Use of chitosan-based crosslinked polymers for soil fertilization

Abstract

The present invention relates to the use of at least one chitosan-based crosslinked polymer for soil fertilization, to a fertilizing composition comprising said polymer as a superabsorbent polymer, to a method for preparing said fertilizing composition, to the use of the fertilizing composition for improving plant growth, and to a method for fertilizing soils using such a fertilizing composition. Figure to be published: no figure

Description

Title of the invention: use of chitosan-based crosslinked polymers for soil fertilization

[0001] The present invention applies to the field of superabsorbent polymers used in the field of fertilization. In particular, the invention relates to the use of at least one chitosan-based crosslinked polymer for soil fertilization, to a fertilizing composition comprising said polymer as a superabsorbent polymer, to a method for preparing said fertilizing composition, to the use of the fertilizing composition for improving plant growth, as well as to a method for fertilizing soils using such a fertilizing composition.

[0002] Superabsorbent polymers (SAP) or superabsorbent hydrogels are three-dimensional networks of hydrophilic polymer chains, which have the capacity to absorb and retain large quantities of fluid (eg water) in their structures, without dissolving.Their applications extend to various fields: separation techniques as molecular sieves (electrophoresis, size exclusion chromatography), hygienic products (baby diapers, feminine hygiene products), food packaging (humidity control agent), medical and biomedical field (implants in plastic surgery, vascular tissue engineering, biomimetic materials for the manufacture of corneas and cartilage), electrical and optical wiring as sealing agent, pharmaceutical field particularly in controlled release systems, agriculture (fertilizers), treatment of polluted water (adsorbents of toxic metals, dyes), cosmetics, etc.

[0003] The SAPs most frequently found in industrial production are prepared from synthetic polymers of petrochemical origin (petroleum-sourced SAPs), via a polymerization-crosslinking process of hydrophilic vinyl monomers such as acrylic acid (AA), acrylamide (AAm), and their sodium salt derivatives. The most widely used synthetic SAP in all fields is sodium polyacrylate. However, this is not used in agriculture because it is not biodegradable.

[0004] Other superabsorbent polymers based on synthetic polymers have been proposed and generally exhibit good performance, particularly in terms of absorption capacity and mechanical strength. However, the majority of them are quite expensive, are not renewable, and are not biodegradable. In particular, for agricultural applications, the synthesis and / or release into the soil of monomers constituting synthetic SAPs should be avoided from an ecological point of view.

[0005] Research teams have therefore been interested in solutions and al sustainable alternatives compatible with agricultural requirements such as the manufacture of superabsorbent polymers produced from bio-sourced substances.

[0006] For example, Jamnongkan et al., J. Polym. Environ., 2010, 18, 413-421 describe the preparation of hydrogels based on chitosan, polyvinyl alcohol (PVA) or a chitosan / PVA mixture crosslinked with glutaraldehyde. Glutaraldehyde is very toxic. In addition, the best results concerning the absorption and release of a solution of water and fertilizer were obtained with PVA, which is a non-biodegradable petroleum-based polymer. Furthermore, the water absorption capacity of chitosan-based hydrogels is very low (0.7 g / g).

[0007] Naturally derived SAPs generally have more limited water absorption properties than synthetic SAPs, and their production cost can be higher. In addition, in the case of agricultural applications, their excessively rapid degradation can represent a major drawback.

[0008] Thus, the SAPs known to date, whether of natural or synthetic origin, are not fully satisfactory for their agricultural use.

[0009] There is therefore a need for new superabsorbent polymers usable in the field of agriculture, particularly in regions with low rainfall and sandy soil, in order to retain water as well as water-soluble fertilizers and redistribute it at the plant's request.

[0010] Thus, the aim of the present invention is to overcome the drawbacks of the aforementioned prior art and to provide a superabsorbent polymer, preferably biodegradable and / or biosourced, having good performance in terms of absorption and / or water retention, without dissolving, so that it can be used in the agricultural field, and easy to prepare.

[0011] The object of the invention is achieved by the superabsorbent polymers which will be described below.

[0012] The present invention thus has as its first object the use of at least one crosslinked polymer based on chitosan, as a superabsorbent polymer in soil fertilization, characterized in that said crosslinked polymer based on chitosan is obtained by crosslinking in a neutral or acidic aqueous medium a chitosan comprising amine functions in the presence of at least one crosslinking agent comprising at least two functions reactive with said amine functions, said reactive functions being chosen from epoxide, vinyl ketone, vinyl ester functions, and one of their mixtures.

[0013] The inventors of the present application have discovered that the chitosan-based crosslinked polymer according to the first subject of the invention has a water absorption capacity and mechanical properties suitable for use in applications agricultural applications, and more particularly in soil fertilization. Indeed, the chitosan-based crosslinked polymer according to the first subject of the invention absorbs water despite soil pressure and / or the presence of fertilizers or other additives. Furthermore, this polymer is easy to prepare.

[0014] In the present invention, soil fertilization encompasses plant growth by soil amendment.

