Fertilizer and its manufacturing method

Abstract

To provide a fertilizer that can be obtained by a safe and simple method, and a method of producing the same.SOLUTION: The fertilizer includes a substance containing a phosphate of low solubility, and a high molecular weight compound comprising a carboxy group and / or a carboxylate group in its molecule, in which the total amount of the carboxy group and the carboxylate group, is not less than 5 mmol / g.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to fertilizers and a method for producing the same.

Background Art

[0002] Phosphorus is a nutrient necessary for plant growth, along with nitrogen, potassium, etc., and the supply of these is often carried out by applying chemical fertilizers or the like to agricultural land. In recent years, from the viewpoints of reducing environmental loads and economically producing phosphorus fertilizers, it has been studied to recover water-soluble phosphoric acid from wastes such as sludge incineration ash, sludge, soil (including industrial waste soil, facility horticulture waste soil, etc.), steelmaking slag, livestock manure, and food waste, and use it as a fertilizer. When recovering phosphoric acid by conventional techniques, a step of making the pH of the treatment target acidic or alkaline may be required (see

[0003] of Patent Document 1). The devices used in the above steps are prone to corrosion, and in the method including the above steps, the safety of those involved in the phosphoric acid recovery business is a concern. Furthermore, since the treatment target with an acidic or alkaline pH cannot be discharged into the environment as it is, a treatment for neutralizing the treatment target with an acidic or alkaline pH is also necessary. In the technology for recovering phosphoric acid, improvement in simplification is required and being studied (see Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] According to the method described in Patent Document 1, phosphoric acid can be recovered by blowing carbon dioxide into an aqueous suspension of incinerated sludge containing heavy metals, then separating the solid and liquid, and recovering the filtrate. Therefore, according to the method described in Patent Document 1, phosphoric acid can be recovered by a simpler method than conventional techniques. However, this method requires steps such as preparing an aqueous suspension of incinerated sludge, blowing in carbon dioxide, and separating solids and liquids, and there was room for improvement in terms of simplification. The object of the present invention is to provide a fertilizer that can be obtained by a safer and simpler method than conventional methods, and a method for producing the same. [Means for solving the problem]

[0005] The inventors of this invention arrived at this present invention as a result of diligent research to solve the above problems. In other words, the present invention is as follows: [1] A fertilizer comprising a substance containing a poorly soluble phosphate and a polymer compound having a carboxyl group and / or a carboxylic acid base in its molecule, wherein the total amount of carboxyl groups and carboxylic acid bases contained in the polymer compound having a carboxyl group and / or a carboxylic acid base in its molecule is 5 mmol / g or more. [2] The fertilizer according to [1], wherein the polymer compound having a carboxyl group and / or a carboxylic acid base in the molecule is a polymer containing constituent units derived from (meth)acrylic acid and / or (meth)acrylate. [3] The fertilizer described in [1] or [2], further prepared by mixing with water. [4] A method for producing fertilizer comprising step 1 of mixing a substance containing a poorly soluble phosphate with a component containing a polymer compound having a carboxyl group and / or a carboxylic acid base in its molecule, wherein the total amount of carboxyl groups and carboxylic acid bases contained in the polymer compound having a carboxyl group and / or a carboxylic acid base in its molecule is 5 mmol / g or more. [5] A method for producing fertilizer according to [4], wherein the polymer compound having a carboxyl group and / or a carboxylic acid base in the molecule is a polymer containing constituent units derived from (meth)acrylic acid and / or (meth)acrylate. [6] A method for producing fertilizer according to [4] or [5], wherein in step 1, water is further added to the components and mixed. [7] The method for producing fertilizer according to [6], further comprising the step of adding water to the components in step 1 and mixing, and then allowing the mixture to stand at a temperature of 10°C to 80°C. [Effects of the Invention]

[0006] According to the present invention, it is possible to provide a fertilizer that can be obtained by a safe and simple method, and a method for producing the same. [Modes for carrying out the invention]

[0007] <fertilizer> The fertilizer of the present invention comprises a substance containing a poorly soluble phosphate and a polymer compound having a carboxyl group and / or a carboxylic acid base in its molecule, wherein the total amount of carboxyl groups and carboxylic acid bases contained in the polymer compound having a carboxyl group and / or a carboxylic acid base in its molecule is 5 mmol / g or more. In this specification, carboxyl groups and / or carboxylic acid bases are also referred to as "carboxylic acid (salt) groups."

[0008] In the present invention, there are no particular limitations on substances containing sparingly soluble phosphates, as long as they contain sparingly soluble phosphates. Examples of substances containing sparingly soluble phosphates include phosphate compounds such as aluminum phosphate, iron(III) phosphate dihydrate, and tricalcium phosphate; and resources such as soil, rice bran, rice husks, rice husk char, livestock manure (cow manure, pig manure, chicken manure), livestock manure char, sludge, sludge incineration ash, steelmaking slag, and food waste.

[0009] In this invention, "poorly soluble phosphate" means "all phosphates other than water-soluble phosphates." In this invention, "water-soluble phosphate" means "phosphates dissolved in water at 25°C." In this invention, "dissolved in water at 25°C" means "dissolves 0.1 mg or more in 1000 g of water at 25°C."

[0010] In the present invention, the polymer compound having a carboxylic acid (salt) group in its molecule has a total amount of carboxyl group and carboxylic acid base of 5 mmol / g or more. The total amount of carboxyl group and carboxylic acid base in the compound is 5 mmol / g or more, which allows the compound to sufficiently solubilize sparingly soluble phosphates in substances containing sparingly soluble phosphates. Even with polymer compounds having a carboxylic acid (salt) group, if the total amount of carboxyl group and carboxylic acid base is less than 5 mmol / g, the compound will not sufficiently solubilize sparingly soluble phosphates in substances containing sparingly soluble phosphates. From the viewpoint of being excellent at solubilizing sparingly soluble phosphates in substances containing sparingly soluble phosphates, polymer compounds having a carboxylic acid (salt) group in the molecule are preferably those in which the total amount of carboxyl groups and carboxylic acid bases is 9 mmol / g or more, and more preferably those in which it is 10 mmol / g or more.

