In general, the biological cycle of the circuit can be represented as a dynamic system which consists of four parts: the circuit capacity, removing the nourishing elements with the basic yield and the by-products, returning nutrients to the soil with plant and root remnants, stock of mobile forms of nutrients in the plow soil layer.

Estimation of biological circuit units parameters are based on the results of agrochemical analysis of nitrogen, phosphorus and potassium in crops individual structural components and crop rotation biomass.

The crop rotation study indicates that total capacity under red clover growing without fertilizers was 89.8 kg / ha. Applying fertilizer increases the circuit capacity by 38-154% with the maximum rate of 228 kg/ha under triple norm of fertilizers treatment. Accumulation of nitrogen in the productive part of the harvest exceeds its returning to the soil with plant remains in the control and in the variants with minimal fertilizing. Nitrogen return factor is 0,46-0,47 . On the background of binary and ternary fertilizer rate the nitrogen accumulated in plant debris exceeds the production commodity output and nitrogen return ratio increases to 0,51-0,52 .

Growing winter wheat without the fertilizers use causes reduced capacity of nitrogen circuit by 1,3 times in comparison with clover and nitrogen return to the soil is reduced by 8.3 times. Applying mineral fertilizers increases the capacity of the nitrogen circuit up to 105-181 kg/ha , but its rate of return is very low (0,08-0,09) regardless of fertilization rate.

Nitrogen circulation capacity in sugar beet crops without fertilizers is 77.3 kg / ha. 53.6 kg / ha ( 69%) of soil nitrogen reserves is removed from the soil with harvest roots, and its return to the soil with plant remains makes 23.7 kg/ha ( 31%). Sugar beet organic mineral fertilizing system enhances both the removal of nitrogen from crop roots and its returning o the soil with plant remains, but the rate of return is low and makes 0,31-0,32 .

Nitrogen circulation capacity in peas crops is the highest among the rotation crops, making 94 kg / ha under zero fertilizing rate. Applying fertilizer increases the absorption capacity by 35-93 kg/ha, but the nitrogen alienation with the production yield crop increases to 83-87 %. Nitrogen return factor is 0,14-0,16 .

Barley crops are characterized with minimum nitrogen circulation capacity in both the control and mineral fertilizers treatment ( 51,9-116 kg / ha ), but with the maximum amount of nitrogen alienation from production yield ( 88-94 %). Nitrogen return with the crop remains factor is ,08-0,1.

The amount of phosphorus inclusion into the biological circuit is much lower than that of nitrogen due to the low concentration of the element in crops. Phosphorus content in the crop rotation biomass without fertilizer ranged from 20.2 kg / ha in crops of red clover and 26 kg / ha of sugar beet. Application of fertilizers enhances the capacity of circulation of phosphorus by 2,4-3,3 times. Cereals and legumes remove 88 %-95 % of the basic yield and by-product from the soil, red clover - 59-63 %, sugar beet - 36-58 % of the phosphorus involved in the biological cycle of the circuit .

Applying fertilizers increases the accumulation of phosphorus in crops and root debris by 4-22 %. Phosphorus return factor for grains is 0,06-0,12, for peas - 0,12-0,16 ; for sugar beet - 0,36-0,41 , for red clover - 0,37-0,40.

The volume of potassium circuit circulation of occupies a middle position between the nitrogen and phosphorus one. Growing crops without fertilizers provides the circuit capacity of 19.4 - 38.2 kg / ha, peas - 41.1 kg / ha, red clover - 68.3 kg /ha, sugar beet - 96.5 kg / ha.

Under systemic fertilizers applying 2,3-3,2 times more potassium is involved in the circuit. Most potassium is accumulates in the biomass under triple fertilizer level rate: red clover - 217 kg / ha, sugar beet - 271 kg / ha winter wheat - 96.5 kg / ha, peas - 88.6 kg / ha , barley - 66 8 kg / ha. The largest amount of potassium is extracted from the soil with the basic crop yield and the by-products in under winter wheat and barley 84 and 90 % respectively, peas - 71%, sugar beet - 63 % red clover - 55% in non-fertilized variants. Under fertilizing the amount of detached potassium is reduced by 3-5%. Potassium return with crops remains ratio for cereals is 0,16-0,20; peas - 0,25-0,29 ; beet sugar - 0,32-0,37, red clover - 0,44-0,49 .

Features of crop rotation plants nutrition, of nutrients biogenic migration in the soil-crop system necessitate generalizing the nutrients circulation parameters for the entire rotation.

The natural rate of soil fertility without using fertilizers provides nitrogen capacity circuit of 76.2 kg / ha phosphorus - 22.7 kg / ha and potassium 52.8 kg/ha. A minimum level of fertilizers use of 4 t / ha manure + N16 P25 K25 increases the capacity of nitrogen circulation to 113 kg / ha, that one of phosphorus – to 35.2 kg / ha, of potassium – to 82 kg / ha. Maximum accumulation of nutrients in biomass is provided under triple level rate of fertilizers - 12 t / ha manure + N48 P75 K75: nitrogen - 188 kg / ha, phosphorus - 63.8 kg / ha, and potassium - 148 kg / ha. Nitrogen production removal from a hectare of crop rotation area is 74-76 %, phosphorus - 76-78 %, potassium - 65-67 %. Crops remains return rate for nitrogen is 0,24-0,26; phosphorus - 0,22-0,24; potassium - 0,33-0,35. It allows to predict the possibility of nitrogen and potassium soil store mobilizing both under the minimum and maximum of fertilizers use level in crop rotation.

The defined parameters of the nutrients biological circuit are the basis for the calculation of static and dynamic balance of nutrients in the crop rotation and of fertilizing optimization systems.

Key words: crop rotation, fertilizer, nutrition elements, biological circuit.