[0015] The chitosan-based crosslinked polymer of the invention is a superabsorbent polymer or SAP. It is therefore defined by a water absorption capacity (well known according to the Anglicism "Water Absorption Capacity" or WAC) of at least 30, preferably at least 50, and particularly preferably at least 100. This corresponds to the capacity of a polymer, in the dry state, to spontaneously absorb at least 30 times, preferably at least 50 times, and particularly preferably at least 100 times, its mass of liquid, in particular water, and preferably distilled water. The liquid thus absorbed is integrated into a matrix represented by the superabsorbent polymer. The WAC is expressed in grams of liquid per gram of superabsorbent polymer. It can be determined according to the standard NF EN ISO 62 / ASTM D570.

[0016] In particular, the WAC can be obtained by introducing a known mass ml (in g) of dry polymer (for example 1 g) into 500 times the volume of water of ml (in ml) (for example 500 ml of water for 1 g) and leaving the polymer to absorb the water for 24 hours. The gel obtained is filtered and then weighed to obtain a mass m2 (in g). The WAC corresponds to the value of the ratio (m2-ml) / ml.

[0017] In the present invention, a "dry state" of a polymer or a "dry polymer" means a residual water content of said polymer of at most approximately 30% by mass, preferably at most approximately 20% by mass, and particularly preferably at most approximately 10% by mass, relative to the total mass of polymer and residual water. The content can be measured using thermogravimetric analysis.

[0018] The chitosan-based crosslinked polymer of the invention preferably has a three-dimensional network. This three-dimensional network is obtained in particular by said crosslinking. Indeed, the presence of the crosslinker allows the creation of chemical bonds between the macromolecular chains of the chitosan.

[0019] In the invention, the amine functions of chitosan are also called primary amine functions or -NH2.

[0020] Said chitosan-based crosslinked polymer of the invention is preferably biodegradable. In other words, the polymer of the invention achieves a biodegradability rate of at least 60% at 60 days on soil according to the OECD 301F method. This corresponds to the “readily biodegradable” category (also known as the Anglicism "readily biodegradable"). The OECD 301F method is based on the measurement of oxygen consumed by bacteria during the mineralization of a given substance.

[0021] The chitosan-based crosslinked polymer of the invention is preferably biosourced. In other words, it is derived from biomass or raw materials of plant and / or animal origin, preferably animal (and not of fossil or petrochemical origin). It is in particular obtained by extraction of biomass or raw materials of plant and / or animal origin, preferably animal; or obtained by reactions applied to biomass or raw materials of plant and / or animal origin, preferably animal.

[0022] In contrast, a product of "fossil or petrochemical origin" corresponds to any product manufactured at least partially from organic compounds derived from petroleum or coal or from petroleum or coal derivatives.

[0023] A product of bio-sourced origin can be differentiated from a product synthesized from fossil or petrochemical raw materials by the methods described in the ASTM D6866 standard.

[0024] According to a preferred embodiment, the chitosan-based crosslinked polymer of the invention has a compressive strength ranging from approximately 1 kPa to 100 kPa.

[0025] According to a preferred embodiment, the chitosan-based crosslinked polymer of the invention has a Young's modulus ranging from approximately 2 kPa to 200 kPa in the swollen state, and particularly preferably from approximately 4 kPa to 100 kPa in the swollen state.

[0026] The compressive strength can be measured using a crushing load on the chitosan-based crosslinked polymer of the invention in the swollen state. The behavior of the latter is evaluated, under load (e.g. load of 100 N), by measuring the Young's modulus. The swollen state is obtained by immersing the chitosan-based crosslinked polymer in distilled water for 24 hours.

[0027] According to a preferred embodiment, the chitosan-based crosslinked polymer of the invention has a stress swelling capacity ranging from approximately 10 to 60 g / g under 0.9 Psi, and particularly preferably from approximately 25 to 40 g / g under 0.9 Psi (i.e. under 0.062 bar or 6208.28 Pa) (in a saline medium).

[0028] The stress swelling capacity can be measured using a stress absorption test, carried out in particular with a rheometer (eg Rheometer “Haake Mars 60”, Thermo Fischer Scientific).

[0029] The chitosan-based crosslinked polymer of the invention preferably has an insoluble content of at least approximately 90% by mass, and particularly preferably ranging from approximately 95% to 99% by mass.

[0030] Thanks to its water retention capacity, the chitosan-based crosslinked polymer of the invention makes it possible to improve the useful water reserve of the soil by capturing water gravity that is not absorbed by plant roots. It returns water to the soil by suction effect when the evapotranspiration phenomenon of plants reduces the water content of the soil. This water retention capacity in the soil thus makes it possible to extend the wilting point of plants, particularly during periods of water stress. Plant water stress is understood to be stress experienced by a plant placed in an environment such that the quantity of water used and evapotranspirated by the plant is greater than the quantity it absorbs. This stress is encountered during periods of drought, but also when the salinity of the environment increases or during periods of cold.

[0031] The crosslinker comprises at least two functions reactive with the amine functions of the chitosan. In other words, the reactive functions of the crosslinker react with the amine functions of the chitosan during crosslinking, to form covalent bonds [eg formation of bonds: -NH (of the chitosan) - Carbon (of the polyepoxide crosslinker)].

[0032] According to a preferred embodiment of the invention, the crosslinker is a bio-sourced compound. In other words, it is derived from biomass or raw materials of plant and / or animal origin, preferably animal (and not of fossil or petrochemical origin). It is in particular obtained by extraction of biomass or raw materials of plant and / or animal origin, preferably animal or obtained by reactions applied to biomass or raw materials of plant and / or animal origin, preferably animal.

