Iron reducing agent containing humic acid extract, use of humic acid extract, and reduction method

Abstract

The present embodiment addresses the problem of providing a material capable of improving the state of iron in alkali soil.　The present embodiment pertains to an iron reducing agent for reducing trivalent iron in soil to divalent iron during alkali soil cultivation, the iron reducing agent containing a humic acid extract which contains at least one selected from among fulvic acid and humic acid and is extracted from a humic acid crude product obtained through the nitric acid oxidation of young charcoal.

Description

Iron reducing agent containing humic acid extract, use of humic acid extract, and reduction method

[0001] The present disclosure relates to iron reducing agents, including humic acid extracts, uses of humic acid extracts, and reduction methods.

[0002] Various growth disorders caused by trace element deficiencies in agricultural crops have long been known, and the importance of trace elements for crop growth has been highlighted. Meanwhile, approximately one-third of the world's land area is covered by alkaline soils, particularly alkalized calcareous soils (calcareous alkaline soils), which are considered unsuitable for agriculture. Iron is present in large quantities on the earth's surface as iron ore and iron oxide. However, in calcareous alkaline soils, iron, a trace element essential for plant survival, exists as iron (III) hydroxide, an insoluble form that is insoluble in water. Because insoluble trivalent iron is difficult for plants to absorb, plants in calcareous alkaline soils are prone to iron deficiency due to their inability to absorb iron. The problem of trace element deficiency in calcareous alkaline soils, which account for 35% of cultivated land, is difficult to solve through fertilization.

[0003] Solubilizing this insoluble iron is an extremely difficult task. Iron is an essential trace element for plants, contributing to various functions such as chlorophyll biosynthesis, photosynthesis, and respiration. However, because crops cannot absorb iron in alkaline soils, they die from iron deficiency (chlorosis), resulting in reduced yield and quality.

[0004] Therefore, providing a technique for improving iron absorption by crops in alkaline soil is important for enabling normal growth of crops on infertile farmland and achieving increased food production.

[0005] Generally, measures to prevent iron deficiency in agricultural crops include foliar spraying or soil drench of chelated iron such as ferric ethylenediaminetetraacetate (Fe-EDTA), Fe-EDDHA, or Fe-HBED. Chelated iron is a form of iron that is easily absorbed by crops, and is therefore commonly used to improve iron deficiency (Patent Document 1).

[0006] Patent No. 5388861

[0007] Chelated iron, such as Fe-EDTA, which is used to supply iron to crops, has excellent iron supplying properties. However, chelating compounds such as Fe-EDTA have low biodegradability, resulting in unabsorbed chelated iron remaining in the environment. Furthermore, among these chelating compounds, ethylenediaminetetraacetic acid (EDTA) captures and solubilizes harmful heavy metals in the environment when released into the environment. The resulting heavy metal chelates are a concern as they may cause contamination of groundwater and other areas, and are subject to regulation in Europe. Furthermore, EDDHA and HBED, which are also used as iron chelating agents, are very expensive and have limited versatility as agricultural materials. Therefore, the development of materials that can be used in place of these chelating compounds is desirable.

[0008] Therefore, an object of the present disclosure is to provide a material that can improve the iron form in alkaline soil.

[0009] The present inventors conducted extensive research to solve the above-mentioned problems and discovered that a specific humic acid extract has an excellent ability to reduce trivalent iron in alkaline soil to divalent iron, leading to the present disclosure.

[0010] Therefore, examples of aspects of this embodiment are as follows. (1) An iron reducing agent for reducing trivalent iron to divalent iron in soil cultivated in alkaline soil, comprising a humic acid extract containing at least one selected from fulvic acid and humic acid extracted from a crude humic acid obtained by oxidizing young charcoal with nitric acid. (2) The iron reducing agent according to (1), wherein the young charcoal is lignite. (3) The iron reducing agent according to (1) or (2), wherein the crude humic acid has a melanic index of 2.0 to 3. (4) The iron reducing agent according to any one of (1) to (3), wherein the cultivation in alkaline soil is the cultivation of grasses. (5) The iron reducing agent according to any one of (1) to (4), wherein the alkaline soil is soil containing calcareous components. (6) Use of a humic acid extract containing at least one selected from fulvic acid and humic acid extracted from a crude humic acid obtained by oxidizing young charcoal with nitric acid, for reducing trivalent iron in soil under alkaline soil cultivation to divalent iron. (7) The use according to (6), wherein the young charcoal is lignite. (8) The use according to (6) or (7), wherein the alkaline soil cultivation is the cultivation of grasses. (9) The use according to any one of (6) to (8), wherein the alkaline soil is soil containing calcareous components. (10) The use according to any one of (6) to (9), wherein the melanic index of the crude humic acid is 2.0 to 3. (11) A method for reducing trivalent iron in soil under alkaline soil cultivation to divalent iron, the method comprising a step of applying to the soil a humic acid extract containing at least one selected from fulvic acid and humic acid extracted from a crude humic acid obtained by oxidizing young charcoal with nitric acid. (12) The method according to (11), wherein the young charcoal is lignite. (13) The method according to (11) or (12), wherein the melanic index of the humic acid crude product is 2.0 to 3. (14) The method according to any one of (11) to (13), wherein the alkaline soil cultivation is the cultivation of a grass family plant. (15) The method according to any one of (11) to (14), wherein the alkaline soil is soil containing a calcareous component. (16) An iron-reducing agent for reducing trivalent iron in soil to divalent iron in alkaline soil cultivation, comprising a humic acid extract containing at least one selected from fulvic acid and humic acid extracted from humic acid crude product.(17) An iron-reducing agent comprising a humic acid extract containing at least one selected from fulvic acid and humic acid. (18) An iron-reducing agent comprising a humic acid extract for reducing trivalent iron to divalent iron in soil cultivated in alkaline soil. This specification incorporates the disclosure of Japanese Patent Application No. 2023-146640, from which the present application claims priority.

[0011] The present disclosure provides materials that can improve iron morphology in alkaline soil.

[0012] This embodiment is an iron reducing agent for reducing trivalent iron to divalent iron in soil cultivated in alkaline soil, which contains a humic acid extract containing at least one selected from fulvic acid and humic acid extracted from a crude humic acid obtained by oxidizing young charcoal with nitric acid.

