Attached from NATURE 26/11/ issue

Henry Gee

Organic farming is not only kinder to the environment than conventional, intensive agriculture - but it has comparable yields of both products and profits. Without loss of production, organic farming can rebuild spent soils, prevent pollution and even combat the greenhouse effect. An extract from a radical 'green' manifesto? No - these are the implications of a sober report in Nature the result of a 15-year study to compare the performance of organic with conventional farming.

In the report, Laurie Drinkwater and colleagues from the Rodale Institute, Kutztown, Pennsylvania, sought to test the contention - a tenet in land management for decades - that the amounts of carbon and nitrogen in the soil are controlled by their net inputs, irrespective of their sources, artificial or organic. This seems odd to ecologists, who have long known that the passage of carbon and nitrogen through an ecosystem is profoundly affected by the numbers and types of species present, the quality of the organic 'litter' available for decomposition, and so on.

Their new report is dramatic proof that what works in a rainforest holds true down on the farm. The researchers simulated three kinds of farm: the first was a conventional farm, in which maize and soybeans were rotated, a mineral nitrogen fertilizer was applied before the maize was planted, and pesticides were freely used.

The other two 'farms' were organic. In one, legumes and grasses were grown to feed beef cattle, and the resulting manure was used as fertilizer for a maize crop. In the other, legumes were grown and then ploughed back, to provide a source of nutrition for a maize crop. Legumes, such as clover, peas and beans, harbour bacteria in their roots that extract nitrogen from the atmosphere, returning it to the soil.

The conventional farm grew just maize and soybeans. The other two systems were less intensive, as other grains and legumes were grown in addition. The conventional farm produced more crops - in terms of the amount of available carbon turned into plant material - than either of the other two systems. On the face of it, the conventional farm seemed more productive. But was it? between 1986 and 1995, the average annual maize yields from all three systems were very similar: 7,140, 7,100 and 7,170 kilograms per hectare in the manure-based, ploughed-in-legumes and conventional systems respectively. Over the past ten years, the economic profitability of the three systems has been comparable.

But the organic systems have further benefits. Soils on organic farms in which a mixture of crops are grown are healthier and have better structure than on conventional one-crop farms. They are better at retaining nitrogen and carbon in a variety of different forms. Because nitrogen fertilizers are not added all at once - as they are in a conventional farm - but released from soil stores throughout the year, less nitrogen is needed, less energy is used, and less nitrogen is washed out of the soil to end up polluting rivers. Over a five-year period, 60% more nitrogen (as nitrate) was washed into groundwater from the conventional system than from either of the two organic systems. These results show that the passage of nutrients through a patch of ground is influenced by the plants grown in it, and how they are grown.

And the benefits don't end there - the greater ability of a soil in an organic farm to sequester carbon than soil in a conventional farm could, if these practices were applied over the entire maize-soybean growing region of the United States, account for up to 2% of the carbon released annually in that country from the combustion of fossil fuels. Given that the United States has agreed to reduce average carbon dioxide concentrations to 7% below 1990 levels by 2008-2012, organic farming could make a significant dent in global warming.

The sad thing is that these results seem so remarkable. The 'green revolution' in which the use of cheap fertilizers and pesticides has kept the world's burgeoning population fed, despite a doubling in the past four decades, has had such immense success that we think it can go on forever and that nothing else is possible.

But the green revolution has exacted a considerable toll. The use of mineral fertilizer now amounts to the annual addition of 70 million tonnes of nitrogen to the soil - the equivalent to all natural additions of nitrogen - and the ecological consequences are being felt. Old-fashioned farms grew a variety of plants and livestock in a kind of interdependent ecosystem, where there was little waste and few deleterious effects outside the farm. Intensive agriculture has led to a decoupling of sources from sinks - it is easier to dump the manure from a million-strong chicken farm into the river than to cart it many miles to spread on another farmer's wheat-only farm. This would never happen in a natural ecosystem. As ecologist David Tilman of the University of Minnesota writes in Nature, "sustainable and productive ecosystems have tight internal cycling of nutrients, a lesson that agriculture must relearn."

c Macmillan Magazines Ltd 1998 - Nature News Service