THE INFLUENCE OF SOYBEAN PLANTING DENSITY ON DINITROGEN FIXATION AND YIELD

We investigated the effect of planting density on soybean (Glycine max (L.) Merr.) yield in glasshouse and field experiments. Because net canopy photosynthesis increases with increasing plant density, we hypothesized that increasing planting density would result in increasing rates of dinitrogen fixation in soybeans and higher yields per unit land area. In glasshouse studies, Wayne variety soybeans were planted in 10-cm diameter pots, 1 plant pot-1 in matrices of 10-, 15-, 20-, 25-, or 30-cm equidistant intervals. Bradyrhizobium japonicum inoculum was added to half of the plants in each treatment. Replicate measurements of total stem height, internode lengths, leaf mass, stem mass, root mass, nodule number, nodule mass, and nitrogenase activity were obtained at 3, 6, and 9 weeks post-emergence. Fruits were harvested and counted at week 14. As planting density increased there were (1) altered morphology and growth rates, (2) increased apparent specific nodule activity (SNA), (3) decreased nodule number and mass, and (4) nearly constant fruit and seed production/plant. Expressed on a unit area basis, nitrogen influx and yield increased geometrically as planting density increased, with maximum values observed for 10-cm plantings. Field studies of Wayne, Stein, Williams, and Gold Harvest soybean varieties were made in 1985. Plots were established containing 100 plants spaced at 10-, 20-, and 30-cm distances. Measurements made during the growing season and at harvest established the same relative trends identified from the glasshouse studies. Increasing plant densities resulted in higher yields per unit land. Varietal differences were almost significant.