The synopsis of the data on soil study and long-term trials with fertilizers in crop rotation with grain crops allowed studying the dynamics of humus condition in grey forestry soils of the Predkamie Area of the Republic of Tatarstan in the intensive agriculture. Statistic assessment of 99 soil profiles showed that arable (topsoil) horizon contains up to 4% of humus. The detailed research in the controlled conditions of a field trial showed that intensive use of soils in a field rotation for 20 years without fertilizing increased humus decay, its content in soil reduced on 0.28-0.30%. In the variants with systematic use of nitrogen mineral fertilizers the humus mineralization has slowed down, its content and reserves have stabilized, the content of its movable forms at the general humus decrease has increased. The humus loss was 7.28 t/ha (without fertilizing) and 1.6-2.5 t/ha (which is 3-4 times less) in the variants with fertilizing. The study of group and fraction compound of humus has established that the long-term use of soil (during 20 years) without fertilizing results in decrease of humus acids in respect to the initial content of them in soil, which indicates an increase in the mineralization of humus. At a long-term application of large doses of nitrogen mineral fertilizers in field rotation the content of movable fractions of GK and FK in humus increases. The increase of humus movable fractions in grey forestry soils at the fertilizing gives the productivity increase of crop rotation. In four rotations the productivity was 250 hwt/ha (without fertilizing) and 666 hwt/ha (with fertilizing).

Studies of the group and fractional composition of humus have determineded that the long-term use of soil (for 20 years) without the use of fertilizers (control) leads to a decrease in the content of humic acids and fulvic acids relative to the initial soil, which indicates an increase in mineralization of the soil humus. Under the influence of a long application of high doses of mineral fertilizers, the content of mobile fractions of humic and fulvic acids in the field rotation increases in the humus content. In systems of agriculture that are not balanced by organic matter, which are predominant in most farms of the Republic of Tatarstan, the use of very high doses of potassium fertilizers is not justified energetically. To compensate for losses of humus and its energy potential in calculating organic fertilizers on backgrounds with high doses of mineral fertilizers, the humification coefficients of organic residues should be increased by 30-40% during the rotational period of 5-6 years, which will reduce the loss of energy reserves and thereby improve the ecological stability of soils and the stability of agricultural landscapes