The Significance of Organic Fertilizers for Increasing Agricultural Productivity in Ukraine

For any farm, increasing the efficiency of soil fertility is of great importance. Soils contain a vast amount of nutrients—concentrated in humus, organic plant residues, and mineral compounds. The approximate reserves of humus, nitrogen, and phosphoric acid for different soils are as follows (Table 1).

Nitrogen in the soil is primarily concentrated in humus, averaging 5-6%; a small portion is present in the form of mineral compounds. A significant amount of phosphorus is also contained in organic compounds. For example, in podzols, organic phosphorus constitutes 20-25% of the total amount, while in chernozems, it is about 50%. Potassium in soils is mainly in mineral form. The total content of potassium, calculated as K₂O, reaches 60-80 tons per hectare.

If we consider that for a wheat grain yield of 40 centners per hectare, 80-120 kg of nitrogen, 40-50 kg of phosphorus, and 56-60 kg of potassium are used, then the reserves in the 20-centimeter layer of chernozems can ensure such a yield for 80-120 years, while the reserves of podzols and serazems can last for 30-50 years. A one-meter layer of podzols and serazems can provide nitrogen for high agricultural yields for 130-150 years, the same layer of chernozems for 500-700 years, and drained soils for up to 2500-2800 years. There are even more years' worth of phosphorus and potassium reserves in mineral soils.

A certain amount of nitrogen, phosphorus, potassium, and other nutrients in the soil is contained in plant residues (roots, post-harvest residues). The largest amount of these residues (up to 250 tons per hectare) is found in virgin soils. Although the amount of nutrients in plant residues is relatively small, it is valuable that these substances are quickly transformed into compounds that are readily available to plants.

Activating the transformation of humus and plant residues through proper soil cultivation and agricultural techniques is a crucial factor in increasing yields in the country. However, the main means of increasing the total harvest of agricultural products, especially in areas with podzolic and leached soil varieties, are organic fertilizers—manure, peat, and various composts. Their application often doubles and even triples the yield of agricultural crops.

Content and Composition of Organic Matter in Soil

It is known that the sources of organic matter in soils are plant residues and microorganisms, as well as manure and composts. These materials, which later form soil humus, are chemically composed of carbohydrates and related substances (monosaccharides, di- and trisaccharides, polysaccharides), lignin, nitrogenous compounds (proteins, amino acids, amides, alkaloids, chlorophyll, glucosides), fats and related substances (palmitic, stearic, oleic, linoleic, and linolenic acids, lecithins, phytosterols, wax), resins and terpenes, suberin, cutin, sporopollenins, and ash substances of plants.

Plant residues in the soil, undergoing various transformation processes, decompose into simpler compounds (H₂O, CO₂, NH₃, HNO₃, etc.) or transition into more inert forms of organic matter.

The beginning of a more in-depth study of the composition and nature of humus was laid by Sprengel in 1826, who achieved significant success in his research on humus. Further study of the composition and nature of soil humus is associated with the names of Berzelius, Mulder, Herman, Schreiner, Shori, Trusov, Sven-Oden, Shmuk, Williams, Waksman, Tyurin, Kononova, and many others.

Humic substances in the soil belong to a unique class of high-molecular-weight organic compounds with a very complex chemical composition that has not yet been precisely deciphered; they are formed by soil bacteria and fungi.

Depending on the biological type of synthesis and decomposition of organic matter in the soil, V.R. Williams distinguishes three groups of humic organic acids:

1. Humic (the group of black-colored humic acids) — formed by aerobic bacteria.
2. Ulminic (brown-colored humic acids) — formed by anaerobic bacteria.
3. Humic, or fulvic acids (the group of colorless humic acids) — synthesized by fungi characteristic of forest plantations, and under certain conditions, these acids can transition into apocrenic acids.

The process of soil humus formation includes the decomposition of plant residues and other forms of dead organic matter into simpler oxidized or reduced compounds; on the other hand, it involves the synthesis of very complex humic acids. The processes of humus formation — decomposition and synthesis — are the result of the enzymatic activity of soil microorganisms.

Specialists in this field assert that soil humus is a very complex and dynamic complex of numerous and highly diverse compounds in terms of their chemical nature. This complex includes four groups of substances: organic matter of undecomposed plant and animal residues, organic matter of living and undecomposed dead microorganisms, intermediate products of the breakdown of complex organic compounds, humic substances, and asphalts as products of unique physicochemical and extracellular enzymatic synthesis processes.

The chemical composition of humus is characterized by the following groups of compounds:

Humic Substances

This group includes the characteristic dark-colored soluble and insoluble substances in alkalis that are typical of soil humus, representing high-molecular-weight oxycarboxylic acids with pronounced colloidal properties. A common property of these substances is their high resistance to acid hydrolysis and insolubility in acetyl bromide, which sharply distinguishes these substances from most plant-derived substances, including lignin, with which humic substances share the property of resistance to acid hydrolysis.

The lowest content of humic substances (about 45-50% of the total humus) corresponds to acid podzolic soils, while the highest (70-90%) corresponds to chernozems and meadow-bog soils; humus-carbonate soils and forest brown soils occupy an intermediate position.

