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Fluid Journal : Fluid Journal 1993-1995
2 Fluid Journal Fall 1995 Planting early, on the other hand, poses equal risk. The grower's strategy to have sorghum head prior to mid- season heat and drought stress can be lost when cool soils slow crop emergence and growth. Again, starters can be helpful since they improve early- season growth in cool soils. Our objective in this experiment was to evaluate no-till sorghum response to several NP starter combinations at two planting dates. Effects of N:P ratio Yield. When averaged over three years of our experiment, starter combinations that provided either 30 or 90 lbs/A of N with 30 lbs/A of phosphate (1: 1 or 3:1 N:P2O5 ratio) increased yields 18 percent over the 90 lbs/A of N and 30 lbs/A of phosphate broadcast check (Figure 1). In two of the three years, the starter that supplied 10 lbs/A of N and 30 lbs/A of phosphate (1:3 N:P2O5 ratio) also increased yield over the broadcast check treatment. When averaged over three years, however, the 1:1 and 3:1 N:P2O5 ratios both yielded 10 bu/A more than the 1:3 N:P2O5 ratio. Even under very poor growing conditions (1991), starters at either the 1: 1 or 3:1 N:P2O5 ratio increased yields over the broadcast check 3:1 N:P2O5 ratio. In 1991, rainfall was 41 percent of normal (Table 1). Note also only 0.74 inch fell in July. When averaged over all treatments, yields were only 23 bu/A. Growing conditions turned nearly ideal in 1992, when total rain doubled that of 1991 (Table 1). Even the unfertilized check plot exceeded 100 bu/ A. Again, starters at either the 1:1 or 3: 1 N:P2O5 ratio improved yields when compared to broadcast check. Note also that in 1993 nearly 18 inches of rain fell in July alone (Table 1). Dry matter. When averaged over three years of the study, only starters with 1:1 or 3:1 N:P2O5 ratio resulted in greater early-season dry matter production (6- leaf stage) than broadcast check (Figure 2). Treatments that supplied starters at 1:1 and 3:1 N:P2O5 ratios increased dry matter production over broadcast check by 18 and 21 percent, respectively. P uptake at the 6-leaf stage followed the same trend as dry matter production. When averaged over three years, the 1:1 and 3:1 N:P2O5 ratios increased P uptake over broadcast check by 23 and 26 percent, respectively. No other starter treatment resulted in any improvement in P uptake. It has been shown that when P is banded, N also is needed in the band for stimulation of P uptake. In our experiment, starter at the 1:3 N:P2O5 ratio did not appear to provide enough N for maximum stimulation of P uptake. Starter at the 1:1 N:P2O5 ratio was no more effective at increasing P uptake than that at the 3:1 N:P2O5 ratio. Leaf concentrations. In 1992 and 1993, starters at 1:1 and 3:1 N:P2O5 ratios gave greatest leaf tissue P concentrations at heading. Leaf N concentrations were greater with starters applied at these same ratios when compared to broadcast check, but not significantly different from those with other combinations. Maturity. When compared with broadcast check, starters applied at 1:1 700 Yields - lbsA 500 600 400 300 200 100 0 0 10N 30P 30N 30P 90N 30P 90N noP N:P O ratio 25 Figure 2. Effect of starter N:P O ratio on 6-le af stage whole-plant dry wieght of sorghum, Gordon and Whitney, Kansas State University, average. Of 1991-93. 25 30N noP noN 30P 90N 30P Broadcast 110 100 90 80 70 60 50 40 30 20 May June Planting Date Figure 3. Effect of planting date on grain sorghum yield, Gordon and Whitney, Kansas State University, average. Of 1991-93.
Fluid Journal 1996-1998