Profitability of using nitrogen fertilizer or inoculating soybean seed

Abstract

Abstract Applying supplemental fertilizer N or inoculating soybean [Glycine max (L.) Merr.] seed with Bradyrhizobium japonicum led to mixed yield responses. We assessed the profitability of seed inoculation, fertilizing with N, or both, across different planting dates using maturity group (MG) IV and MG V soybean cultivars under irrigated conditions. Planting dates ranged from late May to late June at Fayetteville, AR, and Pine Tree, AR, from 2017 to 2019. Urea fertilizer was applied at R2 at 0 or 56 kg N ha−1 using inoculated soybean. Similarly, non‐inoculated treatments ranged from 0, 28, 56, 112, to 168 kg N ha−1. Although fertilizer did increase yield, the response varied by planting date, was small, and only significant at p = .141 and .171 for linear and non‐linear effects of N, respectively. Seed inoculation had a small negative impact (89 kg ha−1) on yield (p = .0157). Using historical prices over the last 10 yr, optimal fertilizer rates ranged from 3.6 to 6.6 kg N ha−1 with an attendant average yield increase of 32 kg ha−1 for early planting and from 2.6 to 4.1 kg N ha−1 for late planting with an average yield gain of 24 kg ha−1. Regardless of timing of planting, the added yield less attendant fertilizer cost resulted in an average net gain of US$7.39 and $6.00 ha−1, for early and late planting, respectively. That net gain was insufficient to cover additional custom fertilizer application charges of $17.30 ha−1. Hence, inoculation and fertilizer N are not recommended.