Organic amendments application downweight the negative effects of weed competition on the soybean yield

•We examine how fertilization types influence the redroot pigweed growth and soybean seed yield.•Weed growth was more for the inorganic application than for the organic application.•The organic and inorganic application improved seed yield compared to the unfertilized control.•The combined application of organic and inorganic inputs resulted in the greatest seed and oil yield.

Applying organic nitrogen (N) sources have been recommended to increase soil fertility and crop growth in organic production systems; however, less information is available on the effects of organic inputs on crop growth in the presence of competing weeds. Field experiments were conducted across two locations during 2012 to investigate the impact of organic and chemical N sources on weed growth and grain and oil yield of soybean. We hypothesized that competition from redroot pigweed against soybean would be weaker when N was supplied by organic sources than when chemical N sources was applied in weed-infested systems. This hypothesis was tested under field conditions with different weed pressures in a randomized complete block design with a split-plot treatment arrangement. Across all fertilization treatments, seed yield losses were 37% for the low weed density and 67% for the high weed density compared with weed-free conditions, the lowest weed biomass obtained for the application of organic amendments. For the high weed density, seed yield loss due to redroot pigweed competition was 9% more for the application of inorganic fertilizer alone than for the application of organic amendments in the combined and alone forms. Soybean grown under the combination of organic and inorganic inputs produced the greatest or equivalent to the greatest LAI, leaf chlorophyll, seed and oil yield for all weed densities. We conclude that weed interference in the soybean cropping system can be reduced by using organic amendments as nutrient sources.