A system for converting unavailable phosphate into available phosphate in soil and its utilization

Abstract

To provide a technique that can efficiently use phosphorus resources as fertilizer.SOLUTION: A system for converting unavailable phosphate in the soil to available phosphate includes a controller, a positive electrode electrically connected to the controller, and a negative electrode that is electrically connected to the controller and brought into contact with the soil. The controller controls the electrode potential of the negative electrode to be -4.5 V or more and -3.0 V or less through the passing of an electrical current.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a system for converting unavailable phosphate in soil to available phosphate. [Background technology]

[0002] Phosphorus has long been known as one of the essential elements for plant growth. The majority of phosphorus in soil is in the form of unavailable phosphate, which cannot be directly utilized by plants. On the other hand, available phosphate, which can be directly utilized by plants, is scarce. Therefore, available phosphate, which is prone to shortage in agricultural soil, is often replenished as fertilizer. However, in Japan, phosphate rock, a raw material for fertilizer, is not produced and the country relies on imports, so there is a need for the reuse of phosphorus resources. For example, Patent Document 1 discloses a technology for recovering phosphorus in the form of magnesium ammonium phosphate by passing an electric current through organic wastewater containing phosphate ions. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-011375 Summary of the Invention [Problem to be solved by the invention]

[0004] However, in the technology described in Patent Document 1, when the recovered phosphorus is used as fertilizer, it is necessary to redissolve the phosphorus to make it in a form suitable for fertilizer, so there is room for improvement in terms of efficient use of phosphorus resources. Therefore, there has been a demand for a technology that allows phosphorus resources to be used efficiently as fertilizer. [Means for solving the problem]

[0005] The present invention can be realized as the following aspects.

[0006] (1) One aspect of the present invention provides a system for converting unavailable phosphate in soil to available phosphate. The system includes a control unit, a positive electrode electrically connected to the control unit, and a negative electrode electrically connected to the control unit and brought into contact with the soil, and the control unit controls the electrode potential of the negative electrode to between −4.5 V and −3.0 V by passing a current through the positive electrode. The system of this aspect can convert unavailable phosphate in the soil around the negative electrode to available phosphate by controlling the electrode potential of the negative electrode to between −4.5 V and −3.0 V.

[0007] (2) The system according to (1) above may further include a reference electrode electrically connected to the control unit, and the control unit may control the electrode potential of the negative electrode relative to the electrode potential of the reference electrode. This system allows the electrode potential of the negative electrode to be adjusted. As a result, non-available phosphate in the soil around the negative electrode can be stably converted into available phosphate.

[0008] (3) In the system described in (1) or (2), the control unit may include a potentiostat. In this system, the potentiostat can easily maintain a constant electrode potential, thereby maintaining a constant electrode potential at the negative electrode. As a result, non-available phosphate in the soil surrounding the negative electrode can be stably converted into available phosphate.

[0009] (4) In the system according to any one of (1) to (3), the negative electrode may be made of carbon fiber. This system allows the specific surface area of ​​the negative electrode to be easily increased, thereby efficiently converting non-available phosphate in the soil surrounding the negative electrode into available phosphate.

[0010] (5) In the system according to any one of (1) to (4), the control unit controls the negative electrode to have an electrode area of ​​1 m 2A current of 600 mA or more and 1000 mA or less may be passed per unit area. According to this type of system, the electrode potential of the negative electrode can be maintained at −4.5 V or more and −3.0 V or less, so that unavailable phosphate in the soil around the negative electrode can be converted into available phosphate.

[0011] (6) In the system according to any one of (1) to (5), the control unit may control the electrode potential of the negative electrode to be in the range of −3.5 V to −3.1 V. This system can efficiently convert unavailable phosphate in the soil around the negative electrode into available phosphate.

[0012] (7) According to another aspect of the present invention, there is provided a method for converting unavailable phosphate in soil to available phosphate. This method includes a step of controlling the electrode potential of a negative electrode in a range of −4.5 V to −3.0 V while the negative electrode is in contact with the soil. According to this aspect of the method, by including the step of controlling the electrode potential of the negative electrode in a range of −4.5 V to −3.0 V, unavailable phosphate in the soil around the negative electrode can be converted to available phosphate.

