Spatial variability of aggregate stability, size distribution, erosion and runoff in selected soils in South Africa

Abstract

Soil erosion and runoff are a major threat to soil productivity since it is associated with the removal of the top layer, depletion of essential plant nutrients, and reduces infiltration of water into the soil. Soil aggregate stability and size distribution are important physical factors in the assessment of soil erosion and runoff. A study was carried out in an approximately 1 ha field in different soils to ascertain the spatial variability of soil erosion, runoff, aggregate stability and size distribution. The spatial variability approach provides insight into the search for soil management strategies to reduce soil erosion and runoff. Twelve soil samples were collected at 0 – 150 mm depth for measurements of soil aggregate stability, size distribution, erosion and runoff at the University of Venda agricultural farm and at the Agricultural Research Council – Vegetable and Ornamental Plants (ARC-VOP) farm. The University of Venda agricultural farm falls under low veld climate and had deep well-drained red soil with high clay content and the soil is classified as Hutton form which is equivalent to Rhodic Ferralsol. ARC-VOP farm falls under humid subtropical climate and is characterised by sandy clay loam soil classified as Clovelly soil form, which is equivalent to Luvisols/Cambisols. Soil aggregate stability and size distribution were measured following the wet sieving method. Macro-aggregate (>0.25 mm) and micro-aggregates (<0.25 mm) were considered in this study. Soil erosion and runoff were measured using a rotating disc rainfall simulator at a rainfall intensity of 45 mm/h. Semi-variogram analysis was used to determine the spatial variability of soil aggregate stability, size distribution, erosion and runoff. A spatial distribution map was produced using ordinary kriging method in ArcMap of ArcGIS 10.5 software. The results showed very weak spatial variability of soil erosion and runoff at both sites. This could have resulted from weak variability of soil infiltration rate, soil crust strength, porosity. Moreover, the weak variation of soil loss could also have resulted from the weak variation of runoff across the measured site in this study. A very weak spatial variability was recorded with 100% spatial ratio for soil aggregate stability at University of Venda agricultural farm while moderate variability with 42.98% spatial ratio was observed at Agricultural Research Council farm. Similarly, microaggregates had very weak variability with 100% spatial dependence at University of Venda agricultural farm whereas Agricultural Research Council farm was characterised with moderate variability with 66.67% spatial dependence. In this study, strong variability was observed on macroaggregates at Agricultural Research Council farm with a spatial dependence of 17.39% whereas weak variability was observed at University of Venda agricultural farm. The effects of the extrinsic factors mainly tillage could be one on the main reason the landscape was characterized with a very weak and moderate spatial variability in this study. However, soil intrinsic factors could have played a role on macroaggregates at Agricultural Research Council farm. Therefore, the spatial analysis showed great importance to be applied in the assessment of soil erosion, runoff, aggregate stability and size distribution