Intercropping

From http://www.scidev.net/News/index.cfm?fuseaction=readNews&itemid=3751&language=1
"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.
..........
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."