[0013] (8) Another aspect of the present invention provides a method for producing available phosphoric acid from unavailable phosphoric acid in soil. This method includes a step of controlling the electrode potential of a negative electrode in a range of −4.5 V to −3.0 V while the negative electrode is in contact with the soil. This aspect of the production method includes a step of controlling the electrode potential of the negative electrode in a range of −4.5 V to −3.0 V, thereby converting unavailable phosphoric acid in the soil around the negative electrode into available phosphoric acid, thereby producing available phosphoric acid.

[0014] The present invention can be realized in various forms, such as an apparatus for converting non-available phosphate in soil into available phosphate, a method for producing soil containing available phosphate, etc. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is an explanatory diagram illustrating a schematic configuration of a system. [Figure 2] 1 is a process diagram showing the steps of a method for converting unavailable phosphate into available phosphate. [Figure 3] FIG. 1 is an explanatory diagram showing a schematic configuration of a system according to an embodiment. [Figure 4] FIG. 4 is an explanatory diagram showing the relationship between current density and electrode potential. [Figure 5] FIG. 1 is an explanatory diagram showing changes in the redox state of a soil layer. [Figure 6] FIG. 1 is an explanatory diagram showing the relationship between current density and phosphate concentration in pore water. [Figure 7] FIG. 1 is an explanatory diagram showing the relationship between electrode potential and phosphate concentration in pore water. [Figure 8] FIG. 1 is an explanatory diagram showing the relationship between current density and ammonium concentration. DETAILED DESCRIPTION OF THE INVENTION

[0016] A. Embodiment FIG. 1 is an explanatory diagram showing a schematic configuration of a system 10 according to one embodiment of the present invention. The system 10 of this embodiment converts non-available phosphate in soil into available phosphate by performing electronic control, which will be described later. In other words, the system 10 of this embodiment produces available phosphate from non-available phosphate in soil by performing electronic control, which will be described later. The soil is not particularly limited, but agricultural soil, for example, may be a suitable target.

[0017] As used herein, "available phosphate" refers to phosphate present in soil in a form that can be absorbed by plants. Available phosphate is essential for plant growth, and a lack of available phosphate affects nucleic acid synthesis and protein synthesis, resulting in growth inhibition and delayed flowering and maturation. Furthermore, as used herein, "unavailable phosphate" refers to phosphate present in soil in a form that cannot be absorbed by plants, and corresponds to phosphate bound to metal ions.

[0018] A system 10 (hereinafter simply referred to as "system 10") for converting unavailable phosphate in soil to available phosphate includes a positive electrode 20, a negative electrode 30, a reference electrode 40, and a control unit 50. The positive electrode 20, the negative electrode 30, and the reference electrode 40 are each electrically connected to the control unit 50. For this connection, a conductor made of, for example, titanium, copper, aluminum, or an alloy thereof may be used.

[0019] The positive electrode 20 corresponds to an electrode paired with the negative electrode 30. The negative electrode 30 corresponds to an electrode paired with the positive electrode 20. The negative electrode 30 is placed in contact with the soil. The reference electrode 40 corresponds to an electrode that provides a reference point for the potential. In this embodiment, the positive electrode 20 and the reference electrode 40 are placed in contact with the soil, similar to the negative electrode 30, but they may also be placed in any other location, such as in water, that is electronically connected to the soil.

[0020] In this embodiment, both the positive electrode 20 and the negative electrode 30 are made of a carbon material. More specifically, they are formed in a brush-like shape using carbon fibers. The negative electrode 30 is made of carbon fibers, which increases its specific surface area, allowing for efficient conversion of unavailable phosphate in the soil surrounding the negative electrode 30 to available phosphate. Carbon fiber electrodes may be produced, for example, by burning carbon cloth of a desired area at a temperature of approximately 500°C for approximately two hours. The reference electrode 40 may be, for example, a commercially available Ag / AgCl reference electrode.

[0021] In this embodiment, the positive electrode 20 and the negative electrode 30 are both formed of the same material, but they may be formed of different materials. Furthermore, the positive electrode 20 and the negative electrode 30 are not limited to carbon fiber, but may be formed of any carbon material, such as carbon cloth. Furthermore, the positive electrode 20 and the negative electrode 30 are not limited to carbon material, but may be formed of any conductive material, such as metal. From the viewpoint of excellent durability and conductivity, the positive electrode 20 and the negative electrode 30 are preferably formed of a carbon material, such as carbon fiber or carbon cloth. Carbon materials are relatively inexpensive, so they are also desirable from the viewpoint of suppressing increases in the cost required for electrode fabrication. Furthermore, in this embodiment, the positive electrode 20 and the negative electrode 30 have the same electrode area, but they may have different electrode areas.

