POTASSIUM AND PHOSPHORUS UPTAKE BY COMPETING PINE AND GRASS - OBSERVATIONS AND MODEL VERIFICATION

A computer model (COMP8) was recently presented that simulates nutrien t uptake by competing and single root systems. However, the model need ed independent verification. Our objectives here were to: (i) determin e if uptake by each species was a simple function of each species' roo t surface area, and (ii) verify COMP8 for K and P uptake by slash pine (Pinus elliottii Engelm. var. elliottii) seedlings in competition wit h Panicum aciculare Desv. ex Poir. in Lam. grass plants. Pine and gras s plants were grown together at low and high initial soil-solution con centrations. Observed and predicted uptake values were compared by lin ear regression and by t tests using variances predicted by Monte Carlo analysis. Far high initial solution conditions, we quantitatively pre dicted 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 L ow initial solution concentrations, only one case of pine and no cases of grass uptake were predicted accurately. For neither K nor P was up take by pine (expressed as a percentage of that taken up by pine and g rass combined) in a 1:1 relationship with the percentage of surface ar ea in pine roots. We conclude that uptake by the competing root system s is not a simple function of their relative root surface areas and th at COMP8 provides an adequate description of nutrient uptake by compet ing and contrasting root systems under certain, well-defined condition s.