Soil Fertility and Its Factors  

1, Soil fertility types: soil fertility is the ability of the soil to meet the water, nutrients, air, heat necessary for the growth and development of crops and called it. Soil fertility is divided into natural fertility and artificial fertility; potential fertility and effective fertility. Natural fertility refers to the fertility of natural soil before it is reclaimed and used; man-made fertility refers to the fertility created by people who cultivate, fertilize, irrigate and other agricultural technology measures on the soil.

Therefore, any soil, the longer the cultivation of crops, the more perfect agricultural technology measures can be used, the greater the proportion of man-made fertility. Therefore, the soil is the object of labor, and is the product of labor. The so-called effective fertility, refers to the cultivation of crops, the part of the crop to be absorbed by the season fertility; potential fertility refers to the existence of the soil, can not be immediately used by the crops of the season those fertility. Potential fertility and effective fertility, under the implementation of appropriate agricultural technology measures, can be transformed into each other.

2, Soil fertility factors: soil moisture, nutrients, air and temperature, known as the four factors of soil fertility. The level of soil fertility is not only affected by the number of each fertility factor appropriate or not, but mainly depends on the degree of coordination between water, fertilizer, air and heat under certain conditions. Therefore, it is necessary to study and master the condition of each fertility factor of the soil and their interrelationship.

(1) Soil moisture conditions. "Water is the lifeblood of agriculture", first of all, the growth and development of crops need a lot of water. This is because: the general crop to obtain a point yield, must consume 500-1000 points of water, all this water is supplied from the soil; crop absorption of nutrients also need to dissolve in water before being used; soil microbial activities and the decomposition and transformation of soil nutrients need water.

(2) Water plays a direct role in constraining the condition of soil air and heat, and also influences the properties of soil such as expansion and contraction, adhesion, bonding and tillage. This shows that soil moisture is not only necessary for crop growth and development, but also can be controlled to harmonize fertilizer, air and heat relations.

①Soil moisture types: soil moisture is generally divided into three types according to the different forces acting on it.

A, Bound water: This is the water that is tightly bound around the soil grains under the action of gravitational force on the surface of the soil grains and is called. This water moves very slowly in the soil, and a part of the surface of the soil grain does not move, so it is difficult to be absorbed by the crop. When the water content of the soil reaches a level where only bound water is available, the crop withers. As the finer the soil grain, the more water is absorbed, so the bound water of clay soil is greater than sand.

B, Capillary water: this is in the soil capillary gravitational effect, kept in the twisted micro soil pores of water and called. This water can move along the capillary pores in all directions up and down and left and right. The law of movement is from the soil layer with high humidity to the soil layer with low humidity. It is the most suitable for crop absorption and utilization of water in the soil. Since the water has various crop nutrients dissolved in it, it in turn provides nutrients for crops.

Oil sand soil, tide sand soil, the emergence of the "back tide" or "back to the wet" phenomenon, is the capillary water upward movement, the groundwater to the tillage layer of the reason. But the capillary water movement will bring surface evaporation constantly occur, resulting in soil moisture loss, so production often take in the tillage loosening, which has cut off the soil capillary, reduce the role of soil moisture evaporation.

C, Gravity water: this is in the soil moisture content exceeds the range of action of the soil capillary force, excessive water leakage downward by the influence of gravity, this leakage of water is called gravity water. It is the most effective water for rice. Although the seepage effect has caused the leakage of water and fertilizer, but regardless of the water field or dry soil, the appropriate seepage is necessary, it is conducive to the renewal of soil air and the downward movement of harmful reducing substances and leaching loss.

② Rice soil moisture status: rice soil in the period of flooding, the tillage layer moisture shows supersaturation, due to gravity, continuous vertical leakage. According to the characteristics of vertical water seepage, rice soil is divided into three types.

A, Groundwater type: This type of rice soil has a high water table (the water table is within 60 cm from the surface), poor drainage, irrigation water layer and groundwater connected, poor permeability, and low mud temperature, such as cold-soaked fields, poured mud fields and deep-footed duck-shit mud soils.

B, Surface water type: In this type of rice soil, the water table is very deep (more than 150 cm) and the irrigation water infiltration cannot reach the groundwater layer, although the drainage is good, it is not resistant to drought. Such as high bank field, Tianshui field and most of the terraced fields belong to it.

C, Good water type: In this type of rice soil, the water table is between 60-150 cm, and the irrigation water layer is not connected with the water table, but the soil capillary water can be circulated up and down, and these fields are generally distributed above the ridge fields or the first and second row fields.

