Relationship Between Soil Hydro-Physical Properties and the Persistence of Soil Water Repellency in Agricultural Soils of Kenya: The Case of Murang’a and Makueni Counties

Abstract

Soil Water Repellency (SWR) is the reduction in the rate of wetting and retention of water in the soil caused by presence of hydrophobic organic matter in the soil. Knowledge of the threshold values at which organic carbon and soil moisture content becomes effective and asserts a negative impact on soil properties especially soil wettability is limited in Kenya. Therefore, this study aimed at characterizing the persistence of SWR using Water Drop Penetration Time Test (WDPT), evaluating the SWR curve as a function of gravimetric water content from the WDPT results and finally developing relationships between SWR parameters. Eighty-four soil samples at 0-15cm and 15-30cm soil depths were collected from agricultural lands in Makueni and Murang’a Counties in Kenya and taken to the laboratory for analysis. The actual SWR was estimated for the samples in their field conditions using the Water Drop Penetration Time Test (WDPT) method. A total of 29% of the soils investigated were water repellent. The results revealed that the WDPT was non-linearly correlated (r = -0.712 and r = -0.238, p<0.01) to soil moisture content (smc) in Murang’a and Makueni soils respectively. The critical soil moisture content (Wc) above which soils with sandy clay loam texture in Murang’a became wettable was found to be between 8 and 16.6% while in soils with loamy sand texture it was 6.2 and 9.0%. In Makueni, for sandy clay loamy soils, the critical soil moisture content was found to be between 3.05% and 7.05 %. The results revealed an extremely strong negative relationship (r =-0.987 and -0.982) between saturated hydraulic conductivity and porosity. The total degree of soil water repellency (SWRAREA) ranged from 8.38second/% smc to 24.91 seconds /% smc. Total Organic Carbon (TOC) was the most important soil property in explaining SWRAREA and Wc. Inclusion of clay and silt in the Multiple Linear Regression (MLR) expression of SWRAREA significantly improved the prediction of SWRAREA from 82% to 85%. Further, an upper limit critical water content of 14.6g of water/kg of soil and 7g of water /kg of soil in Murang’a and Makueni County soils respectively was derived from the simple relationship between the Wc and TOC. Murang’a soils were wettable between 2 and 4% TOC and became repellent above 4.0% TOC while Makueni soils were wettable below 2% TOC and became repellent above 3.4% TOC. These thresholds could be used to derive a critical soil moisture content above which SWR and related limitations to soil functions could be eliminated during irrigation.