RESPONSE OF 1ST-YEAR SOYBEANS TO ROW SPACING, INOCULATION TREATMENTS, AND NITROGEN-FERTILIZATION

Soybeans [Glycine max (L.) Merr.] continue to be grown in fields with no history of soybean production, yet little research has focused on management of a first-year soybean crop. Experiments were conducted in Michigan to determine the effects of inoculant type (seed- vs. soil-applied) and rate, row spacing, and N fertilizer application on nodulation and grain yield of first-year soybeans. Field trials were established at two locations during two consecutive years. Seedings were made on Zilwaukee clay [fine, mixed (calcareous), mesic Typic Haplaquolls] and Shebeon sandy loam (fine-loamy, mixed, mesic Aeric Ochraqualfs) soils in 1985 and on Zilwaukee clay and Kilmanagh loam (fine-loamy, mixed, nonacid, mesic Aeric Haplaquepts) soils in 1986. Soybeans were planted in a split-split plot arrangement with row spacing (10, 20, 30 in.) as main plots, N fertilizer (0, 120 lb/acre) as subplots and inoculant treatments as sub-subplots. Three rates of both powdered seed-applied inoculant (1x, 2x, 4x) and granular soil-applied inoculant (1x, 1/2x, 1/4x) were used, where 1x represents the manufacturer's recommendation. Row spacing effects were noted only on the Zilwaukee clay, where soybean yields were 53.7, 63.7, and 77.0 bu/acre for the 30-, 20-, and 10-in. spacings, respectively. Higher rates of soil-applied inoculant were associated with greater nodule number at both locations; however, inoculant treatment influenced grain yield only on the loam soils. All soil-applied and the 4x seed-applied treatments were associated with higher grain yield (xBAR = 33.7 bu/acre) than the other inoculant treatments, which were not different from the uninoculated control (xBAR = 30.1 bu/acre). Also on the loam soils, the addition of 120 lb N/acre was associated with grain yield increases of 10.8 and 2.9 bu/acre in the 2 yr of the experiment. Economic evaluations suggest that, in some cases, increases in net income may result from applying greater amounts of seed-applied inoculant (4 x), lesser amounts of soil-applied inoculant (1/4x), or N fertilizer to first-year soybeans.