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Fluid Journal : Fluid Journal 2005-2007
Early Spring 2005 Fluid Journal 2 application, and 2) evaluate the cost- effectiveness of different timings and application methods on soybean growth in claypan soils. Objectives of our 2003 experiments, with both low to medium and high soil test K, were to 1) determine soybean yield response and salt injury from different foliar-applied K sources, 2) assess if the K source affects weed control when mixed with a glyphosate- based herbicide, and 3) evaluate the cost-effectiveness of applying K with glyphosate-based herbicides for soybean production. 2001 to 2002 results Salt injury is common with foliar fertilizer applications. However, no foliar crop injury was observed 3 or 7 days aftertheV4,R1toR2,andR3toR4 application timings of K2SO4 or the foliar control, MgSO4 (data not presented). Grain yields were generally higher in 2001 (Figure 1) compared to 2002 (Figure 2), which was probably due to better rainfall distribution in 2001. Soybean yields in both 2001 and 2002 were also 14 to 15 bu/A greater with preplant K compared to foliar-applied treatments. Foliar-applied K at 20 or 39 lbs/A of K2O increased average grain yield 6 to 10 bu/A across all foliar application dates when compared to the untreated or sulfur control in 2001 (Figure 1). However, soybean yields were more responsive to foliar K applications from 10 to 39 lbs/A of K2O under relatively drier conditions in 2002. Grain yields increased 8 to 11 bu/A when compared to the untreated or sulfur control (Figure 2). Soybean grain yield was maximized (an increase of 11 bu/A compared to the control) at a foliar K rate of 39 lbs/A of K2O applied at the R1 to R2 stages of development in 2001 when drought stress conditions were minimal. In contrast, the maximum grain yield increase observed in 2002 was 12 bu/A at the V4 application timing. A foliar K application may be more effective when applied from the V4 to the R1 to R2 stages of development to obtain optimal yields in years with good rainfall distribution. However, substantial yield increases were observed at low rates when conditions were less optimal. Differences in soybean response to foliar K may be affected by climate, since lower soil water content may reduce K uptake through the roots and thereby increase the relative crop response to foliar applications. Cost-effectiveness. The cost- effectiveness of treatments evaluated in this study was ranked preplant K at 300 lbs/A of K2O = preplant K at 150 lbs/A of K2O > preplant K at 600 lbs/A of K2O = foliar K applied at the V4 and R1 to R2 stages at 39 lbs/A of K2O followed by additional foliar treatments and the untreated control. All treatments except the V4 at 10 lbs/A of K2O and R3 to R4 timing at 10, 20, and 39 lbs/A of K2O increased gross margins when compared to the untreated control. Conclusions. A couple of conclusions were reached from this preliminary research at a low-to-medium soil test K site with a claypan soil. First, foliar K applications in soybeans may be a possible management tool to mitigate reduced yields caused by K deficiency. However, optimal soybean grain yields and gross margins were obtained with preplant K fertilizer application timings and foliar treatments
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