EFFECTS OF INOCULATION AND FERTILIZER N LEVELS ON N2 FIXATION AND YIELDS OF SOYBEANS IN ONTARIO

Abstract

Soybeans (Glycine max (L.) Merrill) often do not give yield responses to added fertilizer nitrogen (N) because high soil N levels inhibit fixation of atmospheric N2. Yield responses to N fertilizer applied at planting usually indicate that N2 fixation is less than optimal. The effects of inoculation with Rhizobium japonicum, and fertilizer N levels, on soybean N2(C2H2) fixation and seed yields in Ontario were investigated in ’ 1976 and 1977. Three locations were used each year, representing areas where soybeans had been grown for many years (Ridgetown), for only a few years (Elora), or not at all (Woodstock). Treatments were (a) Uninoculated + 0 N, (b–e) Inoculated + 0, 50, 100 or 200 kg N/ha. Results indicated that inoculation increased seed yields only when soybeans were introduced into new areas. Fertilizer N applications at planting time did not increase yields in areas where soybeans had been grown several times previously, indicating that N2 fixation could support maximum yields. Nodule number and mass, and N2(C2H2) fixation rates were all decreased by fertilizer N. An increase in nodule efficiency, later in the season, in high N treatments was most marked at Ridgetown.