[0022] The control unit 50 has a function of controlling the electrode potential of the negative electrode 30 by passing a current through it. More specifically, the control unit 50 controls the electrode potential of the negative electrode 30 to be between −4.5 V and −3.0 V by passing electrons, i.e., a current, through the soil. By performing such electronic control, a reduction reaction occurs at the negative electrode 30, and phosphoric acid bound to metal ions (hereinafter also referred to as a “phosphate complex”) present around the negative electrode 30 in the soil can be dissociated, thereby converting non-available phosphoric acid in the soil into available phosphoric acid.

[0023] The control unit 50 of this embodiment controls the electrode potential of the negative electrode 30 relative to the electrode potential of the reference electrode 40. By controlling in this manner, the electrode potential of the negative electrode 30 can be kept constant. As a result, non-available phosphate in the soil surrounding the negative electrode 30 can be stably converted to available phosphate. In other words, the system 10 of this embodiment can adjust the electrode potential of the negative electrode 30 more easily than a system that does not include a reference electrode 40 and controls the electrode potential of the negative electrode 30 relative to the electrode potential of the positive electrode 20.

[0024] The control unit 50 of this embodiment includes a potentiostat. By including a potentiostat, the electrode potential of the negative electrode 30 can be easily maintained constant. As a result, non-available phosphate in the soil surrounding the negative electrode 30 can be converted to available phosphate more stably. Note that the control unit 50 is not limited to a potentiostat, and may include any device capable of controlling the electrode potential of the negative electrode 30. Furthermore, the control unit 50 may be configured to include a battery, such as a solar cell.

[0025] As described above, the control unit 50 controls the electrode potential of the negative electrode 30 to between −4.5 V and −3.0 V. From the viewpoint of promoting dissociation of phosphate complexes in the soil and efficiently converting unavailable phosphate to available phosphate, the control unit 50 preferably controls the electrode potential of the negative electrode 30 to between −3.1 V and −3.1 V. Furthermore, from the viewpoint of efficiently converting unavailable phosphate to available phosphate without interfering with the dissociation of phosphate complexes in the soil, the control unit 50 preferably controls the electrode potential of the negative electrode 30 to between −4.0 V and −3.5 V, more preferably −3.5 V or higher. For example, by controlling the electrode potential of the negative electrode 30 to between −3.5 V and −3.1 V, the control unit 50 can efficiently convert unavailable phosphate in the soil surrounding the negative electrode 30 to available phosphate.

[0026] As will be shown in the examples below, a correlation is observed between the magnitude of the current applied by the control unit 50 and the electrode potential of the negative electrode 30. The magnitude of the current applied by the control unit 50 is determined based on the electrode area of ​​the negative electrode 30 per 1 m 2 In the following explanation, this is referred to as the electrode density (mA / m 2 The larger the current supplied by the control unit 50, that is, the higher the current density, the more the electrode potential of the negative electrode 30 can be reduced.

[0027] The magnitude of the current density in the electronic control by the control unit 50 is not particularly limited as long as it can control the electrode potential of the negative electrode 30 to be −4.5 V or more and −3.0 V or less. The magnitude of the current density is determined based on the electrode area of ​​the negative electrode 30 per m2 from the viewpoint of promoting dissociation of phosphate complexes in the soil and efficiently converting unavailable phosphate into available phosphate. 2 The current density is preferably 300 mA or more, more preferably 400 mA or more, even more preferably 600 mA or more, and particularly preferably 700 mA or more per m2 of the electrode area of ​​the negative electrode 30. By setting the current density at or above the lower limit, it is possible to prevent the time required to reach a potential at which the phosphate complex can dissociate from becoming excessively long. Furthermore, the magnitude of the current density is set at 1 / m2 per electrode area of ​​the negative electrode 30 from the viewpoint of efficiently converting non-available phosphate into available phosphate without interfering with the dissociation of the phosphate complex in the soil. 2 The control unit 50 controls the current flow rate of the negative electrode 30 to be, for example, 1200 mA or less, more preferably 1000 mA or less, further preferably 900 mA or less, and particularly preferably 800 mA per 1 m of electrode area of ​​the negative electrode 30. 2 By passing a current of 400 mA or more and 1000 mA or less, preferably 600 mA or more and 1000 mA or less, per negative electrode 30, unavailable phosphate in the soil surrounding the negative electrode 30 can be efficiently converted into available phosphate.

