Sir

There is an upside to the potential future phosphorus shortage highlighted in your News Feature 'The disappearing nutrient' (Nature 461, 716–718; 2009). Long before a global phosphorus crisis occurs, declining supplies may present a windfall for water quality and sustainable agriculture.

As you point out, declining phosphorus supplies will encourage efficient recycling of phosphorus-rich livestock manure. The full potential of livestock manure fertilizers is not being achieved, owing to unfavourable economics and nutrient ratios that are incongruent with crop needs. The phosphorus-to-nitrogen ratio of livestock manure (roughly 1:3) greatly exceeds the phosphorus-to-nitrogen ratio required by common cereal crops such as maize and rice (roughly 1:6). And on multi-operation farms, manure is free.

As a result, manure fertilizer applications often exceed crop phosphorus requirements to meet crop nitrogen demands. The excess phosphorus can be transported to surface waters, fuelling eutrophication of freshwater lakes and low-salinity estuaries such as Lake Victoria and the Stockholm archipelago.

Declining phosphorus supplies coupled with increasing phosphorus demand will encourage sustainable agricultural practices that include more effective manure-fertilizer management. For example, livestock manure can be supplemented with synthetic nitrogen fertilizer to narrow the gap between manure and crop nutrient ratios. Better matching of available-nutrient supply with crop-nutrient demand promises to extend global phosphorus supplies and improve water quality throughout the world.