Wetenschap - 27 september 2001

Brazilian farmers use trees against soil erosion

Brazilian farmers use trees against soil erosion

Trees can make a difference

Farmlands in south-eastern Brazil are degrading rapidly due to soil erosion. Growing trees in combination with crops might be the solution, says PhD student Irene Cardoso.

Much of the spectacular Atlantic coastal rainforest of Brazil has been lost. Timber companies have destroyed thousands of hectares in the last two hundred years, and farmers have also taken their share. "We cannot get the whole forest back, but we can at least improve the situation by using more trees in agriculture," says Cardoso who is conducting PhD research at Wageningen University.

The agronomist feels close to the coastal region near Rio de Janeiro, where many farmers cultivate coffee, maize and other crops on relatively small plots. Her father was one of them. "This is the region where the Portuguese arrived five hundred years ago. At that time the rainforest was still intact, but now only seven percent is left."

Nutrients

The trees no longer stand in the way of farming, but there is another problem: the soils, although sometimes thirty metres deep, are very poor and they get washed away by the rain. "Together with the soil, the nutrients flow down the hills and the soil gets poorer and poorer."

The farmers might consider using trees to protect the soil, thinks Cardoso. In the past ten years scientists have increasingly come to realise that a way to prevent soil degradation is to plant trees within agricultural fields, in between the crops. Farmers can then do something for the environment without having to compromise their income.

Several types of cropping systems enable farmers to find space for lines of trees. Farmers who grow coffee for example, can grow trees, says Cardoso. Working for the Agricultural University of Vi?osa, in Zona da Mata of Minas Gerais, south-eastern Brazil, Cardoso collaborated with an NGO to examine the effects of farmers' agroforestry experiments in Zona da Mata. Coffee growers experimented with several trees. "It actually worked. There was less erosion. Crop production did not go up, although farmers would like to see that also."

Different trees

We still need to learn a lot more about the effectiveness of different trees in agroforestry systems says Cardoso. Some trees are better than others at fixing nitrogen for example. Also the mycorrhiza fungi growing in the soil and on tree roots are very important because they take up nutrients and give them to the trees.

She examined these aspects more closely in a greenhouse of the Sub-department of Soil Quality at Wageningen University. "When I go back to Brazil, I will work further together with farmers and NGOs and try for example to select the best trees for agroforestry. I have high hopes because I know agroforestry is not only an issue of money for the farmers. They sincerely want to preserve the environment."

Hugo Bouter

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