Degree Type


Date of Award


Degree Name

Master of Science



First Advisor

Antonio P. Mallarino


Research is needed to better identify reasons for high temporal soil-test K (STK) variation. This study investigated K uptake by corn (Zea mays L.) and soybean [Glycine max (L.) Merr.], recycling of K from plant materials to the soil, and impacts on STK of soil samples collected in the fall immediately after grain harvest or in spring before planting the next crop. Soil and plant samples were collected from plots of a control (no K) and a non-limiting K rate from 19 already established Iowa field trials resulting in 33 and 14 site-years of data for corn and soybean, respectively. Potassium was analyzed in grain and above-ground vegetative plant tissue at physiological maturity (PM) and at grain harvest, and also in crop residue at roughly 45-day intervals until early spring. For both crops and regardless of the grain yield response to K, the relative K accumulation increase from fertilization was larger for residue than for grain (60 and 9% for soybean, and 57 and 7% for corn). The largest loss of K accumulated in soybean vegetative tissue at PM occurred at late fall (80%) but was much less for corn (46%). By early spring, April, soybean lost 87% of the total K accumulated at PM and corn only 67%. The K loss decreased exponentially to a minimum as precipitation increased for both crops (R2 0.84 for soybean and 0.68 for corn), but a minimum plateau was reached for soybean with about 200 mm of precipitation whereas for corn no minimum was reached with 600 mm, which was the maximum observed for that period. The residue K loss from harvest to spring correlated linearly with the STK increase during that period (R2 0.41, P < 0.01 across crops). This study quantified the amount and rate of K recycling from corn and soybean from PM until spring, the effects of precipitation, and the recycled K influence on STK variation from fall to spring. Consideration of the results will be useful to improve the capacity of K soil testing to assess crop-availability of K and predict crop response to fertilization.

Abbreviations: PM, physiological maturity; STK, soil-test K

Copyright Owner

Ryan Richard Oltmans



File Format


File Size

50 pages