Planting trees that improve soil quality can help boost crop yields for African farmers, an assessment shows. 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
and head of the World Agroforestry
Center. 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.
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 (dertilizer trees systems).
Where the farms are larger, for example in Zambia,
it is possible to rest arable land and allow it to lie fallow, he observed. But
in a place 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 (as pigeon pea).
‘’A lot of the nitrogen was being stored in the trees and
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.’’
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.
However, Place acknowledged that there were a number of
challenges that had to be addressed in order to maximize 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. ut
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.
As well as helping to boost yields, the use of trees in
agriculture has other benefits - such as helping to ‘’climate proof’’
agricultural land. One example 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).
