Applicability of Soil Organic Carbon: Clay Ratio Thresholds for Structural Quality in Northwestern India

Abstract

Soil structural quality plays a fundamental role in determining overall soil quality, as it directly affects aggregate stability, pore space, water and air movement, root growth, and the soil’s resilience against degradation. The soil organic carbon (SOC):clay ratio provides a simple indicator of structural quality by expressing the amount of organic carbon relative to the clay fraction and has been proposed as a basis for evaluating the adequacy of SOC for maintaining favourable soil structure. The present study assessed SOC:clay ratio-based structural quality under contrasting Land Use/Land Cover (LULC) systems in Ballowal Saunkhri–Takarla Watershed of the Shivalik foothills of Punjab, India. Nearly 200 surface soil samples were collected from agricultural land, agroforestry, forest, and open scrub and analysed for SOC and clay content and their SOC:clay ratios were evaluated using the structural-quality thresholds. Mean SOC content followed the order of open scrub < agriculture < agroforestry < forest, with values of 2.86, 4.03, 4.41 and 4.68 g kg-1, respectively. Forest, agroforestry and agricultural soils showed very good structural-quality class, accounting for 80.85, 77.89 and 76.74% of the samples, respectively. In contrast, open scrub had 46.66% of samples in the very good class and the highest proportion (33.33%) in the bad class. The mean SOC:clay ratio was higher under agriculture, agroforestry and forest (0.02) than under open scrub (0.01), indicating differences in the relative status of organic carbon across LULC systems. Overall, SOC:clay ratio effectively differentiated structural-quality conditions among the LULC systems, with forest and agroforestry showing relatively favourable conditions and open scrub had greater structural-quality constraints. The findings demonstrate the potential of SOC:clay ratio-based assessment as a simple indicator for evaluating soil structural quality and identifying LULC systems requiring greater emphasis on organic carbon restoration and soil-management interventions.