Sir

L. E. Drinkwater et al.1 report promising isolated studies on environmental quality and productivity in crop production. But these studies should not be used to discredit high-yield crop production technologies globally.

We do not dispute David Tilman's call, in his News and Views article2 accompanying the paper by Drinkwater et al., for a greener agriculture based on ecological principles. But we do not think that as things stand we have the ability to cope with the probable doubling in global food demand in the twenty-first century with only organic nutrient inputs.

The organic nitrogen production schemes investigated by Drinkwater et al. used essentially twice the land area of their ‘conventional’ system to produce equivalent amounts of grain. But the best arable land is already used for crop production. Therefore expansion of agriculture to produce the required organic nitrogen would necessitate use of marginal land or further encroachment on natural ecosystems, resulting in soil degradation and loss of biological diversity.

Additionally, agronomic technology has advanced considerably beyond the ‘conventional’ practices used by Drinkwater et al. In industrialized countries, the mould-board plough has been largely replaced by no-till and conservation tillage practices. Single applications of fertilizer are being replaced by multiple applications with amounts and timing matched to crop requirements. These practices result in improved soil conservation and quality; greater efficiency in fertilizer use; higher and more stable yields; and they also minimize the land area required for crop production.

Progress towards a ‘greener’ agriculture will come from continued improvements in modern high-yield crop-production methods combined with sophisticated use of both inorganic and organic nutrient sources, water, crop germplasm, pest management and beneficial organisms.