Method for reducing dinitrogen monoxide emission, kit for reducing dinitrogen monoxide emission, and soil composition

Abstract

The present disclosure provides a method for reducing dinitrogen monoxide emission from a soil composition containing a nitrogenous fertilizer. The soil composition contains a base soil and the nitrogenous fertilizer. The method for reducing dinitrogen monoxide emission comprises mixing decomposed leaf matter with the base soil and adjusting the mixed proportion of the decomposed leaf matter to 50 vol% or more of the total of the base soil and the decomposed leaf matter.

Description

Method for reducing nitrous oxide emissions, kit for reducing nitrous oxide emissions, and soil composition

[0001] The present disclosure relates to a method for reducing nitrous oxide emissions, a kit for reducing nitrous oxide emissions, and a soil composition.

[0002] Climate change caused by the increase in the concentration of greenhouse gases in the atmosphere is a global environmental problem. Nitrous oxide (N2O), one of the greenhouse gases, has a global warming potential about 300 times that of carbon dioxide (CO2) and contributes significantly to global warming after CO2 and methane (CH4). In addition, N2O is currently regarded as the main cause of ozone layer depletion, and reducing its emissions and atmospheric concentration is an urgent issue for global environmental conservation.

[0003] The main emission source of N2O is agricultural land, especially derived from nitrogen fertilizers used in crop cultivation. A part of the nitrogen fertilizer applied to agricultural land is absorbed by crops, but at the same time, N2O is generated in the nitrogen cycling process (nitrification and denitrification) of soil microorganisms and released into the atmosphere. In addition, since some soil microorganisms have the ability to reduce N2O to nitrogen gas (N2), soil is both a source and a sink of N2O.

[0004] In order to effectively reduce N2O emissions from agricultural land, it is necessary to achieve either suppression of N2O production in soil, increase in N2O consumption, or both. Conventionally, as technologies for suppressing N2O emissions from soil, application of nitrification inhibitors, use of coated fertilizers, and utilization of soybean rhizobia are known. However, there is a concern that the chemicals and plastic films contained in nitrification inhibitors and coated fertilizers may flow out into the environment and have an adverse impact on the ecosystem. In addition, there is an issue that the application of the method using soybean rhizobia is limited to leguminous plants.

[0005] For example, Non-Patent Document 1 quantitatively evaluates the effect of reducing N2O emissions from agricultural land and reports that the average reduction rate of fertilizers containing nitrification inhibitors is 38%. Non-Patent Document 2 shows that rhizobia (Bradyrhizobium ottawaense) efficiently reduces N2O to nitrogen gas (N2) through high nosZ gene expression, suggesting the possibility of increasing N2O consumption in the soil. However, these methods have not completely solved the problems of environmental impact and limited scope of application.

[0006] Therefore, there is a strong need to develop more versatile and environmentally friendly N2O emission reduction technologies that can solve these problems.

[0007] Hiroko Akiyama, Kazuyuki Yagi, Xiaoyuan Yan, "Quantitative evaluation of the reduction effect of nitrous oxide generated from agricultural land - Average reduction rate of fertilizers containing nitrification inhibitors is -38% -", National Institute for Agro-Environmental Sciences, Research Results Information, FY2009, Vol. 26, http: / / www.affrc.go.jp / Sawa Wasai-Hara, Manabu Itakura, Arthur Fernandes Siqueira, Daisaku Takemoto, Masayuki Sugawara, Hisayuki Mitsui, Shusei Sato, Noritoshi Inagaki, Toshimasa Yamazaki, Haruko Imaizumi-Anraku, Yoshikazu Shimoda, Kiwamu Minamisawa, "Bradyrhizobium ottawaense efficiently reduces nitrous oxide through high nosZ gene expression," Scientific Reports, 2013, 3:18862

[0008] This disclosure is made to solve the above-mentioned problems and aims to provide a method for reducing nitrous oxide emissions, a kit for reducing nitrous oxide emissions, and a soil composition.