Humic substances are further divided into three groups:

1. Humic substances insoluble in alkalis.
2. Humic acids soluble in alkalis and insoluble in ethanol.
3. Hematomelanin acid soluble in ethanol.

In relation to the total humus content in Ukrainian soils, humic acids account for 10.1% (leached forest soils) to 40% (leached chernozems).

Humic acid in soils and peats contains carbon, oxygen, hydrogen, nitrogen, and ash. The nitrogen content is 3.5-4%. In the hydrolysis products of humic acid, Suzuki and Shmuk found amino acids (alanine, aminovaleric acid, proline, leucine, aspartic acid, glutamic acid, tyrosine, histidine), amides, and nitrogen of the non-hydrolyzable residue. However, only half of the total nitrogen in humic acid is capable of hydrolytic cleavage.

It is believed that, in terms of ash-free and nitrogen-free substances, humic acid in soils contains 58.8% carbon, 36.1% oxygen, 5.1% hydrogen, while hematomenelanin acid contains 59% carbon, 36% oxygen, and 5% hydrogen.

Non-Humic Substances

(lignin, cellulose, hemicellulose, proteins, low-molecular-weight breakdown products). In relation to the total humus content, lignin in mineral soils constitutes 5-10 percent.

Compared to hemicelluloses, cellulose is present in much smaller amounts in soils, which is explained by the property of plant residue cellulose to decompose. In the upper horizons of soils, cellulose content in humus ranges from 1 to 6-7 percent.

The total nitrogen content in humus averages about 5%. Only a small portion of organic nitrogen dissolves in water. Although a significant part of nitrogen (as amino acids and amides) enters the acid solution after hydrolysis, it is believed that part of the nitrogen is bound in compounds other than proteins of plant and animal origin.

Asphalts

(fats, resins, waxes, fatty acids, etc.). The content of these substances in soils in relation to the total humus content ranges from 5% (in aerated soils) to 15-20% (in soils where decomposition occurs under anaerobic conditions).

Shori and Martin, Waksman, Norman, Bartolomew concluded that 10 to 30% of the carbon in organic matter is present in soils in the form of polyuronides. Kodjima showed that about 25% of the nitrogen in soil organic matter is in compounds that do not dissolve upon hydrolysis, about 30% is in the form of amino acids, 10% of nitrogen is in the form of ammonia, and the remaining 20% is in other forms in solution.

The content of leucine, isoleucine, and valine in relation to the total nitrogen of amino acids is 33%, oxycarboxylic amino acids are about 20%, and dicarboxylic amino acids are about 25%. Aspartic acid, glutamic acid, and possibly oxoglutaric acids account for about 50% of dicarboxylic amino acids.

E. Russell noted that among organic compounds of phosphorus in soils, phytic acid, compounds of nucleic acid, and nucleotides are encountered. In Iowa soils, about 40-50% of organic phosphorus compounds were hydrolyzed to phosphates, of which about 66% was phytic acid. It is believed that sulfur in soils is present in the form of cysteine and cystine, which are part of proteins from plant residues.

Since Liebig's time, scientists have asserted that soil humus serves as a direct source of mineral nutrients. Among these substances, in addition to CO₂, NH₃ and nitrates, phosphates, sulfates, and other mineral compounds necessary for plant nutrition are of great significance.

In chernozems, the amount of released CO₂ ranges from 15 to 77 kg/ha per day, while during the decomposition of the turf layer of meadow soils, it is 20.0-240 kg/ha. Recently, it has been shown that soil carbon dioxide is assimilated by the root systems of plants.

Research by many scientists has shown that the decomposition of humus on fallow fields in podzolic soils leads to the accumulation of nitrates by the time of sowing in amounts of 0.5-1 ton, while in chernozems, it can reach up to 2.5 tons of saltpeter per hectare.

Various organic compounds, a significant portion of which can form in the soil as intermediate breakdown products, such as creatine, arginine, histidine, guanine, xanthine, hypoxanthine, and nucleic acid, can be assimilated by plants instead of nitrates and ammonia, sugars — as a source of carbon, lecithin, and cystine — as sources of phosphorus and sulfur.

The content of organic matter in the soil increases proportionally to the input of plant residues but depends on the nature of the input. The most important source of organic matter in well-cultivated soils is manure and composts.

The rate of decomposition of organic matter in the soil depends on the composition of plant residues or organic fertilizers, aeration, moisture, temperature, and the chemical and physical properties of the soil.

Different soils contain varying amounts of organic matter, as can be seen from the data in Table 2.

The fundamental task of agriculture is the rational use and regulation of humus content in the soil through tillage, crop rotation, application of manure and composts, green manuring, peat application, drainage and irrigation, and the use of artificial organic and organo-mineral fertilizers.

Tillage intensifies the decomposition of humus, which is particularly noticeable in the cultivation of row crops. Different plants increase the content of organic matter in the soil. Systematic application of manure or composts significantly raises the humus content in the soil.