Economic and biological removal

Biological removal — removal of nutrients from the soil by all parts of the plant: main and by-products, crop residues, roots, fallen leaves remaining in the field.

Economic removal — removal of nutrients with the harvest removed from the field as main and by-products.

The nutrient requirement of plants is determined from economic removal per unit mass of main product, taking into account the amount of by-product. Economic removal is used to calculate application rates of fertilizers for the planned crop yield.

Maximum consumption — the greatest amount of nutrients required to produce a unit of yield. It is always greater than biological removal.

The removal of nutrients with marketable products is influenced by growing conditions.

Differences in the nutrient requirements of crop species and varieties under the same soil and climatic conditions are explained by the different material composition of plants. All environmental factors have a significant influence on the chemical composition of plants and, consequently, on the nutrient requirements of any crop.

Differences in crop nutrient requirements are clearly represented by the values of biological, and more often economic, removal of elements with harvests or in the form of their expenditure per unit of main product with the corresponding amount of by-product.

For determining optimal fertilizer doses and ratios for crops in crop rotations, economic removal and nutrient expenditure per unit of main product with the corresponding amount of by-product are of practical importance.

Root and crop residues are part of biological removal. But remaining in the field, after mineralization they are also an additional source of nutrients for the crops following them after harvest. Taking this into account, in practice economic removal is determined, and from it the nutrient expenditure per unit of production.

The intensification of agriculture, accompanied by an increase in the productivity of cultivated crops, leads to an increase in the economic removal of nutrient elements with harvests.

Long-term average data on nutrient expenditure per unit of production for each variety under specific soil and climatic conditions, as a genotypic trait, are relatively constant and serve as the basis for calculating economic removals, nutrient balances, and optimal fertilizer doses at any productivity level of that variety.

Each crop in its development passes through a nutrient consumption cycle characteristic only of it, therefore fertilizers can be used to regulate processes at different stages of plant growth and development.

The first stage — seed germination and emergence of seedlings — is characterized for all crops by a relatively low requirement for nutrient elements. However, it is precisely during this period that crops are most sensitive to deficiency, excess, and increased concentration of salts in the soil solution. At this stage, crops do not have a developed root system or significant root exudates, therefore they need small amounts of elements, approximately 5–20 kg/ha a.i., in an available water-soluble form.

The second stage — the period of intensive growth and development of vegetative mass. For most crops, it is characterized by intensive uptake of nitrogen, and to a lesser extent phosphorus and potassium. For potassium-demanding crops, potassium exceeds phosphorus as needed. At this and subsequent stages of development, fertilizers may be in the form of salts, but they must be located in the zone of the actively absorbing root system.

The shortest period of nutrient consumption, ending by the end of the second growth stage, is observed in hemp, flax, rice, and spring grain crops. For this reason, for these crops fertilizers must be applied before sowing or at sowing. Top dressing is possible only for part of the nitrogen fertilizers in order to reduce nitrogen losses in zones of sufficient and excessive moisture or to increase the protein content of grain. The vast majority of other crops have an extended period of nutrient consumption, with a maximum occurring in the second stage. In some crops, such as cotton and tomato, nutrient consumption is extended until the end of the growing season, with maximum consumption in the third stage of growth and development. For such crops, in addition to pre-sowing (phosphorus-potassium and part of nitrogen) and at-sowing (phosphorus and/or phosphorus-nitrogen) fertilizer, top dressings with nitrogen, micronutrient, and nitrogen-potassium fertilizers are carried out.

The third stage — fruiting or formation of reproductive organs. For most crops, it is characterized by a decrease in nutrient consumption with a simultaneous change in minima: the requirement for phosphorus and potassium increases and that for nitrogen decreases. At this stage, an intensive redistribution of previously absorbed elements occurs in plants: outflow from leaves to seeds, fruits, and root and tuber crops. The consumed substances must be in the zone of active absorption by the root system, that is, they must have been applied earlier as pre-sowing or post-sowing fertilizer.

The ratios of nutrient elements in crops differ. Thus, the N:P₂O₅:K₂O ratio in economic yields averages 3:1:2.5 for grain crops, 3:1:4 for root and tuber crops, and 4:1:2 for grain legumes. Even in one crop and variety, the ratio of elements can change over time, since as the plant grows and develops, the masses and composition of the organs formed change. Thus, with an increase in the share of straw in the biological yield of grain crops, more nutrients are expended to create 1 t of grain. Grain, as a rule, contains approximately 4 times more nitrogen and phosphorus than straw, whereas potassium and calcium in straw are 2–3 times greater than in grain.

The dynamics of nutrient consumption also depend on the early maturity of varieties. Early varieties, which have a short growing season, consume nutrients more intensively and, consequently, make higher demands on nutritional conditions. In contrast, mid- and late-ripening varieties consume nutrients over a longer period, usually in larger amounts, so fertilizers are applied to them in several applications.

To determine optimal rates and methods of fertilizer application for crops, reference data and recommendations of zonal research institutions are used, as well as agronomic characteristics of released varieties. In the absence of data, they are determined experimentally using field trials. For this, economic removal and nutrient expenditure per unit of main product with the corresponding amount of by-product are determined. The results obtained are thereby maximally close to local soil and climatic conditions.