[0009] According to one aspect of the present disclosure, a method for reducing nitrous oxide emissions from a soil composition containing a nitrogenous fertilizer is provided, wherein the soil composition comprises a base soil and the nitrogenous fertilizer, and the method comprises mixing leaf mold with the base soil and adjusting the mixing ratio of the leaf mold to 50% by volume or more of the total of the base soil and the leaf mold. According to one aspect of the present disclosure, a kit for reducing nitrous oxide emissions from a soil composition containing a nitrogenous fertilizer is provided, comprising leaf mold and instructions, wherein the instructions include instructions for using the kit to reduce nitrous oxide emissions compared to a soil composition combined with the nitrogenous fertilizer by combining a mixture obtained by mixing leaf mold with the base soil and adjusting the mixing ratio of the leaf mold to 50% by volume or more of the total of the base soil and the leaf mold with the nitrogenous fertilizer. According to one aspect of the present disclosure, a soil composition is provided which comprises a mixed soil consisting of base soil and leaf mold, and a nitrogenous fertilizer, wherein the leaf mold accounts for 50% by volume or more of the mixed soil.

[0010] According to this disclosure, a method for reducing nitrous oxide emissions, a kit for reducing nitrous oxide emissions, and a soil composition can be provided.

[0011] Figure 1 shows the change over time in soil-derived N2O emissions when urea is added as a nitrogen source at a concentration of 200 mg N / kg soil to a mixture of black soil and leaf mold. NC (black soil): autoclaved black soil, black soil: black soil only, black 8 leaf mold 2: mixture of 80% black soil and 20% leaf mold, black 5 leaf mold 5: mixture of 50% black soil and 50% leaf mold, black 2 leaf mold 8: mixture of 20% black soil and 80% leaf mold. Error bars indicate the standard deviation (n=3). Figure 2A shows the change over time in soil-derived N2O emissions when urea is added as a nitrogen source to a mixture of 80% black soil and 20% leaf mold. N0 (8 parts black soil, 2 parts leaf mold): No urea added, N200 (8 parts black soil, 2 parts leaf mold): 200 mg N / kg urea added to soil, N400 (8 parts black soil, 2 parts leaf mold): 400 mg N / kg urea added to soil, N1000 (8 parts black soil, 2 parts leaf mold): 1000 mg N / kg urea added to soil. Figure 2B shows the change over time in soil-derived N2O emissions when urea is added as a nitrogen source to a soil mixture of 50% black soil and 50% leaf mold. N0 (5 parts black soil, 5 parts leaf mold): No urea added, N200 (5 parts black soil, 5 parts leaf mold): 200 mg N / kg urea added to soil, N400 (5 parts black soil, 5 parts leaf mold): 400 mg N / kg urea added to soil, N1000 (5 parts black soil, 5 parts leaf mold): 1000 mg N / kg urea added to soil (n=3).

[0012] <Method for Reducing Nitrous Oxide Emissions> In one embodiment, a method for reducing nitrous oxide emissions from a soil composition containing a nitrogenous fertilizer is provided, wherein the soil composition comprises base soil and the nitrogenous fertilizer, and the method includes mixing leaf mold with the base soil and adjusting the mixing ratio of the leaf mold to 50% by volume or more of the total of the base soil and the leaf mold. According to the method for reducing nitrous oxide emissions in this embodiment, it is possible to reduce N2O emissions and absorb N2O from the environment by a simple and safe soil treatment method of mixing leaf mold with soil.

[0013] The soil composition in this embodiment includes a base soil and a nitrogenous fertilizer. The soil composition as a whole may have appropriate physical properties such as moderate water retention, aeration, and drainage. In this disclosure, "base soil" may mean soil that functions as a base material for soil used in crop cultivation and has physical and chemical properties suitable for crop growth. As the base soil, general field soil, paddy field soil, or horticultural soil can be used. Specifically, black soil (black volcanic soil) or lowland soil can be used as the base soil. The pH of the soil composition may be in a range suitable for crop growth, approximately 5.5 to 7.0. In this embodiment, the base soil may include black soil or lowland soil. Those skilled in the art will understand that black soil mainly originates from volcanic ash, has a high phosphate absorption coefficient, low bulk density, and is a light soil (Japan Soil Inventory, naro.go.jp). More specifically, for example, black soil is a black topsoil that sits on top of red soil such as Kanto loam, and can be a fertile and soft soil composed of volcanic ash and decomposed fallen leaves. Lowland soil is mainly distributed around rivers and is understood by those skilled in the art to be soil whose parent material is sediment deposited by river floods (Japan Soil Inventory, naro.go.jp).

