Potassium and Phosphorus Uptake by Competing Pine and Grass: Observations and Model Verification

A computer model (COMP8) was recently presented that simulates nutrient uptake by competing and single root systems. However, the model needed independent verification. Our objectives here were to: (i) determine if uptake by each species was a simple function of each species' root surface area, and (ii) verify COMP8 for K and P uptake by slash pine ( Pinus elliottii Engelm. var. elliottii ) seedlings in competition with Panicum aciculare Desv. ex Poir. in Lam. grass plants. Pine and grass plants were grown together at low and high initial soil‐solution concentrations. Observed and predicted uptake values were compared by linear regression and by t tests using variances predicted by Monte Carlo analysis. For high initial solution conditions, we quantitatively predicted pine uptake for all cases of P and for three of four cases for K. Grass K uptake was accurately predicted in all three cases; however, none of the three cases of grass uptake of P were correct. For the low initial solution concentrations, only one case of pine and no cases of grass uptake were predicted accurately. For neither K nor P was uptake by pine (expressed as a percentage of that taken up by pine and grass combined) in a 1:1 relationship with the percentage of surface area in pine roots. We conclude that uptake by the competing root systems is not a simple function of their relative root surface areas and that COMP8 provides an adequate description of nutrient uptake by competing and contrasting root systems under certain, well‐defined conditions.