[0011] The amount of carboxylic acid (salt) groups in a polymer compound containing a carboxylic acid (salt) within its molecule can be determined by the following method. (1) Place the sample in a beaker, weigh it accurately (1g), add 50ml of deionized water and stir to dissolve or swell the sample and obtain the sample solution. (2) Immerse the electrode of the potentiometric titrator in the sample solution and, while stirring, dropwise add a mixture of reagent primary hydrochloric acid (35%) and ion-exchanged water (35% hydrochloric acid:ion-exchanged water = 1:1) until the pH of the sample solution is 3 or less. (3) Add 0.1 mol / L potassium hydroxide methanol standard solution little by little, stirring until the potential difference becomes constant, and record the potential difference and the amount added when the potential difference becomes constant. Repeat this procedure until the endpoint is reached. The endpoint is determined by plotting the volume of standard solution added and the potential difference on graph paper, and marking the inflection point of the titration curve as the endpoint of the titration. (4) Perform the operations in (1) to (3) on two samples, and also perform a blank test. The total amount of carboxy groups and carboxylate groups contained in the molecule is calculated from the following formula, and the average value of the two samples is taken as the total amount of carboxy groups and carboxylate groups (mmol / g) contained in the molecule. The amount of carboxy groups and carboxylate groups contained in the molecule (mmol / g) = (A - B) × f × 0.1 / S A: The dropping amount (ml) of the 0.1 mol / L potassium hydroxide methanol standard solution required for this test B: The dropping amount (ml) of the 0.1 mol / L potassium hydroxide methanol standard solution required for the blank test f: The titer of the 0.1 mol / L potassium hydroxide methanol standard solution S: The sample collection amount (g)

[0012] Examples of the polymer compound having a carboxylic acid (salt) in the molecule used in the present invention include polymers containing structural units derived from (meth) acrylic acid and / or (meth) acrylate (also referred to as "polymer X"). In this specification, acrylic acid and / or (meth) acrylate are also referred to as "acrylic acid (salt)".

[0013] Polymer X contains structural units derived from (meth) acrylic acid (salt). Examples of (meth) acrylic acid (salt) that form the structural units of polymer X include acrylic acid, acrylate, methacrylic acid, methacrylate, and combinations thereof. In the present invention, (meth) acrylic acid means methacrylic acid and / or acrylic acid. Examples of the salt include alkali metal (such as lithium, sodium (also referred to as Na), and potassium) salts, alkaline earth metal (such as magnesium and calcium) salts, or ammonium (NH4) salts. Among these salts, from the viewpoint of absorption characteristics, etc., alkali metal salts and ammonium salts are preferred, more preferably alkali metal salts, and even more preferably sodium salts.

[0014] Specific examples of polymer X include polyacrylic acid, polyacrylate salts, polymethacrylic acid, polymethacrylate salts, copolymers of (meth)acrylic acid (salt) and starch, and copolymers of (meth)acrylic acid (salt) and other monomers.

[0015] The starch that constitutes the copolymer of (meth)acrylic acid (salt) and starch is not particularly limited, and examples include wheat starch, rice starch, corn starch, and potato starch.

[0016] The other monomers that constitute the copolymer of (meth)acrylic acid (salt) and other monomers mean monomers other than (meth)acrylic acid (salt). Specifically, (meth)acrylic acid 2-hydroxyethyl, (meth)acrylic acid n-methyl, (meth)acrylic acid n-ethyl, (meth)acrylic acid n-butyl, (meth)acrylic acid n-hexyl, polyethylene glycol mono(meth)acrylate, polypropylene glycol mono(meth)acrylate, polybutylene glycol mono(meth)acrylate, methoxypolyethylene glycol mono(meth)acrylate and other (meth)acrylic acid esters having a hydroxy group, an alkyl group, a polyoxyalkylene group, etc., 2-vinylpyridine, 4-vinylpyridine, diethylaminoethyl methacrylate, dipropylaminoethyl methacrylate, dimethylaminopropyl (meth)acrylamide, allyldimethylamine, allyldiethylamine, allyldipropylamine, (meth)acryldimethylamine, (meth)acryldiethylamine, (meth)acryldipropylamine, acrylamidopropyltrimethylammonium chloride, methacrylamidopropyltrimethylammonium chloride, methacrylic acid dimethylaminomethyl ethyl chloride, dimethylaminoethyl methacrylate, N-methacryloyloxyethyl-N,N-dimethylammonium-α-N-methyl carboxybetaine, cationic vinyl monomers such as dimethyldiallylammonium chloride, itaconic acid, aconitic acid, hydroxyacrylic acid, maleic anhydride, fumaric acid, vinylsulfonic acid, allylsulfonic acid, (meth)acrylic sulfonic acid, styrenesulfonic acid, 2-acrylamido-2-methylpropane sulfonic acid, anionic vinyl monomers such as acrylamidodimethylpropane sulfonic acid, vinyl acetate, vinyl propionate and other vinyl esters, ethylene, propylene, isobutylene, vinyl chloride, vinylidene chloride and other olefins, styrene, α-methylstyrene and other styrene-based monomers, butadiene, isoprene, chloroprene and other diene-based monomers, acrylonitrile, methacrylonitrile and other nitrile-based monomers can be mentioned.

[0017] Polymer X may be a crosslinked polymer. Examples of crosslinking agents that constitute a crosslinked polymer include N,N'-methylenebis(meth)acrylamide, ethylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, glycerin di(meth)acrylate, glycerin tri(meth)acrylate, trimethylolpropane tri(meth)acrylate, triallylamine, triallyl cyanurate, triallyl isocyanurate, tetraallyloxyethane, pentaerythritol triallyl ether, ethylene glycol diglycidyl ether, glycerin diglycidyl ether, and sorbitol polyglycidyl ether.

[0018] The amount of crosslinking agent can be appropriately selected according to the desired crosslinking density. For example, it may be 0.005 mol% or more, or 0.5 mol% or more, or 2.0 mol% or less, or 1.5 mol% or less, relative to the monomers constituting the crosslinked polymer.

[0019] Polymer X can be produced, for example, by a method comprising the steps of polymerizing (solution polymerization, emulsion polymerization, suspension polymerization, etc.) a composition containing constituent monomer components [(meth)acrylic acid (salt) and other monomers used as needed], a solvent, a polymerization initiator, and components used as needed (crosslinking agent, neutralizing agent, etc.) to obtain a polymer (polymerization step), drying the polymer, and further neutralizing the polymer with a neutralizing agent as needed.

