Replenishment of Humus due to Crop-Root Residues in Flax and Crop Rotations

In the Central region of the Non-Chernozem zone of Russia on sod-podzolic light loamy soil in a long stationary experiment in flax crop rotations, the process of replenishing humus due to crop-root residues was studied. Alternation of crops in the crop rotation in 2004–2011: pure steam – winter rye with sowing of perennial grasses (red clover and timofeevka) – perennial grasses of the 1^st year of use – perennial grasses of the 2^nd year of use – flax – potato – barley – oats. In 2012–2018, potatoes were excluded from the crop rotation. The contribution of cultivated crops to the accumulation of soil organic matter varied. The largest amount of crop-root residues (6.8 t/ha) remained in the soil after harvesting perennial grasses, which provided a positive humus balance of +1.35 t/ha. In the crop rotation with potatoes without the use of fertilizers, the humus deficit was –0.20 t/ha, with the exclusion of potatoes from the crop rotation, the deficit decreased to –0.05 t/ha per year. The use of fertilizers increased the productivity of crop rotations by an average of 75%, the amount of crop-root residues increased by 10.6 t/ha or 58%, which allowed to obtain a positive balance of humus in the soil. The use of fertilizers at a dose of (NPK)200 (manure 5 t/ha + (NPK)135 and manure 5.7 t/ha + (NPK)120) in an 8-pole crop rotation with pure steam and potatoes when saturated by 25% with perennial grasses provided a humus balance of +0.17 t/ha per year, in 7-full crop rotation with pure steam and without potatoes at saturation by 28.5% with perennial grasses – +0.43 t/ha per year.