Pigeon peas improve soil fertility

Maize is a staple food in Malawi. It is the country’s most important food crop and is widely grown by subsistence farmers. But crop yields tend to be mediocre in many regions, notably due to a lack of phosphorous and nitrogen.

Following a devastating drought in 2005, the Malawi government started a fertiliser programme aimed at boosting maize yields nation-wide. After some years of a dramatic increase in crop yields, maize productivity stagnated and even declined. Experts say that the use of artificial fertilisers is very inefficient because the phosphorous quickly binds to metal ions such as iron and aluminium, so that the nutrient is no longer available to the plants.

Simple but effective alternative

This was the initial situation upon which, Gina Garland, a Ph.D. student at the Swiss ETH in Zürich, started her studies for her doctoral thesis. Garland looked for an alternative long-term solution to solve the problem of the low phosphorous content in the soil. She came up with a relatively simple and low-cost approach: changing the method of cultivation. Growing two or more types of basic food crop in close proximity – intercropping – improves the soil structure, which in turn has a beneficial effect on soil fertility.

For her field trial, Gina Garland opted for a mixed crop of maize and pigeon pea (Cajanus cajan). This legume grows in symbiosis with rhizobia: bacteria with the ability to fix atmospheric nitrogen. The nitrogen also benefits other plants growing nearby. Earlier studies indicate that the pigeon pea can have a positive effect on the soil’s phosphorous content.

In field trials in Malawi and experiments at the ETH greenhouse, the researcher could prove that, intercropped with maize, the pigeon pea plant had a positive effect on the soil structure after just one growing season: the soil aggregate or “crumb” had definitely improved. These soil crumbs retain nutrients and prevent them from binding with metal ions. For the plants, this means that phosphorous in particular is more readily available for longer periods. The soil samples that Garland took from the polycultures showed much higher concentrations of phosphorus, and organic phosphorus in particular, than the maize monocultures.

More nitrogen for the maize plants

However, maize biomass turned out to be no greater than when maize is grown on its own. “One season is likely not enough to produce a significant increase in yields,” says Garland. But there is still potential for boosting crop yields in future, particularly as the maize plants grown as an intercrop exhibited a higher nitrogen uptake thanks to the pigeon pea and its nitrogen-fixing bacteria. “This is a very promising outcome, as our trial shows that nitrogen enrichment occurs very soon after intercropping is introduced,” Garland stresses.

The pigeon pea has an additional benefit: the peas are edible and enhance the diet of the Malawi population. Since its pea pods ripen between the two maize harvests, they also fill a gap in the food supply. Pigeon peas are very hardy plants capable of withstanding drought, Garland says.

(ETH/wi)