The Earth as a planet of solar system exists for about 4.6 billion years. Further, the life on it emerged 800 to 1000 million years ago. The scientists have discovered traces of activity of the primitive humans, which date back 600 to 700 thousand years while the age of agriculture totals only 17 thousand years.
For millions of years water, winds, and live organisms destroyed and crushed rocks of the earths crust. Decaying organisms formed humus or as it is called by scientists, decomposed organic matter. It mixed up with the crushed rock, glued and cemented it. Thus soil appeared on the planet. The first soil formed a basis for development of subsequent larger plants which, in their turn, promoted new accelerated formation of humus. Even with this acceleration the process of soil formation began to proceed with emergence of animals, especially soil dwellers. Different species of bacteria promoted transformation of organic matters into humus. Formation and disintegration of organic substances in the ground is considered to be the main reason of soil formation.
Thus, soil consists of mineral and organic (humus) parts. The mineral part makes from 90 up to 99% and is the dominant component in soil. Its structure includes almost all elements of the periodic system of Dmitriy Mendeleyev. However the basic components of the mineral part of soil are oxygen in its many forms, silicon, aluminium and iron. These four elements make up about 93% of the mineral part content. Humus is the basic source of nutrients for plants. Due to vital activity of microorganisms living in soil there is mineralization of organic substance, which release in accessible to plants form of nitrogen, phosphorus, sulphur and other chemical elements necessary for plants. The organic substance provides big influence on formation of soil and change of its properties.
With decomposition of organic substances in soil carbon dioxide, which fills up the ground part of atmosphere, is assimilated by plants during photosynthesis. Nevertheless no matter how rich the soil is, sooner or later it starts depleting. Therefore for maintaining its fertility it is necessary to bring nutrients (fertilizers) of organic or mineral origin to the soil. Apart from the fact that fertilizers supply plants with nutrients, they improve physical, physicomechanical, chemical and biological properties of the soil. Organic fertilizers substantially improve water, air and thermal properties of soil. An important characteristic of the soil is its ability to absorb water vapours and gaseous substances from the environment. Due to this characteristic the soil detains moisture, and also ammonia formed as the result of decomposition of organic
substances and acting as an important nutrient.
Ground possesses ion-exchange properties similar to properties of pitch. Because of this ground detains cation and anion salts and gradually replaces them by others, acting from the outside. With abundance of moisture these anions are easily washed away from superficial layers of soil and are transferred to deeper layers. Underground waters take away up to 13% of nitrogen contained in fertilizers brought in fields. Therefore nitrate fertilizers are applied to the soil during crop or during development of plants by sprinkling.
The ground as ion-exchange for cations is «charged» mainly by ions of calcium Ca2
+, less by magnesium Mg2
+ and less by ions of ammonium NH2
+, sodium Na+ and Potassium K+. Ions of calcium Ca2
+ and magnesium Mg2
maintain strong structure of ground. By the term «structure of ground» the agriculture specialists define its ability to separate lumps. Ions K+ or NH4
+ and especially Na+, on the contrary, promote destruction of structural units of ground and increase the washing away of humus and mineral substances. In the damp condition such ground becomes sticky, and when dry, it turns into blocks, which do not yield to cultivation (solonetz).
Water flowing from such ground has the colour of tea, which indicates the loss of humus.
The chemical linkage by means of anions of some acids is very important. Nitrate NO3
- and chloride Cl1
- anions do not form hard-soluble compounds with cations, usually contained in the ground. On the contrary, anions of phosphoric, coal, sulphuric acids form hard-soluble compounds with ions of calcium. It is also the reason for chemical absorbing ability of soil.
Many various chemical elements are necessary for development and growth of plant. Plants get them mainly from soil. At first soil gets depleted of nitrogen, phosphorus and potassium. These chemical elements are consumed by plants in the greatest quantity and consequently it is necessary to add corresponding fertilizers to maintain fertility of fields.
During millennia agriculture knew only organic fertilizers various waste products and first of all manure. However even in the balanced farms where agriculture is combined with livestock industry, applying manure on the fields does not provide enough nitrogen and phosphorus taken away from soil with the crops.
Agricultural products are divided into commodity and non-commodity. For example, grain and vegetables are commodity products. They go to the consumer and chemical elements contained in them basically do not come back to the fields. Straw and not consumable parts of vegetables, as a rule, do return back to the ground. Straw returns to the ground as manure, and not consumable parts of vegetables and other waste products are buried by plough. The commodity products contain a lot of nitrogen and phosphorus, and non-commodity products contain a lot of potassium. Thus, as a result of circulation of substances in agriculture potassium can be returned to the ground, but the return of nitrogen and phosphorus is not provided even with applying manure.
Therefore whatever the biases against mineral fertilizers are they are necessarily to be added to the ground in scientifically proven quantities.
It is established, that each ton of corn removes about 55 kg of these nutrients from the ground, a ton of grain crops about 60 kg, and a ton of cotton almost 120 kg. Such figures allow to calculate the amount of fertilizers to be added to the ground. Certainly, various losses of fertilizers should be taken into account.
Nitrogen compounds (oxides and nitric acid) are created in small quantities in the atmosphere. Owing to electric locking nitrogen reacts with oxygen according to the equation
N2 + O2 = 2NO
Nitrogen oxide further is oxidized up to dioxide:
2N╬ + O2 = 2NO2
At the presence of oxygen and water the last turns into nitric acid:
4NO2 + O2 + 2H2╬ = 4HNO3
One hectare of soil receives from 2.5 to 4 kg of bound nitrogen. Accounting for soil dwelling bacteria and fungi (nitrogen binding) assimilating atmospheric nitrogen, 1 hectare of soil annually receives from 5 up to 15 kg of bound nitrogen. Considering that the crop of winter wheat is 25 tons, it consumes about 70 kg of the bound nitrogen. It becomes clear, that soil nitrogen is not sufficiently renewed naturally. However, with respect to this matter, the tubercle bacteria of leguminous plants and especially leguminous grasses supply from 100 to 200 kg of the bound nitrogen annually per 1 hectare. Grain leguminous though supply the ground a little bit less (up to 70 kg), but nevertheless it allows to forgo nitric fertilizers.
So when using lucerne (alfalfa) and clover we can achieve the optimal nitric balance of the soil.
If the soil can be filled with bound nitrogen in various ways, sources of natural supplying of soil with phosphorus do not exist. It is necessary to apply phosphorus fertilizers.
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