Topsoil and Subsoil Potassium Calibration with Leaf Potassium for Fertility Rating

Plants obtain K from both topsoil and subsoil, but soil testing is usually performed on surface soil. Interpretation of subsoil tests for available K has also been limited by insufficient calibration of subsoil K level with growth response. Leaf K response was used to calibrate topsoil and subsoil K levels for fertility rating. A 4-year corn (Zea mays L.) — soybean (Glycine max L.) rotation on Marlboro loamy sand (Typic Paleudult) using various rates of K showed a buildup of K in the top 4 cm of B₂ horizon with moderate annual K fertilization, but little change in Ap horizon K. Leaf K contents were sufficient where soil tests showed low topsoil and medium subsoil K. No yield response to K was obtained even after 4 years without K fertilization. A 1-year corn experiment on Orangeburg sand and Faceville loamy sand (Typic Paleudults) using various rates of K showed increased K in both Ap and subsurface horizons 50 and 70 days after K application. No yield response to K additions was obtained on either soil, even though the Orangeburg Ap horizon tested low in available K. The lack of yield response at low K ratings suggested a need for adjustment in the rating. A 3-year survey of the relationships among corn and soybean leaf K, and available K in Ap and top 10 cm of B₂ horizons was made in the Middle Coastal Plain. Most of the soils were Typic Paleudults except for eight Psammentic Paleudults. A range of K application rates was not used at each site, preventing examination of relative yield response to soil K. Correlations were found between leaf K in both corn and soybeans and K in Ap and B₂ horizons. To examine the contribution of K from each horizon, soil test calibration was achieved by multiple linear regression of leaf K on Ap and B₂ available K values. Soil K fertility ratings were established from the regression equation using values for critical and sufficient leaf K to mark low and high soil test values. This work indicated that soybeans may obtain K from B₂ horizons at greater depths than corn and that available K in the B₂ horizon should be considered along with Ap soil test K to characterize the soil fertility status, particularly in cases where coarse-textured topsoils occur over finer-textured subsoil material.