[0014] The nitrogenous fertilizer in this embodiment may be a fertilizer that supplies the nitrogen necessary for crop growth. Examples of nitrogenous fertilizers include inorganic fertilizers such as urea, ammonium nitrate, ammonium sulfate, calcium nitrate, potassium nitrate, and ammonium phosphate. Other examples of nitrogenous fertilizers include animal fertilizers such as fish meal, oil cake, bone meal, and blood meal, or plant fertilizers such as compost and green manure. The amount of nitrogenous fertilizer to be added can be appropriately determined by those skilled in the art, but for example, it may be 200 mgN or more, 400 mgN or more, or 1000 mgN or more per 1 kg of soil (base soil, or a mixture of base soil and leaf mold). The amount of nitrogenous fertilizer to be added may be, for example, 2000 mgN or less per 1 kg of soil.

[0015] In the soil composition of this embodiment, the addition of nitrogenous fertilizer to the soil composition can be selected from the following methods, depending on the cultivation period or growth stage of the crop: mixing it with the base soil and leaf mold in advance as a base fertilizer, applying it as a top dressing during the growth period, or a combination of these methods. When applying top dressing, the mixing ratio of leaf mold in the soil composition of this embodiment may be maintained at 50% by volume or more.

[0016] The leaf mold in the embodiment may be formed from the decomposition and accumulation of plant matter such as fallen leaves, or from the artificial fermentation and decomposition of plant matter. In nature, leaf mold can be formed when plant matter such as fallen leaves or dead leaves is decomposed by soil microorganisms and accumulates. Furthermore, commercially available leaf mold as a horticultural material may be manufactured by using fallen leaves as the main raw material, piling them up, and promoting fermentation and decomposition by microorganisms while adjusting moisture content and aeration. The leaf mold used in the embodiment may be leaf mold formed in nature or leaf mold manufactured artificially.

[0017] In the soil composition of this embodiment, the mixing of leaf mold into the base soil during the preparation of the soil composition can be done by uniformly mixing the base soil and leaf mold. The mixing may be done using mechanical means such as a tiller, or by hand. The method for reducing nitrous oxide emissions in this embodiment includes adjusting the mixing ratio of leaf mold to 50% or more by volume of the total of the base soil and leaf mold. Furthermore, the method for reducing nitrous oxide emissions in this embodiment may include adjusting the mixing ratio of leaf mold to 60% or more by volume, 70% or more by volume, or 80% or more by volume of the total of the base soil and leaf mold. Alternatively, the method for reducing nitrous oxide emissions in this embodiment may include adjusting the mixing ratio of leaf mold to 90% or less by volume of the total of the base soil and leaf mold. By adjusting the mixing ratio to 50% or more by volume of the total of the base soil and leaf mold, N2O emissions can be efficiently suppressed. While the mechanism by which the effects of this disclosure manifest is not limited, it is believed that the humus in the soil composition can reduce nitrous oxide emissions by suppressing the generation of nitrous oxide or absorbing the generated nitrous oxide.

[0018] In the embodiment of the method for reducing nitrous oxide emissions, "reduction of nitrous oxide emissions" may mean reducing the amount of nitrous oxide released into the atmosphere from the soil composition. Specifically, "reduction of nitrous oxide emissions" may mean reducing the amount of nitrous oxide emissions compared to a soil composition in which base soil alone is combined with a nitrogenous fertilizer, without the addition of leaf mold. In other words, compared to a standard soil composition in which base soil alone is combined with a nitrogenous fertilizer, a soil composition in which a mixture of base soil and leaf mold (the same volume as base soil alone in the standard soil composition) is combined with a corresponding amount of nitrogenous fertilizer results in reduced nitrous oxide emissions. This exceeds the reduction rate that can be explained simply by the dilution of the base soil by leaf mold, as shown in the example, and suggests that leaf mold has a more active effect on reducing nitrous oxide emissions from the base soil. The amount of nitrous oxide emissions can be measured, for example, by the closed chamber method or by a method in which the soil composition is contained in a sealed container such as a gas chlorovial. In the closed-chamber method, the gas phase inside a chamber placed on the soil composition is collected, and its concentration is measured using a nitrous oxide gas analyzer. Alternatively, in laboratory evaluations, the soil composition can be contained in a sealed container such as a gas vial, and the nitrous oxide concentration in the gas phase can be measured using a nitrous oxide gas analyzer. The measured value can be calculated as the nitrous oxide-nitrogen emissions per gram of soil composition every 24 hours.