[0033] Epoxide reactive functions are preferred. This makes it possible to obtain a crosslinked polymer based on entirely biosourced chitosan and having good performance in terms of absorption and mechanical properties.

[0034] The crosslinker is then a polyepoxide, in particular a polyepoxide comprising at least two terminal epoxide functions.

[0035] More preferably, the crosslinker is a polyepoxide corresponding to the following formula (I):

[0036] [Chem.l] in which x is greater than or equal to 2, and R is an at least divalent monomeric, oligomeric, or polymeric radical, and even more preferably an at least divalent monomeric, oligomeric, or polymeric, aliphatic radical.

[0037] x denotes the number of epoxide functions within the crosslinker of formula (I).

[0038] In the invention, an aliphatic radical is an open-chain (linear or branched) hydrocarbon radical, optionally comprising one or more non-aromatic rings. The aliphatic radical may be saturated or unsaturated, and preferably saturated.

[0039] In other words, the polyepoxide is preferably an aliphatic polyepoxide.

[0040] According to a preferred embodiment of the invention, the radical R comprises one or more heteroatoms chosen from an oxygen atom, a nitrogen atom, and one of their mixtures.

[0041] According to a preferred embodiment of the invention, the radical R comprises from 2 to 17 carbon atoms, and more particularly preferably from 3 to 6 carbon atoms.

[0042] The crosslinker of formula (I) can correspond to the following formula (1-1) or (1-2):

[0043] [Chem.2] in which: - 2 < xl < 20, and preferably 2 < xl < 5, - 2 < x2 < 20, and preferably 2 < xl < 5, - R1 is a polyether radical (i.e. comprising several ether functions), said polyether radical being able to further comprise one or more hydroxyl functions (-OH), and - R2 is a polyester radical (i.e. comprising several ester functions), said polyester radical being able to further comprise one or more hydroxyl functions (-OH).

[0044] R1 (respectively R2) is a subgroup of the group R as defined in the invention. R1 (respectively R2) is preferably an aliphatic radical. R1 (respectively R2) preferably comprises from 2 to 17 carbon atoms, and more particularly preferably from 3 to 6 carbon atoms.

[0045] Biosourced crosslinkers (1-1) and (1-2) are preferred.

[0046] The crosslinker corresponding to formula (1-1) may be a polyglycidyl ether of a polyol such as glycerol, a polyglycerol, 1,4-butanediol, 1,3-propanediol, ethylene glycol, sorbitol, isosorbide, or a polyethylene glycol,

[0047] The crosslinker corresponding to formula (1-1) can then be obtained from epichlorohydrin and a polyol as defined above. Epichlorohydrin can itself

[0048]

[0049] can even be obtained by chlorination of glycerol, which is a co-product of the transesterification of vegetable oils for the preparation of biodiesel. Examples of such crosslinkers of formula (I-1) in which R1 is a polyether include the following crosslinkers of formula (I-1'): [Chem. 3]

[0050]

[0051]

[0052]

[0053]

[0054] cm in which: 0 < n < 5, and preferably 0 < n < 3, and 0 < m < 5, and preferably 0 < m < 3. Such crosslinkers corresponding to the formula (1-1) or (I-1 ') are for example those marketed by NAGASE under the reference "DENACOL" or by KUKDO under the reference "KF EPIOL". The crosslinker corresponding to formula (1-2) can be obtained by reaction of a compound of formula (1-1) or (1-1') with a monomeric, oligomeric or polymeric polycarboxylic acid, in particular by polycondensation. The polyacid is preferably chosen from muconic acid, citraconic acid, mesaconic acid, aconitic acid, itaconic acid, fumaric acid, succinic acid, malonic acid, sebacic acid, octadec-9-ene dioic acid, polyacids obtained by radical polymerization from at least one unsaturated dicarboxylic acid, such as polyitaconic acid. The crosslinker corresponding to formula (1-2) can thus be a crosslinker of formula (1-2') obtained according to the following reaction between a compound of formula (1-1) and a monomeric diacid: [Chem. 4] in which: - formula (1-1) is as defined previously, - R3 is an alkylene or alkenylene radical comprising from 2 to 15 carbon atoms, and more particularly preferably from 3 to 6 carbon atoms.

[0055] The compound of formula (1-1) can be used in excess relative to the monomeric, oligomeric or polymeric diacid.

[0056] The biosourced crosslinker preferably represents from 0.01% to 20% by mass approximately, and particularly preferably from 1% to 9% by mass approximately, relative to the total mass of chitosan comprising amine functions.

[0057] Preferably, the biosourced crosslinker represents from 0.01 to 20% by mole approximately, and more preferably from 1 to 10% by mole approximately, relative to the number of moles of amine functions of the chitosan.

[0058] The biosourced crosslinker preferably has a molar mass of at most approximately 1200 g / mol, and particularly preferably of at most approximately 314 g / mol.

[0059] The neutral or acidic aqueous medium can have a pH ranging from approximately 4.5 to 7.

[0060] Chitosan comprising amine functions can have a molar mass ranging from 10 to 250 KDa approximately, and preferably ranging from 70 to 200 KDa approximately.

