Influence of lime on nitrogen fixation by tropical and temperate legumes

Summary

A nitrogen deficient Oxisol which had been fertilized with P, K, Zn and Mo received CaCO₃ at rates which increased continuously from zero to 22 tons/ha. Liming produced a range of pH in the saturation paste from 4.7 to 7.1; a range of calcium in the saturation extract from 0.3 to 3 meq/l; and a significant decline in available manganese. Responses of 9 tropical and 7 temperate legumes were compared across the pH variable.

For Stylosanthes spp. nodule numbers and weight and plant yield declined above pH 5.5. Arachis hypogaea and Vigna sinensis, whose yield increased by only 30%, showed no clear improvement in nodulation or nodule effectiveness (acetylene reduction rate per unit nodule weight).

Increased nodule effectiveness could account for most of the growth increase in Dolichos axillaris and Glycine max var. Kahala. Both the number and effectiveness of nodules increased for Desmodium intortum, Glycine wightii, Medicago sativa, and Trifolium subterraneum. Nodule size and effectiveness increased for G. max var. Kanrich. Only in one species, Coronilla varia, could increase in nodule numbers alone account for the increased growth associated with liming, although increased numbers of nodules probably accounted for much of the response by Lotus corniculatus, and by Desmodium canum and D. intortum up to pH 5.3.

Increased nodulation and plant N contents were consistent with the conclusion that for most species improved N₂-fixation was the cause of growth improvement associated with liming. However, percent N was high in leaves of Vigna and Phaseolus vulgaris at all lime levels. In Phaseolus, variation in nodulation and growth were unrelated. The growth improvement was associated with decline in leaf N, suggesting that something other than N nutrition was limiting.