[0019] In the embodiment of the method for reducing nitrous oxide emissions, the nitrous oxide emissions may be the amount of nitrous oxide emissions after 200 hours or more have elapsed since the nitrogenous fertilizer was combined with the mixture of the base soil and leaf mold. Immediately after adding the nitrogenous fertilizer, the amount of nitrous oxide emissions may temporarily increase, but by measuring after a period of 200 hours or more (for example, at 250, 300, or 350 hours after adding the nitrogenous fertilizer), a more stable reduction effect can be confirmed. This period setting makes it possible to more accurately evaluate the effect of the soil composition containing leaf mold on sustained suppression of nitrous oxide emissions.

[0020] <Kit for Reducing Nitrous Oxide Emissions> In one embodiment, a kit for reducing nitrous oxide emissions from a soil composition containing a nitrogenous fertilizer is provided, comprising leaf mold and instructions, the instructions including instructions for using the kit to reduce nitrous oxide emissions compared to the soil composition combined with the nitrogenous fertilizer by mixing the leaf mold with the base soil and adjusting the mixing ratio of the leaf mold to 50% by volume or more of the total of the base soil and the leaf mold, and then combining the mixture with the nitrogenous fertilizer. The elements of this embodiment (nitrogenous fertilizer, soil composition, reduction of nitrous oxide emissions, leaf mold, etc.) may be described in the section <Method for Reducing Nitrous Oxide Emissions>. In this disclosure, "combining" a mixture of base soil and leaf mold (mixed soil) with a nitrogenous fertilizer means providing a state in which the mixture is at least in contact with the nitrogenous fertilizer, preferably a state in which the mixture is also mixed with the nitrogenous fertilizer. Therefore, it is not necessarily limited to mixing the base soil and leaf mold first and then adding nitrogen fertilizer; the combination of base soil and leaf mold with nitrogen fertilizer can also be achieved by mixing these three elements in a different order or simultaneously. Similarly, by mixing the three elements of base soil, leaf mold, and nitrogen fertilizer in a different order or simultaneously, a soil composition can be provided that includes a mixture of base soil and leaf mold, and nitrogen fertilizer.

[0021] In the kit of the embodiment, the instructions may include instructions to use the kit to reduce nitrous oxide emissions compared to a soil composition in which the base soil alone is combined with the nitrogen fertilizer, by mixing the leaf mold with the base soil and adjusting the mixing ratio of the leaf mold to 60% or more, 70% or more, or 80% or more of the total of the base soil and leaf mold. The instructions may also include instructions to use the kit to reduce nitrous oxide emissions compared to a soil composition in which the base soil alone is combined with the nitrogen fertilizer, by mixing the leaf mold with the base soil and adjusting the mixing ratio of the leaf mold to 90% or less of the total of the base soil and leaf mold, by combining the mixture with the nitrogen fertilizer.

[0022] The instructions in the kit of the embodiment may include a description explaining the effect of reducing nitrous oxide emissions as the purpose of using leaf mold. The instructions may also include a description of a method for mixing leaf mold with base soil, such as a method of adding 50% or more, 60% or more, 70% or more, or 80% or more by volume to a predetermined volume of base soil, and a method of uniformly mixing them.

[0023] The instructions in the embodiment kit may include instructions on preparing the necessary materials, such as how to select the base soil according to the type of crop to be cultivated, the time of cultivation or growth stage of the crop, how to calculate the required amount of leaf mold, and how to select and use nitrogenous fertilizer. Furthermore, the instructions may include a procedure for mixing the leaf mold, such as how to measure the volume of the base soil and leaf mold, how to adjust the mixing ratio, and specific steps for achieving uniform mixing. In addition, the instructions may include one or more of the following regarding the use of nitrogenous fertilizer: a mixing procedure when used as a base fertilizer, a method of application when used as a top dressing, the timing of application, or the amount to be applied. To provide the kit, the leaf mold and instructions can be combined in various forms. For example, the instructions may be provided physically separately from the bag or container containing the leaf mold (e.g., in the form of an accompanying paper document, electronic media, etc.), or they may be provided printed on the bag or container containing the leaf mold. The kit may further include nitrogenous fertilizer and / or base soil.

[0024] <Soil Composition> In one embodiment, a soil composition is provided which comprises a mixed soil consisting of base soil and leaf mold, and a nitrogenous fertilizer, wherein the leaf mold accounts for 50% or more of the mixed soil by volume.

