Characteristics of soils under seasonally flooded wetlands (oshanas) in north-central Namibia

Lowland wetlands generally have a high agricultural production potential and can be local hot-spots for biodiversity. Specific seasonal wetland system is largely distributed in north-central Namibia. Seasonal wetlands consist of seasonal river wetlands (locally known as oshanas). However, studies on soil fertility, salinity and sodicity in seasonal river wetlands are still limited in this area. The objective of this study was hence to investigate the soil fertility status of seasonal wetlands and evaluate their potential for agricultural production and consider sustainability of the land use system. Soil samples were collected from 102 different spots of the flood plain within 3 major seasonal rivers, and analyzed for their physico-chemical properties and salinity and sodicity. The findings for average soil organic carbon (1.94 g kg) and average clay contents (102.3 g kg) of seasonal rivers were drastically lower than the wetland of semi-arid Africa regions (organic carbon, 5.8 g kg; clay contents, 340 g kg), and organic carbon and clay content significantly (p<0.05) decreased at the lower part of each seasonal river. Most of the seasonal river soil’s’ electrical conductivity of saturated paste extract (ECe) and the sodium adsorption ratio of the saturated paste extract (SAR) were more than 4 dS m and 13, respectively. However, there were large differences in electrical conductivity of saturated paste extract (ECe) and the sodium adsorption ratio of the saturated paste extract (SAR) values among the sampling spots. These findings suggest the high agricultural importance to improve the soil organic matter and clay contents, and land selection to avoid the strongly high saline-sodic soil sites in seasonal river.