Soybean Yields with Direct and Residual Nitrogen Fertilization¹

Abstract

Abstract The objective of this research was to determine the effect on soybean ( Glycine max (L.) Merrill) yields of N added at different rates by different times and methods of application, as direct and residual, and as inorganic and organic sources. A number of studies were conducted over a period of several years at 10 field locations in Illinois. Nitrogen at rates up to 360 kg/ha added for corn ( Zea mays L.) the preceding year had no effect on soybean yields. Neither were soybean yields increased by organic sources of N such as manure or alfalfa ( Medicago sativa L.), or by combinations of organic and inorganic sources. Fertilizer N added for soybean as plow‐down, disked‐in, and side‐dressed at early flowering and at pod filling did not increase yields. Nitrogen added for soybeans planted on four dates did not increase yields. High rates of N (1800 and 1440 kg/ha), broadcast and disked‐in in the spring, decreased yield due to germination and seedling injury. Considering all the studies, yields were significantly increased in only 3 out of 133 instances and these occurred at high, uneconomical rates of N fertilizer. It is concluded that N available to the plant is not the growth factor that presently limits soybean yields in Illinois.