably also provided with a heating device 39. This may be an electric heating device arranged in the water buffer reservoir 19 and comprising an electric heating resistor. The electric heating device comprises however preferably a heat pump. When heating the water in the water buffer reservoir 19, the valve 23 is preferably closed so that the tanks 24 are disconnected. Moreover, the water table in the water impermeable reservoirs 2A, 2B is preferably first adjusted to a minimum level so that only a minimum amount of water has to be heated in these reservoirs 2A, 2B. The water heated in the water buffer reservoir 19 is then circulated by means of the pump 22 over the water impermeable reservoirs 2A, 2B. Preferably these reservoirs are insulated by installing a thermal insulation layer, for example a foam layer, underneath the water impermeable foil 1. Also the water buffer reservoir 19 is preferably thermally insulated.The hydroponic grass field system illustrated in Figure 3 also comprises sprinklers 40 recessed in the upper surface of the root zone layer 12. These sprinklers 40 are connected via pipes, not shown in the drawings, to a pipe 41 which is provided with a shutoff valve 42 and which is connected to the water feeding pipe 20 downstream the pump 22. The pump 22 is preferably a variable frequency driven pump so that the water pressure generated by the pump can be increased when the water is to be sprayed by means of the sprinklers 40. Such a pump 22 also enables to reduce the flowrate through the water feeding pipe 20 when a predetermined level of the water table is nearly obtained in the water impermeable reservoir 2A, 2B. ExampleThe hydroponic grass field system according to this example comprises four layers of different granular materials as illustrated in Figure 1. These layers consist of the root zone layer 12, having a thickness of 20 cm, the capillary layer 10, having a thickness of 7 cm, the intermediate layer 8, having a thickness of 5 cm and the water flow layer 6, having a thickness of 10 to 11 cm. The different layers consist of granular materials discussed hereabove with reference to Figures 4 and 5. The root zone layer 12 consists of the “mixed sand”, the capillary layer 10 consists of the “capillary sand 1”, the intermediate layer 8 of the “fine gravel” and the water flow layer 6 of the “coarse gravel”. The roots of the grass growing on these layers extended in the entire root zone layer 12 and also to some extent in the capillary layer 10.Four moisture sensors were arranged in the root zone layer 12 at depths of respectively 5, 10, 15 and 20 cm. In the steady state, the level of the water table was maintained at such a level that at a depth of 10, 15 and 20 cm the target moisture content was about 20 vol.%. In this way, the open porosity was high, in particular about 15 to 20 vol.%. The root zone layer was thus well aerated so that the roots could grow through the entire root zone layer even down to the capillary layer. However, the roots were prevented from growing into the capillary layer by keeping the root zone layer sufficiently humid. In general, growth of roots in the capillary layer is preferably to be prevented to avoid changing the capillary properties of the capillary layer.Figure 6 shows the precipitation on a rainy day on the hydroponic grass field system as well as the level of the water table in the water flow layer 6. At that day, the steady state level of the water table was 9.0 cm above the water impermeable foil 1. The upper threshold value was 9.5 cm and the lower threshold value was 8.5 cm. When the water table reached 9.5 cm, it was lowered to 9.0 cm. When it dropped to 8.5 cm, which it didn’t during that rainy day, it was raised to 9.0 cm. It can be seen that during that day the water table rose slowly, and upon heavier rainfall, somewhat more quickly. This shows that the water did not accumulate in the different layers, especially not in the root zone layer, and that the water was thus quickly drained from the upper layers of the grass field system into the water flow layer 6. The total amount of precipitation was equal to 23.6 mm. The accumulated distances over which the water table was lowered that day by draining water was substantially larger, namely about twice as large. This is due to the fact that the water flow layer has only a porosity which can be estimated at about 45 vol.% so that less water is needed in this water flow layer to raise the water table. A further test was done during about one month, from July 26, 2023 to August 30, 2023. During that month, the maximum (day) temperatures fluctuated between 17 and 28°C and the minimum (night) temperatures between 10 and 18°C. The amount of rainfall is shown in Figure 7. In the following table, the moisture contents measured weakly at a depth of 5, 10, 15 and 20 cm are indicated. Table 2: Moisture contents in vol.% at different depths in the root zone layerDate-5 cm-10 cm-15 cm-20 cm26 July17.920.624.623.12 August12.719.224.821.39 August11.618.523.622.216 August14.918.323.221.223 August18.018.324.519.730 August15.315.817.519.6Average15.118.523.021.2 The measurements of the moisture sensors showed that, in the steady state, the moisture contents in the lower portion of the root zone layer 12 can be kept relatively constant at a level of around 20 vol.%, notwithstanding some rainy days. At the lower surface of the root zone layer 12, namely at a depth of 20 cm, the average moisture content was somewhat lower than the average moisture content at a depth of 15 cm. Apparently, the capillary layer 10 exerts a suction on the lower portion of the root zone layer 12 so that in this lower portion the moisture content doesn’t increase exponentially but instead decreases towards its lower surface. The roots can thus also develop well in the lower portion of the root zone layer 12. Due to the limited maximum height of capillary rise of the intermediate layer 8, this intermediate layer 8 also exerts a suction on the lower portion of the capillary layer 10 so that also the bottom portion of the capillary layer is not saturated with water when the water table is at its steady state level in the water flow layer 6.