Three types of rice soils, with good water type soil fertility is the best, generally high-yielding and stable rice fields. Proper seepage is necessary for rice soils, which helps renew the soil air and eliminate toxic substances. Of course, it should not be too large, so as not to cause nutrient leaching. Generally in the irrigation of 1 inch of water can be saved for three days as the limit, that is, the leakage amount of 0.5-1.0 cm / 24 hours is most appropriate.

(2) Soil air conditions: soil air has a close relationship with soil microbial activity and nutrient transformation, and also has an impact on crop root development. Crop growth and development at all times have certain requirements for soil air.

① Composition of soil air: Part of the air in the soil comes from the atmosphere; part of it is produced by biochemical processes in the soil. Due to the influence of biological (crop roots and microorganisms) life activities in the soil and the decomposition of organic matter, constantly consuming oxygen and producing carbon dioxide and other gases, resulting in a significant difference between the composition of soil air and the atmosphere: the content of oxygen in soil air is lower than the atmosphere, while the content of carbon dioxide is higher than the atmosphere; in addition, soil air is often saturated with water vapor, atmospheric humidity generally only reaches 50 -90%; soil air sometimes contains small amounts of reducing gases, such as methane, hydrogen, ammonia and hydrogen sulfide.

② Characteristics of rice soil air conditions: Rice soil is often in a reduced state due to seasonal or perennial flooding, and gas exchange between the soil air and the atmosphere is isolated by the water layer. The oxygen consumed by crop life activities can only be imported into the roots by the oxygen transporting tissues of crop stems and leaves from the atmosphere, and then secreted out by the roots, causing an inter-root micro-domain oxidizing environment and preventing the rice roots from being poisoned by the surrounding reducing substances. This is the secret of rice being able to grow in an oxygen-deficient environment.

Therefore, the characteristics of soil air conditions in paddy fields are distinctly hierarchical and micro-domain. In the surface of the tillage layer a few millimeters to 1 cm for the oxidation layer, because the iron into high-valent compounds condition, the soil color is yellow-brown or yellow-brown. Below the oxidation layer, the tillage layer is a reduction layer, and the soil color is greenish-gray or blue-gray because iron becomes a low-valent compound. However, in the soil near the inter-root area, rust spots and rust patterns often appear due to the oxygen secretion of the rice root group.

③ The position of soil air in soil fertility: soil air supplies oxygen needed by the respiration of crop roots. If there is a lack of oxygen, the root development is affected and the water and fertilizer absorption function is weakened or even died. This is especially true during seed germination and seedling stage. Although rice has aeration tissue, the soil should also have a certain degree of aeration performance, in order to facilitate the growth of rice roots.

In addition, soil air conditions affect the activities of soil microorganisms and nutrient conversion. Hypoxic microbial activity is mainly suspicious of the air, so that the decomposition of organic matter is slow, resulting in nutrient deficiencies, and even cause nitrogen loss, and at the same time, also produce adverse to the crop nutrition of reductive toxic substances, such as acetic acid, butyric acid, hydrogen sulfide, etc.. In addition, poor soil aeration is conducive to the breeding of pathogens, causing crop infections and diseases, affecting crop growth and reducing yields. Therefore, paddy fields are often regulated by draining open fields and sunny fields.

(3) Soil temperature: soil temperature has a great impact on crop fertility and microbial activities in the soil, as well as the transformation of various nutrients, soil water evaporation and movement. Crops need certain temperature conditions from sowing to maturity, such as barley and wheat can germinate at 1-2°C, while rice and cotton germinate only at 10-12°C. So the right time to sow different crops is determined by the soil temperature. General soil microbial life, the soil temperature of 25 ℃ - 37 ℃ is appropriate, the minimum is 5 ℃, the maximum does not exceed 45 ℃ - 50 ℃. Soil temperature is too low, microbial activity is weakened, or even completely stopped, organic matter is difficult to decompose, the lack of effective nutrients. This is the case with cold-soaked fields, so it is necessary to exclude cold-soaked water and increase the application of pig and cow pen manure, lime, grass ash and fire soil ash to raise the soil temperature.

① Factors affecting soil temperature: Temperature is a manifestation of heat. Soil heat mainly comes from solar radiation heat, followed by the decomposition of organic matter by microorganisms, releasing a certain amount of heat to increase the soil temperature.
There are many factors affecting the variation of soil temperature, such as latitude, altitude of sea level, topography and slope direction. However, the main factor is the soil thermal properties of the soil itself, such as soil heat capacity, thermal conductivity, heat absorption and heat dissipation. Especially, heat capacity and thermal conductivity are the most important internal factors to determine soil temperature.

