Soil acidity in relation to groundnut-Bradyrhizobium symbiotic performance

Effects of soil acidity on groundnut-Bradyrhizobium symbiotic performance were studied in a potted, sandy soil in a glasshouse in Zimbabwe. The soil was limed to soil-pH levels of 5.0 and 6.5. Soil acidity negatively affected plant development, measured as leaf area and plant dry weight, while nodulation was enhanced. This acidity-enhanced nodulation was most evident when nodulation was caused by the indigenousBradyrhizobium population. Effects of soil acidity differed between groundnut cultivars andBradyrhizobium spp. strains, the former having greater importance. TwoArachis hypogaea L. Spanish-type cultivars, Falcon and Plover, performed equally well at neutral soil pH, but Falcon was more acid tolerant. Comparison of the symbiotic performance in neutral versus acid soil of twoBradyrhizobium spp. strains, MAR 411 (3G4b20) and MAR 1510 (CB 756), showed that MAR 411 performed superiorly in neutral soil, but MAR 1510 in acid soil. The indigenousBradyrhizobium population was more effective than was inoculation with strains MAR 411 or MAR 1510. Comparison of twelveBradyrhizobium spp. strains for their symbiotic performance in acid soil showed that some strains were totally ineffective under acidity stress (MAR 253, MAR 967 and MAR 1506), while others performed well.Bradyrhizobium spp. strain MAR 1576 (32 H1) ranked highest for nitrogen accumulation, plant dry weight and leaf area, with strains MAR 1555 (TAL 11) and MAR 1510 following closely. Nitrate fertilisation of groundnut plants led to soil alkalinisation, while nitrogen fixation resulted in soil acidification. Soil acidity in combination with soil sterilisation gave rise to symptoms associated with Al and Mn toxicity.