Soil Organic Carbon to Clay Ratio as an Indicator of Structural Soil Quality: A Case Study of South Bihar, India

Abstract

Soil is a valuable and non-renewable resource for agricultural production, and it is essential to
establish indicators to monitor its condition. Soil organic carbon (SOC) is a key indicator of soil quality and
health since it regulates important physical, chemical, and biological soil properties. Hence, a simple measure
of appropriate levels of soil organic matter is needed for soil evaluation, management and monitoring,
based on readily measurable soil properties. In this study, an index of soil organic matter based on the SOC
to clay ratio, defined by thresholds of SOC/clay ratio for specified levels of soil structural quality, was tested.
We assess the index using data from the land resource inventory of WDC2.0/III/Gaya watershed under agricultural
land, forest land and open scrub. The results showed that SOC was ranged between 2.40 to 10.40 g
kg-1 (surface) and 0.70 to 6.10 g kg-1 (sub-surface) for agriculture; 1.80 to 12.90 g kg-1 (surface) and 1.0 to
5.10 g kg-1 (sub-surface) for forest and for the open scrub, it ranged from 1.30 to 10.20 g kg-1 (surface) and
0.70 to 5.90 g kg-1 (surface). Mean clay content increased in the order of agriculture > forest > open scrub
in the surface layer and increased in sub-surface layer across the LULC. Thresholds of SOC/clay ratio of 1/
8, 1/10 and 1/13 indicated the boundaries between “very good”, “good”, “moderate” and “degraded”
levels of structural condition. On this scale, 92.0, 77.8 and 71.4% of agricultural land, open scrub and forest
sites, respectively, were degraded. The index gives a method to assess and monitor soil organic matter at
national, regional or sub-regional scales based on two routinely measured soil properties.