Abstract

Irrigation schemes in old flood plains of the Okavango River were identified as major non-point sources of sediment nutrients and leachates despite lack of supporting evidence from studies that measured nutrient levels in the river’s mainstream using grab samples. Hence this study sought to check for evidence of loss and transport of nitrates from an irrigated field into the uncultivated riparian zones of the Okavango River. Soil nitrates were tested for using an Eutech ion 6+ pH/mV meter and a nitrate ion selective electrode, in soil samples taken from an irrigated field, a control site and a depression receiving storm water drained from the irrigated field at the Mashare commercial farm. Based on analysis of farm records’ fertilizer application rates and soil nitrates results, it was inferred that maize crops grown in the rainy summer seasons contributed more nitrogen fertilizer losses compared to wheat crops planted in dry winter seasons. The top soil derived from Kalahari sandy soils retained more nitrates compared to the subsoil which had high contents of light-coloured calcrete, which contained low nitrate levels especially when dry. High nitrate levels in horizons 150 cm below the root zone, at a 240 cm depth, and more than twice nitrate levels in the vlei compared to the irrigated field and an uncultivated field proved that there was leaching of nitrates from the irrigated into the uncultivated riparian zones of the Okavango River.

Key words: Calcrete, irrigated field, leachate, maize, nitrates loss, Okavango River, wheat.