The distribution of organic carbon content in the soils of Koppal taluk was low in 21,765 ha (18%), medium in 44,730 ha (37%) and high in an area of about 31,302 ha (26 %) (Figure 3). The presence of organic carbon status in the soils of Yadgir taluk revealed that, low, medium and high in an area of about 22,621 ha (13%), 64,372 ha (38%) and 53,910 ha (31%), respectively (Figure 4).The reason for low organic carbon content in these soils may be attributed to the prevalence of arid condition, where the decomposition of organic matter occur at a faster rate coupled with little or no addition of organic manures, low vegetative cover on the fields and varied cropping pattern, thereby leaving less chances of accumulation of organic carbon in the soils (Dowuona et al. 1998; Nye and Stephans 1962). Baseline survey of households in Sujala-III Project area revealed that, the quantity of FYM applied by the farmers is less than one tonne per hectare at an interval of 3-4 years. Intensive cropping is also one of the reasons for low organic carbon content in soils. The results are in confirmation with those reported by Patil et al. (2016, 2017a, 2017b, 2018a, 2018b, 2018c).
The soil organic carbon (OC) content of Koppal and Yadgir taluk ranged from 0.01 to 2.91 and 0.01 to 4.08 per cent with a mean of 0.68 and 0.73 per cent respectively and S.D. of 0.36 and 0.42 per cent. The variance of 0.13 and 0.18 and C.V. of 52.64 and 57.21 per cent respectively (Table 1). The values were positively skewed and had positive kurtosis value. The soil organic carbon (OC) content was medium to high in majority of soil samples due to regular addition of organics in the form of FYM and compost. Similar results were obtained by Rajendra Hegde et al. (2021). According to Rao et al. (2008) higher clay content in soil was responsible for maximum organic carbon content in soils.
Variogram and model parameters (Geo-statistics)
The lag size and range of soil organic carbon of Koppal and Yadgir taluk was 627.71 and 90.28 m and 5043.69 and 608.96 m respectively with variogram expressed in exponential model fits the experimental semivariogram for soil nutrients with low RMSE values (Figure 5 and Figure 6) similar results were obtained by Rajendra Hegde et al. (2019). The spatially dependence was moderate in both the taluks with nugget of 0.09 and 0.08, partial sill of 0.03 and 0.07, and sill of 0.12 and 0.15 respectively. The Nugget to Sill ratio (N:S ratio) of 0.75 and 0.54 respectively (Table 2). It had low nugget effect which suggests that random variance of variable is low in the study area. This means that near and away samples have similar and different values, respectively. In other words, a small nugget indicated a spatial discontinuity between neighboring points and showed weak spatial dependence at the same grid points.
The variogram of soil organic carbon was described by spherical model. The soil organic carbon showed moderate spatial variability which might be attributed to extrinsic (fertilization and cultivation practices) and intrinsic factor (soil forming processes). This observation is in conformity with Reza et al. (2012).
The organic carbon status in soils is to be enhanced with following integrated approaches such as, application of recommended dose of FYM for each crop grown, green manuring using pre monsoon rainfall, growing of sunhemp as an intercrop and incorporation to soil at flowering stage in wider row spaced crops like maize, cotton, sunflower, fruit and plantation crops, growing of legume as an intercrop to increase organic carbon content of soil due to shedding of leaves at maturity, crop rotation with legume to increase organic carbon content of soil due to shedding of leaves at maturity and crop residue management using microbial consortium. The suggested above practices will help in arresting land degradation, improving physical, chemical and biological properties and fertility status of soils to achieve sustainable production.