Influence of fertilizer use on the phosphorus and potassium status of sandy soils

Factorially combined rates of 0, 19.6, 39.1, and 58.7 kg/ha P and 0, 37.2, 74.4, and 111.6 kg/ha K were applied annually to plots on soils of different fertility levels. The soil types were Brady and Fox sandy loams. Cropping reduced the levels of phosphorus and potassium in the soil when these elements were omitted from the fertilizer. The decline in phosphorus and potassium levels was more rapid at high fertility levels than low, and much more rapid for potassium than for phosphorus. Where these elements were added as fertilizer, phosphorus and potassium levels in the soil were increased. The increase was greater for phosphorus than for potassium. These changes in soil test levels are described by simple polynomial equations that are used to project the amounts of fertilizer required to maintain certain nutrient levels in the soil. The equations can also be used to predict the amount of fertilizer required to raise an existing nutrient level to some other arbitrarily selected level.