[0013] By reducing trivalent iron to divalent iron, the insoluble trivalent iron can be reduced to divalent iron, making it easier to solubilize. The insolubilization of trivalent iron refers to the formation of compounds insoluble or hardly soluble in water, such as hydroxides or oxides, by iron with a valence of +3 (trivalent iron), and the solubilization of divalent iron refers to the ionization of iron with a valence of +2 (divalent iron) to form Fe ２＋ This refers to the leaching out of iron as a water-soluble compound. Divalent iron exists in soil as water-soluble compounds such as hydroxide compounds and oxide compounds. Examples of soluble divalent iron compounds include iron(II) chloride, iron(II) sulfate, and iron(II) hydroxide. Examples of insoluble trivalent iron compounds (insoluble trivalent iron) include iron(III) chloride, iron(III) oxide, and iron(III) hydroxide.

[0014] The iron reducing agent according to the present embodiment includes a humic acid extract containing at least one selected from fulvic acid and humic acid extracted from a crude humic acid obtained by oxidizing young charcoal with nitric acid. Because the humic acid extract possesses excellent iron-reducing power, application to alkaline soil can effectively reduce trivalent iron in alkaline soil to divalent iron, which is readily absorbed by crops. Therefore, application of the iron reducing agent according to the present embodiment to alkaline soil can reduce trivalent iron to divalent iron, improving the iron form and increasing the availability of iron to crops. As a result, iron deficiency in crops in alkaline soil can be treated, ameliorated, prevented, suppressed, or alleviated, and crop quality can be improved. Furthermore, unlike compounds such as Fe-EDTA, which are difficult to decompose in the environment, the iron reducing agent according to the present embodiment can be produced from naturally occurring raw materials (young charcoal), thereby minimizing environmental impact. Furthermore, the iron reducing agent according to the present embodiment can be produced inexpensively, making it advantageous in terms of cost.

[0015] This embodiment will be described in detail below.

[0016] In the iron reducing agent according to this embodiment, alkaline soil refers to soil with a pH of greater than 7.

[0017] The pH of the soil can be obtained by adding 25 ml of distilled water to 10 g of air-dried soil, shaking the mixture for 1 hour, and measuring the pH of the resulting suspension.

[0018] Alkaline soils include not only inherently alkaline soils but also soils that have been converted from non-alkaline soils into alkaline soils through, for example, fertilization or desertification. Examples of inherently alkaline soils include soils containing various calcareous components, such as shell fossil soil, calcareous soil, and coralline soil. These calcareous components may be contained in a single type or in combination with two or more types. Furthermore, the term also includes mixed soils that are alkaline soils overall, consisting of alkaline soils containing these various calcareous components and non-alkaline soils.

[0019] The pH of alkaline soil is 7 or higher, more preferably greater than 7, preferably 7.5 or higher, and preferably 8 or higher. When the pH of alkaline soil is 7.5 or higher, the iron component exists as trivalent iron, and the iron reducing agent according to this embodiment can be effectively used. The pH of alkaline soil is, for example, 11 or lower, preferably 10 or lower. A pH of alkaline soil of 11 or lower is preferable from the viewpoint of promoting plant growth.

[0020] As used herein, alkaline soil cultivation simply refers to the cultivation of plants in alkaline soil. Soil cultivation refers to cultivation using natural or artificial soil. Soil cultivation includes, for example, open-field cultivation, greenhouse cultivation, and vinyl greenhouse cultivation. Soil cultivation also includes flooded cultivation, in which plants are grown in water-filled soil.

[0021] The iron reducing agent according to this embodiment includes a humic acid extract containing at least one selected from fulvic acid and humic acid extracted from a crude humic acid obtained by oxidizing young charcoal with nitric acid.

[0022] Young coal is coal with a lower carbon content than bituminous coal and the like, and is defined as having a carbon content of 83% by mass or less. The carbon content of young coal is preferably 78% by mass or less. When the carbon content is 78% by mass or less, coalification is still in progress, the amount of aromatic condensed rings in the components is still small, the proportion of condensation polymerization is small, and the proportion of aliphatic hydrocarbon side chains and main chains is high, making it susceptible to oxidative decomposition by nitric acid.

[0023] Examples of young coal include brown coal (lignite), peat, and subbituminous coal. One type of young coal may be used alone, or two or more types may be used in combination. Of these, brown coal (lignite) can be preferably used as young coal.

[0024] The humic acid extract contained in the iron reducing agent according to this embodiment is obtained by extracting fulvic acid and / or humic acid from a crude humic acid product obtained by oxidizing young charcoal with nitric acid.

[0025] Humic acid can be classified into humin, which is a fraction insoluble in both acid and alkali, humic acid, which is alkali-soluble but acid-insoluble, and fulvic acid, which is acid-soluble. In this embodiment, the humic acid fraction and the fulvic acid fraction are important, and both fractions, which have almost continuous characteristics, are collectively referred to as humic acid.

[0026] Nitric acid oxidation of young coal can be carried out, for example, by adding 50 to 200 parts by mass of nitric acid (anhydrous equivalent) per 100 parts by mass of young coal and mixing. Preferably, nitric acid oxidation can be carried out by adding 60 to 180 parts by mass of concentrated nitric acid (anhydrous equivalent) per 100 parts by mass of young coal and mixing. That is, the blending ratio (mass ratio) of young coal to nitric acid is, for example, 1:0.5 to 1:2, preferably 1:0.6 to 1:1.8. The reaction time is, for example, 20 to 180 minutes, preferably 30 to 120 minutes. After the reaction, the water may be removed by drying or the like, and the crude humic acid product may be obtained in a solid state.

[0027] By adjusting the amount of nitric acid added and the reaction time, the melanic index of the resulting humic acid crude product can be set in the range of 2.0 to 3. In the nitric acid oxidation of young charcoal, if the melanic index of the resulting humic acid crude product is 2.0 or higher, a large amount of water-soluble humic acid having active groups such as alcoholic hydroxyl groups and methoxyl groups is produced, and the extraction rate is also improved. Furthermore, if the melanic index is 3 or less, excessive oxidation reactions can be suppressed and the cost of using nitric acid can be reduced. Note that the melanic index of natural humic acid (humic acid extracted from young charcoal without nitric acid oxidation) is generally 1.7 or less.

[0028] Here, the melanic index is an index used to classify soil humic acid, and is the ratio of absorbance at wavelengths of 450 nm and 520 nm in the absorption spectrum of a sodium hydroxide extract (A ４５０ / A ５２０ ) (Kyōichi Kumada, Chemistry of Soil Organic Matter, 2nd Edition, Academic Press Center (1981), Japanese Journal of Soil Science and Plant Nutrition, Vol. 71, No. 1, pp. 82-85 (2000)).