[0020] The acidic groups of the constituent units constituting polymer X [for example, carboxyl groups of constituent units derived from acidic group-containing monomers such as (meth)acrylic acid] are preferably partially neutralized with a base, from the viewpoint of solubilizing sparingly soluble phosphates and facilitating the liberation of phosphate ions and / or available phosphate. As the base for neutralization, alkali metal hydroxides such as sodium hydroxide and potassium hydroxide, or alkali metal carbonates such as sodium carbonate, sodium bicarbonate, and potassium carbonate can be used. Neutralization may be performed at any of the following timings in the production of polymer X: before the polymerization process, during the polymerization process, after the polymerization process, or a combination thereof. Specifically, examples include neutralizing the acidic group-containing monomers before the polymerization process, and neutralizing the polymer obtained after polymerization in a water-containing gel state.

[0021] As a polymer compound having a carboxylic acid (salt) in its molecule, from the viewpoint of easily solubilizing poorly soluble phosphates and releasing phosphate ions and / or available phosphate, it is preferably a polymer containing constituent units derived from (meth)acrylic acid (salt), and more preferably polyacrylic acid (salt).

[0022] The fertilizer of the present invention comprises a substance containing sparingly soluble phosphate and a polymer compound having a carboxylic acid (salt) group in its molecule. When the fertilizer of the present invention is used in an environment where water is provided, the substance containing sparingly soluble phosphate and the polymer compound having a carboxylic acid (salt) group in its molecule mix with the water, solubilizing the sparingly soluble phosphate in the substance and releasing phosphate ions and / or available phosphate. The phosphate ions and available phosphate are components that can be absorbed by plants and function as fertilizer. In other words, according to the present invention, phosphate ions and / or available phosphate can be released simply by using it in an environment where water is provided, thus enabling it to function as a fertilizer in a simple and safe manner.

[0023] The amount of polymer compound having a carboxylic acid (salt) group in the molecule is preferably 0.01 to 200 parts by weight, and more preferably 0.1 to 100 parts by weight, per 100 parts by weight of the substance containing the sparingly soluble phosphate, from the viewpoint of easily solubilizing the sparingly soluble phosphate.

[0024] The fertilizer of the present invention may be obtained by further mixing water with a substance containing sparingly soluble phosphate and a polymer compound having a carboxylic acid (salt) group in its molecule. In this embodiment, because water is mixed in, the substance containing sparingly soluble phosphate and the polymer compound having a carboxylic acid (salt) group in its molecule contained in the fertilizer mix with the water, solubilizing the sparingly soluble phosphate in the substance and releasing phosphate ions and / or available phosphate, so that it functions as a fertilizer as is.

[0025] In the present invention, when the fertilizer is obtained by further mixing water with a substance containing a sparingly soluble phosphate and a polymer compound having a carboxylic acid (salt) group in its molecule, the amount of water to be mixed is preferably 10 to 50,000 parts by weight, and more preferably 1,000 to 30,000 parts by weight, per 100 parts by weight of the substance containing the sparingly soluble phosphate, from the viewpoint of ease of mixing.

[0026] The fertilizer of the present invention may contain other components besides those mentioned above (substances containing poorly soluble phosphates, polymer compounds having a carboxylic acid (salt) group in the molecule, and water used as needed). Examples of such other components include nitrogen sources such as ammonium sulfate, ammonium chloride, ammonium nitrate, urea, Chilean saltpeter, and calcium cyanamide; potassium sources such as wood ash, seaweed ash, potassium chloride, and potassium sulfate; and inorganic or organic soil conditioners such as zeolite, activated carbon, vermiculite, perlite, peat moss, and coco peat.

[0027] <Method of manufacturing fertilizer> The present invention provides a method for producing fertilizer, comprising step 1 of mixing a substance containing a sparingly soluble phosphate with a component containing a polymer compound having a carboxylic acid (salt) group in its molecule, wherein the total amount of carboxyl groups and carboxylic acid bases contained in the polymer compound having a carboxylic acid (salt) group in its molecule is 5 mmol / g or more.

[0028] Step 1 is a step of mixing a substance containing sparingly soluble phosphate with a polymer compound having a carboxylic acid (salt) group in its molecule. The substance containing sparingly soluble phosphate used in Step 1 is the same as the "substance containing sparingly soluble phosphate" described in the "fertilizer" section. The polymer compound having a carboxylic acid (salt) group in its molecule used in Step 1 is the same as the polymer compound having a carboxylic acid (salt) group in its molecule described in the "fertilizer" section.

[0029] In step 1, the amount of polymer compound having a carboxylic acid (salt) group in the molecule to be mixed is preferably 0.01 to 200 parts by weight, and more preferably 0.1 to 100 parts by weight, per 100 parts by weight of the substance containing the sparingly soluble phosphate, from the viewpoint of easily solubilizing the sparingly soluble phosphate.

[0030] In step 1, from the viewpoint of facilitating the mixing process and easily solubilizing the poorly soluble phosphate in the substance, it is preferable to further add water to a component containing a substance containing a poorly soluble phosphate and a polymer compound having a carboxylic acid (salt) group in its molecule and mix them. In step 1, the amount of water to be mixed is preferably 10 to 50,000 parts by weight, and more preferably 1,000 to 30,000 parts by weight, per 100 parts by weight of the substance containing the sparingly soluble phosphate.

[0031] The temperature conditions for carrying out Step 1 can be set considering the type and amount of substance containing sparingly soluble phosphate, the type and amount of polymer compound having a carboxylic acid (salt) group in its molecule, and the amount of water used as needed. From the viewpoint of making it easier to solubilize the sparingly soluble phosphate, it is preferable to carry out Step 1 at a temperature of 10°C to 80°C.

[0032] In the present invention, the method for producing fertilizer is preferably such that, from the viewpoint of sufficiently solubilizing the sparingly soluble phosphate in a substance containing sparingly soluble phosphate, a step (contact step) is performed in which, after mixing with water in step 1 to obtain a mixture, the substance containing sparingly soluble phosphate in the mixture is brought into contact with a polymer compound having a carboxylic acid (salt) group in its molecule. Methods for bringing the substance containing sparingly soluble phosphate into contact with the compound having a carboxylic acid (salt) group in its molecule include stirring the mixture obtained by performing step 1, and letting the mixture obtained by performing step 1 stand for a predetermined time.

