Soybean Inoculation Trial

Bob Henson

A field experiment was conducted at the North Dakota State University Carrington Research Extension Center to evaluate the response of soybean to commercial inoculants and to compare this response to varying levels of soil nitrogen (N). The trial was sown to RoundupReady soybean cultivar ‘RG200RR’ (Maturity Group 0.0) on 29 May at the rate of 200,000 live seeds / acre in 7” rows. A soil sample the previous fall tested 37 lbs NO₃^--N / acre, with adequate phosphorus. In addition to an absolute control (no inoculum, no N fertilizer), N fertilizer treatments were included to study total N levels (soil test + fertilizer) of 50, 75, and 150 lbs N / acre without inoculation. Additional inoculant treatments of interest were also included. Plots measured 10’ x 25’ and were arranged in a randomized complete block design with four replicates. Weeds were controlled with herbicides and hand weeding. No other pesticides were applied.

Although soybean had been grown previously in nearby fields, the plot area did not have a known history of soybean, as evidenced by the sparse nodulation in the control treatment (Table 1). Visual nodulation scores indicated that 20 of the 38 inoculation treatments were significantly better than the control. No statistically significant differences among treatments were detected in days to physiological maturity, height, or lodging (data not shown).

Although no statistically significant differences were detected in grain yield, all inoculation and N fertilizer treatments resulted in numerically higher yields, with some yield increases greater than 30% (Table 1). Starter levels of N fertilizer (50 and 75 lbs total N / acre) numerically increased yield over the control, but only 150 lbs N / acre approached the yield of the best inoculant treatments. Future research will include starter N in combination with inoculation. Seed oil concentration tended to decrease with increased yield, while a highly significant positive correlations existed among seed N concentration, grain yield, and grain N yield. The positive yield response to all inoculation / N fertilizer treatments and the positive correlation between grain yield and seed N concentration are indications that N was a limiting factor to soybean production at this site.

Averaged across all treatments within a category, all inoculant formulations numerically improved nodulation, grain yield, seed protein concentration, and grain N yield over the uninoculated (including the N fertilizer) treatments. Grain yield and quality characteristics were similar among inoculant formulations, but visual nodulation scores tended to be better with seed-applied inoculants than with granular products applied in-furrow. Future trials will include a combination of seed-applied and granular inoculants, which is an option growers should consider on new soybean ground.