[0029] The melanic index is a method used to classify soil humic acids, but it can also be applied to the analysis of crude humic acid products. The melanic index in this specification is calculated using the following method: Using a mortar and a 250 μm sieve, a sample is crushed to a 250 μm sieve size. Next, approximately 0.10 g of the 250 μm sieve size and 45 ml of 0.5 mol / L sodium hydroxide are placed in a 50 ml centrifuge tube and shaken back and forth at approximately 100 rpm for approximately 1 hour at room temperature (20°C). The tube is then centrifuged at 3,500 × g for approximately 10 minutes. The supernatant after centrifugation is filtered through filter paper (e.g., Advantec No. 5C). The absorbance of the filtrate at 450 nm and 520 nm is measured, using distilled water as a blank. In this case, if the absorbance at 450 nm is 1.0 or more, add 0.1 mol / L aqueous sodium hydroxide to adjust the absorbance to 0.8 or more but less than 1.0, and then measure the absorbance at 520 nm. The ratio (absorbance at 450 nm / absorbance at 520 nm) is calculated and used as the melanic index.

[0030] The melanic index can be adjusted by the amount of nitric acid and reaction time during the production of humic acid crude product. For example, the more nitric acid added, the higher the melanic index tends to be. As the nitric acid oxidation of young charcoal progresses, more humic acid containing active groups such as alcoholic hydroxyl groups and methoxyl groups is produced. The average molecular weight of the compounds produced by oxidation also decreases, and the melanic index of the resulting humic acid crude product tends to increase. For example, when the melanic index is 2.3 or higher, the humic acid becomes smaller in molecular weight and more soluble, resulting in a higher amount of humic acid and fulvic acid extracted. However, when the melanic index of the humic acid crude product exceeds 3.0, the produced humic acid may be further decomposed, resulting in a decrease in the humic acid content (extraction amount). Therefore, it is preferable that the melanic index of the humic acid crude product be 3.0 or less. Using the melanic index, the physical properties of the humic acid crude product can be evaluated without performing complicated analytical procedures such as measuring the mass average molecular weight of the humic acid, facilitating the production of the humic acid extract in this embodiment.

[0031] The humic acid extract in this embodiment contains fulvic acid and / or humic acid extracted from the above-mentioned humic acid crude product.

[0032] The extraction of humic acid from humic acid crude product is not particularly limited, but can be performed, for example, at a pH of 8 or less (preferably 7 or less). Specifically, humic acid can be extracted by adding water to the humic acid crude product and, while measuring the pH, adding an alkali such as potassium hydroxide appropriately to adjust the pH to 5 to 8 (preferably 6 to 7). The liquid temperature is not particularly limited, but is, for example, 40 to 90°C. The extraction time is not particularly limited, but is, for example, 0.5 to 3 hours. Extraction can be performed under stirring. After extraction, it is preferable to cool the liquid temperature to 40°C or less. The unreacted residue can be subjected to solid-liquid separation using centrifugation, a filter press, or the like to obtain an extract. The obtained extract may be used as a humic acid extract itself, or it may be optionally purified, concentrated, and / or dried before use as a humic acid extract. In particular, the humic acid extract of this embodiment has high solubility in water even when dehydrated and powdered, and can be used after being powdered by a spray dryer, freeze-drying, etc. The humic acid extract of this embodiment may be in a liquid form (extract) or a solid form, and the concept includes both liquid and solid forms.

[0033] An extract containing fulvic acid and a humic acid fraction can be obtained by extracting humic acid from a crude humic acid product at, for example, a pH of 6 to 8 (preferably, a pH of 6.5 to 7.5). Also, by extracting humic acid from a crude humic acid product at, for example, a pH of 1 to 2, a humic acid fraction can be obtained as a precipitate, and a fulvic acid fraction can be obtained as a supernatant.

[0034] From the viewpoint of being particularly excellent in iron-reducing power, the humic acid used in this embodiment preferably contains humic acid extracted from a crude humic acid having a melanic index of 2.3 to 2.8, and specifically, it preferably contains humic acid obtained by the following production method: (1) a step of mixing young charcoal and nitric acid in a mass ratio of 1:0.8 to 1:1.6 (nitric acid on an anhydrous basis) to prepare a crude humic acid having a melanic index of 2.3 to 2.8, (2) a step of adding water and an alkali to the crude humic acid to obtain an extract at a pH of 6 to 8, and (3) a step of performing solid-liquid separation of the extract obtained in (2) to obtain a humic acid extract.

[0035] In the (1) preparation step of crude humic acid, the mixture of young charcoal and nitric acid can be reacted, for example, at 40 to 95°C (preferably 60 to 90°C) for 0.5 to 6 hours (preferably 1 to 3 hours).

[0036] In the step (1) of preparing a crude humic acid product, it is preferable to mix young charcoal and nitric acid in a mass ratio of 1:1 to 1:1.5 (nitric acid is anhydrous).

[0037] The melanic index of the crude humic acid product (1) is preferably 2.4 or more, and is preferably 2.7 or less or 2.6 or less.

[0038] In the extraction step (2), the solid-liquid ratio of humic acid crude product to water is preferably 1:3 or more. The solid-liquid ratio of humic acid crude product to water is preferably 1:3 to 1:12, and more preferably 1:5 to 1:10. The solid-liquid ratio is the amount of extraction solvent relative to the amount of raw young charcoal. For example, when 100 g (100 mL) of solvent (water) is added to humic acid crude product prepared from 10 g of young charcoal, the solid-liquid ratio is 1:10.

[0039] In the extraction step (2), the extraction temperature is, for example, 40 to 95°C (preferably 60 to 90°C), and the extraction time is, for example, 0.5 to 24 hours.

[0040] The total organic carbon (TOC) concentration of the extract can be measured, for example, by the following method. The supernatant obtained by centrifuging the humic acid extract at 3,000 × g is measured using a total organic carbon analyzer (TOC-L, manufactured by Shimadzu Corporation) using a combustion catalytic oxidation method. Note that the total organic carbon concentration is not a direct quantitative value of humic acid. Compared to the complicated humic acid quantification methods (such as the International Humic Substances Society method described above), TOC, which is easily quantifiable, can be used as an indicator of humic acid concentration. There is a strong correlation between the total organic carbon concentration and the humic acid concentration, and although this may depend on factors such as the type of lignite used as raw material, the amount of humic acid can generally be estimated to be 1.4 to 1.8 times the total organic carbon concentration. The total organic carbon concentration of the extract can be adjusted by changing the solid-liquid ratio of the humic acid crude product to the solvent. Furthermore, when producing humic acid crude product, increasing the melanic index increases the extraction rate, and this method can also be used to adjust the organic carbon content of the extract.

[0041] The iron-reducing agent according to this embodiment contains the humic acid extract and can be used to reduce trivalent iron to divalent iron in soil cultivated in alkaline soil. The iron-reducing agent according to this embodiment may be a composition containing the humic acid extract.

