Topsoil and subsoil potassium as affected by long-term potassium fertilization of corn-soybean rotations

Long-term potassium (K) fertilization practices are likely to affect the K content of soils. This study assessed the effect of long-term K fertilization strategies for corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] rotations on extractable K in the soil profile of a major Iowa soil type at two locations. The soil type was a Webster fine-loamy, mixed, mesic, Typic Haplaquoll at both sites. Soil samples were collected from the 0-15, 15-30, 30-60. and 60-90 cm depths after 17 years (Site 1) or 19 years (Site 2) of K fertilization with combinations of two initial rates and four annual rates. The initial rates were 0 and 1,344 or 1,120 kg K ha-1 at Site 1 and 2, respectively, and the annual rates ranged from 0 to 100 kg K ha-1. Samples were analyzed for ammonium acetate-extractable K (STK) and nitric acid (HNO3)-extractable nonexchangeable K (HNO3-K). Concentrations of STK and HNO3-K in the top 0-15 cm soil layer at the two sites were higher for the high initial K rates and were linearly related with the annual K rate. Results for the subsoil layers varied between sites and extractants. At Site 1, annual rates of 30 kg K ha-1 or higher resulted in a relative accumulation of HNO3-K in the 15-30 cm layer. At Site 2, these rates resulted in relative accumulations of STK in the 30-60 cm layer and of HNO3-K in the 60-90 cm layer, but with relative depletions of STK in the 15-30 and 60-90 cm layers. Thus, use of one extractant may not always be sufficient to evaluate cropping and fertilization effects on subsoil K. Long-term K fertilization of corn and soybean rotations affected extractable K of both the topsoil and subsoil. The effects on subsoil K, however, were smaller compared with effects on the topsoil and varied markedly between sites, subsoil layers, and extractants.