The purpose of the research is to study the effect of long-term application of fertilizers on the dynamics of humus in the sod-podzolic medium-loam soil of the Republic of Mari El. It was found that in the control grain-grass crop rotation on the natural soil fertility for 24 years, the humus content in the soil gradually decreased. By the end of the second rotation, it decreased by 0.03%, in the third-by another 0.08 % and the fourth-by 0.02 %. The content of humus in the soil of the first fruit-bearing crop rotation decreased even more intensively than in the control. In 2001, the content of humus in the natural soil fertility decreased by 0.05% compared to the initial state. In 2007, at the end of the second rotation, humus in the first crop rotation decreased by 0.09…0.10 % from the initial state. In 2013 - by 0.16…0.17 %. The lowest humus content- 1.55 % for all the time of research was in the soil of the first fruit-bearing crop rotation at the end of the Quad rotation against the background of natural soil fertility. The difference compared to the initial state in this variant was -0.17 %. The highest humus content was in the soil of the second crop rotation, where rotted manure was applied under potatoes-80 t / ha. By the end of the first rotation, the humus content in the soil of the crop rotation increased by 0.03…0.04 %. By the end of the second-by 0.11% relative to the first rotation, the third-by 0.10…0.12 %, the fourth-by 0.20…0.21 %. Against a background without fertilizers, the humus content was always higher than when using N60P60K60. The most stable humus content was in the soil of the third crop rotation. In this rotation of six fields, two fields were under perennial legumes. During three rotations, the content of humus in the soil in this crop rotation was within the range of 1.72…1.74 %. Only after the fourth rotation, the humus content decreased to 1.70…1.73 %.