[0042] The iron reducing agent according to the present embodiment may be in a solid form or a liquid form, for example, a concentrated liquid form that can be diluted before use.

[0043] The iron reducing agent according to this embodiment may contain only the humic acid extract, or may further contain any other material in addition to the humic acid extract, as long as the effect of this embodiment is not substantially impaired. Examples of other materials include solvents, carriers, pH adjusters, surfactants, fertilizer components such as minerals, pesticide components, binders, and bulking agents. These materials may include, for example, components commonly used in pesticides or fertilizers. These materials may be used alone or in combination of two or more.

[0044] As the carrier, for example, when the iron reducing agent according to this embodiment is in a solid form, inorganic substances such as vermiculite, talc, diatomaceous earth, kaolin, calcium carbonate, clay, calcium hydroxide, white clay, silica gel, etc., or solid carriers such as wheat flour, starch, etc. When the iron reducing agent according to this embodiment is in a liquid form, the solvent as another material is preferably water or an alcohol (e.g., ethanol or ethylene glycol).

[0045] When the iron reducing agent according to this embodiment is in a solid form, it may be dissolved or dispersed in a solvent such as water or alcohol before use. When the iron reducing agent is in a liquid form, it may be diluted with these solvents.

[0046] When the iron reducing agent according to the present embodiment is in a solid form, its shape and size are not particularly limited. However, from the viewpoints of ease of production, ease of application to soil, etc., it is preferable that the iron reducing agent be in a granular form having a particle size of 1 to 10 mm (preferably 2 to 4 mm).

[0047] The humic acid extract used in the iron reducing agent of this embodiment is more useful when transporting or manufacturing secondary products with a higher concentration. However, to prevent the extract from gelling due to a decrease in temperature or pH, a total organic carbon concentration of 5,000 to 50,000 mg / L is preferred, and 10,000 to 45,000 mg / L is more preferred.

[0048] The iron reducing agent according to this embodiment can reduce trivalent iron in alkaline soil to divalent iron by application to alkaline soil. The humic acid extract according to this embodiment has excellent iron-reducing activity, so it can effectively reduce trivalent iron to divalent iron. As a result, easily solubilized divalent iron is produced, and Fe, which is a form available to plants, is obtained. ２＋This increases iron deficiency levels, making it possible to treat, ameliorate, prevent, suppress, or alleviate iron deficiency symptoms in plants. For example, various iron deficiency symptoms in crops grown in alkaline soil containing calcareous components can be ameliorated, prevented, suppressed, or alleviated. Iron deficiency symptoms refer to various growth disorders that occur when the iron content, an element essential for plant survival, is insufficient, or when iron is present in a form that is difficult for plants to utilize due to oxidation or the formation of compounds with other substances, making it impossible for plants to utilize iron for vital activities. Common symptoms include chlorosis, in which leaves turn yellow and white due to the inhibition of chloroplast synthesis, and reduced crop yield and quality caused by nutrient deficiency due to chlorosis.

[0049] Application to soil is not particularly limited, and can be carried out by, for example, spraying, dripping, irrigation, or mixing into soil.

[0050] The plants to be cultivated are not particularly limited, and may be monocotyledonous or dicotyledonous. They may also be trees or herbs. They may also be wild plants or plants used in agriculture, horticulture, or forestry. Plants used in agriculture, horticulture, or forestry include, for example, crops (including fruit vegetables), grains, ornamental plants, flowers, fruit trees, etc.

[0051] Specifically, the target plants are not particularly limited, but examples thereof include Gramineae, Liliaceae, Palmaceae, Solanaceae, Brassicaceae, Apiaceae, Asteraceae, Amaryllidaceae, Amaranthaceae, Fabaceae, Cucurbitaceae, etc. Examples of these plants include the following: Poaceae: rice (Oryza sativa), corn (Zea mays), barley (Hordeum vulgare), wheat (Triticum aestivum), rye (Secale cereale), Job's tears (Coix lacryma-jobi var. ma-yuen), bamboo (Phyllostachys), sugarcane (Saccharum officinarum), foxtail millet (Setaria italica), barnyard millet (Echinochloa esculenta), sorghum (Sorghum bicolor), napier grass (Pennisetum pupureum), Erianthus ravenae, miscanthus (Miscanthus virgatum), switchgrass (Panicum), and oat (Avena fatua). Liliaceae: tulip (Tulipa gesneriana) and lily (Lilium sp.). Palm family: oil palm (Elaeis guineensis, Elaeis oleifera), coconut palm (Cocos nucifera), date palm (Phoenix dactylifera), wax palm (Copernicia). Solanaceae: tomato (Solanum lycopersicum), eggplant (Solanum melongena), pepper (Capsicum annuum), potato (Solanum tuberosum). Brassicaceae: cabbage (Brassica oleracea), bok choy (Brassica rapa). Apiaceae: carrot (Daucus carota). Asteraceae: lettuce (Lactuca sativa). Amaryllidaceae: onion (Allium cepa), leek (Allium fistulosum). Amaranthaceae: spinach (Spinacia oleracea). Legume family: soybean (Glycine max), adzuki bean (Vigna angularis).Cucurbitaceae: cucumber (Cucumis sativus), pumpkin (Cucurbita sp.).

[0052] The application amount of the iron reducing agent according to this embodiment is not particularly limited, and can be appropriately set depending on the concentration of the active ingredient (humic acid extract), application time, weather conditions, application frequency, plant type, cultivation density, growth stage, application method, etc.

[0053] The iron reducing agent according to this embodiment may be applied together with or mixed with fertilizers (three-element fertilizer, dolomitic lime, shell fossils, calcium silicate, bark compost, etc.) and soil conditioners (zeolite, diatomaceous earth, green tuff, humic acid materials, vermiculite, peat moss, etc.) typically used in agricultural work.

[0054] One aspect of this embodiment is the use of a humic acid extract containing at least one selected from fulvic acid and humic acid extracted from a crude humic acid product obtained by oxidizing young charcoal with nitric acid, for reducing trivalent iron to divalent iron in soil cultivated in alkaline soil. Because the humic acid extract of this embodiment has excellent iron-reducing power, it can be used to reduce trivalent iron to divalent iron in soil cultivated in alkaline soil.

[0055] Furthermore, one aspect of this embodiment is a method for reducing trivalent iron to divalent iron in soil cultivated in alkaline soil, the method comprising a step of applying to the soil a humic acid extract containing at least one selected from fulvic acid and humic acid, which is extracted from a crude humic acid obtained by oxidizing young charcoal with nitric acid. The humic acid extract in this embodiment has excellent iron-reducing power, and by applying this humic acid extract to soil cultivated in alkaline soil, trivalent iron can be reduced to divalent iron.

