GRAIN-SORGHUM RESPONSES TO AMMONIUM-NITROGEN AND NITRATE-NITROGEN FERTILIZATION

Few researchers have contrasted the effect of ammonium (NH4+) versus nitrate (NO3-) on the growth and grain yield of grain sorghum [Sorghum bicolor (L.) Moench]. A field experiment was conducted in northeastern Kansas to evaluate the effects of time and form of applied N on fertilizer band NH4+ / NO3- ratios and on the N nutrition, dry matter accumulation and grain yield of two grain sorghum hybrids. Urea, urea-ammonium nitrate, urea + calcium nitrate, ammonium nitrate and calcium nitrate solutions having NH4+ / NO3- ratios of 1:0, 3:1, 1:1, 1:1 and 0:1, respectively, were used at equivalent N rates. The nitrification inhibitors, nitrapyrin and dicyandiamide, as well as two-way and three-way timing of application schemes were used to minimize the conversion of NH4+ to NO3-. Soil NH4+-N and NO3--N concentrations in the fertilizer band indicated that split applications of N with nitrification inhibitors were effective in slowing the conversion of NH4+ to NO3- from planting through early grainfill. Regression analyses suggested that dry matter accumulation was not closely related to ratio of soil NH4+ / NO3-. Combined over hybrids, split applications of N and years, values for dry matter accumulation and N uptake were generally greatest for sorghum supplied with the N sources that contained both NH4+ and NO3-. These effects were more prevalent after boot stage than before. Also, in the combined analysis, no significant differences were observed among the N sources for grain yield, kernel number or kernel weight. Overall, relatively high soil NH4+ / NO3- ratios appeared not to be as important for the growth and productivity of sorghum as the availability of some NH4+ with NO3-.