LONG-TERM PHOSPHORUS AND POTASSIUM APPLICATION TO CORN ON COASTAL-PLAIN SOILS

The effect of long-term fertilizer applications on soil test levels, y ield, and crop removal depends on soil texture and mineralogy. The obj ectives of this study were to measure the fluctuations in soil P and K in soils of varying texture and to evaluate the usefulness of the Meh lich-1 soil extractant in predicting yield response. A field experimen t was conducted for 11 yr on two soils: a Tifton loamy sand (fine-loam y, siliceous, thermic Plinthic Kandiudults) and a Greenville sandy cla y loam (clayey, kaolinitic, thermic Rhodic Kandiudults). Phosphorus tr eatments were 0, 92, 183, and 275 lb P2O5 lb/acre in the first year fo llowed by annual applications of 0, 37, 73, and 0 lb/acre, respectivel y. Potassium treatments were 0, 48, 96, and 144 lb K2O/acre in the fir st year followed by annual applications of 0, 48, 96, and 0 lb/acre, r espectively. Soil samples (0-6 in.) were collected each year at tassel ing of the corn (Zea mays L.) crop, P and K concentrations were determ ined in soil and grain samples, and yield was measured. Initial applic ation rates influenced initial soil test levels, and annual applicatio n rates affected the rate of change of soil test levels with time. All significant yield increases on the Greenville soil were due to P appl ication. Phosphorus moves rapidly from solution into less available fo rms in the Greenville soil due to the soil's high P fixing capacity. Y ield increases on the Tifton soil resulted from K applications, presum ably due to greater leaching losses on the sandier textured soil. The Mehlich-1 extractant provides a questionable measure of soil P availab ility for corn. Particularly in areas where P runoff potential is high , P fertilizer should not be applied to irrigated corn in the Southeas t unless the Mehlich-1 extractable soil P concentration less than or e qual to 12 lb/acre. Data from this long-term study show that accountin g for soil K below the plow layer may improve the use of Mehlich-1 ext ractant as a measure of K availability for corn.