[0056] The present embodiment will be described below using examples. Note that the specific aspects of the humic substances according to the present embodiment are not limited to those in the examples, and the configuration can be changed as appropriate within the scope of the present disclosure.

[0057] 1. Preparation of Humic Acid Extract [Example 1] (Humic Acid Extract E1) In a draft chamber, 500 g of lignite with a carbon content of 77% by mass, crushed to a total size of 250 μm, was placed in a 2-liter beaker, and 630 g of nitric acid with a concentration of 48% by mass was added. An oxidation reaction with nitric acid was carried out in a 70°C water bath for approximately 1 hour, followed by drying at 105°C to obtain a crude humic acid product. The melanic index was measured using the aforementioned method and found to be 2.2. Next, as an extraction step, approximately 900 mL of aqueous potassium hydroxide solution (0.5 mol / L) was added to 100 g of this crude humic acid product, and an appropriate amount of aqueous potassium hydroxide solution (1.0 mol / L) was added to adjust the pH to 6.5. Water was then added to achieve a solid-liquid ratio of 1:10, and extraction was carried out at 80°C for 1 hour. The extracted liquid was centrifuged at 3,000 × g, and the supernatant was obtained as humic acid extract E1.

[0058] [Example 2] (Humic Acid Extract E2) The amount of nitric acid added was 1,350 g, and during extraction, about 100 mL of potassium hydroxide aqueous solution (0.5 mol / L) was added, and an appropriate amount of potassium hydroxide aqueous solution (1.0 mol / L) was added to adjust the pH to 2.0. An extract was prepared in the same manner as in Example 1 (Humic Acid Extract E2). The melanic index of the humic acid crude product was 2.5.

[0059] [Examples 3 and 4] (Humic Acid Extract E3 and Humic Acid Extract E4) Humic acid extracts were prepared in the same manner as in Example 1, except that 1,350 g of nitric acid was added. The resulting humic acid extract was then purified according to the International Humic Substances Society (IHSS) method, and fractionated into a humic acid fraction (Example 3) or a fulvic acid fraction (Example 4). Hereinafter, the humic acid fraction may also be referred to as purified humic acid, and the fulvic acid fraction may also be referred to as purified fulvic acid.

[0060] Specifically, 6N hydrochloric acid was added to the obtained humic acid extract to adjust the pH to approximately 1.0, and the mixture was stirred for 1 hour and then centrifuged (3000 g, 10 minutes) to obtain a precipitate fraction. Next, distilled water was added to the precipitate fraction, and the mixture was centrifuged at 3000 g for 10 minutes to obtain a precipitate fraction. This washing process was repeated three times. The precipitate fraction obtained after three washes was dissolved in water to obtain a purified humic acid fraction (humic acid extract E3) (Example 3).

[0061] Specifically, the supernatant obtained by the centrifugation (3000 g, 10 minutes) was passed through a column (volume Φ2.5 × 20 cm) packed with XAD-8 resin to adsorb the colored substances, and the resin was washed with 0.65 column volumes of distilled water. The flow rate was 15 bed volumes per hour, and the non-adsorbed fraction and wash fraction were discarded. Next, one column volume of aqueous sodium hydroxide solution (0.1 N) was passed through the column, followed by 2 to 3 column volumes of distilled water to desorb the adsorbed fraction, and a purified fulvic acid fraction (humic acid extract E4) was obtained (Example 4).

[0062] [Comparative Example 1] (Humic Acid Extract C1) A humic acid fraction was obtained from the above lignite according to the International Humic Substances Society method (IHSS method) and designated as humic acid extract C1. The melanic index of the lignite was 1.7. As shown in Table 29 in "4. Measurement of iron reducing power" described below, C1 does not have iron reducing power. For C1, the lignite was not oxidized with nitric acid.

[0063] The TOC (total organic carbon) concentrations of the humic acid extracts E1 and E2 prepared in Examples 1 and 2, the humic acid extract E3 (purified humic acid) prepared in Example 3, the humic acid extract E4 (purified fulvic acid) prepared in Example 4, and the humic acid extract C1 prepared in Comparative Example 1 are shown in Table 1. The concentrations are listed as TOC concentrations.

[0064]

[0065] 2. Evaluation of plants using humic acid extract in hydroponic cultivation under alkaline conditions In this test, reagents manufactured by Kanto Chemical Co., Inc. were used, and the hydroponic solution was prepared by dissolving each material in ion-exchanged water so as to have the composition shown in Table 2.

[0066]

[0067] [Test 1] (Test 1-1: Humic Acid Extract E1) Rice (variety: Hitomebore) was used as a test crop, and a cultivation test of the humic acid extract was carried out by hydroponic cultivation.

[0068] For the first 7 days, 3.5 L of ion-exchanged water was placed in a 5 L plastic container, and the seeds that had been germinated were sown on a stainless steel cultivation plate. From the 7th day after sowing, the crops were grown in 3.5 L of the above hydroponic solution (Table 2).

[0069] From the 14th day after sowing, iron(III) chloride was added to the above hydroponic solution (Table 2) to a concentration of 50 μM, and the humic acid extract E1 prepared in Example 1 was added to a TOC concentration of 20 mg / L, and the pH was adjusted to 8.0 with sodium hydroxide solution. The cultivation period was 28 days, and the pH was adjusted to 8.0 every day with sodium hydroxide solution, and the hydroponic solution was replaced with the same hydroponic solution every 3 days.

[0070] The crops were evaluated after the completion of cultivation (28 days after sowing). Specifically, the SPAD value (measured using a Konica Minolta SPAD-502Plus chlorophyll meter), the number of tillers, plant height, and dry weight were measured as evaluation items, and the total iron concentration in the upper and lower leaves of the plant was also measured (evaluation of iron absorption).

[0071] The total iron content of the leaves was measured using the following method. Five upper leaves and five lower leaves were crushed to prepare samples. 30 to 50 mg of the crushed sample was weighed into a beaker, 5 ml of nitric acid and 0.2 ml of sulfuric acid were added, and the mixture was heated and decomposed on a hot plate, after which the volume was adjusted to 10 ml to prepare a test solution. The iron content of this test solution was measured using ICP atomic emission spectrometry (Agilent 5110VDV).

[0072] (Test 1-2: Humic Acid Extract E2) Crops were grown hydroponically and evaluated in the same manner as in Test 1-1, except that instead of adding the humic acid extract E1 prepared in Example 1 to the hydroponic solution so that the TOC concentration was 20 mg / L, the humic acid extract E2 prepared in Example 2 was added to the hydroponic solution so that the TOC concentration was 20 mg / L.