[0025] The soil composition of this embodiment contains a nitrogenous fertilizer in a mixture of base soil and leaf mold. The mixing ratio of leaf mold may be 60% or more by volume, 70% or more by volume, or 80% or more by volume of the soil mixture. Alternatively, the mixing ratio of leaf mold may be 90% or less by volume of the soil mixture. By using such mixing ratios, the emission of nitrous oxide can be efficiently suppressed. The explanation in the section on "Method for Reducing Nitrous Oxide Emissions" may be applied to the various elements of this embodiment (nitrogenous fertilizer, soil composition, leaf mold, etc.).

[0026] The method for determining the mixing ratio of leaf mold in this embodiment is not limited. The mixing ratio of leaf mold can be determined by quantifying the amount of organic matter in the mixed soil, taking advantage of the characteristics that leaf mold mainly consists of organic matter and base soil mainly consists of inorganic matter. Specifically, first, a step can be performed to prepare several standard samples by mixing leaf mold and base soil in predetermined volume ratios. The standard samples can be prepared, for example, in increments of 10 volume%, with the leaf mold mixing ratio ranging from 20 volume% to 100 volume%. Next, a step can be performed to measure the amount of organic matter for each standard sample. The amount of organic matter can be measured by drying the standard sample, then strongly heating it at a high temperature, for example, 600°C, and calculating the loss on ignition from the weight change before and after ignition. Quantification of the amount of organic matter may include a step of creating a calibration curve by plotting the obtained amount of organic matter on the vertical axis and the mixing ratio of leaf mold on the horizontal axis. The proportion of leaf mold in a soil mixture can be determined by measuring the amount of organic matter in the mixture and comparing it with a calibration curve. Note that the method for measuring organic matter is not limited to loss on ignition; other methods for quantifying organic matter, such as measuring carbon content, can also be used. It is also possible to determine the proportion of leaf mold by microscopic observation of soil samples.

[0027] In the soil composition of this embodiment, nitrogenous fertilizer may be pre-mixed with the soil as a base fertilizer, or it may be applied later as a top dressing. The type and amount of nitrogenous fertilizer used can be appropriately selected according to the type of crop to be cultivated or its growth stage. The soil composition of this embodiment can be used for both greenhouse cultivation and open-field cultivation, and is applicable to the cultivation of various crops.

[0028] In the soil composition of this embodiment, the base soil may include black soil or lowland soil.

[0029] In the soil composition of this embodiment, the leaf mold may be 60% by volume, 70% by volume, or 80% or more of the mixed soil. In the soil composition of this embodiment, the leaf mold may be 90% by volume or less of the mixed soil.

[0030] The following examples illustrate the concept, but this disclosure is not limited to the examples described below.

[0031] Soil Preparation: In this experiment, commercially available black soil and leaf mold (both manufactured by EMATA) were used. The black soil was passed through a 2 mm mesh sieve and then air-dried at 30-35°C. The leaf mold was similarly air-dried at 30-35°C, then crushed in a mill mixer and passed through a 2 mm mesh sieve. These prepared soils were mixed in four different volume ratios of black soil:leaf mold = 10:0, 8:2, 5:5, and 2:8.

[0032] Preparation of experimental containers: 100 mL gas chlorophyll vials, sterilized by autoclaving, were used for the experiment. Each vial was filled with 10 g of air-dried soil prepared using the method described above.

[0033] For each vial of nitrogenous fertilizer, a sterilized urea solution was added at a nitrogen content of 200 mg N / kg soil. Sterilized water was also added to reach 60% of the soil's water content. After these additions, the vials were sealed and left to stand in a 25°C incubator.

[0034] N2O concentration was measured eight times at 24, 48, 72, 96, 192, 216, 264, and 336 hours after urea addition. At each measurement, air was collected from the gas phase of the vial using a syringe, and the concentration was measured with an N2O gas analyzer. After measurement, the vial was opened to replace the gas phase with fresh air, resealed, and left to stand until the next measurement. The obtained values ​​were converted to N2O-N emissions per gram of soil every 24 hours and summarized in Figures 1 and 2.

[0035] Figure 1 shows the results of an investigation into the effect of the leaf mold mixing ratio on N2O emissions. In the plot with only black soil, N2O emissions remained almost constant throughout the experimental period. On the other hand, the plot with 20% leaf mold mixed in showed consistently high N2O emissions. In the plots with a high proportion of leaf mold mixed in, N2O emissions decreased over time, and N2O absorption was confirmed after 96 hours in the 80% leaf mold plot and after 192 hours in the 50% leaf mold plot.

