A kind of acidic soil heavy metal stabilization soil repair agent and preparation method thereof

Abstract

The present invention relates to a kind of acidic soil heavy metal stabilized soil remediation agent and preparation method thereof, the soil remediation agent includes a biological kernel, a fertilizer middle layer and an inorganic outer layer from the inside to the outside, the biological kernel includes an organic carrier loaded with microorganisms, the fertilizer middle layer includes calcium magnesium phosphate fertilizer, and the inorganic outer layer includes oyster shell powder, nano manganese dioxide and ammonium chloride. The organic carrier includes soy milk residue and humus soaked in a nutrient solution, the solute of the nutrient solution includes an organic carbon source and an inorganic salt, and the solute is water. The soil remediation agent is based on 100 parts by mass of oyster shell powder, and the dosage of each component raw material is as follows: 20-30 parts of the organic carrier, 20-33 parts of calcium magnesium phosphate fertilizer, 8-18 parts of biochar, 3-8 parts of nano manganese dioxide, and 10-20 parts of ammonium chloride.

Description

Technical Field

[0001] The invention belongs to the technical field of soil remediation, and in particular relates to an acidic soil heavy metal stabilized soil remediation agent and a preparation method thereof. Background Art

[0002] Soil is the material basis for sustainable economic and social development and is related to the health of the people. At present, the overall condition of my country's soil environment is worrying, and pollution is relatively serious in some areas. Soil acidification is a common problem. Acidified soil is often accompanied by the risk of excessive toxic heavy metal content. Soil acidification is a natural soil formation process, but in recent years, farmers have blindly increased the use of chemical fertilizers in order to quickly increase crop yields, and improper irrigation methods have exacerbated acidification. Soil acidification seriously affects the normal production of crops. Its main manifestations are: leading to a decrease in soil organic matter, a loss of basic substances such as calcium and magnesium, the destruction of soil aggregate structure, and the compaction of the soil, accompanied by the risk of increased toxic heavy metal compounds. It is reported that the area of ​​soil acidification in my country has reached more than 40% of the cultivated land area. Since the 1980s, the pH value of farmland soil in the country has dropped by 0.5 units.

[0003] In terms of improving or repairing acidified and heavy metal soils, the commonly used method is to apply chemicals such as lime. This method has the advantages of fast improvement and obvious effect, but in the long run, it may cause greater soil compaction and reduced soil fertility, which is not conducive to the growth of crops. Therefore, people are more willing to use other environmentally friendly materials that will not cause secondary pollution to improve soil acidification. Summary of the invention

[0004] In view of the above problems, the present invention provides an acidic soil heavy metal stabilized soil remediation agent and a preparation method thereof. The soil remediation agent uses calcium magnesium phosphate fertilizer, nano manganese dioxide and oyster shell powder as a base material, which can increase the pH value of the soil and improve the physical and chemical properties of the soil. At the same time, it can adsorb heavy metal elements, which is beneficial to the stabilization of heavy metal pollutants such as cadmium and lead in the soil, reduce their mobility in the soil, and reduce the risk of crops absorbing pollutants and transferring them to the food chain.

[0005] In a first aspect, the soil remediation agent includes a biological inner core, a fertilizer middle layer and an inorganic outer layer from the inside to the outside, wherein the biological inner core includes an organic carrier loaded with microorganisms, the fertilizer middle layer includes calcium magnesium phosphate fertilizer, and the inorganic outer layer includes oyster shell powder, nano manganese dioxide and ammonium chloride.

[0006] Optionally, the microorganism is a bacterium beneficial to the growth of plants or crops, selected from one or a combination of two or more of Bacillus laterosporus, Bacillus colloidus, mycorrhizal fungi, round brown nitrogen-fixing bacteria, brown nitrogen-fixing bacteria, lactic acid bacteria, Bacillus coagulans, and Aspergillus oryzae. Those skilled in the art can independently select suitable strains according to the types and characteristics of plants or crops on the soil to be repaired.

[0007] Optionally, the organic carrier comprises soymilk residue and humus soaked in a nutrient solution, the mass ratio of the soymilk residue to the humus is 1:(1-3), the solute of the nutrient solution comprises an organic carbon source and an inorganic salt, and the solute is water;

[0008] The organic carbon source is selected from one or a combination of two or more of glucose, soy milk, sucrose and fructose; the inorganic salts include but are not limited to water-soluble potassium salts, sodium salts, calcium salts and ammonium salts.

