Assessment of plant-available potassium for no-till, rainfed soybean

Temporal and spatial availability of K can influence soybean [Glycine max (L.) Merr.] productivity. This study quantified the impact of initial soil K concentrations, soil-water content, and soybean K uptake on soil K pools (water-extractable solution-phase K [K_sol], 1 mol L^-1 NH₄OAc-extractable K, referred to as exchangeable K (K _exch), and 5-min sodium tetraphenylboron [NaTPB] extractable K [K_TPB]) and compared predictions of plant K availability using the NaTPB vs. NH₄OAc tests. Soil and soybean samples were collected five times between the VE and R6 development stages in 2003 and 2004 from fallow and cropped no-till Toronto-Millbrook silt loam Alfisols at the Throckmorton Purdue Agricultural Center. Gravimetric soil water content was measured weekly during the growing season. Low-, medium-, and high-K fertility plots were replicated four times. Initial K_exch levels ranged from 60 to 290 and from 50 to 90 mg kg^-1 at the 0- to 10- and 10- to 20-cm depths, respectively. Medium- and high-fertility soils had the highest grain yields. The 0- to 5-cm soil layer had the highest K_exch levels, water availability, and soybean K uptake. Compared with K_exch, K_sol levels were less stratified and the surface layer was less dynamic. The NaTPB extraction was a better predictor of soybean K uptake in 1 yr, but across both years, NH ₄OAc was superior. Potassium measurements in the 0- to 10-cm soil layer provided slightly better estimates of plant K uptake than those in the 0- to 20-cm layer. Greater water availability and K uptake in the 0- to 5-cm soil layer suggest that under evenly distributed, intermittent rainfall conditions in somewhat poorly drained soils, vertical soil K stratification might not be a concern for no-till soybean production.