A, Soil heat capacity: The number of heat calories (cal/cm3/degree) required to increase the temperature of dry soil by 1°C per 1 cm3 is called the soil heat capacity. The heat capacity of water is 1; air is 0.0003; soil particles between the two, about 0.5-0.6. Since the solid part of the soil changes very little, therefore, the size of the soil heat capacity is mainly determined by the amount of soil water and air, where the water is more gas less soil, the heat capacity is large, slow temperature increase, cooling is also slow, the temperature change is small; conversely, the soil temperature change is large. Therefore, rice field management, early spring daytime drainage to increase temperature, night irrigation insulation; summer use of deep irrigation to reduce temperature.

B, Soil thermal conductivity: soil thermal conductivity refers to the performance of heat conduction from the higher temperature soil layer to the lower temperature soil layer. Its size is related to the ratio of soil solid, liquid and gas phase composition. The thermal conductivity of soil minerals is 100 times that of air; water is 25 times that of air; organic matter is 5 times that of air; and air hardly transfers heat. It is clear that the thermal conductivity of soil depends on the relative ratio between air and water. Therefore, mid-tillage loosening has reduced soil thermal conductivity, so that the temperature of topsoil is not easily transferred downward and the temperature of deep soil is not easily dissipated upward.

② Regulation of soil temperature changes: soil temperature often changes with the influence of meteorological factors, in order to meet the needs of crop growth and development, we must take effective measures around the goal of increasing soil temperature in early spring, reducing soil temperature in summer, and maintaining soil temperature in autumn and winter.

A, Reasonable irrigation: early spring during the cold snap more irrigation, irrigation deep water, to avoid a sudden drop in soil temperature, enhance the ability of seedlings to resist low temperatures; general weather during the use of shallow water between irrigation, warming and ventilation, to promote crop growth. In summer to enhance soil heat dissipation, take a combination of short-term irrigation deep water and regular irrigation open field, to achieve the purpose of heat dissipation, ventilation, water supply, and promote crop growth and development. In autumn and winter, generally combined with fertilization, the implementation of pre-frost irrigation to reduce crop frost damage.

B, Reasonable fertilization: Under the premise of ensuring adequate fertilization, increase the application of organic fertilizers, such as fire soil ash, rotted pig and cow pen silt, etc., to improve soil temperature. First, deepen the soil color, increase the soil heat absorption; second, the organic fertilizer decomposition in the release of heat; third, soil loosening, increase the air capacity, reduce the soil heat capacity. In addition, it also directly improves the nutrition of crops.

C, The implementation of mulching: early spring and autumn and winter low-temperature season, the use of grass ash, chopped grass seeds (purple clouds), dry (wet) cow dung, moss, plastic film, etc. cover the ground, can improve the soil heat absorption, reduce heat loss, insulation and frost protection; summer and autumn during the high temperature and drought, the use of straw or other crop straw cover the ground, there is shade and sun protection, reduce the role of soil temperature, but also to reduce water evaporation and eliminate weeds D. Mid-tillage loosening

D, Mid-tillage loosening: this is conducive to the increase in soil air capacity, reducing the role of downward heat transfer from the topsoil and rising soil temperature in the lower layers. Therefore, in early spring, the clay heavy tight soil to loosen the soil to improve soil temperature, speed up the seed germination; summer mid-tillage loosening, moderate the root system activity layer soil temperature is too high, to promote the growth of crop roots.

In addition, the use of wind barriers, windbreaks, smoke and the application of chemical temperature enhancers, etc., can regulate soil temperature, can be applied according to local conditions.

(4) Soil nutrients: most of the nutrients needed by crops come from the soil, but most of the nutrients in the soil exist in insoluble minerals and organic matter, which are late-effective and difficult for crops to absorb and use. The ionic fast-acting nutrients that can be absorbed and used by the crop in season only account for 0.005-0.1% of the soil weight and are present in aqueous solutions and adsorbed on the surface of soil colloids. However, this late-effective and fast-acting nutrients can be converted into each other under certain conditions.

① Conversion of organic carbon compounds: Organic matter such as cellulose, starch, disaccharides, monosaccharides and fats in the soil do not contain nitrogen. They are transformed in the soil in two ways.

One is that when well aerated, they are rapidly decomposed by aerobic bacteria and fungi and finally produce CO2 and H2O, and give off a lot of heat. This heat is the driving force of soil biochemistry and the source of energy required for the life activities of soil microorganisms.

