Effects of pH on growth and nodulation of two forage legumes

A study was conducted to compare the responses to acid pH of Medicago sativ a and Lotus glaber, two forage legumes with different environmental require ments, either supplied with inorganic nitrogen (N) or inoculated with diffe rent strains of their nod-ale bacteria. Medicago sativa showed, in both tre atments, a significant reduction in total dry weight at pH below 6.0. In co ntrast, Lotus glaber grew equally well at all the pHs assayed in the presen ce of adequate N. Under inoculated conditions, in the absence of N supply, plant growth was dependent on the bacterial strain used. When the ability o f each strain to multiply in culture medium was examined, it was observed t hat Sinorhizobium meliloti strains showed a pH-sensitive response that inhi bited growth at pH 4.0, whereas Mesorhizobium loti strains showed normal gr owth at this pH. These results suggest that for the effectiveness of Mesorh izobium loti-Lotus glaber symbiosis in acid soils the major factor to be co nsidered is the tolerance of the bacterial strain to acid conditions, while the limiting factors for the Sinorhizobium meliloti-Medicago sativa symbio sis are the sensitivity to low pH of both the plant and its bacteria.