[0033] The temperature conditions in the contact process can be set considering the type and amount of the substance containing the sparingly soluble phosphate, the type and amount of the polymer compound having a carboxylic acid (salt) group in its molecule, and the amount of water. From the viewpoint of simplifying the apparatus, the temperature conditions are preferably the same as those in process 1 (for example, 10°C to 80°C). From the viewpoint of sufficiently solubilizing the sparingly soluble phosphate in the substance containing the sparingly soluble phosphate, the contact time in the contact process is preferably 1 hour or more, more preferably 3 hours or more, preferably 240 hours or less, and more preferably 168 hours or less. As a contact process, from the viewpoint of being a simple method, it is preferable to allow the mixture obtained by performing step 1 to stand at a temperature of 10°C to 80°C.

[0034] If water is not mixed in step 1, when a mixture of components containing the sparingly soluble phosphate obtained in step 1 and a polymer compound having a total amount of carboxyl groups and carboxylic acid bases of 5 mmol / g or more in its molecule is used in an environment where water is provided, the phosphate-containing substance and the polymer compound having carboxylic acid (salt) groups in its molecule in the mixture mix with the water, solubilizing the sparingly soluble phosphate in the substance and releasing phosphate ions and / or available phosphate. Therefore, the mixture can function as a fertilizer when used in the presence of water.

[0035] When water is mixed in step 1, the mixture after water is added, or after the contact step if necessary, contains phosphate ions and / or available phosphate released from the sparingly soluble phosphate. Therefore, the mixture can be used as fertilizer as is. The solid obtained by removing water from the liquid component obtained by filtering the aforementioned mixture may be used as fertilizer, and furthermore, the precipitate (calcium phosphate, etc.) obtained by adding a calcium salt (e.g., calcium hydroxide, calcium chloride, etc.) to the aforementioned liquid component may also be used as fertilizer. [Examples]

[0036] The present invention will now be specifically described with reference to examples, but the present invention is not limited to these examples unless it deviates from the spirit of the invention. Unless otherwise specified, parts refer to parts by weight, and % refers to weight percent. Temperatures below include a range of ±2°C.

[0037] The components used in the examples and comparative examples are as follows: [Substance (A) containing poorly soluble phosphate: also referred to as "substance (A)"] (A-1) "Aluminum Phosphate" (manufactured by Junsei Chemical Co., Ltd.) (A-2) "Iron(III) phosphate dihydrate" (manufactured by Sigma-Aldrich) (A-3) "Tricalcium Phosphate" (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) (A-4) "Rice Husk Charcoal" (Manufactured by Rakuyo Co., Ltd.) (A-5) "Chicken Manure Charcoal" (Made by Takenochikara Rakuten Market Store) (A-6) "Soil" (Brown lowland soil collected in Kyoto Prefecture, which has been continuously fertilized with chemical fertilizers for 20 years) (A-7) "Soil" (Black volcanic soil collected in Saitama Prefecture) For substance (A), the total phosphorus concentration and the water-soluble phosphorus concentration were measured using the method described below, and the weight percentage of water-soluble phosphorus based on the total phosphorus weight was calculated. The results are shown in Table 1.

[0038] [High molecular weight compound (B) containing a total amount of carboxyl groups and carboxylic acid bases of 5 mmol / g or more within the molecule: [Also called compound (B)] (B-1) "Calibon L-400" [Sodium polyacrylate, manufactured by Sanyo Chemical Industries, Ltd.: weight-average molecular weight 10,000, solids content 43%] (B-2) Sodium polyacrylate obtained in Production Example 1 (B-3) Sodium polyacrylate crosslinked polymer obtained in Production Example 2 (B-4) Sodium polyacrylate crosslinked polymer obtained in Production Example 3 (B-5) Starch-acrylic acid copolymer obtained in Production Example 4 (B-6) Acrylic acid copolymer obtained in Production Example 5

[0039] [Compounds (B') in which the total amount of carboxyl groups and carboxylic acid bases contained within the molecule is less than 5 mmol / g: [Also called compound (B')] (B'-1) Carboxymethylcellulose sodium (manufactured by Fujifilm Wako Pure Chemical Corporation) (B'-2) Acrylic acid copolymer obtained in comparative manufacturing example 1 For compounds (B) and (B'), the solid content and carboxylic acid (salt) group content were calculated using the method described below. The results are shown in Table 2.

[0040] <Method for measuring the total phosphorus concentration in substance (A)> The total phosphorus concentration in substance (A) was measured by the following method. The phosphorus content (mass%) of the target substance was measured by X-ray fluorescence analysis. Using this measurement, the total phosphorus concentration (mgP / g) in the substance containing sparingly soluble phosphate was calculated using the following formula. [Total phosphorus concentration in substance (A) (mgP / g)] = [Phosphorus content (mass%)] × 10 Of the substances (A) to be measured, those other than reagents [(A-4), (A-5), (A-6), and (A-7)] were subjected to ashing treatment under the ashing conditions described below, and the results were measured and converted to their pre-ash composition ratios using the following formula. [Total phosphorus concentration in substance (A) before ashing (mgP / g)] = [Total phosphorus concentration in substance after ashing (mgP / g)] × [Amount remaining after ashing (%)] / 100 (Measurement conditions for the X-ray fluorescence apparatus: The same conditions apply to both ashing-treated and non-ashing-treated samples.) Device name: Supermini200 [Manufactured by Rigaku Corporation] Method: Wavelength dispersion type Measurement method: SQX analysis (FP method) Sample measurement diameter: 30 mm [Measurement conditions for (A-1), (A-2), and (A-3)] Measurement atmosphere: Helium Residue (main component): None Sample preparation method: press molding or loose powder method Film used: PP (polypropylene film) Sample amount: 2-3g [Measurement conditions for (A-4), (A-5), (A-6), and (A-7)] Measurement atmosphere: Vacuum Residue (main component): None Sample preparation method: Loose powder method Film used: PP Sample amount: 2-3g [Ashing treatment for (A-4), (A-5), (A-6), and (A-7)] Of the substances (A) to be measured, those other than the reagents [(A-4), (A-5), (A-6), and (A-7)] were subjected to ashing treatment using the following method. (a) The sample was finely ground in a mortar. (b) The finely ground sample was pre-dried using a circulating air dryer at 105°C for 3 hours to obtain a pre-dried sample. (c) 10 g of the pre-dried sample was accurately weighed into an alumina crucible whose tare weight had been measured (S1). It was placed in an electric furnace and heated at 600°C for 1 hour, then at 1050°C for 1 hour, and its weight was measured after it returned to room temperature (25°C) (G1). (d) The amount of remaining ashing (%) was calculated using the following formula. Ashing amount (%) = [{Weight after returning to room temperature (G1) - Tare weight of crucible} / Sample weight (S1)] × 100