[0028] As will be described in the examples below, electronic control by the control unit 50 increases both the phosphate concentration and the ammonium concentration in the soil surrounding the negative electrode 30. From the viewpoint of converting non-available phosphate in the soil to available phosphate while promoting the production of ammonium in the soil, the control unit 50 preferably controls the electrode potential of the negative electrode 30 to between −3.5 V and −3.0 V, more preferably between −3.2 V and −3.0 V, and even more preferably between −3.1 V and −3.0 V. By controlling the electrode potential of the negative electrode 30 to a value equal to or higher than the lower limit, the progress of denitrification around the negative electrode 30 can be suppressed, thereby suppressing a decrease in the amount of ammonium produced in the soil.

[0029] As will be described in the examples below, the control unit 50 performs electronic control, which causes the ORP to decrease and the pH to increase in the soil surrounding the negative electrode 30. This is because hydrogen ions are consumed by a reduction reaction in the soil. Therefore, the electronic control described above can prevent the soil from becoming excessively oxidized, and as a result, the acidification of the soil can be suppressed.

[0030] The period during which the control unit 50 performs electronic control is not particularly limited, but is preferably one day or more, more preferably three days or more, and even more preferably one week or more, from the viewpoint of stabilizing the electrode potential of the negative electrode 30. During the period during which the control unit 50 performs electronic control, the electrode potential of the negative electrode 30 may be fixed at an arbitrary value of −4.5 V or more and −3.0 V or less, or may fluctuate within the range of −4.5 V or more and −3.0 V or less.

[0031] FIG. 2 is a process diagram showing the steps of a method for converting unavailable phosphate in soil to available phosphate (hereinafter also simply referred to as the "method for converting available phosphate"). The method for converting available phosphate is achieved by the same steps as the method for producing available phosphate from unavailable phosphate in soil (hereinafter also simply referred to as the "method for producing available phosphate"). In other words, FIG. 2 can also be said to be a process diagram showing the steps of a method for producing available phosphate from unavailable phosphate in soil. The method for converting available phosphate and the method for producing available phosphate are achieved by the above-mentioned system 10.

[0032] The method for converting available phosphate includes a step (step P110) of controlling the electrode potential of the negative electrode 30 to between −4.5 V and −3.0 V while the negative electrode 30 is in contact with the soil. Similarly, the method for producing available phosphate includes a step (step P110) of controlling the electrode potential of the negative electrode 30 to between −4.5 V and −3.0 V while the negative electrode 30 is in contact with the soil. Step P110 promotes dissociation of phosphate complexes in the soil around the negative electrode 30. As a result, non-available phosphate in the soil can be converted to available phosphate. That is, step P110 allows available phosphate to be produced from non-available phosphate in the soil.

[0033] According to the system 10 of the first embodiment described above, it is possible to convert unavailable phosphorus in the soil into available phosphorus. In other words, it is possible to produce available phosphorus from unavailable phosphorus in the soil. Therefore, the unavailable phosphorus present in the soil can be used as a phosphate source, and the amount of phosphorus fertilizer applied to the soil can be reduced. Furthermore, because it is possible to directly convert unavailable phosphorus present in the soil into available phosphorus, it is possible to omit the steps of recovering phosphorus from the soil and redissolving the recovered phosphorus to make it in a form suitable for fertilizer. Therefore, it is possible to efficiently utilize phosphorus resources in the soil.

[0034] Furthermore, according to the system 10 of this embodiment, unavailable phosphorus in soil can be converted to available phosphorus using an apparatus with a simple structure, which prevents an increase in the manufacturing cost of the system 10. Furthermore, according to the system 10 of this embodiment, unavailable phosphorus in soil can be converted to available phosphorus through a simple operation, which prevents the process of converting to available phosphorus from becoming complicated. Furthermore, according to the system 10 of this embodiment, unavailable phosphorus in soil can be converted to available phosphorus through a reduction reaction at the negative electrode 30, and the oxidation reaction at the positive electrode 20 can also be used in composting and wastewater treatment.

