Summary: With a grant funded by the Delaware Soybean Board, we studied whether split application of K for soybeans would benefit yield on sandy soils. Yield increases through split application were not seen, although yield had a positive correlation with greater tissue K levels, and split applied plots had the highest leaf tissue K.
Discussion: With the lower cation exchange capacity of sandy soils, it is possible that some pre-plant applied K could leach below the root zone prior to crop uptake. We set up three treatments, no K, all pre-plant (70 lbs K2O/acre), and a 50/50 split. For the split applied treatment, the first half was applied pre-plant while the remaining amount was done just prior to reproductive stages. Potassium (0-0-62) was applied as dry granules with a Valmar spreader.
The initial soil tests showed that K was optimum across most of the plots, but the goal of this project was to observe if even those numbers are adequate in sandy soils. There were no yield differences by treatment in this study, although the highest absolute yield (62.9 bu/acre) was in the split-applied plots. There was a significant block effect, indicating that field variability contributed to yields. Interestingly, pre-harvest soil tests did not correlate to yield, but post-harvest soil tests did. Even though yield did not vary by treatment, higher soil test levels at the end of the experiment still supported greater yields, indicating the importance of K in soybean yields.
The highest leaf tissue K concentrations were from the split-applied soybean plots (2.21%), while the other two treatments had similar amounts. Soybean leaf tissue levels taken prior to the split application did not correlate to yield, but tissue tests following application had a positive relationship. Just like soil tests, yields were higher where soybean tissue levels had greater K. This may have been improved through later season applications, but uptake during this period from other soil sources can’t be ruled out. More work should be done to uncover this relationship, particularly in soils with lower soil test levels. In sandy soils, soil test level K should be observed more often to ensure proper levels. This project will be repeated in 2020.