[0041] <Method for measuring the concentration of water-soluble phosphorus in substance (A)> The concentration of water-soluble phosphorus in substance (A) was determined by the following method. (1) 1 g of the sample was mixed with 99 g of deionized water and allowed to stand at room temperature (25°C) for 3 hours. Using a 2.5 ml Terumo syringe, the supernatant of the sample solution after standing at room temperature was filtered through a 0.45 μm syringe filter and the filtrate was collected. (2) Take 1.5 ml of calibration aqueous solution (prepared using a phosphate standard solution and diluted to 0.2 ppm, 0.5 ppm, 1 ppm, and 5 ppm as phosphate ions) into each of the attached cells of Pack Test (R) Phosphate / Phosphate Phosphate WAK-PO4 (Kyoritsu Chemical Research Institute), add 4 drops of the attached sulfuric acid, close the lid, and shake four times. (3) Following the instructions for the phosphate pack test, the color developing solution in the individual packaging was mixed, the individual packaging was cut open with scissors, and the contents were returned to the cell. (4) The absorbance was measured 3 minutes after the calibration curve aqueous solution was drawn up. The measurement was performed using the apparatus described below and under the following conditions. Absorbance measuring device: UV-Vis spectrophotometer UV-1280 [(manufactured by Shimadzu Corporation)] Measurement conditions: Photometric measurement, wavelength 650 nm, optical path length 5 mm (5) The absorbance of the sample filtrate was measured using the same method as described in (2) to (4). At this time, if the value exceeded the calibration curve of 5 ppm, it was diluted and remeasured until the absorbance was between 0.2 ppm and 5 ppm. If the value was less than 0.2 ppm, it was considered to be below the detection limit. (6) The concentration of phosphate ions in the filtrate of the sample was determined from the calibration curve. The concentration of water-soluble phosphorus in a substance containing poorly soluble phosphate was calculated by substituting the phosphate ion concentration into the following formula. [Concentration of phosphate phosphorus in the filtrate (ppm)] = 31 × [Concentration of phosphate ions in the filtrate (ppm)] / 95

[0042] <Method for calculating the weight percentage of water-soluble phosphorus based on the total phosphorus weight [100 × (water-soluble phosphorus / total phosphorus)]> The weight percentage of water-soluble phosphorus in substance (A), based on the total phosphorus weight (indicated as "percentage of water-soluble phosphorus" in Table 1), was calculated using the following formula. Weight percentage of water-soluble phosphorus based on total phosphorus weight (%) = 100 × [Concentration of water-soluble phosphorus in substances containing sparingly soluble phosphates (mgP / g)] / [Total phosphorus concentration in substances containing sparingly soluble phosphates (mgP / g)]

[0043] [Table 1]

[0044] <Manufacturing Example 1: Manufacturing of Sodium Polyacrylate (B-2)> 500 parts of deionized water were charged into a reaction vessel equipped with a stirrer, dropping funnel, nitrogen gas inlet tube, thermometer, and reflux condenser. The system was stirred, replaced with nitrogen gas, and the temperature was raised to 85°C. At the same temperature (85°C), a mixture of 200 parts acrylic acid, 0.07 parts dodecyl mercaptan, 1.4 parts 2,2'-azobis(2,4-dimethylvaleronitrile), and 16 parts isopropyl alcohol was added dropwise over 3 hours, and the reaction was continued at the same temperature for another hour. The reaction mixture was cooled to 30°C, 396 parts of 28% NaOH water were added to adjust the pH to 10.0, and then further diluted with 1063 parts of deionized water to obtain a diluted solution containing sodium polyacrylate (B-2). The weight-average molecular weight of sodium polyacrylate (B-2) was 1,000,000, and the solid content in the diluted solution was 12%.

[0045] <Manufacturing Example 2: Manufacturing of Sodium Polyacrylate Crosslinked Polymer (B-3)> To 1,000 parts of an aqueous solution containing 25% monomer components consisting of 72 mol% sodium acrylate, 27.95 mol% acrylic acid, and 0.05 mol% N,N-methylenebisacrylamide, 0.01 parts hydrogen peroxide and 0.02 parts ascorbic acid were added, and adiabatic polymerization was carried out in a nitrogen atmosphere at a polymerization initiation temperature of 10°C for 5 hours to obtain a gel-like polymer. This gel-like polymer was dried in a belt-type hot air dryer set to 150°C, and then pulverized to a particle size of 20-60 mesh to obtain polysodium acrylate crosslinked polymer (B-3). Polysodium acrylate crosslinked polymer (B-3) was insoluble in water, and its weight-average molecular weight could not be measured.

[0046] <Manufacturing Example 3: Manufacturing of Sodium Polyacrylate Crosslinked Polymer (B-4)> In Production Example 2, the same procedure as in Production Example 2 was performed except that the gel-like polymer was pulverized to a particle size of 60-150 mesh to obtain sodium polyacrylate crosslinked polymer (B-4). (B-4) was insoluble in water, and its weight-average molecular weight could not be measured.

[0047] <Manufacturing Example 4: Production of Starch-Acrylic Acid Copolymer (B-5)> Ten parts corn starch, 200 parts water, and 1,000 parts methanol were charged into a reaction vessel equipped with a stirring rod, nitrogen blowing tube, and thermometer, and stirred at 50°C for 1 hour under a nitrogen atmosphere. After cooling to 30°C, 20 parts acrylic acid, 80 parts sodium acrylate, 1 part N,N-methylenebisacrylamide, and 0.5 parts azobisvaleric acid as a polymerization catalyst were added, and polymerization was carried out by stirring at 60°C for 6 hours, yielding a white turbid liquid [I]. After filtering this white turbid liquid [I], the resulting powder was washed with a water-methanol mixed solution (water to methanol weight ratio 2:10), dried under reduced pressure at 60°C for 3 hours, and then pulverized to obtain 49 parts of powdered starch-acrylic acid copolymer (B-5). Starch-acrylic acid copolymer (B-5) was insoluble in water, and its weight-average molecular weight could not be measured.