Two is when poorly ventilated, by the role of gas-susceptible bacteria, slow decomposition, only a small amount of heat and CO2, and the accumulation of a large number of organic acids (acetic acid, butyric acid), methane, hydrogen and other reducing substances, crop growth and development obstacles. Such as rice "turn autumn" or "soluble root" phenomenon, is butyric acid damage. Therefore, the paddy field turning green fertilizer, combined with the application of lime, is to neutralize the organic acid, to eliminate the poisonous paddy field.

② Conversion of nitrogen in soil: organic nitrogen accounts for more than 99% of the soil, inorganic nitrogen is less than 1%; the total nitrogen content of paddy fields is about 0.1-0.2%, and inorganic nitrogen is even less. The vast majority of nitrogen absorbed by crops from the soil is converted from organic nitrogen. There are four main types of conversion formation.

A, Ammonification: nitrogen-containing organic matter in the soil, such as protein, urea and chitosan (chitin), etc. under the action of ammonification bacteria, gradually decompose and release ammonia, called ammonification. This process can take place regardless of good or bad aeration. Ammonia combines with acid roots in the soil to form ammonium salts, which are absorbed and used by crops, or are preserved by adsorption of soil colloids.

B, Nitrification: The process of converting ammonia or ammonium salts into nitric acid under well-aerated conditions, by the action of nitrite bacteria, nitrate bacteria, etc., is called nitrification. NO3-N is a good effective state nutrient for crops, but can not be adsorbed by the soil colloid, easy to lose with water, so deep plowing and loosening the soil to keep it moist, favorable nitrification and prevent the dispersion of ammonia in the soil.

C, Denitrification: When the soil is poorly aerated and contains a large amount of fresh organic matter and nitrate in the soil, under the action of denitrifying bacteria, the nitrate is reduced to nitrogen that is not available to the crop and lost, a process called denitrification. This action is detrimental to crop nutrient uptake and growth and must be prevented. The use of shallow inter-irrigation, open field aeration and application of ammonium nitrogen fertilizer, dry soil loosening after the rain, can prevent the occurrence of denitrification.

D, Biological nitrogen capture: Inorganic nitrogen in the soil (such as ammonium salts, nitrate) is partially absorbed and used by microorganisms, weeds and soil animals to synthesize biological organisms, so that the effective state of soil nitrogen is reduced, called biological nitrogen capture. Especially the most prominent microbial nitrogen capture, when the soil with a large number of fresh, organic fertilizer containing cellulose and other environmental conditions and suitable, microbial activity and reproduction, consuming the effective nitrogen in the soil, thus leading to a lack of nitrogen nutrients or serious shortage of crops. Therefore, where straw is returned to the field or the application of a large number of unrotted organic fertilizers containing more fiber, must be combined with the application of appropriate quick-acting nitrogen fertilizer to supplement the effective soil nitrogen for crop uptake.

However, biological nitrogen capture is temporary until the decomposition of organic fertilizers will stop, while the nitrogen is still returned to the soil after the death of microorganisms for crop uptake and utilization. So this is completely different from the nitrogen loss caused by denitrification.

③ The conversion of phosphorus in the soil: the total amount of phosphoric acid (calculated as P2O5) in the general soil is between about 0.05-0.2%. Red and yellow soil is only about 0.06%, according to this calculation, these phosphorus is also enough for a number of years of crop harvest needs. However, the soil can be well absorbed by crops to use water-soluble phosphorus (such as Na, K, NH4 and other phosphate and calcium phosphate) and weak acid-soluble phosphorus (such as dicalcium phosphate) is very little; and most of the insoluble phosphorus (dicalcium phosphate) and very insoluble phosphorus (such as iron phosphate, aluminum phosphate) and organic phosphorus. They need to be transformed in various ways before they can be absorbed and used by crops.


The conversion of soil inorganic phosphorus is mainly influenced by soil reactions. In strongly acidic soils, phosphorus and iron, aluminum ions combined to produce insoluble iron phosphate, aluminum phosphate precipitation and fixed by the soil; in calcareous soils, phosphorus becomes tricalcium phosphate is fixed by the soil. Only when the soil reaction is in neutral or near-neutral (pH 6.5-7.5) conditions, the effectiveness of phosphorus increases.

Transformation of soil organic phosphorus. In the soil, the main organic phosphorus compounds are nucleoproteins, nucleic acids, lecithin, phytochemicals and other phosphorus-containing compounds in plants. They are hydrolyzed to release phosphoric acid by the action of soil microorganisms. This phosphoric acid, like hydrolytic phosphorus, then undergoes various transformations in the soil into effective phosphate for crop uptake and use.