Determining the impact of lablab growth on the nitrogen content and nutrient status of savanna soil at Motshephiri Village, Limpopo, South Africa

Abstract

It is inevitable that the prolonged soil disturbance by crop cultivation with little or no nutrient supplementation through the application of manure or organic fertilizers results in the depletion of the nutrients necessary for plant growth. Low soil fertility levels in savanna soils have been linked to low crop yields, with nitrogen (N) being the most important limiting factor in crop yield. Subsistence farmers, without sufficient means to purchase chemical fertilizers for nutrient replenishment such as those in Motshephiri village may benefit from legume crops that are efficient at returning nutrients to the soil, particularly N. To determine the impact of lablab growth on savanna soil, N and other nutrient elements in a pot experiment was conducted in a greenhouse laid in a Randomized Complete Block Design consisting of four treatments (control, Bradyrhizobium japonicum inoculant, superphosphate and Bradyrhizobium japonicum inoculant + superphosphate) where superphosphate was applied at three different levels (45, 60 and 75 kg/ha). The soil used in this pot experiment comprised low total N, low NO3- and high NH4+. The results of the study indicated that lablab growth significantly increased total N and NO3-, and reduced NH4+ relative to the soil pre-lablab growth. However, the N content (total N, NO3- and NH4+) did not differ significantly amongst different levels of superphosphate with or without Bradyrhizobium japonicum inoculant. Lablab growth, in the absence of Zn has been proven to have a significant impact on both the soil macro- (P, K, Ca, Mg, and Na) and micronutrient levels (Fe, Mn, Cu, B and Cl). Additionally, plant growth parameters were measured at week 5 and 10 to assess their performance in response to the treatments. Plant height, leaf area, number of leaves and leaf chlorophyll content were not significantly influenced by Bradyrhizobium japonicum inoculant and/or superphosphate treatments at week 5 and 10 except for leaf chlorophyll content which was significantly higher in control at week 5 relative to the rest of the treatments. This study suggests that lablab has the potential to improve soil N and the overall soil fertility within a short period of time without the need for superphosphate fertilizer or Bradyrhizobium japonicum inoculant.