Predicting the effects of weeds on K and P uptake by young slash pine on a Spodosol

Observed uptake of K and P from a Spodosol by slash pine ( Pinus elliottii Engelm. var. elliottii) grown alone or in competition with weeds (mainly species of Panicum, Dichanthelium and herbaceous dicotyledons) was compared with uptake predicted by a model based on solute transport theory. Two 35 day periods were simulated during which there was significant observed uptake by pine in weed-free areas (mmol K and mmol P), but little or no uptake by pine in weed areas. Regressions of predicted versus observed uptake of either K or P by pine were not significantly different from a 1:1 line. Hence, the model quantitatively predicted the effect of weeds on uptake of these nutrients by pine. However, uptake of both nutrients by weeds was over predicted by 300–400%, which resulted in the under-prediction of soil solution concentrations in some cases. These results verify the model for predicting uptake of K and P by slash pine (but not for weeds) under these conditions. In weed-free areas, model results suggested that soil solution concentrations were lowest in the 10–26 cm depth directly under the stem, and that most uptake was from the 10–50 cm depth of soil within a 30 cm radius of the stem. Model results are presented that illustrate the potential utility of this approach to studying nutrient uptake by young plantations.