[0009] Optionally, the ratio of the concentration of the organic carbon source to the number of microorganisms in the nutrient solution is (20-45) g / ml: 1×10 5 CFU / mL; the ratio of the concentration of inorganic salts in the nutrient solution to the number of microorganisms is (2-10) g / ml: 1×10 5 CFU / mL; the ratio of the sum of the mass of soy milk residue and humus to the volume of the nutrient solution is 1g:(10-20)ml.

[0010] Those skilled in the art can flexibly select appropriate organic carbon sources and inorganic salts according to the loaded microorganisms, and adjust the concentration of the nutrient solution.

[0011] Optionally, the middle layer of the fertilizer includes calcium magnesium phosphate fertilizer and biochar. Biochar is the solid residue produced by pyrolysis or incineration of agricultural straw. Both calcium magnesium phosphate fertilizer and biochar can provide nutrition for plants or crops on the soil to be repaired, and cooperate with microorganisms to provide richer nutrition for microorganisms.

[0012] Optionally, the particle size of the oyster shell powder is 60-80 mesh, the particle size of the biochar is 80-100 mesh, the particle size of the calcium magnesium phosphate fertilizer is 60-100 mesh, and the particle sizes of the soy milk residue and the humus are both 150-200 mesh.

[0013] Optionally, the soil remediation agent is based on 100 parts by mass of oyster shell powder, and the dosage of each component raw material is as follows: 20-30 parts of the organic carrier, 20-33 parts of calcium magnesium phosphate fertilizer, 8-18 parts of biochar, 3-8 parts of nano manganese dioxide, and 10-20 parts of ammonium chloride.

[0014] The soil remediation agent described in the present invention is granular and includes three layers, the outermost layer is an inorganic outer layer, the nano manganese dioxide therein has the characteristics of small particles and large specific surface area, has good adsorption performance for heavy metal ions in the environment, and can effectively fix heavy metal pollutants in the soil; oyster shell powder is an alkaline mineral fertilizer, the main component of which is calcium carbonate, and has a porous structure, which has a certain effect on improving the physical and chemical properties of the soil and reducing the migration capacity of heavy metals; ammonium chloride is not only a fertilizer, but also can react with excess calcium carbonate after being dissolved in water to play a role in stripping off part of the inorganic outer layer; after holes appear in the inorganic outer layer, acidic substances produced by plants in the soil can enter the middle layer of the fertilizer and utilize calcium magnesium phosphate fertilizer; after the calcium magnesium phosphate fertilizer is continuously released slowly, the middle layer of the fertilizer partially disintegrates, and the internal biological core can release the microorganisms, which is beneficial to the growth of plants or crops. Before the inorganic outer layer and the fertilizer middle layer disintegrate or have holes, the microorganisms rely on organic carriers, calcium magnesium phosphate fertilizers and biochar to grow. For aerobic microorganisms, air can penetrate into the biological inner core through the porous structure of the inorganic outer layer and the fertilizer middle layer for use by the microorganisms.

[0015] In a second aspect, the preparation method of the soil remediation agent comprises the following steps:

[0016] (1) uniformly mixing the crushed soy milk residue and humus, preparing the nutrient solution with the organic carbon source and the inorganic salt, and soaking the mixture of the soy milk residue and the humus in the nutrient solution to obtain a kernel culture medium;

[0017] (2) After the microorganisms are cultured to a good state, the microorganisms are inoculated into the inner core culture medium obtained in step (1), and after the microorganisms are stably immobilized and grown on the soymilk residue and humus, the mixture of the soymilk residue and humus is removed, drained until no water is dripping, and sphericalized in a wet state to obtain a biological inner core;

[0018] (3) Mixing calcium magnesium phosphate fertilizer and biochar evenly, adding water, and mixing into a mud-like state, and covering the outer layer of the biological inner core to obtain intermediate particles;

[0019] (4) The oyster shell powder, nano manganese dioxide and ammonium chloride are mixed evenly, and then ethanol is added to form a mud, which is covered on the outer layer of the wet intermediate particles, and the soil remediation agent is obtained after natural drying.