Claims

1. A hydroponic grass field system having a sand substrate growing medium (11), which hydroponic grass field system comprises at least one water impermeable reservoir (2) which contains water forming a water table and, arranged in said at least one water impermeable reservoir (2):a plurality of successive layers of different granular materials arranged on top of each other, including:a root zone layer (12) wherein grass (50) is rooted, which root zone layer (12) is composed of said sand substrate growing medium (11) and having an upper and a lower surface, anda water flow layer (6) arranged underneath the root zone layer (12) and configured to enable water to flow into and out of said water impermeable reservoir (2); anda water table control system configured to raise and to lower said water table by feeding water into and removing water out of said water impermeable reservoir (2) via said water flow layer (6),characterised in thatthe granular materials of said successive layers each have a D15 sieve size value and a D85 sieve size value, the D15 sieve size value of each of the layers which are situated underneath another one of said successive layers being smaller than 8.0 times the D85 sieve size value of the layer which is situated on top of the respective layer;the granular materials of said successive layers each have a maximum height of capillary rise and the successive layers an average thickness, the average thickness of each of said successive layers, different from said water flow layer (6), being smaller than the maximum height of capillary rise of the granular material of the respective layer;said root zone layer (12) has an average thickness TRZL of at least 15.0 cm, a saturated hydraulic conductivity KRZL, measured in accordance with ASTM F1815-97, of at least 20.0 cm / h and the granular material (11) of the root zone layer (12) has a maximum height of capillary rise hc,RZL of at least 20.0 cm and a D15 sieve size value D15,RZL, a D85 sieve size value D85,RZL and a D90 sieve size value D90,RZL;said plurality of successive layers includes a capillary layer (10) which is situated underneath the root zone layer (12) adjacent the lower surface thereof and which is configured to feed water by capillary rise of said water from the capillary layer (10) into the root zone layer (12), which capillary layer (10) has an average thickness TCL of between 4.0 cm and 15.0 cm and a saturated hydraulic conductivity KCL, measured in accordance with ASTM F1815-97, of at least 40 cm / h and the granular material of the capillary layer (10) has a maximum height of capillary rise hc,CL which is larger than 6.0 cm but smaller than hc,RZL and a D15 sieve size value D15,CL, a D85 sieve size value D85,CL and a D90 sieve size value D90,CL; andsaid water flow layer (6) has an average thickness TWFL of at least 5.0 cm and a saturated hydraulic conductivity KWFL, measured in accordance with ASTM F1815-97, of at least 500 cm / h and the granular material of the water flow layer (6) has a maximum height of capillary rise hc,WFL of at least 1.0 cm and a D15 sieve size value D15,WFL, a D85 sieve size value D85,WFL and a D90 sieve size value D90,WFL.

