Yield of soybeans as influenced by genetic characteristics, climatic conditions, and nitrogen nutrition

Eleven soybean cultivars, including a set of nodulating and non-nodulating isogenic lines, were tested in a field of Hokkaido University with and without nitrogen application (300 kg N/ha) for 2 to 5 years, and the data of grain yield and yield components were collected. The following results were obtained. 1. Among the cultivars, there was a positive correlation between the grain number per unit field area and the grain yield, while there was a negative correlation between the grain number per unit field area and the 1,000 grain weight. 2. Under favorable climatic conditions the grain yield was higher because the grain number was larger. 3. The yield increase associated with the improvement of the nitrogen status of the plants was due to increases of both the grain number per unit field area and the 1,000 grain weight. The increase of yield at low yield levels was considered to be more related to the increase of the 1,000 grain weight, while that at high levels was more associated with the increase of grain number per unit field area. 4. Thus, it is concluded that varieties which produce high yields by taking advantage of favorable climatic conditions and nitrogen fertilizer application are more frequent among cultivars with a large number of pods and small grains, which are mostly of the indeterminate type.