Cattle manure and intercropping effects on soil properties and growth and yield of pearl millet and cowpea in Namibia

The impoverished Namibian soils can very well be enriched with nutrients by applying cattle manure, especially when resource-poor farmers are unable to purchase expensive chemical fertilizers. In this study, effects of cattle manure on different cropping patterns, i.e., mono- and mixed-cropping of pearl millet (Pennisetum glaucum L.) and cowpea (Vigna unguiculata L.) were compared across three cropping seasons at manure application rates of 0, 31 and 62 Mg ha⁻¹. The experimental plots were laid out in a randomized complete-block design with four replications. The parameters measured were growth and yield of the crops, soil organic carbon (SOC), total soil N, available soil P, and exchangeable soil K. The SOC was determined by the Black-Walkley protocol, total N by the modified Kjeldahl method, available P by the Olsen method, and exchangeable K by ammonium acetate extraction. Application of 31 Mg ha⁻¹ cattle manure increased SOC, total soil N, available P, and exchangeable K 1.3, 1.2, 1.2, and 1.4 times, respectively, over the control; the respective increase by the addition of 62 Mg ha⁻¹ was 1.7, 1.5, 1.3, and 2.1 times. However, differences between 31 and 62 Mg ha⁻¹ manure were not significant for SOC, total soil N, and available P, except for exchangeable K. Both grain yield and biomass yield of pearl millet increased with the application of 31 Mg ha⁻¹ manure, but declined with the application of 62 Mg ha⁻¹ manure, whereas grain yield and biomass yield of cowpea declined with the addition of 62 Mg ha⁻¹ manure. Both cowpea and pearl millet yielded more when intercropped than when grown as monocultures.