2. The hydroponic grass field system according to claim 1, wherein said plurality of successive layers include a number of successive layers which are arranged on top of the water flow layer (6) and each of which has an average thickness selected to allow, in case the water flow layer (6) is completely saturated with water, rise of water by capillary action from the water flow layer (6) upto a height which is at most 5.0 cm, preferably at most 3.0 cm, underneath the upper surface of the root zone layer (12).

3. The hydroponic grass field system according to claim 1 or 2, wherein D15,RZL is smaller than 230 µm, preferably smaller than 210 µm and more preferably smaller than 180 µm, with D15,RZL being preferably larger than 100 µm.

4. The hydroponic grass field system according to any one of the claims 1 to 3, wherein D15,WFL is larger than 10 mm and the ratio D90,WFL / D15,WFL is smaller than or equal to 3.0, preferably smaller than or equal to 2.5 and more preferably smaller than or equal to 2.0.

5. The hydroponic grass field system according to claim 4, wherein D15,WFL is smaller than 8.0 times D85,CL, preferably smaller than 7.0 times D85,CL and more preferably smaller than 6.0 times D85,CL.

6. The hydroponic grass field system according to claim 5, wherein said plurality of successive layers consist of the root zone layer (12), the capillary layer (10) and the water flow layer (6), the capillary layer (10) being arranged directly on top of the water flow layer (6) and the root zone layer (12) being arranged directly on top of the capillary layer (10).

7. The hydroponic grass field system according to any one of the claims 1 to 6, wherein the granular material of said capillary layer (10) comprises a porous granular material (13), preferably a granular lava material, with D15,CL being larger than 1.25 mm, preferably larger than 1.75 mm and more preferably larger than 2.0 mm.

8. The hydroponic grass field system according to any one of the claims 1 to 7, wherein said plurality of successive layers include an intermediate layer (8) arranged between said capillary layer (10) and said water flow layer (6), which intermediate layer (8) has a D15 sieve size value D15,IL, a D85 sieve size value D85,IL and a D90 sieve size value D90,IL, with D15,IL being smaller than 8.0 times D85,CL, preferably smaller than 7.0 times D85,CL and more preferably even smaller than 6.0 times D85,CL, and with D15,WFL being smaller than 8.0 times D85,IL, preferably smaller than 7.0 times D85,IL and more preferably even smaller than 6.0 times D85,IL.

9. The hydroponic grass field system according to claim 8, wherein the intermediate layer (8) has an average thickness TIL of between 3.0 and 10.0 cm.

10. The hydroponic grass field system according to claim 8 or 9, wherein the granular material (7) of the intermediate layer (8) has a maximum height of capillary rise hc,IL of between 4.0 and 12.0 cm, preferably between 5.0 and 10.0 cm.

11. The hydroponic grass field system according to any one of the claims 8 to 10, wherein D15,IL is larger than 1.0 mm, preferably larger than 1.5 mm and more preferably larger than 2.0 mm, but smaller than 5.0 mm, preferably smaller than 4.0 mm and more preferably smaller than 3.0 mm, with D85,IL being preferably smaller than 10.0 mm, more preferably smaller than 8.0 mm and most preferably smaller than 6.0 mm.

12. The hydroponic grass field system according to any one of the claims 8 to 11, wherein the D90,IL / D15,IL ratio is larger than 1.4, preferably larger than 1.5 and smaller than or equal to 3.0, preferably smaller than or equal to 2.5.

13. The hydroponic grass field system according to any one of the claims 1 to 12, wherein the D90,CL / D15,CL ratio is larger than 1.5, preferably larger than 2.0 and more preferably larger than 2.5, but smaller than or equal to 5.5, preferably smaller than or equal to 4.5.

14. The hydroponic grass field system according to any one of the claims 1 to 13, wherein the D90,RZL / D15,RZL ratio is larger than 1.5 and preferably larger than 2.0, but smaller than or equal to 5.5, preferably smaller than or equal to 4.5.

