Effect of supplemental irrigation on soil chemical properties and growth of Acacia Saligna in Northwestern Kenya

Abstract

The aim of the experiment was to evaluate the effects of supplemental irrigation using brackish (3.15 dsm') and fresh waters (0.30 dsm") on soil chemical properties and growth of Acacia saligna grown on runoff collection fields. The experiment involved supplementing natural precipitation and harvested runoff with irrigation using the two water quality levels during dry spells. The irrigation treatments were applied in a randomised complete block design with three replicates to one year old Acacia saligna trees which, had been raised under harvested runoff. Tree growth and biomass accumulation were determined using stem girth method. The double ring infiltrometer with a falling head was used to measure the rates of water infiltration into the soil. Tree phyllodes were wet digested and cation contents determined using an atomic absorption spectrophotometer while chloride was determined using a chloridometer. Irrigation with brackish water increased the levels of soil Mg2+ and ESP but depressed the average and final rates of water infiltration into the soil. Supplemental irrigation with fresh water improved the soil permeability to water. Supplementary irrigation with the two water types did not affect both the ionic content of phyllodes and the growth and biomass accumulation of Acacia saligna trees. Acacia saligna tolerated salinity of irrigation water of 3.15 dsm-1and excluded chloride ions from its phyllodes. Natural precipitation and harvested runoff water sufficed to grow Acacia saligna without supplemental irrigation during the dry spells under conditions at our experimental site. The brackish water of 3.15 dsm-1 can safely be used for supplemental irrigation of Acacia saligna during dry spells at the experimental site. Long-term studies need to be conducted to evaluate the sustainability of these results given the highly variable rainfall pattern of the region, which often leads to several dry months.