[0061] Chitosan can have a degree of acetylation ranging from about 1% to 50%, and preferably ranging from 5% to approximately 20%. These ranges thus provide an indication of the content of amine functions in chitosan.

[0062] The crosslinked chitosan polymer in accordance with the first subject of the invention can be obtained according to a process comprising at least the following steps: (i) providing a neutral or acidic aqueous solution comprising a chitosan containing amine functions, ii) adding to said neutral or acidic aqueous solution a crosslinking agent comprising at least two functions reactive with said amine functions, said reactive functions being chosen from epoxide, vinyl ketone, vinyl ester functions, and one of their mixtures, iii) stir to obtain a homogeneous aqueous dispersion, iv) heating the aqueous dispersion until gelation in a static and hermetic environment, iv) let cool.

[0063] The chitosan provided in step i) is as defined in the first subject of the invention. The bio-sourced crosslinker provided in step ii) is as defined in the first subject of the invention.

[0064] Step i)

[0065] The chitosan provided in step i) may have a molar mass ranging from approximately 10 to 250 KDa, and preferably ranging from approximately 70 to 200 KDa.

[0066] The chitosan provided in step i) may have a degree of acetylation ranging from approximately 1% to 50%, and preferably from approximately 5% to 20%. These ranges thus make it possible to give an indication of the content of amine functions in the chitosan.

[0067] The neutral or acidic aqueous solution may have a pH ranging from approximately 4.5 to 7.

[0068] The acidic aqueous solution preferably comprises a chitosan, at least one acid, and water.

[0069] The acid may be a mineral or organic acid, and preferably an organic acid.

[0070] Examples of mineral acids that may be mentioned include hydrochloric acid.

[0071] Examples of organic acids that may be mentioned are acetic acid, formic acid, or lactic acid.

[0072] In particular, the acidic aqueous solution comprising a chitosan provided in step i) may comprise from 1% to 10% by mass approximately of organic acid, relative to the total mass of the acidic aqueous solution comprising a chitosan.

[0073] The neutral or acidic aqueous solution provided in step i) may have a viscosity ranging from approximately 3 to 5 x 106 mPa.s, and preferably from approximately 48 to 3.8 x 105 mPa.s. The viscosity may be determined using a rheometer.

[0074] The neutral or acidic aqueous solution provided in step i) may comprise from 0.1% to 20% by mass approximately of chitosan, and preferably from 1% to 10% by mass approximately of chitosan, relative to the total mass of the neutral or acidic aqueous solution comprising a chitosan.

[0075] The acidic aqueous solution provided in step i) may be prepared according to a step iA) comprising the addition of a chitosan to an acidic aqueous solution, or a step iB) comprising the addition of an acid to an aqueous dispersion of chitosan, and particularly preferably said step iA) (respectively said step iB) is carried out with stirring.

[0076] Step iA)

[0077] The amount of acid in the acidic aqueous solution is preferably defined so that it results in an acidic aqueous solution comprising a chitosan having the pH as defined in the invention.

[0078] Step iA) is preferably carried out with stirring.

[0079] Step iA) can last from 30 min to 2 hours.

[0080] Step iA) is preferably carried out at a temperature ranging from approximately 20°C to 40°C.

[0081] According to a preferred embodiment, stirring is maintained at the end of step iA) and during step ii).

[0082] Step iB)

[0083] The amount of acid to be added to the aqueous chitosan dispersion is preferably defined so that it results in an acidic aqueous solution comprising a chitosan having the pH as defined in the invention.

[0084] Step iB) is preferably carried out with stirring.

[0085] Step iB) can last from 30 min to 2 hours.

[0086] Step iB) is preferably carried out at a temperature ranging from approximately 20°C to 40°C.

[0087] According to a preferred embodiment, stirring is maintained at the end of step iB) and during step ii).

[0088] Step ii)

[0089] The crosslinking agent is preferably added during step ii) at a rate of approximately 0.01% to 20% by mass, and particularly preferably approximately 1% to 9% by mass, relative to the total mass of chitosan.

[0090] Preferably, the crosslinking agent represents from 0.01 to 20% by mole approximately, and more preferably from 1 to 10% by mole approximately, relative to the number of moles of amine functions of the chitosan.

[0091] The crosslinker preferably has a molar mass of at most approximately 1200 g / mol, and particularly preferably of at most approximately 314 g / mol.

[0092] The crosslinker can be prepared by methods well known to those skilled in the art, such as for example by reaction of a polyol (eg as defined above in the first subject of the invention) with epichlorohydrin to form a chlorohydrin, followed by the elimination of HCl in a basic medium to form an epoxide.

[0093] Step ii) preferably involves the dropwise addition of the crosslinking agent with stirring.

[0094] Step ii) can last from 30 min to 2 hours.

[0095] Step ii) is preferably carried out at a temperature ranging from approximately 20°C to 40°C.

[0096] Step ii) may be carried out with an aqueous dispersion of said crosslinker. The crosslinker may therefore be diluted beforehand in water, to form said aqueous dispersion.

[0097] Step iii)

[0098] Step iii) may consist of continuing stirring after the addition of the crosslinking agent.

[0099] Step iii) may last at least 5 min.

[0100] Step iv)

[0101] During step iv), the aqueous dispersion is heated until gelation in a static and hermetic medium.

