Status of humus in soil under various long-term tillage systems

The content and fractional composition of humus in soil exposed to conventional, shallow and minimum tillage for 40 years was investigated on Cambisols. Humus status depended on tillage methods and depth. In cultivated soil, conventional annual tillage can be replaced by shallow or minimum tillage, since this significantly increased soil humus content in the 0–10 and 10–20 cm soil layers. In both experiments with lower (Experiment 1) and higher (Experiment 2) application rates of mineral fertilizer, in the whole 0–30 cm layer in the minimum soil tillage systems (MT1 and MT2), the content of humus significantly increased, and in the shallow tillage system (ShT), it did not differ significantly, compared with conventional tillage (CT). In both experiments, the content of humus in the 0–10 and 10–20 cm layers of the shallow tillage treatment was significantly higher than that in the 20–30 cm layer. Having replaced the conventional soil tillage by shallow or both minimum tillage systems, humus quality (characterized by the parameters of humus fractional composition) remained similar. The data of humus fractional composition showed that abandonment of conventional tillage results in a significant increase in stable soil humus forms. Application of shallow and minimum soil tillage systems significantly increased the content of humic acids strongly bound with the soil clay minerals in the 0–30 cm layer in both experiments, except ShT in Experiment 2. In Experiment 1, with lower application rates of mineral fertilizer, minimum soil tillage systems (MT1 and MT2) significantly increased the E4:E6 ratio of humic acids, which indicated suitable conditions for humification and the existence of high molecular humic acids in the soil.