[0020] Optionally, in step (1), the mixture of soy milk residue and humus is stirred while being immersed in the nutrient solution to promote full contact between the solid and the liquid. The stirring speed is 50-80 rpm, that is, the stirring speed should not be too fast to avoid excessive bubbles in the nutrient solution. The soaking time is 12-24 hours.

[0021] Optionally, in step (2), if there are two or more microorganisms, each microorganism is cultured separately first, and then inoculated into the inner core culture medium after each microorganism reaches a good state;

[0022] The good state refers to the colony count reaching 1×10 5 Pieces / cm 2 , in step (1), the concentrations of the organic carbon source and the inorganic salt in the nutrient solution are determined according to the inoculation number of the microorganisms in the good state;

[0023] The stable immobilization growth of the microorganisms on the soymilk residue and humus means that the colony count of each microorganism on the soymilk residue or humus reaches 1×10 5 Pieces / cm 2 ;

[0024] The particle size of the biological inner core is 0.8-1.5 mm.

[0025] For example, there are three kinds of microorganisms. The colony counts of the first microorganism and the second microorganism on the soy milk residue both reach 1×10 5 Pieces / cm 2 , while the colony count on humus did not reach 1×10 5 Pieces / cm 2 The colony count of the third microorganism on the humus reached 1×10 5 Pieces / cm 2 , while the number of colonies on soy milk residue did not reach 1×10 5 Pieces / cm 2 This situation belongs to the stable immobilization growth of the microorganisms on soy milk residue and humus.

[0026] Optionally, in step (3), the amount of water added is 1-2 times the total mass of calcium magnesium phosphate fertilizer and biochar; and the particle size of the intermediate particles is 2-4 mm.

[0027] Optionally, in step (4), the amount of ethanol added is 0.3-0.6 times the total mass of oyster shell powder, nano manganese dioxide and ammonium chloride, and the particle size of the soil remediation agent is 6-9 mm. DETAILED DESCRIPTION

[0028] The soil to be repaired in the following examples and comparative examples is the pomelo planting soil in Xiazhai Town, Pinghe County, Fujian Province.

[0029] Example 1

[0030] The soil remediation agent described in this embodiment includes a biological inner core, a fertilizer middle layer and an inorganic outer layer from the inside to the outside, wherein the biological inner core includes an organic carrier loaded with microorganisms, the fertilizer middle layer includes calcium magnesium phosphate and biochar, the biochar is a solid residue produced by pyrolysis of agricultural straw, and the inorganic outer layer includes oyster shell powder, nano manganese dioxide and ammonium chloride.

[0031] The microorganism is Bacillus laterosporus, purchased from Shandong Nuojie Biotechnology Co., Ltd. The organic carrier includes soy milk residue and humus soaked in a nutrient solution, the mass ratio of soy milk residue to humus is 1:1, the solute of the nutrient solution includes an organic carbon source and an inorganic salt, the solute is water; the organic carbon source is glucose, and the inorganic salt is potassium chloride and sodium chloride.

[0032] The ratio of the glucose concentration in the nutrient solution to the number of microorganisms is 20 g / ml: 1×10 5 CFU / mL; the ratio of the concentration of potassium chloride and sodium chloride in the nutrient solution to the number of microorganisms is 2g / ml: 1×10 5 CFU / mL; the ratio of the sum of the mass of soy milk residue and humus to the volume of the nutrient solution is 1g:10ml.

[0033] The particle size of the oyster shell powder is 60 meshes, the particle size of the biochar is 80 meshes, the particle size of the calcium magnesium phosphate fertilizer is 60 meshes, and the particle sizes of the soy milk residue and the humus are both 150 meshes.

[0034] The soil remediation agent is based on 100 parts by mass of oyster shell powder, and the dosage of each component raw material is as follows: 20 parts of the organic carrier, 20 parts of calcium magnesium phosphate fertilizer, 8 parts of biochar, 3 parts of nano manganese dioxide, and 10 parts of ammonium chloride.

