Tilman replies — Andrén and co-authors express scepticism about the possibility that ‘ecological’ methods may help solve agricultural pollution2, but do not dispute the equality of yields from manured versus fertilized crops. I highlighted1 experimental work on ‘ecological’ farming2 not because it offered a definitive solution to agricultural pollution, which it did not do, but because it demonstrated the plausibility of approaches that have been ignored in most conventional research. Any practices that seemingly reduce pollution while maintaining yields and profitability are worthy of study.

Agricultural intensification during the past 35 years has led to a doubling of world grain production, but this required 6.9- and 3.5-fold increases in annual global rates of nitrogen and phosphorus fertilization, respectively, and a doubling of irrigated land3. Use of pesticides also increased markedly, with many of these accumulating far from points of application4. If these trends presage the future, the next doubling of global food production, expected within 35 years, will require a tripling of nitrogen and phosphorus fertilization and a doubling of irrigation3.

Nitrogen and phosphorus losses from agricultural fields are already the major source of nutrient loading into freshwater and nearshore marine ecosystems5, and are a major source of terrestrial nutrient loading6. This pollution is having serious impacts on non-agricultural ecosystems5,6.

Research is needed that pursues all reasonable approaches to this problem. The apparent distrust between conventional and ‘ecological’ schools of agricultural thought must not blind either side to novel insights, nor slow the development of solutions to a global problem.