The effects of a factorial combination of calcium (0.125, 2.0 mM calcium sulphate), pH (4.0, 4.5, 5.0, 6.0) and nitrogen (nil, 2.0mM ammonium nitrate) on the nodulation and growth of some tropical and temperate pasture legumes were determined. Macroptilium lathyroides and Lotononis bainesii were unaffected by low pH and low calcium (100% of plants nodulated at all treatments). In Stylosanthes humilis, Trifoliurn semipilosum, and T. rueppellianum reduced nodulation occurred only at pH 4.0. Desmodium uncinatum and Trifolium repens were sensitive to low pH, and nodulation was markedly reduced at pH 4.0 and below 5.0 respectively. Medicago sativa, M. truncatula, M. scutellata, and Glycine wightii were extremely sensitive, maximum nodulation occurring only at pH 6-0 and 2 . 0 mM calcium. Furthermore, the dry matter yields of plants that nodulated under inferior treatments were less than those of plants nodulated under optimum treatments. Nodulation and growth were strongly controlled by pH, and although interactions occurred with calcium treatment, these were primarily in the intermediate pH range.
Dry matter yields of nodulated plants of S. humilis and L. bainesii were greater at the low calcium sulphate treatment (0.125mM) than at thc high treatment (2.0mM). The remaining legumes gave varying positive responses to the high calcium treatment. Root weight ratios of nodulated plants were decreased by the higher treatments, the minimum occurring at those treatments producing the highest dry matter. In the presence of applied nitrogen (no nodulation), the effects of pH and calcium on dry matter were small, but the trends were similar to those for nodulation and dry matter production of the nodulated plants in the Rhizobium series. Root weight ratios of plants in the nitrogen series were equal to or less than those for the maximum-yielding plants in the nodulation series.
It is considered that both nodulation and subsequent plant growth are dominated by the hydrogen ion effect, and that the beneficial effect of calcium operates within certain pH limits. pH had little effect on plants that were well supplied with nitrogen.
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