[0035] The preparation method of the soil remediation agent comprises the following steps:

[0036] (1) uniformly mixing the crushed soy milk residue and humus, preparing the nutrient solution with the organic carbon source and the inorganic salt, and immersing the mixture of the soy milk residue and the humus in the nutrient solution, stirring at a stirring speed of 80 rpm for 12 h, to obtain a kernel culture medium;

[0037] (2) When the culture of Bacillus laterosporus reaches a good state, that is, the number of colonies reaches 1×10 5 Pieces / cm 2 , inoculating the microorganism into the inner core culture medium obtained in step (1), in step (1), determining the concentration of the organic carbon source and the inorganic salt in the nutrient solution according to the inoculation number of the microorganism in the good state; the cultivation method of Bacillus laterosporus is detailed in patent CN201510700147.6;

[0038] After the microorganisms have been stably immobilized and grown on the soybean milk residue and humus, that is, the number of microbial colonies on the soybean milk residue or humus has reached 1×10 5 Pieces / cm 2 Afterwards, the mixture of soy milk residue and humus is scooped out, drained until no water is dripping, and formed into a ball in a wet state to obtain a biological core, the particle size of which is 1 mm;

[0039] (3) Mixing calcium magnesium phosphate fertilizer and biochar evenly, adding water, and mixing into a mud-like state, and covering the outer layer of the biological inner core to obtain intermediate particles;

[0040] The amount of water added is 1 times the total mass of calcium magnesium phosphate fertilizer and biochar; the particle size of the intermediate particles is 4 mm;

[0041] (4) mixing oyster shell powder, nano manganese dioxide (purchased from McLean, brand number M812765) and ammonium chloride evenly, adding ethanol, mixing into a mud-like state, covering the outer layer of the wet intermediate particles, and drying naturally to obtain the soil remediation agent;

[0042] The amount of ethanol added is 0.3 times the total mass of oyster shell powder, nano manganese dioxide and ammonium chloride, and the particle size of the soil remediation agent is 6 mm.

[0043] Add the above-mentioned soil remediation agent to the surface of the soil to be repaired, till it thoroughly, and irrigate it with unpolluted water. After three months, collect 5-6 soil samples at a depth of 2-20 cm, measure the effective heavy metal concentration and pH value in the soil to be repaired, and then take the average value.

[0044] The detection method of soil pH value adopts the agricultural industry standard of the People's Republic of China "NY / T 1377-2007 Determination of Soil pH"; the detection method of effective heavy metal concentration adopts the "Research on the Determination Method of Extractable State (Effective State) of Heavy Metals in Soil".

[0045] Comparative Example 1

[0046] The soil remediation agent, preparation method and use method of this comparative example are different from those of Example 1 in that the soil remediation agent does not include a biological inner core, but only includes a fertilizer middle layer and an inorganic outer layer. The soil remediation agent is based on 100 parts by mass of oyster shell powder, and the amount of each component raw material is as follows: 20 parts of calcium magnesium phosphate fertilizer, 8 parts of biochar, 3 parts of nano manganese dioxide, and 10 parts of ammonium chloride.

[0047] The preparation method does not include steps (1) and (2), and directly proceeds to step (3), that is,

[0048] (3) mixing calcium magnesium phosphate fertilizer and biochar evenly, adding water, and forming into balls in a wet state to obtain intermediate particles;

[0049] The amount of water added is 1 times the total mass of calcium magnesium phosphate fertilizer and biochar; the particle size of the intermediate particles is 4 mm;

[0050] (4) mixing oyster shell powder, nano manganese dioxide and ammonium chloride evenly, adding ethanol, mixing into a mud-like state, covering the outer layer of the wet intermediate particles, and drying naturally to obtain the soil remediation agent;

[0051] The amount of ethanol added is 0.3 times the total mass of oyster shell powder, nano manganese dioxide and ammonium chloride, and the particle size of the soil remediation agent is 6 mm.

[0052] Comparative Example 2

[0053] The soil remediation agent, preparation method and use method of this comparative example are different from those of Example 1 in that the soil remediation agent does not include a fertilizer middle layer, but only includes a biological inner core and an inorganic outer layer. The soil remediation agent is based on 100 parts by mass of oyster shell powder, and the amount of each component raw material is as follows: 20 parts of the organic carrier, 3 parts of nano manganese dioxide, and 10 parts of ammonium chloride.

[0054] The preparation method does not include step (3). After steps (1) and (2), step (4) is directly performed, that is, oyster shell powder, nano manganese dioxide and ammonium chloride are evenly mixed, ethanol is added, and the mixture is mixed into a mud-like state, which is then covered on the outer layer of the moist biological inner core.