[0102] The temperature can range from approximately 20°C to 100°C, and preferably from approximately 50°C to 70°C.

[0103] Step iv) can last at most 24 hours, and preferably from 5 hours to 15 hours.

[0104] During step iv), the amine functions of the chitosan react with the crosslinker, and in particular with the epoxy functions of the crosslinker.

[0105] The polymerization of the aqueous dispersion in water leads to the production of a crosslinked chitosan gel.

[0106] Step iv) is carried out in a static environment (i.e. without stirring).

[0107] Step iv) is carried out in a hermetic environment (i.e. protected from the outside).

[0108] The process may comprise, after step iii) and before step iv), a step during which the aqueous dispersion is poured into a sealed container.

[0109] Step v)

[0110] Step v) consists of allowing the formed gel to cool.

[0111] The chitosan-based crosslinked polymer obtained can then be shaped according to a step vi), in particular by fractionation.

[0112] The chitosan-based crosslinked polymer obtained at the end of step v) or vi) can be dried according to a step vii).

[0113] Drying vii) may be carried out using an oven or in air, or it may comprise a freezing sub-step followed by a freeze-drying sub-step.

[0114] This step vii) makes it possible to obtain a residual water content of at most 30% by mass, and preferably at most 20% by mass.

[0115] The chitosan-based crosslinked polymer is preferably in solid form.

[0116] The second subject of the invention is a fertilizing composition comprising at least one crosslinked polymer based on chitosan as defined in the first subject of the invention and at least one fertilizer.

[0117] The fertilizer

[0118] The fertilizer or fertilizer can be chosen from organic fertilizers and mineral fertilizers, and preferably from mineral fertilizers.

[0119] Organic fertilizers can be of natural origin (for example, animal or plant origin) or synthetic (for example, urea or urea derivatives). Mineral fertilizers can be synthetic or come from natural deposits.

[0120] Fertilizer or manure is used in agriculture, horticulture, forestry and for gardening or landscaping activities in order to provide plants with additional nutrients, so as to improve their growth and / or increase the yield and quality of crops. Fertilizers may comprise one or more of the following elements: - basic elements such as nitrogen (N), phosphorus (P) and potassium (K); - secondary elements such as calcium (Ca), sulfur (S) and magnesium (Mg); - trace elements such as iron (Fe), manganese (Mn), molybdenum (Mo), copper (Cu), boron (B), zinc (Zn), chlorine (Cl), sodium (Na), cobalt (Co), vanadium (V) and silicon (Si).

[0121] In a preferred embodiment, the fertilizer or fertilizer is chosen from NPK type fertilizers, i.e. comprising at least nitrogen (N), phosphorus (P) and potassium (K), said nitrogen, phosphorus, and potassium each being present at a rate of at least approximately 3% by mass, relative to the total mass of said fertilizer.

[0122] Among NPK type fertilizers, we can cite ammonium salts; in particular ammonium and potassium phosphate, potassium nitrate, ammonium and potassium sulfate, diammonium sulfate, urea, ammonium nitrate, monocalcium phosphate, potassium chloride or potassium sulfate.

[0123] The fertilizing composition preferably comprises at least approximately 50% by mass of the chitosan-based crosslinked polymer, and more particularly preferably from approximately 65% ​​to 99.5% by mass of the chitosan-based crosslinked polymer, relative to the total mass of the fertilizing composition.

[0124] The fertilizing composition preferably comprises from 0.5% to 10% by mass approximately of the fertilizer, and more particularly preferably from 1% to 5% by mass approximately of the fertilizer, relative to the total mass of the fertilizing composition.

[0125] The fertilizing composition according to the second subject of the invention is used in a formulation suitable for its application to the area to be treated.

[0126] The fertilizing composition is preferably in solid form, for example in the form of powder, granules, or beads. This solid fertilizing composition can be provided as such to the user, i.e. ready to use.

[0127] The fertilizing composition may further comprise one or more additives making it possible to improve the absorption of water and / or the release of the fertilizer and its penetration into the plant tissues; and / or one or more growth regulators.

[0128] The additives making it possible to improve the absorption of water and / or the release of the fertilizer and its penetration into the plant tissues can be chosen from wetting agents and surfactants, in particular alcohol ethoxylate, and chelating agents, in particular EDTA (ethylenediaminetetraacetic acid) or DTPA (diethylenetriaminepentaacetic acid).

[0129] The growth regulators may be chosen from maleic hydrazide, chloromequat chloride (for example “Cyclocel®”), auxin derivatives, growth regulators of natural origin, and a mixture thereof.

[0130] Advantageously, the growth regulators of natural origin are salicylic acid, salicylic acid salts such as ammonium salicylate, jasmonates, auxins, gibberellins, cytokinins, lunularic acid, abscisic acid, or a mixture thereof.

[0131] The fertilizing composition according to the invention may also contain other more conventional additional additives such as anti-foaming agents, defoaming agents, etc. integrants, stabilizing agents, humectants, thickeners or pH regulators.