[0048] <Manufacturing Example 5: Manufacturing of Acrylic Acid Copolymer (B-6)> 240 parts of deionized water were charged into a reaction vessel equipped with a stirrer, dropping funnel, nitrogen gas inlet tube, thermometer, and reflux condenser. The system was stirred, replaced with nitrogen gas, and the temperature was raised to 85°C. At the same temperature, a mixture of 87.4 parts acrylic acid, 130.1 parts hydroxyethyl methacrylate, 4.9 parts dodecyl mercaptan, 0.6 parts 2,2'-azobis(2,4-dimethylvaleronitrile), and 16 parts isopropyl alcohol was added dropwise over 3 hours, and the reaction was continued at the same temperature for another hour. The resulting mixture was cooled to 30°C, and 73 parts of 28% NaOH water were added to adjust the pH to 6.0 to obtain acrylic acid copolymer (B-6). The weight-average molecular weight of the acrylic acid copolymer contained in acrylic acid copolymer (B-6) was 12,000, and the solid content in the polymer was 40%.

[0049] <Comparative Manufacturing Example 1: Manufacturing of Acrylic Acid Copolymer (B'-2)> In Production Example 5, an acrylic acid copolymer (B'-2) was obtained in the same manner as in Production Example 5, except that the amount of acrylic acid was changed to 43.2 parts and the amount of hydroxyethyl methacrylate was changed to 169.2 parts. The weight-average molecular weight of the acrylic acid copolymer contained in the acrylic acid copolymer (B'-2) was 15,000, and the solids content was 40%.

[0050] <Method for measuring the weight-average molecular weight of (co)polymers> The weight-average molecular weight of the (co)polymers obtained in the production example and comparative production example was measured by gel permeation chromatography (GPC). The measurement conditions were as follows. (Measurement conditions) Equipment: Gel permeation chromatography ["HLC-8120GPC", manufactured by Tosoh Corporation] Columns: "TSKgelG6000PWxl" and "TSKgelG3000PWxl" [both manufactured by Tosoh Corporation] connected in series. Eluent: 0.5% by weight sodium acetate dissolved in methanol / water = 30 / 70 (volume ratio). Reference substance: Polyethylene glycol (hereinafter abbreviated as PEG) Injection conditions: Sample concentration 0.25 wt%, column temperature 40°C

[0051] <Method for measuring solid content> The solid content of compounds (B)[(B-1~(B-6)] and (B')[(B'-1), (B'-2)] was measured by the following method. A 1g sample was precisely weighed into an aluminum cup (container weight: T) whose weight was measured to four decimal places using a precision balance, and the sample weight (S) before drying was recorded. The aluminum cup containing the 1g sample was placed in a hot air circulating dryer heated to 130°C, dried for 45 minutes, then removed, and the total weight (G) after drying was precisely weighed. The solid content was calculated using the following formula. Solids content (%) = [{Total weight after drying (G) - Container weight (T)} / Sample weight before drying (S)] × 100

[0052] <Method for calculating the total amount of carboxyl groups and carboxylic acid bases contained in compound (B) [(B-1~(B-6)] and compound (B') [(B'-1)~(B'-2)]> (1) The sample was placed in a beaker and its weight (1 g) was accurately measured. 50 ml of deionized water was added and stirred to dissolve or swell the sample and obtain the sample solution. (2) The electrode of the potentiometric titrator was immersed in the sample solution, and while stirring at a speed that did not cause the solution to splash out of the beaker, a mixture of reagent primary hydrochloric acid (35%) and ion-exchanged water (35% hydrochloric acid:ion-exchanged water = 1:1) was added dropwise with a dropper until the pH of the sample solution was 3 or less. (3) A small amount of 0.1 mol / L potassium hydroxide methanol standard solution was added, and the mixture was stirred until the potential difference became constant. The potential difference and the amount added at the point where the potential difference became constant were recorded. This procedure was repeated until the endpoint was reached. The endpoint was determined by plotting the volume of standard solution added and the potential difference on graph paper, and the inflection point of the titration curve was used as the endpoint of the titration. (4) The procedures in (1) to (3) were performed on two samples, and a blank test was also conducted. The total amount of carboxyl groups and carboxylic acid bases contained in the molecule was calculated using the following formula, and the average value of the two samples was taken as the total amount of carboxyl groups and carboxylic acid bases contained in the molecule (mmol / g). The total amount of carboxyl groups and carboxylic acid bases contained in the molecule (mmol / g) = (AB) × f × 0.1 / S A: The amount of 0.1 mol / L potassium hydroxide methanol standard solution dropped into this test (ml) B: Volume (ml) of 0.1 mol / L potassium hydroxide methanol standard solution required for the blank test. f: Titer of 0.1 mol / L potassium hydroxide methanol standard solution S: Sample collection amount (g)

[0053] [Table 2]

[0054] <Examples 1-13, Comparative Examples 1-5> Fertilizers for the examples and comparative examples were prepared, and the following evaluation tests were conducted.

[0055] <Evaluation Test 1: pH of the supernatant after phosphate recovery> The pH of the supernatant was measured using the following equipment under the following conditions. The results are shown in Table 3 or Table 4. Equipment used for pH measurement: Portable pH meter D-71 (manufactured by HORIBA) Measurement conditions: 25±2℃ A pH of 6.0 to 9.0 in the supernatant is preferable because it eliminates the need for a neutralization step.

[0056] <Evaluation Test 2: Percentage of water-soluble phosphorus (phosphate)> (1) Calculation of the phosphorus concentration contained in the mixture The phosphorus concentration in the mixture obtained in the examples and comparative examples [a mixture of substance (A), compound (B) or compound (B'), and deionized water] was calculated using the following formula. [Phosphorus concentration in the mixture (ppm)] = [Amount of substance (A) added (g)] × [Total phosphorus concentration of substance (A) (mgP / g)] × 10 (2) Calculation of the water-soluble ratio of phosphorus (ratio of phosphoric acid) The water-soluble phosphorus ratio in the filtrate obtained from the examples and comparative examples was calculated using the following formula. The results are shown in Table 3 or Table 4. [Ratio of phosphorus solubility in water (%)] = 100 × {[Concentration of phosphate phosphorus in the filtrate (ppm)] / [Concentration of phosphorus contained in the mixture (ppm)]} A higher water-soluble phosphorus ratio is preferable, specifically 0.2% or higher.