[0055] Comparative Example 3

[0056] The soil remediation agent, preparation method and use method of this comparative example are different from those of Example 1 in that the soil remediation agent does not include an inorganic outer layer, but only includes a biological inner core and a fertilizer middle layer. The amount of each component is the same as that of Example 1, but oyster shell powder, nano manganese dioxide and ammonium chloride are not used.

[0057] The preparation method does not include step (4), and the intermediate particles obtained in step (3) are the soil remediation agent.

[0058] Comparative Example 4

[0059] The soil remediation agent, preparation method and use method of this comparative example are different from those of Example 1 in that, in step (2), the mixture of soy milk residue and humus is scooped out, drained until no water is dripping, and then mixed evenly with calcium magnesium phosphate fertilizer, biochar, oyster shell powder, nano manganese dioxide and ammonium chloride at one time to prepare the soil remediation agent.

[0060] Comparative Example 5

[0061] The soil remediation agent, preparation method and use method of this comparative example are different from those of Example 1 in that there is no ammonium chloride in the inorganic outer layer.

[0062] Comparative Example 6

[0063] The soil remediation agent, preparation method and use method of this comparative example are different from those of Example 1 in that the soil remediation agent does not include a biological inner core, but only includes a fertilizer middle layer and an inorganic outer layer. After the Bacillus laterosporus is cultured to a good state in step (2), it is directly inoculated into the mixture of calcium magnesium phosphate and biochar in step (3), and then water is added to make a mud-like state, which is covered on the outer layer of the biological inner core to obtain intermediate particles.

[0064] Example 2

[0065] The soil remediation agent, preparation method and use method of this embodiment are different from those of Example 1 in that the soy milk residue and humus of the organic carrier are not soaked in the nutrient solution, and in step (2), the microorganisms cultured to a good state are inoculated onto the soy milk residue and humus to obtain a loose biological kernel.

[0066] Example 3

[0067] The soil remediation agent, preparation method, and use method of this embodiment are different from those of Example 1 in that the soil remediation agent is based on 100 parts by mass of oyster shell powder, and the amounts of the raw materials of each component are as follows: 30 parts of the organic carrier, 33 parts of calcium magnesium phosphate fertilizer, 18 parts of biochar, 8 parts of nano manganese dioxide, and 20 parts of ammonium chloride.

[0068] Example 4

[0069] The soil remediation agent, preparation method, and use method of this embodiment are different from those of Embodiment 1 in that the mass ratio of soy milk residue to humus is 1:3.

[0070] Example 5

[0071] The soil remediation agent, preparation method, and use method of this embodiment are different from those of Embodiment 1 in that the mass ratio of soy milk residue to humus is 1:4.

[0072] Example 6

[0073] The soil remediation agent, preparation method, and use method of this embodiment are different from those of embodiment 1 in that the ratio of the concentration of inorganic salts in the nutrient solution to the number of microorganisms is 10 g / ml: 1×10 5 CFU / mL.

[0074] Example 7

[0075] The soil remediation agent, preparation method, and use method of this embodiment are different from those of Example 1 in that the ratio of the concentration of inorganic salts in the nutrient solution to the number of microorganisms is 11 g / ml: 1×10 5 CFU / mL.

[0076] Example 8

[0077] The soil remediation agent, preparation method and use method of this embodiment are different from those of Embodiment 1 in that, in step (4), the amount of ethanol added is 0.6 times the total mass of oyster shell powder, nano manganese dioxide and ammonium chloride.

[0078] Example 9

[0079] The soil remediation agent, preparation method and use method of this embodiment are different from those of Example 1 in that, in step (4), the amount of ethanol added is 0.7 times the total mass of oyster shell powder, nano manganese dioxide and ammonium chloride.

[0080] Table 1 Experimental results of embodiments and comparative examples

[0081]

[0082] As can be seen from the above table, the acidic soil heavy metal stabilization soil remediation agent provided by the present invention has a good effect on improving the pH value of acidic soil and fixing heavy metal lead. The soil remediation agent is also effective for heavy metals cadmium and thallium. The passivation rate of the effective state Cd in the above embodiments 1-9 is between 70-90%, and the passivation rate of the effective state Tl is between 70-80%. The raw material components of the soil remediation agent are non-toxic, low in cost, and suitable for wide application.