[0073] (Test 1-3: Control) Crops were grown hydroponically and evaluated in the same manner as in Test 1-1, except that the humic acid extract E1 prepared in Example 1 was not added to the hydroponic solution.

[0074] (Results of Test 1)

[0075] As shown in Table 3, humic acid extracts E1 and E2 were found to maintain normal leaf color when rice was grown under alkaline conditions. Iron deficiency symptoms were observed in the upper leaves, with yellowing and a decrease in SPAD value. While the SPAD value of the upper leaves in the control group decreased, the application of humic acid extracts E1 or E2 maintained a high SPAD value in the upper leaves.

[0076]

[0077] As shown in Table 4, the addition of humic acid extract E1 or E2 increased the number of tillers compared to the control.

[0078]

[0079] As shown in Table 5, application of humic acid extract E1 or E2 was found to have an effect of increasing plant height compared to the control.

[0080]

[0081] As shown in Table 6, the addition of humic acid extract E1 or E2 significantly increased the dry weight of the aboveground parts.

[0082]

[0083] As shown in Table 7, the addition of humic acid extract E1 or E2 significantly increased the dry weight of the underground parts.

[0084]

[0085] As shown in Table 8, the addition of humic acid extract E1 or E2 improved the amount of iron absorbed into the leaves.

[0086] [Test 2] Rice (variety: Hitomebore) was used as the test crop, and a cultivation test was conducted using a humic acid extract in hydroponic cultivation. The hydroponic solution was prepared by dissolving each ingredient in ion-exchanged water to obtain the composition shown in Table 2.

[0087] (Test 2-1: Humic Acid Extract E3) For the first 7 days, 3.5 L of ion-exchanged water was placed in a 5 L plastic container, and the seeds that had been subjected to the germination treatment were sown on a stainless steel cultivation plate. From the 7th day after sowing, the crops were cultivated in 3.5 L of the above hydroponic solution (Table 2).

[0088] From the 14th day after sowing, iron(III) chloride was added to the above hydroponic solution (Table 2) to a concentration of 50 μM, and humic acid extract E3 (purified humic acid) prepared in Example 3 was added to a TOC concentration of 25 mg / L or 50 mg / L, and the pH was adjusted to 8.0 with sodium hydroxide solution. The cultivation period was 28 days, and the pH was adjusted to 8.0 every day with sodium hydroxide solution, and the hydroponic solution was replaced every 3 days.

[0089] After the cultivation was completed (28 days after sowing), the SPAD, number of tillers, plant height, and dry weight were measured and evaluated.

[0090] (Test 2-2: Humic Acid Extract E4) Crops were grown hydroponically and evaluated in the same manner as in Test 2-1, except that instead of adding the humic acid extract E3 prepared in Example 3 to the hydroponic solution so that the TOC concentration was 25 mg / L or 50 mg / L, the humic acid extract E4 prepared in Example 4 was added to the hydroponic solution so that the TOC concentration was 20 mg / L.

[0091] (Test 2-3: Control) Crops were grown hydroponically and evaluated in the same manner as in Test 2-1, except that the humic acid extract E3 prepared in Example 3 was not added to the hydroponic solution.

[0092] (Results of Test 2)

[0093] As shown in Table 9, humic acid extracts E3 and E4 were found to maintain normal leaf color when rice was grown under alkaline conditions. Iron deficiency symptoms were characterized by yellowing and whitening of upper leaves and a decrease in SPAD value. While the SPAD value of upper leaves in the control decreased, application of humic acid extracts E3 or E4 maintained a high SPAD value of upper leaves.

[0094]

[0095] As shown in Table 10, application of humic acid extract E3 or humic acid extract E4 increased the number of tillers compared to the control.

[0096]

[0097] As shown in Table 11, the application of humic acid extract E3 or humic acid extract E4 was found to have an effect of increasing plant height compared to the control.

[0098]

[0099] As shown in Table 12, the addition of humic acid extract E3 or humic acid extract E4 significantly increased the dry weight of the aboveground parts.

[0100]

[0101] As shown in Table 13, the addition of humic acid extract E3 or humic acid extract E4 significantly increased the dry weight of the underground parts.

[0102] [Test 3] Rice (variety: Hitomebore) was used as the test crop, and a cultivation test was conducted using the humic acid extract in hydroponic cultivation. The hydroponic solution was prepared by dissolving each ingredient in ion-exchanged water to obtain the composition shown in Table 2.

[0103] (Test 3-1: Humic Acid Extract E3) For the first 7 days, 3.5 L of ion-exchanged water was placed in a 5 L plastic container, and the seeds that had been germinated were sown on a stainless steel cultivation plate. From the 7th day after sowing, the seeds were cultivated in 3.5 L of the above hydroponic solution (Table 2).

[0104] From the 14th day after sowing, iron(III) chloride was added to the above hydroponic solution (Table 2) to a concentration of 50 μM, and humic acid extract E3 (purified humic acid) prepared in Example 3 was added to a TOC concentration of 25 mg / L, and the pH was adjusted to 8.0 with sodium hydroxide solution. The cultivation period was 28 days, and the pH was adjusted to 8.0 every day with sodium hydroxide solution, and the hydroponic solution was replaced every 3 days.

[0105] After the completion of cultivation (28 days after sowing), the SPAD value, the number of tillers, the plant height, and the dry weight were measured. The SPAD value was measured three times a week from the 14th day after sowing onwards.

[0106] (Test 3-2: Control) Crops were hydroponically cultivated and evaluated in the same manner as in Test 3-1, except that instead of adding the humic acid extract E3 prepared in Example 3 so that the TOC concentration was 25 mg / L, the humic acid extract C1 prepared in Comparative Example 1 was added so that the TOC concentration was 25 mg / L.

[0107] (Test 3-3: Reference) Crops were grown hydroponically and evaluated in the same manner as in Test 3-1, except that instead of adding the humic acid extract E3 prepared in Example 3 to the hydroponic solution, Fe-EDTA, which is known as an iron chelating agent, was added to the hydroponic solution to a concentration of 50 μM, and 50 μM iron (III) hydroxide was not added.

[0108] (Test 3-4: Control) Crops were grown hydroponically and evaluated in the same manner as in Test 3-1, except that the humic acid extract E3 prepared in Example 3 was not added to the hydroponic solution.

