ASSESSMENTS OF THE RELATIVE VALUES OF COMPOUND NITROGEN-PHOSPHORUS FERTILIZERS FOR WHEAT PRODUCTION

A general procedure is developed for calculating optimal rates of application of compound fertilizers, containing several nutrients in fixed composition. The calculations are based on response surface models that have been obtained with single nutrient fertilizers. The relative values of alternative compounds can be assessed from the increases in profit that result from their use at optimal rates and this is preferable to simply comparing fertilizer compositions with that of an ideal mix of single nutrient fertilizers. The relative values of a range of standard nitrogen-phosphorus fertilizers are assessed for wheat production in southern New South Wales. The results indicate the need for a compound with a low nitrogen/phosphorus ratio of about 0.5. For the range of fertilizers considered it is concluded that it will generally be preferable to use superphosphate, supplementing with nitrogen fertilizer where N deficiency is suspected. The assumptions required for the calculation of optimal fertilizer rates are discussed.