Image caption The nitrogen-fixing roots of certain trees provide valuable nutrients to resource-poor arable land

Planting trees that improve soil quality can help boost crop yields for African farmers, an assessment shows.

Fertiliser tree systems (FTS) also help boost food security and play a role in "climate proofing" the region's arable land, the paper adds.

Researchers from the World Agroforestry Centre say poor soil fertility is one of the main obstacles to improving food production in Africa.

The results appear in the International Journal of Agricultural Sustainability.

"In Africa, it is generally agreed that poor soil management - along with poor water management - is most greatly affecting yields," explained co-author Frank Place, head of the centre's Impact Assessment team.

He said that despite chemical fertilisers having been on the market for more than half a century, farmers appeared reluctant or unable to buy them.

"Therefore, there have been a lot of attempts to bring in other types of nutrients from other systems - such as livestock and plants" he told BBC News.

"We have been working quite a lot on what is broadly referred to as 'fertiliser tree systems'."

Although it has been known for centuries that certain plants, such as legumes, "fix" nitrogen in the soil and boost food crop yields, Dr Place said that the centre's researchers had been looking to develop a more active management approach such as FTS.

In TFS across Africa as a whole, yields are doubling or more in two-thirds of cases Frank Place, WAC

"Some farms, for example in Zambia, where the farms are larger, it is possible to rest arable land and allow it to lie fallow," he observed.

"But in places such as much of Malawi, where population densities are higher, they cannot afford to fallow their land; so we came up with alternative management systems where they could intercrop the trees with the (maize)."

While the technique is not new, Dr Place said that some of the nitrogen-fixing species used by farmers were probably not the most effective.

For example, farmers in East Africa had been using Cajanus cajan (also known as pigeon pea).

"A lot of the nitrogen was being stored in the trees' seeds; so there was an effort to use other trees that put a greater volume in the soil, such as Gliricidia sepium (one of its common name is mother of cocoa)," he said.

"A really nice thing about G. sepium is that we have been coppicing some of those trees for 20 years and they still continue to grow back vigorously."

What is 'nitrogen-fixing'? The atmosphere consists of about 80% nitrogen, but plants cannot use it in this form

Certain plants, such as legumes, have bacteria growing in their root hairs that convert it into a form that plants can use

This form of nitrogen is know as "green manure" and is a nutrient that helps plants, such as food crops, to grow (Source: World Agroforestry Centre)

However, he acknowledged that there were a number of challenges that had to be addressed in order to maximise yields.

For example, some systems suggested planting rows of trees between rows of crops with mixed results.

"We realised that there were a few management problems with that sort of system - what tended to happen was that there was too much competition between the crops and the trees," Dr Place explained.

"We developed a new management system where the trees were cut very low to the ground at the time you are planting the crop so then there was no light competition.

"The trees go into a dormant state when you cut them like this, so the root system is not competing straight away for the nutrients, so the maize is free to become established.

"The trees only really start to come out out of the dormant phase when the maize is already tall."

Another challenge was to provide enough seeds in order to have mass-scale planting. He said that balancing the provision of high-quality seeds with large local engagement was another hurdle that had to be overcome.

But the rewards in improved yields were noticeable, he added.

"Some of the studies have shown that in TFS across Africa as a whole, yields are doubling or more in two-thirds of cases."

Where the systems were not delivering such good results, Dr Place said that scientists were looking to refine current practices and modify them to suit the local conditions.

'Climate proofing'

As well as helping to boost yields, the use of trees in agriculture has other benefits - such as helping to "climate proof" agriculture land.

One example, Dr Place said, was the use of Faidherbia albida (common names include winter thorn and apple-ring acacia) in West African arable landscapes.

"It has a deep penetrating tap root, and it can secure a good water supply even in dry years," he explained.

"Generally speaking, tree roots do go much deeper than crop roots, so it is recycling nutrients and water from deeper reaches.

"There are also studies showing that these roots act as conduits and bring up water to surface root systems (such as those belonging to crops)."

The editor-in-chief of the International Journal of Agricultural Sustainability, Professor Jules Pretty from the University of Essex, UK, said the study illustrated that there was a growing movement of agricultural innovations across Africa that were increasing yields and at the same time improving the environment.

"Trees and shrubs in agricultural systems seem to break some of the rules of agriculture - in this case, farmers are using shrubs to create a diverse rotation pattern rather than year-on-year maize," he told BBC News.

"The trees fix nitrogen and improve the soil; the leaves can be fed to livestock; the crops then benefit greatly in subsequent years."