[0132] The third subject of the invention is a process for preparing a fertilizing composition in accordance with the second subject of the invention, comprising one or other of the following sequences S1 or S2: IF : Al) bringing a fertilizer into contact with a chitosan-based crosslinked polymer as defined in the first subject of the invention, or S2: A2) mixing a fertilizer with a neutral or acidic aqueous solution comprising a chitosan containing amine functions and with a crosslinker comprising at least two functions reactive with said amine functions, said reactive functions being chosen from epoxide, vinyl ketone, vinyl ester functions, and one of their mixtures, and crosslinking.

[0133] The fertilizer may be as defined in the second subject of the invention.

[0134] The neutral or acidic aqueous solution comprising a chitosan containing amine functions is preferably as defined in the first subject of the invention.

[0135] The chitosan is preferably as defined in the first subject of the invention.

[0136] The crosslinking agent is preferably as defined in the first subject of the invention.

[0137] The sequence SI is preferably carried out according to a method as defined in the first subject of the invention where the fertilizer is added after step vii).

[0138] Sequence S2 is preferably carried out according to a method as defined in the first subject of the invention where the fertilizer is added between step i) and step ii).

[0139] The fourth object of the invention is the use of a fertilizing composition in accordance with the second object of the invention, to improve plant growth.

[0140] The plants can be chosen from cereals, legumes, market garden plants, aromatic plants, herbaceous plants, woody plants, climbing plants, bulbous plants, trees, shrubs.

[0141] The fifth subject of the invention is a method for fertilizing soils comprising bringing a target area into contact with an effective amount of a fertilizing composition as defined in the second subject of the invention.

[0142] In particular, the fertilizing composition can be dispersed directly onto the target area.

[0143] Preferably, the fertilizing composition as defined in the second subject of the invention is provided to the plant via a culture substrate.

[0144] This culture substrate can be the soil but also the culture media considered for hydroponic cultures.

[0145] Advantageously, the fertilizing composition is applied directly in contact with the culture substrate on the surface of which the plants to be treated develop.

[0146] Thanks to the fertilizing composition as defined in the second subject of the invention, the overdoses of fertilizers which are not absorbed by the plants and which are leached into the soil are eliminated (eg 40% to 70% of nitrogen, 80% to 90% of phosphorus and 50% to 70% of potassium).

[0147] Other characteristics, variants and advantages of the use of the crosslinked chitosan polymer or of the fertilizing composition, or of the process for preparing the crosslinked chitosan polymer or of the fertilizing composition according to the invention will emerge more clearly on reading the following examples of embodiment, given by way of illustration and not limitation of the invention.

[0148] EXAMPLES

[0149] Determination of the quantity of water

[0150] The amount of water is determined using thermogravimetric analysis (TGA). This is an analysis tool comprising a microbalance positioned in an oven. It allows the loss of mass to be recorded as a function of temperature and therefore the presence of water in a sample to be quantified. It is expressed as a mass percentage or in grams.

[0151] Water absorption capacities (WAC) and insoluble content

[0152] The WAC is measured by taking a mass ml of crosslinked polymer or fertilizer composition of around 1 g which is immersed in 500 ml of distilled water for 24 hours. After 24 hours, the gel obtained is separated from the aqueous phase to measure its mass m2. The following formula is applied to determine the WAC:

[0153] [Math.l] m2 - ml ml

[0154] The insoluble rate is evaluated by completely drying the gel of mass m2, giving a mass m3. The following formula gives the insoluble rate:

[0155] [Math.2] Unresolvable Tx =--------7-r;--------:-------* 100 ml — quantity of water in ml

[0156] Compression test

[0157] The principle of the compression test is based on the application of a crushing load on the crosslinked polymer or the fertilizing composition and the evaluation of the behavior of the latter under load by measuring fundamental variables such as compressive stress and strain.

[0158] The test is carried out with an “INSTRON 3366” compression machine. In practice, a piece of crosslinked polymer or swollen fertilizer composition of well-defined dimensions (usually a cube / cylinder of 10 x 10 x 10 mm) is placed on the lower plate of the machine. To obtain the swollen state, the polymer or fertilizer composition is dried in a freeze dryer to a residual water content of approximately 20% (mass “0.1 g), then immersed in 400 ml of distilled water for 24 hours. Then, the polymer or fertilizer composition is cut into small cubes of 1 cm x 1 cm x 1 cm. In the compression test, a compressive load is applied to the crosslinked polymer or fertilizer composition using the upper plate of the machine (being connected to a 100 N force cell) with a displacement speed of 1 mm / min until the maximum load value is reached.During the experiment, the crosslinked polymer or the fertilizer composition undergoes an increasing deformation until reaching the breaking point and the force inducing this deformation is none other than the compressive stress. Thus, by plotting the compressive stress (kPa) as a function of the compressive strain (%), two domains can be distinguished: an elastic domain, where the induced deformation is reversible, and a plastic domain where the deformation is irreversible (definitive). It is from the right slope of the elastic domain that the Young's Modulus, reflecting the elasticity of the crosslinked polymer or the fertilizer composition, is determined.

