NODULATION, ACETYLENE REDUCTION AND YIELD OF FABA BEANS AS AFFECTED BY INOCULUM CONCENTRATION AND SOIL NITRATE LEVEL

Abstract

On low nitrate soils, maximum yields of faba beans (Vicia faba L.) were obtained using the manufacturer’s recommended rate of 4.2 g inoculum/kg seed at two locations in 1975. Up to 16.9 g of inoculum was required in 1976 for maximum yields in the dry seedbeds. A granular inoculum appeared to withstand low soil moisture conditions better than the powder form. On low nitrate soils, symbiotic fixation accounted for a maximum of 67.5–70.0 kg N/ha in 1975, and 146.0 kg N/ha in 1976. In 1976, at two locations with high nitrate soils, acetylene reduction was 37% less than the maximum rate found at the low nitrate location. Inoculum treatments did not increase yields at the high nitrate locations. At one location, yields decreased at 8.4 and 16.9 g powder inoculum and with granular inoculum. It is suggested that the more abundant nodules diverted a significant amount of carbohydrates away from the developing pods. It is concluded that faba beans are essentially self-sufficient in nitrogen from symbiotic fixation in low nitrate soils, but that recommended rates of powder inoculum may be inadequate in dry seedbeds.