[0109] (Results of Test 3)

[0110] As shown in Table 14, humic acid extract E3 was found to be effective in maintaining the leaf color of the newest leaves until the end of cultivation, even under alkaline conditions. This purified humic acid exhibited an effect equivalent to that of Fe-EDTA, which is known to supply iron to plants through its iron chelating effect.

[0111]

[0112] As shown in Table 15, humic acid extract E3 showed the effect of increasing the number of tillers even when cultivated under alkaline conditions.

[0113]

[0114] As shown in Table 16, the humic acid extract E3 showed an effect of increasing plant height when cultivated under alkaline conditions.

[0115]

[0116] As shown in Table 17, humic acid extract E3 increased the dry weight of the aboveground parts compared to the control when cultivated under alkaline conditions.

[0117]

[0118] As shown in Table 18, humic acid extract E3 increased the dry weight of the underground parts compared to the control when cultivated under alkaline conditions.

[0119]

[0120] As shown in Table 19, the iron content in the leaves increased when cultivated under alkaline conditions compared to the control. This demonstrates that application of humic acid extract E3 improves the amount of iron absorbed by the plant. Note that no measurement data was available for Comparative Example 1.

[0121] 3. Plant Evaluation Using Humic Acid Extract in Calcareous Alkaline Soil [Test 4] The cultivated crop was rice, and the variety used was Hitomebore. The soil used was shell fossil soil (pH 8.9, containing 1.7% iron; Nihonkai Fertilizer Co., Ltd.), and the fertilizer used was Ecolong Total 391-40 Day Release Type (hereinafter referred to as ELT40) (J-Cam Agri Co., Ltd.). 1 / 10,000a white pots (Fujiwara Seisakusho) were used for cultivation, and 1 kg of the above soil and 1.75 g of ELT40 per pot were added to the pots for cultivation. The soil pH after addition of the fertilizer was 8.8.

[0122] (Test 4-1: Humic Acid Extract E3) Seedlings grown in water for 5 days were transplanted one by one into one pot. Two pots were placed in a 5 L plastic container, and water was added so that the soil surface was completely submerged. The amount of water was adjusted so that the water level was always 3 cm above the soil surface.

[0123] At the same time as transplanting the seedlings, humic acid extract E3 was added to the water in an amount of 25 mg (25 mg / L addition amount) or 50 mg (50 mg / L addition amount) of TOC per liter of water, thereby containing the humic acid extract in the water. No decrease in pH was observed due to the addition of humic acid extract E3 or Na-EDTA.

[0124] After the completion of cultivation (4 weeks after transplanting), the plant height, number of tillers, and dry weight were measured for each leaf age. SPAD values ​​were measured three times a week after transplanting.

[0125] (Test 4-2: Reference) Crops were grown in soil and evaluated in the same manner as in Test 4-1, except that instead of adding the humic acid extract E3 prepared in Example 3 to the water, Na-EDTA, which is known as an iron chelating agent, was added to the water to a concentration of 30 μM.

[0126] (Test 4-3: Control) Crops were cultivated in soil and evaluated in the same manner as in Test 4-1, except that the humic acid extract E3 prepared in Example 3 was not added to the water.

[0127] (Results of Test 4)

[0128] As shown in Table 20, when cultivated in alkaline calcareous soil, leaf color faded in the control, while leaf color was maintained in the case of application of humic acid extract E3. Furthermore, application of humic acid extract E3 showed an effect equivalent to that of application of Na-EDTA.

[0129]

[0130] As shown in Table 21, when cultivated in alkaline calcareous soil, application of humic acid extract E3 increased the number of tillers compared to the control. Furthermore, application of humic acid extract E3 showed an effect equivalent to that of application of Na-EDTA.

[0131]

[0132] As shown in Table 22, when cultivated in alkaline calcareous soil, application of humic acid extract E3 was found to have an effect of increasing plant height compared to the control. Furthermore, application of humic acid extract E3 showed an effect equivalent to that of application of Na-EDTA.

[0133]

[0134] As shown in Table 23, when cultivated in alkaline calcareous soil, application of humic acid extract E3 increased aboveground dry weight compared to the control. Furthermore, application of humic acid extract E3 showed an effect equivalent to that of application of Na-EDTA.

[0135]

[0136] As shown in Table 24, when cultivated in alkaline calcareous soil, application of humic acid extract E3 increased the dry weight of the underground parts compared to the control. Furthermore, application of humic acid extract E3 showed an effect equivalent to that of application of Na-EDTA.

[0137] [Test 5] The cultivated crop was wheat, and the variety used was Norin 61. The soil used was a mixture of shell fossil soil (pH 8.9, containing 1.7% iron, manufactured by Nihonkai Fertilizer Co., Ltd.) and perlite (Fuyolite No. 2) (Fuyo Perlite Co., Ltd.) in a volume ratio of 7:3, and the fertilizer component was ELT40. The cultivation pots used were polyvinyl chloride pots (φ12 cm, height 10 cm, capacity 830 cm) (Tokai Chemical Industry Co., Ltd.), and the pots were filled with mixed soil (shell fossil soil 620.12 g, perlite 23.39 g: 764.3 cm). ３ The mixture was used for cultivation. The pH after adding the fertilizer components was 8.8.

[0138] (Test 5-1: Humic Acid Extract E3) Wheat seeds were immersed in RO water in a dark place at 10°C for 3 days, and then left under light at 25°C for about 1 day to promote acclimation and germination. Germinated seeds were sown individually in pots, and 10 pots were grown per test plot. Irrigation was performed by submersion, and the water level was maintained at about 0.5 to 1 cm so that the bottom of the pot was always submerged in water.

[0139] When the mixed soil and ELT40 were mixed, humic acid extract E3 was added in an amount to give a TOC content of 50 mg / L or 200 mg / L (50 mg / L addition group and 200 mg / L addition group).

[0140] Starting five days after sowing, SPAD values ​​were measured three times a week, and plant height was measured once a week. Starting 21 days after sowing, the number of tillers was measured four times. After cultivation was completed, the dry weight of the aboveground parts was measured. The average values ​​of 10 individuals are shown in Tables 25 to 28 below as the values ​​for each test plot.

[0141] (Test 5-2: Reference) The plants were cultivated and evaluated in the same manner as in Test 5-1, except that instead of mixing the humic acid extract E3 prepared in Example 3 with the mixed soil, Na-EDTA, which functions as a chelating agent for iron ions, was added to the mixed soil to a concentration of 30 μM.

[0142] (Test 5-3: Control) Cultivation and evaluation were carried out in the same manner as in Test 5-1, except that the humic acid extract E3 prepared in Example 3 was not mixed into the mixed soil.

