of the profit function, however, this ratio is
both misleading and inappropriate if
applied in relation to benefits gained from
fertilizer use, because it tends to
overestimate economic fertilizer rates.
Whatever model of response curve is
chosen, it is important to consider the
model fit as well as sensitivity to price
changes.
The approach described does not take
into account other economic variables
such as labor costs associated with
fertilizer use or interest to be paid on
loans for purchasing fertilizer. These
need to be included for a full economic
assessment.
determination of optimal rates for multiple
nutrients and their interactions. Ali (1999)
presents an approach for evaluating
organic fertilizers.
Colwell (1994) describes the
Further reading
Ali M. 1999. Evaluation of green manure
technology in tropical lowland rice systems.
Field Crops Res. 61:61–78.
Colwell JD. 1994. Estimating fertilizer
requirements: a quantitative approach.
Wallingford: CAB International.
Dawe D, Moya P. 1999. Variability of optimal
N applications for rice. In: Program report for
1998. Manila (Philippines): International Rice
Research Institute. p 15–17,
Helyar KR, Price GH. 1999. Making
recommendations based on soil tests. In:
Peverill KI, Sparrow LA, Reuter DJ, editors.
Soil analysis: an interpretation manual.
Collingwood: CSIRO Publishing. p 331–357.
Pingali PL, Hossain M, Pandey S, Price L.
1998. Economics of nutrient management in
Asian rice systems: towards increasing
knowledge intensity. Field Crops Res. 56:157–
176