[0159] Absorption under stress (AUL)

[0160] Stress absorption tests of said fertilizer composition or crosslinked polymer were carried out according to a measurement method described by Ramazani-Harandi et al. [Polym. Test., 2006, 25, 470-474]. A ThermoFischer “Haake Mars” type rheometer is used to apply a force of determined value to a sample of fertilizer composition or crosslinked polymer. A setup is thus made with a plastic crystallizer (d = 47 mm, h = 70 mm) in which a filter disc of porosity (porosity 0, d = 32 mm, h = 3 mm) is placed on which a sample of cylindrical fertilizer composition (d = 10 mm, h = 13 mm) or crosslinked polymer is deposited.A load of circular geometry (d = 25 mm, h = 1 mm) allows, with the help of the rheometer force sensor, to exert on the sample of fertilizer composition or crosslinked polymer a force of 0.16 N, 0.33 N or 0.49 N (corresponding to 0.3 psi, 0.6 psi and 0.9 psi respectively) by sliding freely in a hollow plastic cylinder (d = 29 mm, h = 70 mm). The amount of distilled water added to the crystallizer is controlled in order to avoid the influence of the hydraulic pressure of the water on the sensor device, and it should be equal to the height of . the sample of fertilizer composition or crosslinked polymer in the inner cylinder for the duration of the tests. During stress swelling, the sample of fertilizer composition or crosslinked polymer pushes upwards the circular geometry load and the change in the height of the fertilizer composition or crosslinked polymer is measured as a function of time. The sample of fertilizer composition or crosslinked polymer is weighed before and after the test to determine the stress absorption.

[0161] Efficacy on plant growth under water stress conditions

[0162] The impact of the presence of the chitosan-based crosslinked polymer of the invention on plant growth under water stress conditions was evaluated according to a specific experimental protocol as detailed below. Corn seeds (Zea mays saccharata) previously soaked in water were sown in trays containing potting soil and left for a period of 72 hours. Then, 2 g of the chitosan-based crosslinked polymer of the invention was mixed with 4 liters of potting soil and the resulting mixture was distributed into 18 pots named PL

[0163] Two comparative control groups were established, a negative control group, comprising 4 liters of potting soil without crosslinked polymer distributed in 18 pots named PCO and a positive control group comprising 4 liters of potting soil mixed with 3.4 g of a superabsorbent polymer marketed by Evonik under the reference “Stockosorb 660” (crosslinked homopolymer of polyacrylic acid partially neutralized with potassium salt) distributed in 18 pots named PCI.

[0164] Then, the germinated plants were introduced into each PI, PCO, and PCI pot.

[0165] To allow swelling of the chitosan-based crosslinked polymer of the invention or the commercial superabsorbent polymer, 1000 ml of water was added to each PI, PCO, and PCI pot.

[0166] Three watering conditions were implemented, namely 0%, 50% and 100%, corresponding to the respective watering frequency. For each condition, 6 replicates (pots) were carried out. 100% watering means watering 300 ml of water every 2 days per pot for a period of 35 days, and 50% watering means watering 300 ml of water every 4 days per pot for a period of 35 days. 0% watering means no watering for a period of 35 days.

[0167] After 35 days, the plants were measured, harvested and the fresh biomass of the roots and leaf parts was recorded. Subsequently, the root and leaf parts were subjected to an oven drying process at 50°C for a period of 7 days, followed by weighing to determine the dry biomass.

[0168] EXAMPLE 1: Synthesis of a crosslinked polymer based on chitosan as defined in the invention PI

[0169] An acidic aqueous solution of chitosan was prepared as follows: a An acidic aqueous solution comprising 2.5% by mass of acetic acid was prepared by mixing 890 g of water with 23 g of acetic acid (supplier: Merck, CAS No. 64-19-7). Then, 75 g of chitosan (supplier: VNF, CAS No. 9012-76-4) was added to the acidic aqueous solution and the resulting mixture was stirred for approximately 1 hour. The resulting mixture was then heated to a temperature of approximately 55°C with stirring continued for approximately 1 hour. Stirring was then stopped and then an aqueous dispersion of a glycerol polyglycidyl ether crosslinker sold under the trade name “KF EPIOL - PE 311E” by KUKDO previously prepared (6.6 g of glycerol polyglycidyl ether in 12 g of water) was added to the previously formed acidic aqueous chitosan solution. Then the resulting mixture is stirred for a few minutes, the stirring is stopped and the resulting mixture is heated to about 70°C and allowed to react until a gel forms.

[0170] The gel obtained is then cut into pieces which are placed in the freezer for 1 night then freeze-dried until reaching a residual water content of approximately 22% by mass.

[0171] The obtained chitosan crosslinked polymer has the properties as illustrated in the following Table 1: Residual water quantity (% by mass) WAC (g / g) Insoluble rate (% by mass) Young's modulus (kPa) AUL under 0.9 psi (g / g) 22 41 90 182 28

[0172] EXAMPLE 2 of use of the crosslinked polymer of Example 1

[0173] A fertilizer of the NPK family based on nitrogen, phosphorus, and potassium was incorporated into the crosslinked polymer in the manner described below.

[0174] 7.5 g of chitosan (supplier: VNF, CAS n°9012-76-4) was added to 88.2 g of water. The resulting aqueous chitosan solution was stirred and then 2.3 g of acetic acid (supplier: Merck, CAS No. 64-19-7) was added to form an acidic aqueous solution comprising a chitosan. The acidic aqueous solution comprising a chitosan was then heated to a temperature of approximately 40°C with stirring continued for approximately 1 hour. Then, 2 g of an NPK fertilizer solution (supplier: Botanic, 10% by mass of NPK) was added and stirring continued for 1 hour. Then, 0.66 g of glycerol polyglycidic ether crosslinker sold under the trade name “KF EPIOL - PE 311E” by KUKDO was added to the previously formed acidic aqueous chitosan solution further comprising NPK. The resulting mixture was stirred for a few minutes, the stirring was stopped and the resulting mixture was heated to about 70°C and allowed to react until a gel forms.

