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[BEIJING] Chinese scientists have discovered why an agricultural practice called 'intercropping' increases crop yields.

The discovery could help farmers cut down the amount of chemical fertiliser they use on their crops, reducing chemical pollution in soils.

The practice of intercropping — which Chinese farmers have practised for thousands of years — involves growing two or more crops in alternate rows in the same place and at same time, and can greatly increase grain yields.

In many intercropping practices, legumes are planted with crops. The legumes fix nitrogen in the soils, which then fertilises the crops grown with them.

But other benefits of legumes in intercropping are not clearly understood.

Li Long, Zhang Fusuo and colleagues at China Agricultural University looked at below-ground biological interactions between faba bean and maize. Their research was published last week (3 July) in the Proceedings of the National Academy of Sciences.

They carried out field trials in the western Chinese province of Gansu over four years, and showed that intercropping with faba bean increased the maize yield by an average of 43 per cent.

"The benefits are obvious when they grow together. The underground biological processes play an important role in yield increase," Li told SciDev.Net.

The researchers found that the roots of the faba bean plant released organic acids into the soil, which increases the solubility of inorganic phosphorus, a plant nutrient. Plants take up soluble phosphorus more readily, which explains the increase in the crops' yields.

Enzymes released by the faba bean plant into the soil also decomposed organic phosphorus into an inorganic form, which could then be used by both plants.

Faba bean yield increased by 26 per cent due to more available phosphorous, its roots being a different length to those of maize, and the crops having different growth seasons.

The greater yields and more efficient land utilisation resulting from intercropping could be a significant factor in meeting the increasing global food demand, said Li.

Shen Qirong, a professor in plant nutrition at Nanjing Agricultural University, told SciDev.Net that by intercropping, farmers could cut down on the amount of phosphorous fertilisers they use, as intercropped plants have more phosphorous available to them.

Shen said that at present, the "use of phosphate fertilisers is only 15 per cent and the remaining 85 per cent stay in soils uselessly."

Link to full paper in the Proceedings of the National Academy of Sciences

Reference: Proceedings of the National Academy of Sciences 104, 11192 (2007)