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Fluid Journal : Fluid Journal 1996-1998
2 Fluid Journal Fall 1998 adequate to supply grain sorghum with nitrogen needed to produce yields that were limited by low rainfall. Lack of response to N fertilization prevented any assessment of efficiency of the fertilizer placement methods. Although yield responses to N fertilizer treatments were not significant at the 10 percent probability level, sorghum grain yields showed trends in yield response to N fertilizer applications on the Kempville loamy fine sand at the New Kent site (Figure 2). The crop reached its maximum yield (112 bu/A) at an N starter band rate of 70 lbs/A in combination with sidedressed N at 80 lbs/A. Beyond this rate, grain yield decreased for all sidedress applications greater or less than 80 lbs/A. Review of grain yield response data (Figure 1) and calculation of grain yields for all starter/band treatments using regression equations, showed that 93 percent of the maximum grain yield was achieved at a much lower rate of starter N and sidedressed N. For example, a starter N rate of 44 lbs/A in combination with 30 lbs/A of sidedress N produced 104 bu/A. Such a rate is advisable when rainfall is highly variable and below average, and when there are agronomically significant levels of soil profile mineral N. Rapid growth and nutrient uptake by sorghum plants normally occur about 35 days after emergence, at the 8-leaf stage. Sidedress application at this stage is practically feasible and could be beneficial for the crop. Objectives 3 and 4 Figures 3 and 4 show yield responses of grain sorghum to varying applications of phosphorus and sulfur. At the Henrico site, grain yield averaged 73 bu/A for P, and 72 bu/A for S. At New Kent, grain yield averaged 108 bu/A for P, and 105 bu/A for S. Mehlich I extractable phosphorus levels were 22 and 60 ppm for the Atlee and Kempsville soils, respectively, which are calibrated as medium to very Figure 2. Yield response to sidedress N applications on no-till grain sorghum, New Kent County, Khosla, et al. , 1997. Figure 1. Mineral-N, SO4-S, and P analysis taken at planting, Henrico County, Khosla, et al., 1997. 60 70 80 LBs/A Mineral-N Sulfate-S P 0 0-6 Soil depth - inches 6-12 12 -24 24-36 10 20 30 40 50 Sidedress N rate (lbs/A) 0 40 60 80 Grain yield (bu/A) 100 120 140 20 40 60 80 100 60+10 lbs N acre-1 70 lbs N acre-1 50 lbs N acre-1 30 lbs N acre-1 10 lbs N acre-1 Starter Band N --Broadcast + Band N 120 140
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