Flood variation and soil nutrient content in floodplain vegetation communities in the Okavango Delta

We investigated the influence of hydroperiod variation on soil nutrient content in the Okavango Delta seasonal floodplains. Soil samples were collected from eight zones of homogenous vegetation cover after low and high floods and analysed for pH, Na, Mg, Ca, K and P content. A Student's t-test was used to test for differences in pH, Na, Mg, Ca, K and P between soils after low and high floods. The Kruskal-Wallis test was used to compare means of flooding duration and depth between low and high floods. Na, K, Mg, P and pH levels in soils were significantly different (p < 0.05) after low floods compared with after high floods. Na content was lower ( p < 0.05) in Zones 2 and 8 and higher ( p < 0.05) in Zones 4, 5 and 6 during high flood than during low flood. Ca content was lower ( p < 0.05) in Zones 1, 2, 3, 4 and 5 and higher in Zone 7 (p < 0.05) under high flood than during low flood. Mg content was lower ( p < 0.05) in Zones 1 and 5 and higher (p < 0.05) in Zones 6, 7 and 8 during high flood than during low flood. K content was lower in Zones 1 and 3 and lower in Zones 6, 7 and 8 (p < 0.05) during high flood than during low flood. pH was significantly lower and higher after a high flood in Zones 1 and 6, respectively, than during low flood. P content was significantly (p < 0.05) higher in all zones after high flood than after low flood. Flooding depth and duration increased (p < 0.05) in all vegetation zones during high flood. Our results have direct implications for molapo (flood recession) farming. We recommend that farmers plough immediately after the onset of flood recession when the soil is still moist and rich in nutrients.