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[NAIROBI] Heavy 'nutrient mining' — the unreplenished removal by crops of soil nutrients such as phosphorous, nitrogen and potassium — by smallholder farmers has left most soils in Eastern Africa's Great Lakes region infertile and unproductive, a new study has shown.

Investigations carried out from 2007 to 2011 in four agro-ecological regions of Rwanda and south-western Uganda looked at the impact on banana plants.

They revealed that poor soil fertility was responsible for diminishing banana yields during that period, with 5 and 30 tonnes produced per hectare compared to a potential yield of over 70 tonnes per hectare.

Measuring the organic and mineral content of soil nutrients, the researchers found that levels of important soil minerals that sustain plant growth were low, and that the little fertility left was mainly from topsoil organic matter.

The study was carried out by Séverine Delstanche, a researcher at the Catholic University of Louvain, Belgium, and led by the International Institute of Tropical Agriculture (IITA) with funding from the Belgian government.

"Nutrient mining reduces yields, leading to food insecurity not only in rural, but also in urban areas as [lower crop surpluses] will have an impact in the cities, and prices will increase," said Piet van Asten, a systems agronomist at IITA in Uganda.

He said the research had concentrated on bananas because they were crucial for food security and rural livelihoods. There are also no fertiliser recommendations for bananas in the region.

Other studies have focused on the impact of nutrient-poor soil on maize or other crops. "Low soil fertility is a problem for all crops," van Asten said.

Peter Okoth, a soil scientist at the International Center for Tropical Agriculture (CIAT) in Nairobi, Kenya, told SciDev.Net that African farmers cultivate crops without paying sufficient attention to what they are actually removing or returning to the soil.

Smallholder farmers "don't return the nutrients back to the soil because they either are not aware of the change in nutrient dynamics caused by crop harvests, or don't know how to do it," he said.

Okoth added that "governments in Africa need to invest heavily in soil analysis laboratories that are manned by qualified technical people to provide advice on what is needed to make the best [use] of the soils."