Effect of Nitrogen Fixation by Legumes on Soil Acidity

The objectives of this study were to determine the quantitative effect of symbiotic N fixation on soil acidity and to compare this effect with the theoretical amount of acidity that should be produced on the basis of the excess-base and N content of the crops. Alfalfa (Medicago sativa L.) and soybeans (Glycine max (L.) Merr.) were grown on two soils in a greenhouse experiment. The amount of acidity that developed was determined by measuring the amounts of calcium carbonate required to bring the pH of the cropped soils back to those of the uncropped. The excess-base and N contents of the plants were obtained by analyzing the tops, stubbles, and roots of alfalfa and the tops, roots, and nodules of soybeans. The amount of N fixed was calculated from the differences between the total N found in the crops and the nitrate accumulation that took place in the uncropped soils during the experimental period. Some information was also obtained on the uptake of N from one of the soils by growing a nonnodulating strain of soybeans. Under the intensive cropping in the greenhouse the amount of N fixed by ‘Vernal’ alfalfa, during a growth period of 167 days (3 cuttings) or by soybeans (Glycine max, T201 and T202, U. S. Regional Soybean Laboratory) during a growth period of 67 to 73 days was sufficient to lower the pH of the soils by more than 1.0 pH unit and to require the addition of up to 2000 ppm of CaCO₃ to bring the soils back to their original pH. On the basis of the data obtained, N fixation from an annual yield of 10 metric tons/ha (4.6 tons/acre) of alfalfa would produce acidity hi the soil equivalent to 600 kg CaCO₃/ha or 535 lb/acre. Relatively good agreement was obtained between the amount of acidity developed in the soils by alfalfa and soybeans and the theoretical amounts calculated from the N and excess-base contents of the plants. Thus, the acidity produced in soils by N fixation is of considerable significance under certain cropping conditions. It is suggested that it may also have been an important factor in soil genesis and development.