15. The hydroponic grass field system according to any one of the claims 1 to 14, wherein hc,CL is larger than 8.0 cm, preferably larger than 10.0 cm.

16. The hydroponic grass field system according to any one of the claims 1 to 15, wherein TRZL is smaller than 30.0 cm and preferably smaller than 25.0 cm, with TRZL being preferably larger than 18.0 cm, more preferably larger than 20.0 cm.

17. The hydroponic grass field system according to any one of the claims 1 to 16, which comprises one or more moisture sensors (18) which are arranged in the root zone layer (12) to measure a moisture content of the root zone layer.

18. The hydroponic grass field system according to claim 17, wherein said moisture sensors (18) comprise a first moisture sensors which is arranged at a first depth in the root zone layer (12) and a second moisture sensor which is arranged at a second depth in the root zone layer (12) which is larger than said first depth.

19. The hydroponic grass field system according to any one of the claims 1 to 18, wherein said water table control system is configured to control capillary feeding of water into the root zone layer (12) by adjusting the water table in the water flow layer (6) to a steady state level wherein at least part of the water loss by evapotranspiration is compensated for by the capillary feeding of water into the root zone layer (12).

20. The hydroponic grass field system according to any one of the claims 1 to 19, wherein said water table control system comprises an ebb and flow system which is configured to control capillary feeding of water into the root zone layer by temporarily raising the water table to a flooding level which is situated above the water flow layer (6) but below the lower surface of the root zone layer (12).

21. The hydroponic grass field system according to claim 20, wherein said ebb and flow system is configured to temporarily lower the water table to a draining level wherein the water is drained out of the water flow layer (6), the ebb and flow system preferably comprising drains (4) which are recessed in the bottom of the water impermeable reservoir (2).

22. The hydroponic grass field system according to any one of the claims 1 to 21, wherein said water table control system comprises at least one water buffer reservoir (19) connected to said water impermeable reservoir (2) and at least one pump (22) for pumping water from said water buffer reservoir (19) into said water impermeable reservoir (2) and / or vice versa.

23. The hydroponic grass field system according to claim 22, which comprises at least two water impermeable reservoirs (2A, 2B) wherein said plurality of layers are arranged, said two water impermeable reservoirs (2A, 2B) being connected to each other and / or to said water buffer reservoir (19).

24. The hydroponic grass field system according to any one of the claims 1 to 23, which comprises a circulation system for circulating water via said water flow layer (6) in at least one direction through said water impermeable reservoir (2), the circulation system being preferably configured to circulate the water in said one direction and subsequently in the opposite direction through the water impermeable reservoir (2).

25. The hydroponic grass field system according to claim 24, wherein said circulation system preferably comprises at least one set of first drain pipes (4A) arranged in a substantially horizontal plane in the water impermeable reservoir (2) and at least one set of second drain pipes (4B) arranged in the same substantially horizontal plane in the water impermeable reservoir (2), the second drain pipes (4B) being interposed between the first drain pipes (4A), when circulating water through the water impermeable reservoir (2), the second drain pipes (4B) being configured to drain water from the water impermeable reservoir (2) when water is fed into the water impermeable reservoir via the first drain pipes (4A) and the first drain pipes (4A) being configured to drain water from the water impermeable reservoir (2) when water is fed into the water impermeable reservoir (2) via the second drain pipes (4B).

26. The hydroponic grass field system according to claim 24 or 25, wherein said circulation system comprises an oxygenator (32) for injecting oxygen, an oxygen containing gas or an ozone containing gas in the water circulated through the water impermeable reservoir (2).

27. The hydroponic grass field system according to any one of the claims 1 to 26, wherein said water is a hydroponic plant nutrient solution which comprises preferably nitrogen, potassium, phosphorous and micro-nutrients including manganese, zinc and boron and preferably also copper and molybdenum, which hydroponic grass field system preferably comprises a dosing apparatus (33) for dosing these plant nutrients in said water.

28. The hydroponic grass field system according to any one of the claims 1 to 27, which is free of any sheet materials between said plurality of successive layers.

Images