|Bananas growing near a homestead. The crop's yields in Eastern Africa are low due to poor soils and low fertilizer - organic or inorganic-application by small-holder farmers.|
Most of the soils in the Great Lakes region of Eastern Africa are poor with very little fertility left in them. This is one of the main reasons behind the low yields in the area, which has one of the lowest rates of fertilizer use in the world and a rapidly increasing population to feed, according to a recent study.
The barren soils are a result of years of mining and insufficient replacement of nutrients by small-holder farmers mostly practicing low-input agriculture. They remain a threat to the future of small-holder farming and the food and income of millions of people in the region if appropriate action is not taken.
The study, which sought to identify and rank the constraints faced by small-holder banana growers in the region, also measured the actual nutrient content left in the soils―in both the organic and mineral part―across different agroecoregions in Rwanda and Uganda.
This was after establishing that poor soil fertility was one of the main causes of the current low banana yield of 5-30 tons/year against a potential yield of over 70 tons/year. It accounted for up to 50% of the yield gap.
The study found that there was little fertility left in most soils and what was there was mostly due to the organic matter in the topsoil. Furthermore, while banana is a very important crop for the region, providing food and income for over 85% of the population, the use of external inputs such as fertilizers was virtually nonexistent and soil fertility was mostly managed by recycling local organic residues.
The study was conducted from 2007 to 2011 in four agroecological regions in Rwanda (Butare, Kibungo, and Ruhengeri) and South-West Uganda (Ntungamo) and looked at the banana plants, various crop management practices, pests and diseases, and the chemical properties of soils.
It was undertaken by Séverine Delstanche as a PhD student of the University of Louvain in Belgium and formed part of the Belgium (DGD)-funded project titled 'Sustainable and profitable banana-based systems for the African Great Lakes Region' led by the International Institute of Tropical Agriculture (IITA). It was also part of the Consortium for Improving Agriculture-based Livelihoods in Central Africa (CIALCA) project.
The study further established that soil fertility and banana productivity varied not only between the regions but also within regions, villages, and at the farm level. The regional differences were due to differences in soils and their parental material whereas at the farm level, they were often the result of differential management practices. The soils close to the homestead were more fertile due to kitchen wastes of ashes and organic residues.
Delstanche says that despite the acknowledgment by farmers and researchers of the importance of soil fertility in agricultural production, little research has been carried out to understand the current state of soils and the impact of past and present farming practices.
Therefore, she says, farmers are unaware of the nutritional status of their soil and how best to make use of the little resources available to them to increase production and productivity.
Dr Piet van Asten, IITA systems agronomist, says the findings of this research are very significant. "We knew that our soils were poor but we did not know just how poor. But now, we've calculated the nutrient stocks and have learned that very little nutrients are left. Moreover, the soil fertility almost entirely depends on the organic matter in the soil."
"The study therefore stresses the importance of recycling crop residues to improve soil fertility. Over 80% of the nutrients in the soil comes from the organic matter and not from the clay or sand itself."
A related study by Van Asten and his team estimated that the over 100 trucks of banana bunches that reach Kampala everyday deplete the soils in the rural areas annually of 1.5 million kg of potassium (K) and 0.5 million kg of magnesium (Mg).
The study supports the Africa Union's Abuja declaration on fertilizers for an African Green Revolution which has stated that efforts to reduce hunger on the continent must begin by addressing its severely depleted soils and recommends countries to increase fertilizer use from the current 8 tons/ha to at least 50 tons/ha by 2015 to boost agricultural production.