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Greener is greener. Tomatoes grown with organic mulch and less chemical fertilizer (right) do better than those raised under more conventional conditions.

Fewer Chemicals, Better Tomatoes?

Replacing chemical with biological fertilizers may extend crop growth and ward off disease, a new study suggests. The finding, from experiments on tomatoes grown with a legume mulch, posits that the biological fertilizers alter gene expression to make crops more robust.

Agricultural output rose dramatically in the 1950s and '60s thanks to the high-yielding techniques of the "green revolution." But the techniques rely on chemical fertilizer and pesticides, which can harm human health, water quality, and wildlife. Some farmers and scientists have looked for ways to replace the chemicals with biological sources of fertilizer such as cover crops that can be plowed under or used as mulch to provide nutrients. While supporters claim more vigorous growth and pest resistance, critics point to a lack of a scientific explanation for that improvement.

To investigate, plant biochemist Autar Mattoo and colleagues at the U.S. Department of Agriculture's Agricultural Research Center in Beltsville, Maryland, grew tomatoes under two types of field conditions. The conventional condition included black plastic row covers for weed control and a full dose of fertilizer. The other tomatoes got a mulch of hairy vetch, a legume that provides nitrogen and controls weeds, and a half dose of fertilizer. The researchers sampled leaves three times during the season and screened for genes whose expression differed under the different growing conditions.

Leaf death and disease onset in tomatoes grown with the legume was delayed by 2 weeks compared with tomatoes grown conventionally. And legume-grown tomato leaves expressed higher levels of nitrogen uptake genes, such as rubisco and glutamine synthetase, and defense genes, such as chitinase and osmotin, the authors report online this week in the Proceedings of the National Academy of Sciences. Mattoo says the research is another prod for farmers to investigate alternative techniques, which he says could increase yields by up to 20%, at least for tomatoes.

"The study has to be done in a lot more systematic manner," says Kulvinder Gill, a crop geneticist at Washington State University in Pullman, who faults the paltry data and statistics presented. Even so, Gill says he'd be surprised if the findings are not borne out by larger, more tightly controlled experiments.

Related sites
Mattoo's site
Gill's site