[0035] C. Working Example The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0036] (1) Experimental method FIG. 3 is an explanatory diagram showing the schematic configuration of a system in an example. For convenience of illustration, FIG. 3 shows an enlarged view of a container in which a negative electrode was placed on the left side of the page. First, soil collected from farmland was placed in a container with a diameter of 12 cm and a height of 15 cm, forming a 3 cm-high soil layer. An electrode for the reduction reaction (hereinafter referred to as the "negative electrode") was placed on the soil layer, and a 2 cm-high soil layer was formed on the negative electrode. The electrode used was a carbon fiber electrode prepared by burning a 10 cm x 10 cm carbon cloth (manufactured by News Company, product name: PL200E) at 500°C for 2 hours. Titanium wire (manufactured by Nilaco, product name: TI-451465) was used as the conductor of the control circuit.

[0037] Next, the container containing the soil layer and electrodes was filled with tap water, and the container was placed in a plastic container (36 cm wide, 30 cm high, and 51 cm long) filled with tap water. To inject electrons into the soil layer (hereinafter referred to as "electronically controlling"), an oxidation electrode (hereinafter referred to as the "positive electrode") was placed near the water surface. The area of ​​the positive electrode was the same as that of the negative electrode. In this example, tap water was used to clarify phosphorus elution from the soil, but soil may be used instead of tap water.

[0038] To perform electronic control, a potentiostat (Hokuto Denko, product name: HA-151B) was used to fix the current at a constant value and to inject electrons into the soil layer. As shown in Figure 3, the potentiostat had the working electrode (WE) terminal connected to the negative electrode, the counter electrode (CE) terminal connected to the positive electrode, and the reference electrode (RE) terminal connected to the reference electrode. Different current densities (250, 400, 600, 800, 1000 mA / m 2) to send electrons into the soil layer, causing a reduction reaction at the negative electrode. The current density was measured by applying a fixed current to the potentiostat with a constant electrode area (0.01 m 2 ) is the value divided by the current density of 250mA / m 2 The experimental period was 2 weeks under the above conditions, and 1 week under the other current density conditions. During the experimental period, the reference electrode and the negative electrode were connected to a voltmeter (manufactured by Sanwa Electric Instruments, product name: PC710), and the potential of the negative electrode was measured over time.

[0039] After the experimental period, the negative electrode was removed from the soil layer, and the soil layer was manually stirred. The pH and oxidation-reduction potential (ORP) of the soil layer were then measured using a pH / ORP meter (Horiba, Ltd., product name: D-73). Next, the soil was removed from the container, and the soil pore water was extracted using a centrifuge (AS ONE, product name: CN2060). The phosphate and ammonium concentrations in the pore water were then measured. The phosphate concentration was measured using Pack Test (Kyoritsu Chemical Research Institute, product names: DPM2-PO4-D and WAK-PO4(D)). The ammonium concentration was measured using Pack Test (Kyoritsu Chemical Research Institute, product names: DPM2-NH4 and KR-NH4-4).

[0040] (2) Changes in electrode potential due to electronic control FIG. 4 is an explanatory diagram showing the relationship between current density and electrode potential under electronic control. In FIG. 4, the horizontal axis represents the current density (mA / m 2 ), and the vertical axis shows the stable electrode potential (V vs. Ag / AgCl). When electrons are injected into the soil layer, a reduction reaction occurs at the negative electrode, causing the electrode potential to decrease over time, but eventually the electrode potential stabilizes. This stable electrode potential is also called the "stable electrode potential." As shown in Figure 4, a high correlation (correlation coefficient: -0.93) was observed between the current density and the electrode potential. When the current density was set to 250 mA / m 2 to 1000mA / m 2By increasing the concentration of electrons to 0.05V, the electrode potential could be reduced from -2.0V to -4.5V. These results show that by injecting more electrons into the soil layer, the reduction reaction at the negative electrode progresses more, and the electrode potential can be reduced further. Furthermore, as the reduction reaction progresses, the soil layer around the negative electrode is affected by reduction, so it is expected that the reduction of the soil layer will progress, i.e., the amount of reduced substances will increase.

[0041] (3) Changes in the redox state of soil layers due to electronic control It is expected that the redox state of the soil layer will change as the reduction reaction progresses at the negative electrode. Based on this expectation, we investigated the changes in the pH and oxidation-reduction potential (hereinafter referred to as ORP) of the soil layer when electronic control was performed at each current density.

