Nitrogen Effects On Plant Populations of Soybean.

Soybean yield response to plant population can vary based on plant growth habit, soil type, moisture supply, fertility, and other environmental factors. Field-scale trials in Indiana have demonstrated optimal soybean stands near 250,000 plants ha^-1 though some trials exhibited no yield difference with stands as low as 125,000 plants ha^-1. One potential source of variable soybean responses could be nitrogen (N) supply from the soil and biological N fixation (BNF). We provided a non-limiting supply of N to determine if the adaptability of soybean to low plant populations was related to N supply. We applied a total of 560 kg N ha^-1 in the forms of urea and polymer coated urea. Forty percent applied at planting and the remaining 60% at V5. Six seeding rates from 123,500 to 617,500 seeds ha^-1 (increments of 98,800 seeds ha^-1) were treated with or without the fertilizer N. Plant height, crop reflectance, seed mass, and grain yield were collected and analyzed.  Soybean plants were taller due to higher populations and not additional N supply. Without supplemental N, grain yield was maximized around 275,000 plants ha^-1and decreased at the highest population.  Lower plant populations (< 60,000 plants ha^-1) yielded more with the non-limiting supply of N than the untreated control. Soybean seed mass was larger with additional N.  Seed mass, one of three grain yield components, contributed to the stable yields of soybean across various plant populations treated with N. In 2012, the drought and late-season pod feeding by bean leaf beetle impacted the overall results. More research is needed to validate the response of low plant populations of soybean to N supply.