Humus quantity and quality control macro- and micronutrient availability in arid and semi-arid calcic soils

Abstract Calcic Chernozem and Light Calcic Calcisol are important agricultural soils of Kazakhstan’s steppe and arid zones. This study evaluated the relationships between humus quantity, humus fractional composition, and macro- and micronutrient availability in four contrasting agricultural soil profiles. Total humus content, humic- and fulvic-acid fractions, exchangeable cations, available macronutrients, DTPA-extractable Cu, Mn, Fe, Mo, and Co, and hot-water-extractable B were determined. The Calcic Chernozem profiles had higher total humus contents, reaching 3.81% in the surface horizon, and a greater relative contribution of humic-acid fractions, with Cₕₐ/C𝒻ₐ ratios of 1.2–2.0. In the Light Calcic Calcisol, surface-horizon humus contents ranged from 1.05 to 1.45%, while fulvic-acid fractions predominated, with ΣFA reaching 73.2% and CHA/CFA ratios of 0.1–0.2. Macro- and micronutrient concentrations varied markedly among profiles and horizons. Particularly low concentrations of hot-water-extractable B and DTPA-extractable Fe were observed in the Light Calcic Calcisol. Exploratory correlation analysis and PCA identified associations among humus fractions, soil chemical properties, and nutrient distribution. The first two principal components explained 82.5% of the total variance in the Calcic Chernozem and 84.5% in the Light Calcic Calcisol. Because each soil–management combination was represented by one field profile and the sites differed in climate, crop rotation, irrigation, fertilizer management, land-use history, and sampling date, the observed patterns should be interpreted cautiously. The findings support soil-specific fertility management focused on maintaining soil organic matter and assessing potential B and Fe deficiencies in low-humus calcareous soils.