A computer program called NPDECIDE has been developed by the Western Australian Department of Agriculture to assist with decisions relating to the use of phosphorus fertiliser on cereal crops. NPDECIDE functions by estimating the response of the crop to combinations of nitrogen and phosphorus fertilisers, and by indicating the likely profit from the use of these materials. In order to carry out this task, NPDECIDE must be able to calculate the availability to the growing plants of nitrogen from the fertilisers and from organic material in the soil
