A dynamical model of soil phosphorus carryover in grazed pasture provides the basis for bioeconomic optimization of fertiliser rates. A linear constraint is introduced to optimise the allocation of limited funds to fertiliser on several farm blocks, each of which represents a different land and/or stock class. The optimal constrained maintenance application of fertiliser to each block is calculated, and an heuristic approach to this equilibrium is suggested. In some cases this involves withholding of fertiliser from unresponsive blocks. The dynamical model and economic optimisation method have been implemented in a commercial fertiliser planning decision support tool Outlook.