Lecture
Soil fertility is the capacity of soil to satisfy plants' requirements for nutrients, moisture and air, and to provide the conditions for their normal life activity. Soil is a source of humanity's material well-being, the greatest gift of nature. Therefore, the protection and reproduction of soil fertility is the primary foundation of highly productive farming and of obtaining high and stable yields. An important indicator of high soil fertility is the presence of a sufficient reserve of the biogenic elements plants need, which occur in a form available to agricultural crops as a result of the mobilization of elements that make up potential fertility, and as a result of fertilizer application. An important property of soil is its absorption (sorption) capacity, by which is meant the capacity of soil to absorb and retain solid, liquid and gaseous substances. Owing to the soil's absorption capacity, nutrient elements, while remaining available to plant roots, are retained against leaching. They accumulate in the soil over centuries, taking part in biochemical cycles and sustaining the life of new generations of plant organisms. High soil fertility ensures optimal plant nutrition, the formation of a high yield, and produce of full quality for human food and animal feed. Such soils are able to accumulate moisture in the necessary quantities and forms and to retain it against percolation through the profile, surface runoff and evaporation into the atmosphere, thereby creating optimal water and air regimes. A good humus content and structural condition of the soil ensure its high moisture capacity. An important property of fertile soils is their biological activity, which characterizes the intensity of the biological processes taking place in the soil. Beneficial soil microorganisms not only take part in the biological cycling of nutrient elements but also secrete enzymes, antibiotics, growth stimulants and other organic substances that have a beneficial effect on cultivated plants.
Well-cultivated, fertile soils have favorable physicochemical properties for the normal growth and development of plants, since they simultaneously provide the root system with sufficient food, air, moisture and heat. The creation of optimal conditions for plant growth and development largely depends * on changes in the physical, chemical and biological properties of the soil; * on the presence in it of a sufficient amount of nutrients assimilable by plants; * on the intensity of the processes by which nutrient elements pass from a form poorly available to plants into a readily available one and vice versa, i.e., the processes of their mobilization and immobilization. All this determines the need of cultivated plants for fertilizers, as well as for the application of a set of agrotechnical and reclamation measures. In other words, there is a constant interrelationship among plants, soil and fertilizers. A fertilizer applied to the soil, as a result of its interaction with the soil and the action of soil microorganisms, undergoes various transformations that affect its ability to move through the soil, the solubility of the nutrient elements it contains, and their availability to plants. These transformations depend on the properties of the soil and of the fertilizer. For example, on sandy soils the rate of decomposition of applied organic fertilizers, other factors being equal, is higher than on loamy and clay soils. The rate and degree of decomposition of organic fertilizers also depend on how rich the soil is in microorganisms, on their composition and biological activity, and on the conditions that determine the life activity of the microorganisms (soil structure, its aggregation and aeration, the hydrothermal regime and physicochemical properties, the presence of nutrients in it, etc.). The intensity of mineralization of organic fertilizers is largely determined by their biogenicity. For example, manure is a biologically active substance; it is rich in microorganisms, with each ton containing up to 13 kg of living microbes. Peat, on the contrary, is poor in microorganisms and therefore decomposes slowly in the soil. To speed up the decomposition process, biologically active substances (manure, liquid manure, feces and others) are added to peat, that is, organic composts are prepared.
In the soil, mineral fertilizers (like the mineral products of the decomposition of organic fertilizers) undergo profound transformations. For example, under the influence of the acid reaction of the soil solution or the acidic root exudates of a crop such as lupin, rock phosphate flour passes into a soluble form available for plant nutrition. Mineral fertilizers can enter into exchange reactions with the solid colloidal particles of the soil and in this way be retained in it; they can be absorbed by microorganisms and temporarily fixed in living plasma, and so on. The rate of the transformation processes of fertilizers applied to the soil depends on * the nature of the fertilizer, * the properties of the soil, * climatic conditions, * and also on the set of agrotechnical measures employed. The interaction of fertilizers and soil can have a positive or a negative effect on plant nutrition, yield formation and product quality. The positive effect of the systematic application of organic and mineral fertilizers consists in changes to the physicochemical properties of the soil. For example, long-term application of manure leads to an increase in the soil's organic matter content and to an increase in the soil's absorption capacity; at the same time, exchangeable and hydrolytic acidity decrease and the degree of base saturation of the soil increases. An example of the deterioration of soil properties under long-term application of mineral fertilizers is a shift in the reaction of the soil solution toward acidification (especially when the soil already has an acid soil-solution reaction), which occurs as a result of the displacement of hydrogen and aluminum ions from the absorbing complex, and also in connection with the physiological acidity of nitrogen and potassium fertilizers. Such negative consequences are, rather, a sign of the improper use of agrochemical agents by humans, since a scientifically grounded fertilizer system makes it possible to avoid the negative consequences of their application. With the correct application of mineral fertilizers in combination with organic ones (manure in particular), together with liming, and with the addition of supplements to neutralize the physiological acidity of the fertilizers, soil acidity not only does not increase but may even decrease.
In addition, the fertilizer also acts on the soil (the reaction of the soil solution, the intensity and direction of microbiological processes, etc.), that is, besides supplying the plant with nutrient elements, fertilizers act on the general conditions of soil fertility. It is therefore very important to know the composition of the soil, its properties and fertility, and the nature and direction of the physicochemical, chemical and biological processes occurring in it. This will make it possible to correctly determine the particular features of fertilizer transformation in the soil and its effect on plant growth, taking into account biological requirements and the specific conditions of cultivation. The greatest effect from fertilizers is obtained under conditions in which plants are provided in the best possible way with all the conditions of life they need — food, water, air, heat, light — when the soil is free of weeds, and when the plants are not affected by pests, diseases, etc. For a fuller understanding of the influence of agrochemistry on soil fertility and properties, the following questions must be considered: * the composition and properties of the mineral and organic parts of the soil; * the absorption capacity and properties of the soil; * the change and optimization of soil fertility and properties under long-term fertilizer application; * the biological cycling and balance of biogenic elements and humus in the agrocenosis. These topics will be considered in sequence in the following subchapters of the textbook.
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