Claims

1. A heavy metal stabilized soil remediation agent for acidic soil, characterized in that: From the inside to the outside, it includes a biological core, a fertilizer middle layer and an inorganic outer layer, wherein the biological core includes an organic carrier loaded with microorganisms, the fertilizer middle layer includes calcium magnesium phosphate and biochar, and the inorganic outer layer includes oyster shell powder, nano manganese dioxide and ammonium chloride; The organic carrier comprises soybean milk residue and humus soaked in a nutrient solution, the mass ratio of the soybean milk residue to the humus is 1:(1-3), the solute of the nutrient solution comprises an organic carbon source and an inorganic salt, and the solvent is water; The ratio of the concentration of inorganic salts in the nutrient solution to the number of microorganisms is (2-10) g / ml: 1×10 5 CFU / mL; The preparation method of the soil remediation agent comprises the following steps: (1) uniformly mixing the crushed soy milk residue and humus, preparing the nutrient solution with the organic carbon source and the inorganic salt, and soaking the mixture of the soy milk residue and the humus in the nutrient solution to obtain a kernel culture medium; (2) After the microorganisms are cultured to a good state, the microorganisms are inoculated into the inner core culture medium obtained in step (1). After the microorganisms are stably immobilized and grown on the soymilk residue and humus, the mixture of the soymilk residue and humus is removed, drained until no water is dripping, and formed into a ball in a wet state to obtain a biological inner core; (3) Mix the calcium magnesium phosphate fertilizer and biochar evenly, add water, and mix them into a mud-like state, which is then covered on the outer layer of the biological inner core to obtain intermediate particles; (4) mixing oyster shell powder, nano manganese dioxide and ammonium chloride evenly, adding ethanol, mixing into a mud-like state, covering the outer layer of the wet intermediate particles, and drying naturally to obtain the soil remediation agent; In step (2), the good state means that the number of colonies reaches 1×10 5 Pieces / cm 2 ; In step (4), the amount of ethanol added is 0.3-0.6 times the total mass of oyster shell powder, nano manganese dioxide and ammonium chloride.

2. The soil remediation agent according to claim 1, characterized in that: The microorganism is selected from one or a combination of two or more of Bacillus laterosporus, Bacillus colloidus, mycorrhizal fungi, round brown nitrogen-fixing bacteria, brown nitrogen-fixing bacteria, lactic acid bacteria, Bacillus coagulans, and Aspergillus oryzae.

3. The soil remediation agent according to claim 1, characterized in that: The organic carbon source is selected from one or a combination of two or more of glucose, soy milk, sucrose and fructose; the inorganic salt is water-soluble potassium salt, sodium salt, calcium salt and ammonium salt.

4. The soil remediation agent according to claim 3, characterized in that: The ratio of the concentration of organic carbon source to the number of microorganisms in the nutrient solution is (20-45) g / ml: 1×10 5 CFU / mL; The ratio of the sum of the mass of soy milk residue and humus to the volume of the nutrient solution is 1g:(10-20)ml.

5. The soil remediation agent according to claim 3, characterized in that: The biochar is a solid residue produced by pyrolysis of agricultural straw.

6. The soil remediation agent according to claim 5, characterized in that: The particle size of the oyster shell powder is 60-80 meshes, the particle size of the biochar is 80-100 meshes, the particle size of the calcium magnesium phosphate fertilizer is 60-100 meshes, and the particle sizes of the soy milk residue and the humus are both 150-200 meshes.

7. The soil remediation agent according to claim 6, characterized in that: The soil remediation agent is based on 100 parts by mass of oyster shell powder, and the dosage of each component raw material is as follows: 20-30 parts of the organic carrier, 20-33 parts of calcium magnesium phosphate fertilizer, 8-18 parts of biochar, 3-8 parts of nano manganese dioxide, and 10-20 parts of ammonium chloride.

8. The soil remediation agent according to claim 7, characterized in that: In step (2), if there are two or more microorganisms, each microorganism is cultured separately first, and then inoculated into the inner core culture medium after each microorganism reaches a good state.

9. The soil remediation agent according to claim 8, characterized in that: In step (1), the concentrations of the organic carbon source and the inorganic salt in the nutrient solution are determined according to the inoculation quantity of the microorganisms in the good state; The stable immobilization growth of the microorganisms on the soymilk residue and humus means that the colony count of each microorganism on the soymilk residue or humus reaches 1×10 5 Pieces / cm 2 .

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