[0042] Figure 5 is an explanatory diagram showing the change in the redox state of the soil layer. In Figure 4, the horizontal axis represents pH, and the vertical axis represents the redox potential (V vs. Ag / AgCl) of the soil layer. The soil layer without electronic control (current density: I = 0 mA / m 2 The redox potential in the soil layer (current density: I = 250-1000 mA / m) was 0.33 V, indicating an oxidized state. 2 The ORP in the soil layer (current density: I = 0 mA / m) decreased to approximately -0.55 V at all current densities. This means that the ORP in the soil layer is independent of the current density and is ultimately maintained at a stable potential. 2 While the pH of the soil layer was 5.72 in the untreated condition, the pH of the soil layer increased to 7.28 or higher with electronic control. This indicates that the acidification of the soil layer was alleviated by electronic control. This is thought to be due to a reduction reaction occurring within the soil layer, which consumed hydrogen ions.

[0043] These results demonstrate that the above-mentioned electronic control can lower the ORP of the soil layer while raising the pH of the soil layer. Here, non-available phosphate in the soil is a complex of phosphate and metal ions, and the stability of the complex is thought to depend on the soil pH and ORP. Furthermore, soil reduction promotes the leaching of phosphate. Furthermore, the form of phosphorus is thought to depend on the pH. Therefore, changing the pH and ORP values ​​of the soil layer using the above-mentioned electronic control is thought to be useful for converting non-available phosphate in the soil to available phosphate.

[0044] (4) Phosphate elution due to electronic control It is expected that the phosphate concentration in the pore water will increase due to the dissociation of phosphate complexes, i.e., unavailable phosphate. Based on this expectation, we investigated the relationship between current density and phosphate concentration in the pore water, and the relationship between electrode potential and phosphate concentration in the pore water.

[0045] FIG. 6 is an explanatory diagram showing the relationship between current density and phosphate concentration in pore water. In FIG. 6, the horizontal axis represents the current density (mA / m 2 ) and the vertical axis represents the phosphate concentration (mg / L) in the pore water. As shown in Figure 6, the soil layer without electronic control (current density 0 mA / m 2 ) and the phosphate concentration in the interstitial water at 250mA / m 2 The phosphate concentration in the pore water of the soil layer was similar to that of the soil layer that had been electronically controlled for two weeks at a current density of 250 mA / m. 2 When electronic control was performed for two weeks at a current density of 84000mA h / m 2 In comparison, 400mA / m 2 The phosphate concentration in the pore water of the soil layer that had been electronically controlled for one week at a current density of 400 mA / m 2 When electronic control is performed for one week at a current density of 67200mA h / m 2From this result, it was found that the dissolution of phosphoric acid, i.e., the dissociation of the phosphate complex, depends on the electrode potential. This means that unless the potential reaches a level at which the phosphate complex can dissociate, the complex will not dissociate even if electrons are injected into the soil layer. In addition, the current density condition was 400mA / m 2 to 1000mA / m 2 In all cases, the phosphate concentration in the pore water increased compared to the soil layer without electronic control.

[0046] Figure 7 shows the relationship between electrode potential and phosphate concentration in pore water. In Figure 7, the horizontal axis represents the stable electrode potential (V vs. Ag / AgCl), and the vertical axis represents the phosphate concentration in pore water (mg / L). As shown in Figure 7, the phosphate concentration increased with decreasing electrode potential and reached a maximum at an electrode potential of -3.3 V. Furthermore, the phosphate concentration at an electrode potential of -4.5 V was lower than that at an electrode potential of -3.3 V. This is thought to be because, at an electrode potential of -4.5 V, dissociation of not only the phosphate complex but also other metal complexes occurs, potentially hindering the dissociation of the phosphate complex.

[0047] The results shown in Figures 6 and 7 reveal the following. Specifically, it was found that controlling the electrode potential of the negative electrode to between -4.0 V and -3.0 V can dissociate phosphate complexes in the soil and convert unavailable phosphate into available phosphate. It was also found that controlling the electrode potential of the negative electrode to between -3.5 V and -3.0 V can more efficiently dissociate phosphate complexes in the soil.

