Plant Uptake and Leaching of Applied and Indigenous Potassium in Dothan Soils1

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AbstractThe K fertilization requirement for corn (Zea mays L.) frequently is overestimated by soil test procedures for Atlantic Coastal Plain soils having sandy Ap horizons and high K concentrations in clayey subsoil. It seemed probable that the lack of response could reflect availability of subsoil K. This field and laboratory research was conducted to evaluate the availability and origin of subsoil K and the downward movement of K in two Dothan soils (Plinthic Paleudults). These soils had low CEC in the Ap, A2, and B21t horizons; fine sandy loam Ap horizons; higher clay contents in the B21t horizons than in the Ap horizons; and very acidic B21t horizons. Mineral suites in the clay‐sized fraction of the two soils were predominantly 14 A intergrade minerals, kaolinite, and quartz. The three horizons in these two soils contained high levels of total K ranging from 6.5 to 12.0 meq/100 g. Over 90% of the total K in the A2 and B21t horizons of the two soils was in feldspar and mica forms indicating that parent material, rather than leaching of applied K, was the main origin of K in these horizons. Decreases in exchangeable K occurred during the first growing season in the A2 and B21t horizons of control plots in field experiments on the two soils. These decreases in exchangeable K indicated K uptake by corn plants from the subsoil horizons. With time, two‐annual broadcast applications of 83 and 249 kg K/ha as KCl increased the exchangeable K in the A2 and B21t horizons of the two soils. These increases were ascribed to leaching of K into the subsoil horizons. This leaching of K suggests that downward movement of applied K could result in an accumulation of K in clayey subsoil. The K in subsoil, either from leaching of applied K or from genetic origin, would be available to plants unless root growth into the subsoil was restricted by adverse chemical and physical properties.