Poor soils savage banana yields across EA
By Farmbiz | Mon 10 Sep, 2012

One of the most comprehensive studies of East African soils ever undertaken has confirmed that farming practices have leeched the soil of nutrients without replenishing them, to such a degree as to be halving many key crop yields, threatening the income and food security of over 85 per cent of East Africans.

The results have come in a study of banana yields, at a time when the Banana Association of Kenya reports that poor soils have caused a 40 per cent drop in yield in the last one year alone. This has led to a 10 per cent increase in banana prices that has pushed customers to alternatives and left many of the country's 400,000 smallhold banana farmers with unsold crops.

However, according to the new research, the damage has been far greater than this latest drop. In a study of banana farms in four agro-ecological regions in Rwanda (Butare, Kibungo, and Ruhengeri) and South-West Uganda (Ntungamo) from 2007 to 2011, poor soil fertility was responsible for diminishing banana yields over that period to between 5 and 30 tonnes per hectare compared to a potential yield of over 70 tonnes per hectare.

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

The researchers said the problem was made more severe in that East Africa has one of the lowest rates of fertilizer use in the world and a rapidly increasing population to feed.

"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 Dr Piet van Asten, a systems agronomist at International Institute of Tropical Agriculture (IITA) in Uganda, commenting on the findings.

The barren soils are the result of years of farming with insufficient replacement of nutrients by small-holders, mostly practising low-input agriculture. But the results now threaten future farming and the food and income of millions of people in the region.

The study, which focused on banana farming, as an important crop for the region, in that it provides food and income for over 85 per cent of the population, was led by Séverine Delstanche of the University of Louvain in Belgium.

Delstanche said that despite the acknowledgment by farmers and researchers of the importance of soil fertility in agricultural production, little research had been carried out to understand the current state of soils, and the impact of past and present farming practices. This had left farmers unaware of the nutritional status of their soil and not knowing how best to make use of the little resources available to them to increase production and productivity.

According to Dr Asten, the findings of this research are particularly 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.”

For this reason, the study stresses the importance of recycling crop residues to improve soil fertility, since over 80 per cent of the nutrients in soil come from organic matter and not from the clay or sand itself.

Another study by Van Asten, related to Delstanche's research, shows that the estimated 100 trucks of banana bunches that reach Kampala every day deplete 1.5 million kg of potassium (K) and 0.5 million kg of magnesium (Mg) from the soils in the rural areas each year.

The study corroborates the Africa Union’s Abuja declaration on fertilizers, 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.

Written by Bob Koigi for African Laughter

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