[0175] The fertilizing composition in the form of a gel obtained was then cut into pieces which were placed in the freezer for 1 night and then freeze-dried.

[0176] The fertilizer composition has the properties as illustrated in the following Table 2: Residual water quantity (% by mass) WAC (g / g) Insoluble rate (% by mass) Young's modulus (kPa) AUL under 0.9 psi (g / g) 30 12 95 12S 19

[0177] EXAMPLE 3 of application: use of the crosslinked polymer for plant growth

[0178] The protocol as described previously made it possible to collect the results as represented in Table 3 below: Average plant height (cm) Fresh leaf biomass weight (q) Fresh root biomass weight (a) Dry leaf biomass weight (q) Dry root biomass weight (q) PI-100% 122.3 81.9 15.2 8.4 1.6 PC0-100% 120.7 80.5 16.2 8.4 1.7 PCI-100% 113.7 63.5 11.65 5.9 L2 Pl-50% 136 115.7 17. 11.9 1.9 PC0-50% 110.3 81.7 aq 8.55 1.4 PC 1-50% 137.5 140.0 1.9.4 13.5 2.1 P1-0% 86.5 16.8 3.0 ÜJ 1.0 PC0-0% 81.2 20.5 3.2 3.8 1.1 PC1-O% 86.8 19.6 3.0 4.1 1.1

[0179] We observe compared to the negative control under water stress with 50% watering (i.e. PC0-50%): - increase in plant size from 18 to 23.3% (Pl-50%), - increase in leaf biomass from 30.5 to 41.7% (Pl-50%), - increase in root biomass from 56.2 to 73.4% (P1-5O%).

[0180] We observe compared to the negative control without water stress with 100% watering (i.e. PC0-100%): - 50% reduction in watering frequency with an increase in plant size of 7.9 to 12.7% (Pl-50%), - 50% reduction in watering frequency with increased biomass foliar from 32.4 to 43.7% (Pl-50%), - 50% reduction in watering frequency with stable root biomass (Pl-50%).

[0181] In conclusion, when related to the quantity of water used for irrigation, the crosslinked polymer of the invention induces a significant increase in plant growth, both in terms of plant size and biomass produced, under water stress conditions. For a dosage of 2 g of crosslinked polymer of the invention per 4 liters of potting soil, this increase varies from 115 to 200%, i.e. more than double the growth observed without the use of superabsorbent polymer.

Claims

1.

2.

3.

4. Claims Use of at least one chitosan-based crosslinked polymer as a superabsorbent polymer in soil fertilization, characterized in that said chitosan-based crosslinked polymer is obtained by crosslinking in a neutral or acidic aqueous medium a chitosan comprising amine functions in the presence of at least one crosslinking agent comprising at least two functions reactive with said amine functions, said reactive functions being chosen from epoxide, vinyl ketone, vinyl ester functions, and one of their mixtures. Use according to claim 1, characterized in that the chitosan-based crosslinked polymer has a water absorption capacity of at least 30. Use according to claim 1 or 2, characterized in that the crosslinker is a polyepoxide comprising at least two terminal epoxide functions. Use according to any one of the preceding claims, characterized in that the crosslinking agent corresponds to the following formula (1-1) or (1-2): [Chem. 5]

5.

6. in which: - 2 < xl < 20, - 2 < x2 < 20, - R1 is a polyether radical, said polyether radical being able to further comprise one or more hydroxyl functions (-OH), and - R2 is a polyester radical, said polyester radical being able to further comprise one or more hydroxyl functions (-OH). Fertilizing composition, characterized in that it comprises at least one crosslinked polymer based on chitosan as defined in any one of the preceding claims and at least one fertilizer. Fertilizing composition according to claim 5, characterized in that the fertilizer is chosen from NPK type fertilizers, i.e. comprising at least less nitrogen (N), phosphorus (P) and potassium (K), said nitrogen, phosphorus and potassium each being present at a rate of at least 3% by mass, relative to the total mass of said fertilizer.

7. Fertilizing composition according to claim 5 or 6, characterized in that it comprises at least 50% by mass of the crosslinked polymer based on chitosan, relative to the total mass of the fertilizing composition.

8. Process for preparing a fertilizer composition according to any one of claims 5 to 7, comprising one or other of the following sequences S1 or S2: S1: A1) bringing a fertilizer into contact with a crosslinked polymer based on chitosan as defined in any one of claims 1 to 4, or S2: A2) mixing a fertilizer with a neutral or acidic aqueous solution comprising a chitosan containing amine functions and with a crosslinker comprising at least two functions reactive with said amine functions, said reactive functions being chosen from epoxide, vinyl ketone, vinyl ester functions, and one of their mixtures, and crosslinking.

9. Use of a fertilizing composition according to any one of claims 5 to 7, for improving plant growth.

10. A method of fertilizing soils comprising contacting a target area with an effective amount of a fertilizer composition according to any one of claims 5 to 7.