Abstract
The aim of the study was to assess the effect of manure, straw, and mineral fertilizers on the humus content and fractional composition of humus in light chestnut soil under a rainfed grain crop rotation in southeastern Kazakhstan. It was found that organic fertilizers had a more pronounced positive effect on the humus status of the soil than the mineral fertilizer system. The most distinct effect was observed after manure application under winter wheat, where humus content increased from 1.67–1.70% to 1.74%. Straw incorporation had a more moderate and not always statistically significant effect on humus content, increasing it from 1.68% to 1.70%. The application of mineral fertilizers did not cause significant changes in this indicator compared with the control. The effect of fertilizers on the fractional composition of humus was less pronounced than their effect on total humus content. The most noticeable changes were recorded under manure application, where the Cha/Cfa ratio decreased to 0.72 compared with 0.74 in the control. It was shown that under rainfed farming conditions, manure is the most effective factor for maintaining the humus status of the soil, while the changes identified at the initial stage of fertilizer application may be regarded as indicators of the direction of subsequent organic matter transformation under the long-term application of organic materials.