Effects of Calcium, Manganese, and Aluminum on Growth of Rhizobia in Acid Media

Abstract

Growth studies were done in defined liquid media to assess effects of Mn toxicity and Ca deficiency associated with soil acidity. The study included 23 strains of cowpea rhizobia previously found capable of growth at 4.5 and 10 strains of Rhizobium japonicum tolerant of pH 4.8. The low level of Ca (50 µM) represented the extreme low range in soil solutions, and the high level of Mn (200 µM) has been found toxic to legume hosts of the strains tested.In a detailed growth study of three cowpea strains at pH 4.6, low P (10 µM) limited maximum population density in all three strains. Low Ca limited it in one strain.A rapid screening method based on attainment of turbidity from a small inoculum was applied to the cowpea rhizobia at pH 4.5 and soybean rhizobia at 4.8. High Mn and low Ca slowed growth of some strains, but Mn stopped growth of none and low Ca stopped growth of only three strains. Neither was as severe a stress as 25–50 µM Al, simultaneously observed and previously reported. All strains tolerant of Al were tolerant of Mn and low Ca.Possible amelioration of Al toxicity by Ca was tested in three cowpea strains, by a factorial experiment with three Ca levels (50–1,000 µM) and four Al levels (0–100 µM), at pH 4.5 in liquid media. Calcium had a statistically significant protective effect against Al in two strains, but the effects were small and probably of no biological or practical significance.In acid soils, Al toxicity and acidity itself are probably more important limiters of rhizobial growth than Mn toxicity and Ca deficiency.