Root and shoot growth, seed composition, and yield components of no-till rainfed soybean under variable potassium

Spatial soil-K availability for no-till soybean [Glycine max (L.) Merr.] has not been studied extensively. We characterize soybean growth- and yield-component and quantify root parameters as a function of soil depth in K-stratified soils with 1 M ammonium acetate extractable-K ranges 60–290 at 0–10 cm increment and 50–90 mg kg−1 at the 10–20 cm increment. Shoots and roots (five depth increments to 50 cm) were collected during development and grain at harvest during 2 years. Soil K at or above the critical level (104 mg K kg−1) increased early-season leaf area and root K-uptake rates early and late in reproductive development. Greater number of seeds plant−1 increased yield for soils with K near the critical level. Soil-K above the critical level increased luxury K-uptake without improving yield, seed-K concentration and accumulation, or seed oil and protein concentration. Greater root length density (>41% of the total) in surface soil coupled with previous results showing greater water content throughout the season in surface soil compared to deeper layers illustrates K stratification caused by no-till may enhance soybean K-uptake.