[0048] (5) Ammonium elution due to electronic control Another effect of electron regulation in soil was investigated: the relationship between current density and ammonium concentration in pore water.

[0049] FIG. 8 is an explanatory diagram showing the relationship between current density and ammonium concentration. In FIG. 8, the horizontal axis represents the current density (mA / m 2) and the vertical axis shows the ammonium concentration (mg / L) in the pore water. As shown in Figure 8, similar to the results for phosphate elution, the soil layer without electronic control (current density 0 mA / m 2 ) and the ammonium concentration in the interstitial water at 250 mA / m 2 The ammonium concentration in the pore water of the soil layer was similar to that of the soil layer that had been electronically controlled for two weeks at a current density of 400 mA / m. 2 The ammonium concentration in the pore water of the soil layer, which was electronically controlled for one week at a current density of 400 mA / m 2 The electrode potential at the current density of 400 mA / m was -3.0 V. 2 to 1000mA / m 2 In all cases, the ammonium concentration in the pore water increased compared to the soil layer without electronic control. 2 ~1000mA / m 2 By fixing the current density at 400 mA / m and injecting electrons into the soil layer, the electrode potential of the negative electrode is maintained at -3.0 to -4.5 V, which results in an increase in the ammonium concentration in the soil. 2 When the electrode potential is less than -3.0 V, the current density is 400 mA / m 2 A decrease in the ammonium concentration was observed when the electrode potential was -3.0 V, compared to when the electrode potential was -3.0 V. The reason for this is thought to be that denitrification occurs around the negative electrode when the electrode potential is less than -3.0 V. Therefore, it was found that when using the electronic control of the present invention to increase the ammonium concentration while increasing the phosphate concentration in the soil, it is desirable to maintain the electrode potential at around -3.0 V.

[0050] The above experimental results showed that applying the electronic control of the present invention to field soil can increase the fertilizer components in the soil. This suggests that phosphorus resources in particular can be used efficiently as fertilizer, allowing for a reduction in the amount of fertilizer applied. Furthermore, applying the electronic control of the present invention to field soil can mitigate soil acidification, enabling more efficient cultivation.

[0051] D. Other Embodiments The configuration of the system 10 in each of the above-described embodiments is merely an example and can be modified in various ways. For example, the system 10 may be configured in such a way that the reference electrode 40 is omitted.

[0052] The present invention is not limited to the above-described embodiments and can be realized in various configurations without departing from the spirit of the present invention. For example, the technical features in the embodiments and examples corresponding to the technical features in each aspect described in the Summary of the Invention section can be appropriately replaced or combined to solve some or all of the above-described problems or achieve some or all of the above-described effects. Furthermore, if a technical feature is not described as essential in this specification, it can be deleted as appropriate. [Explanation of symbols]

[0053] 10... system, 20... positive electrode, 30... negative electrode, 40... reference electrode, 50... control unit

Claims

1. 1. A system for converting unavailable phosphate in soil to available phosphate, comprising: A control unit; a positive electrode electrically connected to the control unit; a negative electrode electrically connected to the control unit and brought into contact with the soil; Equipped with The control unit controls the electrode potential of the negative electrode to be −4.5 V or more and −3.0 V or less by passing a current. system.

2. 10. The system of claim 1, further comprising: a reference electrode electrically connected to the control unit; the control unit controls the electrode potential of the negative electrode relative to the electrode potential of the reference electrode. system.

3. 3. The system according to claim 1 or claim 2, The control unit includes a potentiostat. system.

4. 3. The system according to claim 1 or claim 2, The negative electrode is formed of carbon fiber. system.

5. 3. The system according to claim 1 or claim 2, The control unit determines whether the electrode area of ​​the negative electrode is 1 m ２ A current of 600mA or more and 1000mA or less is passed per system.

6. 3. The system according to claim 1 or claim 2, The control unit controls the electrode potential of the negative electrode to be −3.5 V or more and −3.1 V or less. system.

7. 1. A method for converting unavailable phosphate in soil to available phosphate, comprising: The negative electrode is in contact with the soil, and the electrode potential of the negative electrode is controlled to be −4.5 V or more and −3.0 V or less. method.

8. A method for producing available phosphoric acid from unavailable phosphoric acid in soil, comprising: The negative electrode is in contact with the soil, and the electrode potential of the negative electrode is controlled to be −4.5 V or more and −3.0 V or less. Manufacturing method.

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