Determination of the causes of secondary salinization in Kibwezi

Abstract

Following increased rates of population growth, more and more marginal land has been opened up to meet the ever increasing food demand. Crop production in the arid and semi arid lands suffers from insufficient rainfall. Irrigation is practised where possible to suppliment rainfall in meeting crop water requirement. Irrigation has however been known to responsible for the degradation of somemarginal lands. A piece of land owned by the Tana and Athi Rivers Development Authority . (TARDA) has been abandoned due to secondary salinization. In a bid to find out the causes of this secondary salinization, four existing land uses were randomly selected to represent treatments on KibweziLixisols. The treatments were i) abandoned salinized field (Trt 1), ii) irrigated cropland (Trt 2), iii) virgin natural savanna (Trt 3) and iv) non-irrigated cropland (Trt 4). The first objective of the study was to find out if irrigation water or groundwater qualities resulted in secondary salinization while the second aimed at determining the influence of secondary salinization on physicaland chemical characteristics of the soil. Costat was used for the analyses of variance and Duncan'sMultiple Range Test for the separation of means. It has been observed in the past that some physical and/or chemical properties of the soil increase with depth on irrigated lands but not so on non-irrigated ones. Based on these observations analyses of results obtained from the first and the third horizons were performed. The second horizonshad intermediate values between the two. Theirrigation water had moderate to medium salinity having an electrical conductivity, EC, of 0.94 mS/cm. Its sodium level was low with an adjusted sodium adsorption ratio, SARadj, of 6.72. The soluble sodium percentage, SSP, was as high as 57. The pH was moderately alkaline with bicarbonates and chloride levels at 5.0 and 9.0 me/l respectively. The ratio ofMg:Ca was 4: 1. Thiswater caused sodicity rather than salinity conditions to the soil. The EC of the soil extract was found to be <4 mS/cm and the ESP <15 in the first horizons for all treatments. However the ESP was as high as 59 for Trt 1 in the third horizons although the EC remained <4 mS/cm. There was no significant difference at P=<0.05 in Ee for all treatments and horizons but the ESP was xv significantlydifferent in the third horizons. The cation exchange capacity, CEC, was <24 Cmol/kg andthe base saturation percentage >50 for all horizons and treatments. Soil organic carbon, sac, wasvery low « 1%) for all horizons and treatments. The texture was insignificantly different at P=<O.05for all horizons and treatments. It was mainly sandy with percentage sand >60. The basic infiltration rate was significantly different at P=<O.05 for all treatments in the third horizons.It was highest in Trt 3)md lowest in Trt 1. The saturated hydraulic conductivity, Ksat, wasinsignificantly different for all treatments in the first horizons and insignificantly different for all treatments in the third horizons except Trt 1 which exhibited the lowest Ksat at the same level of significance.Water content values were very low all being <30%. Soil water retention was higher in irrigatedfields than on non-irrigated ones for all treatments and horizons. Thecause of the secondary salinization in lixisols ofKibwezi area was attributed to the high Mg:Ca ratioof the irrigation water which led to greater adsorption of sodium on the soil exchange sites