[0057] (Example 1) In a 100 ml glass beaker, 1 part aluminum phosphate (A-1), a substance containing sparingly soluble phosphate, and 0.03 parts of sodium polyacrylate (B-1) [Sanyo Chemical Industries, Ltd., Carribon N-400], a polymer compound (B) containing 5 mmol / g or more of carboxylic acid groups in its molecule, were measured out in terms of solid content and mixed to obtain a mixture. Then, 99 parts of deionized water were added and the mixture was gently shaken to obtain a mixed solution. The mixed solution was left to stand at room temperature at 25°C for 3 hours. The supernatant was collected using a 2.5 ml Terumo syringe, filtered through a 0.45 μm syringe filter, and the filtrate was recovered. The concentration of phosphate phosphorus in the filtrate was measured using Pack Test(R) Phosphate / Phosphate Phosphorus WAK-PO4 (Kyoritsu Chemical Research Institute) and a spectrophotometer. The concentration of phosphate phosphorus in the filtrate was 10 ppm. Furthermore, the pH of the supernatant remaining in the beaker after the filtrate was collected (the supernatant after liberation of phosphate) was measured and found to be 7.5.

[0058] (Example 2) In a 100 ml glass beaker, 0.5 parts of aluminum phosphate (A-1), a substance containing sparingly soluble phosphate, and 0.03 parts of sodium polyacrylate (B-1), compound (B), were measured out in terms of solid content and mixed to obtain a mixture. Then, 99 parts of deionized water were added and the mixture was gently shaken to obtain a mixed solution. The beaker of this mixed solution was immersed in a 50°C water bath and allowed to stand for 3 hours. The supernatant was drawn up using a 2.5 ml Terumo syringe, and the filtrate was collected using a 0.45 μm syringe filter. Similar to Example 1, the concentration of phosphate phosphorus in the filtrate was measured, and the concentration was 26 ppm. Also, similar to Example 1, the pH of the supernatant remaining in the beaker after collecting the filtrate was measured, and the pH was 7.5.

[0059] (Example 3) In Example 1, the same procedure was followed as in Example 1, except that 0.03 parts of cross-linked polyacrylate (B-4) was added instead of sodium polyacrylate (B-1) to obtain the mixture, and the standing time of the mixture with water was changed from 3 hours to 1 week. In addition, the phosphate concentration of the filtrate and the pH of the supernatant remaining in the beaker after collecting the filtrate were measured using the same method as in Example 1. The phosphate concentration of the filtrate was 35 ppm, and the pH of the supernatant was 7.0.

[0060] (Example 4) In a 100 ml glass beaker, 0.1 parts of aluminum phosphate (A-1), a substance containing sparingly soluble phosphate, and 0.03 parts of sodium polyacrylate crosslinked polymer (B-4), compound (B), were measured out and mixed to obtain a mixture. Then, 99 parts of deionized water were added and the mixture was gently shaken to obtain a mixed solution. The beaker containing the mixed solution was immersed in an 80°C water bath and left to stand for 3 hours. The supernatant was drawn up using a 2.5 ml Terumo syringe, and the filtrate was collected by attaching a 0.45 μm syringe filter. Similar to Example 1, the concentration of phosphate phosphorus in the filtrate was measured, and the concentration of phosphate phosphorus in the filtrate was 30 ppm. Furthermore, similar to Example 1, the pH of the supernatant remaining in the beaker after collecting the filtrate was measured, and the pH of the supernatant was 7.0.

[0061] (Example 5) In Example 1, the same procedure was followed except that 1 part of iron(III) phosphate dihydrate (A-2) was added instead of aluminum phosphate (A-1), and 0.03 parts of sodium polyacrylate (B-2) was added instead of sodium polyacrylate (B-1). Furthermore, the phosphate phosphorus concentration of the filtrate and the pH of the supernatant remaining in the beaker after collecting the filtrate were measured using the same method as in Example 1. The phosphate phosphorus concentration of the filtrate was 12 ppm, and the pH of the supernatant was 9.0.

[0062] (Example 6) In Example 1, the same procedure was followed except that 1 part tricalcium phosphate (A-3) was added instead of aluminum phosphate (A-1), and 0.03 parts of cross-linked polyacrylate (B-3) was added instead of sodium polyacrylate (B-1). Furthermore, the phosphate phosphorus concentration in the filtrate and the pH of the supernatant remaining in the beaker after collecting the filtrate were measured using the same method as in Example 1. The phosphate phosphorus concentration in the filtrate was 11 ppm, and the pH of the supernatant was 7.0.

[0063] (Example 7) In Example 4, the same procedure was followed except that a portion of rice husk charcoal (A-4) was added instead of aluminum phosphate (A-1). Furthermore, the phosphate phosphorus concentration in the filtrate and the pH of the supernatant remaining in the beaker after collecting the filtrate were measured using the same method as in Example 1. The phosphate phosphorus concentration in the filtrate was 3 ppm, and the pH of the supernatant was 8.4.

[0064] (Example 8) In Example 4, the same procedure was followed except that 1 part of chicken manure char (A-5) was added instead of aluminum phosphate (A-1), and 0.03 parts of starch-acrylic acid copolymer (B-5) was added instead of sodium polyacrylate crosslinked polymer (B-4). In addition, the phosphate phosphorus concentration of the filtrate and the pH of the supernatant remaining in the beaker after the filtrate was collected were measured using the same method as in Example 1. The phosphate phosphorus concentration of the filtrate was 55 ppm, and the pH of the supernatant was 9.0.

[0065] (Example 9) In Example 4, the same procedure was followed except that a portion of soil (A-6) was added instead of aluminum phosphate (A-1). Furthermore, the phosphate phosphorus concentration in the filtrate and the pH of the supernatant remaining in the beaker after collecting the filtrate were measured using the same method as in Example 1. The phosphate phosphorus concentration in the filtrate was 2 ppm, and the pH of the supernatant was 7.3.

[0066] (Example 10) In Example 1, the same procedure was followed except that acrylic acid copolymer (B-6) was added in an amount equivalent to 0.03 parts of solid content instead of sodium polyacrylate (B-1). Furthermore, the phosphate phosphorus concentration of the filtrate and the pH of the supernatant remaining in the beaker after collecting the filtrate were measured using the same method as in Example 1. The phosphate phosphorus concentration of the filtrate was 5 ppm, and the pH of the supernatant was 6.0.

[0067] (Example 11) In Example 4, the same procedure was followed except that sodium polyacrylate (B-1) was added in the same amount as in Example 4, in terms of solid content, instead of the sodium polyacrylate crosslinked polymer (B-4). In addition, the phosphate phosphorus concentration of the filtrate and the pH of the supernatant remaining in the beaker after the filtrate was collected were measured using the same method as in Example 1. The phosphate phosphorus concentration of the filtrate was 28 ppm, and the pH of the supernatant was 7.6.

