This brief discussion of the economic aspects
of fertilizer use is restricted to the estimation
of optimal fertilizer rates and profit for a single
rice crop. For nutrients with long-term residual
effects such as P and K, the economic
contribution of applied fertilizer must also be
considered over time periods in years or even
decades.
For a single nutrient, profit ( , gross return
above fertilizer cost) and the increase in profit
compared with no fertilizer applied (An) can
be calculated using