Effects of supplementary nitrogen fertilizer on nodulation, yield and seed characteristics of soybean (Glycine max) on the Darling Downs

Abstract

In field experiments with soybeans (Glycine max) conducted over four seasons at Hermitage Research Station, Queensland, levels of applied nitrogen (as urea) up to 134 kg ha -1 were examined. Nodulated soybeans failed to respond to applied nitrogen in three out of the four seasons. A response was obtained in one season when abnormal climatic conditions of low rainfall and high temperatures resulted in a comparatively lower level of nodulation. The probability of such a response is discussed in relation to climatic data of the area over the past 108 years. Nodulation and nitrogen fertilizer increased seed size and nitrogen content of the seed in soils free of Rhizobium japonicum but nitrogen fertilizer had little effect on these characteristics in soils carrying R. japonicum. Oil content of the seed was negatively correlated with the nitrogen content in rhizobia-free soils. Nitrogen fertilizer had a depressive effect on nodulation. This effect was severe when inoculated soybeans were grown in soil free of R. japonicum particularly under conditions of declining soil moisture and high temperatures. In soils with R. japonicum the depressive effect was comparatively light.