Impact of Paddy Fields on Soil Salinity and Sodicity at Kalimbeza Rice Project, Namibia

Long-term rice (Oryza sativa L.) paddy field cultivation can lead to negative impact on the crop productivity and yield stability. Soil salinity and sodicity which are closely related to unsustainable cultivation management and soil properties that affect plant growth and agricultural productivity. The objective of this study was to evaluate the long-term impact of cultivation of paddy rice on field salinity and/or sodicity accumulation in the Zambezi River floodplains of Namibia. Soil samples were collected from four randomly selected rice paddy fields under cultivation for 5, 10 years and non-cultivated areas under the project. A soil pH of 5.5 was measured in the 10-year paddy fields compared with 6.7 in the non-cultivated area, revealing soil acidification and not salinity or sodicity, resulting from long-term rice cultivation. The electrical conductivity (EC) of 50.5 mS/m found in the 10-year paddy fields was nearly three times (17.6) in the non-cultivated area and 16.8 in the 5-year paddy fields. The highest Na concentration (2.52 meq/L) was found in paddy fields cultivated for 10 years, compared to 1.0 and 0.7 meq/L in the fields cultivated for 5 years and in the non-cultivated areas, respectively. The highest SAR (2.5) was observed in the 10-year paddy fields, compared to 1.32 and 1.46 in the 5-year and non-cultivated areas, respectively. The results revealed an accumulation of salinity and sodicity. However, the EC (<400 mS/m) and SAR (<13) indicated normal soil conditions. EC strongly and positively correlated with Na (r = 0.979), magnesium (r = 0.733), calcium (r = 0.932), and SAR (r = 0.79), and negatively correlated with C:N ration (r = −0.752). SAR strongly and positively correlated with Na (r = 0.844) and negatively with C:N ratio (r = −0.845). This revealed the need for soil management strategies that would prevent acidifying effect due to depletion of soil exchangeable bases including the use of acidifying fertilizers, inefficient use of exchangeable bases leading to excessive leaching, unbalanced plant utilization of exchangeable bases leading to nutrient imbalances and depletion of bases, and inefficient use of bases leading to excessive leaching of exchangeable bases in the soil. These findings can serve as a baseline for selecting sustainable paddy field management practices at the Kalimbeza Rice Project and in similar agro-ecologies.