[0036] Next, Figure 2 shows the results of an investigation into the effect of urea addition on N2O emissions. In the 20% leaf mold mixture, an increase in N2O emissions was observed after 200 hours with increasing urea addition. On the other hand, in the 50% leaf mold mixture, no increase in N2O emissions was observed after 200 hours regardless of urea addition, confirming that N2O was suppressed or absorbed.

[0037] These results demonstrate that mixing leaf mold at a volume of 50% or more into the soil is effective in suppressing N2O emissions, regardless of the amount of nitrogen fertilizer applied.

[0038] This disclosure includes the following embodiments: [Clause 1] A method for reducing nitrous oxide emissions from a soil composition containing a nitrogenous fertilizer, wherein the soil composition comprises a base soil and the nitrogenous fertilizer, and the method comprises mixing leaf mold with the base soil and adjusting the mixing ratio of the leaf mold to 50 volume percent or more of the total of the base soil and the leaf mold. [Clause 2] The method for reducing nitrous oxide emissions according to Claim 1, wherein the base soil comprises black soil. [Clause 3] The method for reducing nitrous oxide emissions according to Claim 1 or 2, further comprising adjusting the mixing ratio of the leaf mold to 80 volume percent or more of the total of the base soil and the leaf mold. [Clause 4] The method for reducing nitrous oxide emissions according to any one of Claims 1 to 3, wherein the nitrous oxide emissions are the nitrous oxide emissions after 200 hours or more have elapsed since the nitrogenous fertilizer was combined with the mixture of the base soil and the leaf mold. [Item 5] A kit for reducing nitrous oxide emissions from a soil composition containing a nitrogenous fertilizer, comprising leaf mold and instructions, wherein the instructions include instructions for using the kit to reduce nitrous oxide emissions compared to a soil composition combining the base soil alone with the nitrogenous fertilizer, by mixing the leaf mold with base soil to adjust the mixing ratio of the leaf mold to 50% by volume or more of the total of the base soil and the leaf mold, and then combining the mixture with the nitrogenous fertilizer. [Item 6] A soil composition comprising a soil mixture consisting of base soil and leaf mold, and a nitrogenous fertilizer, wherein the leaf mold constitutes 50% by volume or more of the soil mixture. [Item 7] The soil composition according to item 6, wherein the base soil comprises black soil. [Item 8] The soil composition according to item 6 or 7, wherein the leaf mold constitutes 80% by volume or more of the soil mixture.

[0039] While this disclosure has been described with reference to several embodiments described above, it is not limited to the examples given in those embodiments. Various modifications can be made to the structure and details of this disclosure within the scope of this disclosure.

Claims

1. A method for reducing nitrous oxide emissions from a soil composition containing a nitrogenous fertilizer, wherein the soil composition comprises a base soil and the nitrogenous fertilizer, and the method comprises mixing leaf mold with the base soil and adjusting the mixing ratio of the leaf mold to 50% by volume or more of the total of the base soil and the leaf mold.

2. The method for reducing nitrous oxide emissions according to claim 1, wherein the base soil includes black soil.

3. The method for reducing nitrous oxide emissions according to claim 1, further comprising adjusting the mixing ratio of the leaf mold to 80% or more by volume of the total of the base soil and the leaf mold.

4. The method for reducing nitrous oxide emissions according to any one of claims 1 to 3, wherein the nitrous oxide emissions are those obtained 200 hours or more after the nitrogen fertilizer has been combined with the mixture of the base soil and leaf mold.

5. A kit for reducing nitrous oxide emissions from a soil composition containing nitrogenous fertilizer, comprising leaf mold and instructions, wherein the instructions include instructions for using the kit to reduce nitrous oxide emissions compared to a soil composition combining the base soil alone with the nitrogenous fertilizer, by mixing the leaf mold with base soil and adjusting the mixing ratio of the leaf mold to 50% by volume or more of the total of the base soil and leaf mold, and then combining the mixture with the nitrogenous fertilizer.

6. A soil composition comprising a mixed soil consisting of base soil and leaf mold, and a nitrogenous fertilizer, wherein the leaf mold constitutes 50% by volume or more of the mixed soil.

7. The soil composition according to claim 6, wherein the base soil includes black soil.

8. The soil composition according to claim 6 or 7, wherein the leaf mold is 80% by volume or more of the mixed soil.

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