Determination of nitrate retention capacity of Kalahari sands using a permeameter: Case study of Mashare farm soil in the Okavango River basin

Abstract

Namibian irrigators in Okavango River basin use high application rates of N fertilizers on Kalahari sandy soils which may leach nitrates and pollute water resources. The capacity of the sandy soils to retain the nitrates against leaching conditions of prolonged water saturation is not well understood. Therefore, in this study the sandy soil was tested for nitrate leaching rate and retention capacity using a constant head permeability meter in laboratory experiments. Eight samples were collected using a soil auger from one irrigated field. Four samples were top soil from 0 to 30 cm depth horizon and four were from the underlying 30–60 cm depth. A sieve analysis showed that the top layer had over 90% sand, less than 10% clay fraction and no silt fraction. The organic matter content in the top soil averaged 0.56% and subsoil had a calcrete gravel fraction. At a soil density of 1.4 g/cm3 the field's topsoil and subsoil had average permeability constants of 0.012 cm/s and 0.014 cm/s respectively. 3660 mg nitrate were applied to the 6 test samples, like irrigators' highest fertigation rates of 66 kg/ha. The nitrate concentration of effluent from the permeability meter was measured using a Eutech ion 6 + pH/mV meter and a nitrate ion selective electrode. The nitrate leachate concentration against time was a quadratic function with two distinct phases; a fast leaching rate with a steep gradient, until the topsoil and subsoil had lost about 21% and 29% respectively, and then a slower leaching rate close to a zero gradient. Under the constant saturation conditions, the slow leaching phase began after 8 and 7 h, for top and subsoil respectively. Therefore, for reducing nitrate leaching from the Kalahari sandy soils, a maximum fertigation rate of 45 kg/ha per single application was recommended for a calcium nitrate fertilizer.