[0068] (Example 12) In Example 1, the same procedure as in Example 4 was followed, except that sodium polyacrylate (B-1) was added in an amount equivalent to 0.002 parts in terms of solid content. Furthermore, the phosphate phosphorus concentration of the filtrate and the pH of the supernatant remaining in the beaker after collecting the filtrate were measured using the same method as in Example 1. The phosphate phosphorus concentration of the filtrate was 5 ppm, and the pH of the supernatant was 7.3.

[0069] (Example 13) In Example 1, the same procedure was followed except that 1 part of soil (A-7) was added instead of aluminum phosphate (A-1), 0.03 parts of cross-linked polyacrylate polymer (B-3) was added instead of sodium polyacrylate (B-1), and the mixture was left to stand at 10°C for 24 hours. The phosphate phosphorus concentration in the filtrate and the pH of the supernatant remaining in the beaker after collecting the filtrate were measured using the same method as in Example 1. The phosphate phosphorus concentration in the filtrate was 2 ppm, and the pH of the supernatant was 7.5.

[0070] (Comparative Example 1) In Example 3, the same procedure was followed except that 0.03 parts of sodium carboxymethylcellulose (B'-1) was added instead of sodium polyacrylate crosslinked polymer (B-4). Furthermore, the phosphate phosphorus concentration of the filtrate and the pH of the supernatant remaining in the beaker after collecting the filtrate were measured using the same method as in Example 1. The phosphate phosphorus concentration of the filtrate was 2 ppm, and the pH of the supernatant was 7.1.

[0071] (Comparative Example 2) In Example 1, the same procedure was followed except that an acrylic acid copolymer (B'-3) was added in the same amount as in Example 1, in terms of solid content, instead of sodium polyacrylate (B-1). Furthermore, the phosphate phosphorus concentration of the filtrate and the pH of the supernatant remaining in the beaker after collecting the filtrate were measured using the same method as in Example 1. The phosphate phosphorus concentration of the filtrate was 2 ppm, and the pH of the supernatant was 6.9.

[0072] (Comparative Example 3) In Example 9, the same procedure was followed except that 0.03 parts of carboxymethylcellulose (B'-1) were added instead of sodium polyacrylate crosslinked polymer (B-4). Furthermore, the phosphate phosphorus concentration of the filtrate and the pH of the supernatant remaining in the beaker after collecting the filtrate were measured using the same method as in Example 1. The phosphate phosphorus concentration of the filtrate was 0.1 ppm, and the pH of the supernatant was 6.9.

[0073] (Comparative Example 4) In Example 9, the same procedure was followed except that the sodium polyacrylate crosslinked polymer (B-4) was not used. Furthermore, using the same method as in Example 1, the phosphate phosphorus concentration in the filtrate and the pH of the supernatant remaining in the beaker after collecting the filtrate were measured. The phosphate phosphorus concentration in the filtrate was 0.1 ppm, and the pH of the supernatant was 6.0.

[0074] (Comparative Example 5) In Example 13, the same procedure was followed except that the polyacrylate sodium crosslinked polymer (B-3) was not used. Furthermore, using the same method as in Example 1, the phosphate phosphorus concentration in the filtrate and the pH of the supernatant remaining in the beaker after collecting the filtrate were measured. The phosphate phosphorus concentration in the filtrate was 0 ppm, and the pH of the supernatant was 6.0.

[0075] [Table 3]

[0076] [Table 4]

[0077] As shown in Tables 3 and 4, the example product containing a substance with poorly soluble phosphate and a polymer compound with a total amount of carboxyl groups and carboxylic acid bases of 5 mmol / g or more in the molecule showed a higher phosphorus solubility ratio than the comparative example product. In other words, it can be seen that phosphoric acid is efficiently dissolved into water simply by using the example product containing a substance with poorly soluble phosphate and a polymer compound with a total amount of carboxyl groups and carboxylic acid bases of 5 mmol / g or more in the molecule in an environment where water is provided. The phosphoric acid dissolved into water is phosphate ions and / or available phosphoric acid, which are components that can be absorbed by plants and function as fertilizer. Furthermore, since the pH of the supernatant after liberating phosphoric acid from the example product is in the range of 6 to 9, according to the present invention, the step of returning the liquid to neutral after liberating phosphoric acid is not essential. In other words, according to the present invention, a substance containing a sparingly soluble phosphate and a compound in which the total amount of carboxyl groups and carboxylic acid bases contained in the molecule is 5 mmol / g or more can be used as a fertilizer simply by using it in an environment where water is provided. As a result, it can be seen that the fertilizer of the present invention is a fertilizer that can be obtained by a safe and simple method.

Claims

1. A substance containing a poorly soluble phosphate, a polymer compound having a carboxyl group and / or a carboxylic acid base in its molecule, and water. With respect to 100 parts by weight of the substance containing the sparingly soluble phosphate, the amount of the polymer compound having a carboxyl group and / or a carboxylic acid base in its molecule is 0.1 to 100 parts by weight, and the amount of water is 1,000 to 30,000 parts by weight. A fertilizer in which the total amount of carboxyl groups and carboxylic acid bases contained in the polymer compound having a carboxyl group and / or a carboxylic acid base in its molecule is 5 mmol / g or more.

2. The fertilizer according to claim 1, wherein the polymer compound having a carboxyl group and / or a carboxylic acid base in the molecule is a polymer containing constituent units derived from (meth)acrylic acid and / or (meth)acrylate salt.

3. Step 1 involves mixing a substance containing a poorly soluble phosphate, a polymer compound having a carboxyl group and / or a carboxylic acid base in its molecule, and water. In step 1, the amount of the polymer compound having a carboxyl group and / or a carboxylic acid base in its molecule is 0.1 to 100 parts by weight per 100 parts by weight of the substance containing the sparingly soluble phosphate. In step 1, 100 to 30,000 parts by weight of water are mixed with 100 parts by weight of a substance containing a sparingly soluble phosphate. A method for producing fertilizer, wherein the total amount of carboxyl groups and carboxylic acid bases contained in the polymer compound having a carboxyl group and / or a carboxylic acid base in its molecule is 5 mmol / g or more.

4. A method for producing fertilizer according to claim 3, wherein the polymer compound having a carboxyl group and / or a carboxylic acid base in the molecule is a polymer containing constituent units derived from (meth)acrylic acid and / or (meth)acrylate salt.

5. A method for producing fertilizer according to claim 4, further comprising the step of allowing a mixture obtained by mixing with water in step 1 to stand at a temperature of 10°C to 80°C.

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