In areas of sub-Saharan Africa, the diet consists largely of maize, which naturally contains low concentrations of zinc. Photo: Shutterstock
The population of sub-Saharan Africa is growing fast, but feeding all those people is a challenge. One problem is that the soils, and therefore the crops grown in them, contain insufficient nutrients. The soil biologist Mirjam Breure investigated whether fertilisation with micronutrients such as zinc and boron can help increase crop yields and food quality.
Most people associate malnutrition mainly with food shortages, but hidden hunger is also a problem in sub-Saharan Africa: people consume enough calories but they don’t get enough nutrients such as zinc. This can lead to serious health problems, says soil biologist Mirjam Breure of Wageningen University & Research. Breure: “Around forty per cent of the population of sub-Saharan Africa probably don’t get enough zinc. Zinc deficiency can lead to growth and developmental disorders in children and it negatively affects resistance to all kinds of infections. There are estimates that globally zinc deficiency accounts for the deaths of 450,000 children under the age of five annually.”
Various factors are responsible for zinc deficiency. “On the one hand, it is caused by an insufficiently varied diet. In some parts of the countryside, the diet consists largely of maize. Grains such as maize naturally contain low concentrations of zinc. However, another reason for the hidden hunger is the low quantities of zinc in many soils. As a result, there is relatively little zinc in the crops people eat.”
Zinc, boron, copper and iron
One of the solutions being considered to increase yields and improve food quality is fertilisation of the crops with micronutrients. Breure: “Plants need various nutrients to be able to grow well. Nitrogen, phosphate and potassium are what are termed macronutrients; plants need them in relatively large quantities. You can control that using manure, compost and artificial fertiliser. Micronutrients are trace elements such as zinc, boron, copper and iron. Plants need them too, but in smaller quantities.”
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Zinc deficiency is responsible globally for an estimated 450,000 deaths annually of children under the age of five
A lab analysis of tropical soils. Photo: Mirjam Breure
Average maize yields are only two tons per hectare in sub-Saharan Africa, compared with six tons per hectare elsewhere in the world. Ghana, Ethiopia and Kenya are now recommending fertilisation with micronutrients to increase yields, says Breure. “But not much research has been done to back up these recommendations. For instance, it is not known which regions or soil types need such fertilisation to achieve higher yields.”
The soil scientist and fellow researchers at the Wageningen Institute for Environment and Climate Research (WIMEK) have therefore sought to develop models that can predict the availability in soils of zinc and of boron, another trace element, which is essential for growth. Breure: “In combination with soil maps, such models could help significantly in identifying regions where fertilisation with micronutrients can boost yields and lead to higher concentrations of zinc in maize.”
Field trials
Field trials in Kenya, Zambia and Zimbabwe involving fertilisation of maize with zinc and boron were used to see whether the bioavailability of these micronutrients – the availability in the soil – could be predicted using soil properties such as the pH and organic matter content. “Such properties have a big influence on the availability of a variety of nutrients in the soil. To develop models of the availability of micronutrients, we therefore selected locations that differ in these soil characteristics. We also specifically looked for locations that had very low concentrations of zinc and boron in the soil.”
However, the effect on yields of fertilisation with micronutrients was disappointing. “Fertilisation with zinc increased yields in only one location in Zambia and one location in Zimbabwe, while boron had no positive effect at all. We even saw a decline in crop yields in some places with boron because we had applied too much fertiliser.”
A collection of grain samples from Zimbabwe. Photo: Regis Chikowo
A field trial in Zambia with zinc. Photo: Mirjam Breure
The fertilisation with zinc did have an effect on zinc concentrations in the maize: zinc levels in the maize were higher after fertilisation in about half the locations. “But that increase is probably not enough to have a significant impact on human health,” says Breure. “The models developed by my colleague, soil chemist Elise Van Eynde, show that zinc concentrations in maize are related to zinc concentrations in the soil. The soil could explain about 25 per cent of the variation in zinc concentration in the grain. We found that other factors were also important, such as the maize variety and local growing conditions. If we take them into account too, we can explain about half the variation in zinc concentrations in maize.”
No quick fix
It therefore seems as if fertilisation with micronutrients cannot offer a quick fix as yet for the problem of malnutrition. Figuring out the effect of fertilisation on yields based on soil characteristics remains a challenge. “Contrary to expectations, our trials show that it is difficult to identify micronutrient deficiencies in crops. Unfortunately, that means we still don’t know whether fertilisation with zinc and boron has the potential to increase yields.” The team thinks that other measures could be more effective in boosting yields. “Many farmers in Africa hardly use any organic fertilisers, so there is a lot to be gained there. Fertilisation with macronutrients such as nitrogen, phosphate and potassium alone could double crop yields.”
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Contrary to expectations, we found it is difficult to identify micronutrient deficiencies in crops
Maize yields are weighed at a field trial in Kenya. Photo: Mirjam Breure
While fertilisation with zinc might improve human health, Breure does not see this as the only option for tackling zinc deficiency in humans. Breure: “Fertilising crops with micronutrients costs the farmer money but crops with higher zinc concentrations don’t fetch a higher price on the market. So there’s no financial incentive for farmers to apply zinc fertiliser. And our research has shown that the increase in zinc concentrations in maize is marginal.” Here too, other measures might be more effective. “Plant breeding could be used to develop crops with higher zinc concentrations. But perhaps even more important is diversification in people’s diets. Animal products such as meat and eggs contain more zinc than plants, and there are big differences between crops too.”
While the research results have given scientists a better understanding of the complexities of fertilisation with micronutrients, Breure is disappointed that fertilisation with micronutrients such as zinc and boron did not have the desired effect: “The fight to end hunger and poverty in Africa has interested me ever since I was young. It is one of the reasons I specialised in tropical arable farming. Africa has a population of 1.4 billion and 20 per cent of them are malnourished. It would be great if we could do something about that by getting a better understanding of the availability of nutrients in the soil.”
Online
Read more about the Micronutrients for better yields project
Read more about the research of the Wageningen Institute for Environment and Climate Research (WIMEK)
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