[0143] (Results of Test 5)

[0144] As shown in Table 25, when cultivated in alkaline calcareous soil, application of humic acid extract E3 showed an improvement in leaf color compared to the control.

[0145]

[0146] As shown in Table 26, when grown in alkaline calcareous soil, application of humic acid extract E3 increased the number of tillers compared to the control and Na-EDTA.

[0147]

[0148] As shown in Table 27, when cultivated in alkaline calcareous soil, application of humic acid extract E3 improved plant height compared to the control and Na-EDTA.

[0149]

[0150] As shown in Table 28, when cultivated in alkaline calcareous soil, application of humic acid extract E3 increased the dry weight of the aboveground parts compared to the control and application of Na-EDTA.

[0151] 4. Measurement of iron reducing power The iron reducing power of the humic acid extracts E1 to E4 and C1 prepared in Examples 1 to 4 and Comparative Example 1 was measured. ＴＭ A FRAP assay kit (Cosmo Bio Co., Ltd.) was used. This assay kit reduces ferric iron to divalent iron using electron donors in the humic acid extract. The amount of divalent iron produced is shown in the table as iron reducing power.

[0152]

[0153] As shown in Table 29, the humic acid extracts E1 to E4 of Examples 1 to 4 exhibited higher iron-reducing power than the humic acid extract of Comparative Example 1, and in particular, the humic acid extract E3 (purified humic acid) of Example 3 exhibited high iron-reducing power. The humic acid extracts of the Examples were found to be iron-reducing agents that increase the availability of iron to plants by reducing insolubilized trivalent iron present in alkaline soils such as calcareous soils to soluble divalent iron.

[0154] 5. Measurement of Divalent Iron Content in Alkaline Soil The change in the divalent iron content in alkaline soil due to the application of the humic acid extract E3 prepared in Example 3 was confirmed.

[0155] To 5 g of shell fossil soil (pH 8.9, containing 1.7% iron, manufactured by Nihonkai Fertilizer Co., Ltd.), 20 ml of ion-exchanged water was added, and humic acid extract E3 was added at the concentration shown in Table 30. After standing for 1 hour, the soil was filtered using filter paper, and the resulting soil samples were used for measuring the iron content.

[0156] 20 ml of 0.5 mol / L hydrochloric acid was added to 2 g of the prepared soil, and the mixture was ultrasonically treated for 10 minutes. The soil was then filtered again, and the ferrous iron content of the resulting solution was measured.

[0157] The divalent iron was measured using orthophenanthroline colorimetry, which utilizes the color produced by the chelate of orthophenanthroline and divalent iron to quantify the amount of divalent iron.

[0158]

[0159] As shown in Table 30, application of humic acid extract E3 increased the divalent iron content in alkaline soil in a concentration-dependent manner.

[0160] The above examples demonstrate that the humic acid extract of this embodiment has excellent iron-reducing activity, reducing insoluble trivalent iron in alkaline soil to divalent iron, thereby solubilizing iron and promoting healthy plant growth. Specifically, application of the humic acid extract in alkaline soil improves chlorosis, a typical symptom of iron deficiency in plants. Furthermore, the humic acid extract has high activity in reducing trivalent iron to divalent iron, increasing the divalent iron content in alkaline soil. Application of humic acid under alkaline conditions improves the iron absorption ability of plants. These findings demonstrate that the humic acid extract of this embodiment has iron-reducing activity in calcareous alkaline soil. Therefore, the humic acid extract of this embodiment can be used as an excellent iron-reducing agent.

[0161] The upper and / or lower limits of the numerical ranges described herein can be arbitrarily combined to define a preferred range. For example, the upper and lower limits of the numerical ranges can be arbitrarily combined to define a preferred range, the upper limits of the numerical ranges can be arbitrarily combined to define a preferred range, and the lower limits of the numerical ranges can be arbitrarily combined to define a preferred range.

[0162] The claims that follow this written disclosure are expressly incorporated into this written disclosure herein, with each claim standing on its own as a separate embodiment. The present disclosure includes all instances in which an independent claim is replaced by its dependent claim. Furthermore, any additional embodiments derived from the independent claim and the subsequent dependent claim are also expressly incorporated into this written specification.

[0163] Although the present embodiment has been described in detail above, the specific configuration is not limited to this embodiment, and any design changes that do not deviate from the gist of the present disclosure are also included in the present disclosure. All publications, patents, and patent applications cited in this specification are incorporated herein by reference in their entirety.

Claims

1. An iron reducing agent for reducing trivalent iron in soil to divalent iron in alkaline soil cultivation, comprising a humic acid extract containing at least one selected from fulvic acid and humic acid extracted from a crude humic acid obtained by oxidizing young charcoal with nitric acid.

2. The iron reducing agent according to claim 1, wherein the young coal is lignite.

3. The iron reducing agent according to claim 1, wherein the melanic index of the humic acid crude product is 2.0 to 3.

4. The iron reducing agent according to claim 1, wherein the alkaline soil cultivation is the cultivation of a grass plant.

5. The iron reducing agent according to claim 1, wherein the alkaline soil is a soil containing calcareous components.

6. Use of a humic acid extract containing at least one selected from fulvic acid and humic acid extracted from a crude humic acid product obtained by oxidizing young charcoal with nitric acid, for reducing trivalent iron in soil to divalent iron in alkaline soil cultivation.

7. The use according to claim 6, wherein the young coal is lignite.

8. The use according to claim 6, wherein the alkaline soil cultivation is the cultivation of a grass plant.

9. The use according to claim 6, wherein the alkaline soil is a soil containing calcareous components.

10. The use according to claim 6, wherein the humic acid crude product has a melanic index of 2.0 to 3.

11. A method for reducing trivalent iron in soil to divalent iron in alkaline soil cultivation, comprising a step of applying to the soil a humic acid extract containing at least one selected from fulvic acid and humic acid extracted from a crude humic acid obtained by oxidizing young charcoal with nitric acid.

12. The method of claim 11, wherein the young coal is lignite.

13. The method according to claim 11, wherein the humic acid crude product has a melanic index of 2.0 to 3.

14. The method according to claim 11, wherein the alkaline soil cultivation is the cultivation of a grass plant.

15. The method according to claim 11, wherein the alkaline soil is a soil containing calcareous components.

16. An iron reducing agent comprising a humic acid extract containing at least one selected from fulvic acid and humic acid extracted from a crude humic acid product, for reducing trivalent iron in soil to divalent iron in alkaline soil cultivation.

17. An iron reducing agent comprising a humic acid extract containing at least one selected from fulvic acid and humic acid.

18. An iron reducer containing a humic acid extract for reducing trivalent iron to divalent iron in soil in alkaline soil cultivation.

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