Response of common bean (Phaseolus vulgaris L.) to nitrogen, phosphorus and rhizobia inoculation across variable soils in Zimbabwe
2018
Chekanai, Vongai | Chikowo, Regis | Vanlauwe, B. (Bernard)
Common bean is an important crop with potential to curb malnutrition in poor Sub-Saharan African populations. Yields of common bean (Phaseolus vulgaris L.) are, however poor, limited by low soil phosphorus (P), nitrogen (N) and poor biological N₂-fixation. On-farm experiments were carried out to study the effect of N, P and rhizobia inoculation on common bean yield and yield components during the 2014/2015 and 2015/2016 cropping seasons in Eastern Zimbabwe. Experiments were conducted on five farmers’ fields located in two agroecologies; three fields were considered to be degraded with soil organic carbon (SOC) < 4 g kg⁻¹ and available P < 6 mg kg⁻¹, while the two non–degraded sites had SOC > 7 g kg⁻¹ and available P > 15 mg kg⁻¹. Two common bean varieties (Gloria and NUA45) were tested in a split-plot arranged in randomized complete block design. The main plot factor was the combination of N (0 and 40 kg ha⁻¹) and P (0 and 20 kg ha⁻¹), and the sub-plot factors were variety (Gloria and NUA 45) and inoculation with Rhizobium tropici strain CIAT899 (+/− inoculum). At planting, both N and P were applied at 20 kg ha⁻¹, with an additional 20 kg ha⁻¹ N top dressing applied at flowering. Analysis of variance indicated common bean did not respond to rhizobia inoculation (P > 0.05) whilst P significantly increased the number of nodules and active nodules per plant (P < 0.001), and grain yield. Application of 40 kg ha⁻¹ N significantly increased the number of pods per plant, number of seeds per pod, and grain yields. A significant NP interaction was only observed on grain yield for non-degraded soils. Co-application of N and P in non-degraded sites increased grain yields from 0.27 to 1.48 Mg ha⁻¹during the first season and from 0.37 to 2.09 Mg ha⁻¹during the second season. On degraded sites, NP application resulted in uninspiring grain yield gains of 0.09 to 0.19 Mg ha⁻¹ during the first season, and from 0.16 to 0.28 Mg ha⁻¹ in the second season. In general, effects of N or P were not significantly different, suggesting that farmers could invest in either of these nutrients for increased common bean grain yields. Strategically, P investments would be more logical as residual P effects to rotational cereals improve overall cropping system performance. The response of common bean to inoculation in Zimbabwe still needs to be widely investigated for these and other varieties.
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