By Grace Zawadi

Scientists have found farmers in the Central Highlands are suffering a nutrient deficiency that’s causing heart disease, lost pregnancies, and newborn deaths due to the way they fertilise their soils.

The scientists found the farmers and the plants they eat were both short of selenium, which is essential to the human body’s health in fighting off infections, regulating hormones and stopping cell damage. Almost 87% of the children and 97% of the women in the study were found short of the mineral in tests of their hair and diets,  with their main source being plants, which also tested short.

Yet their local soils  often contained selenium. The problem, found the researchers, was that low organic materials and fertility in the soils were stopping plants from taking up the mineral, so it was there in the soil, but the poorly fertilised soil had the wrong chemistry for it to make it into plants . The result has been less selenium in the food families eat every day, increasing their disease and preventing children from developing normally.

To track the problem, researchers from Meru University of Science and Technology traced selenium from the soil where the crops were grown into the food people ate and finally into people’s bodies. They collected soil samples from farms, analysed foods commonly eaten in households, recorded what families had eaten over the last 24 hours and tested samples from 159 children and 111 women aged 19 to 39 years from eight farming communities across Kenya’s Central Highlands.

What they found was that having selenium in soil was not enough. Crops could only benefit if the soil was healthy enough to release it, and, on average, only 1.82% of the selenium in the soil was available for the crops in the Central Highlands to absorb. The rest remained locked in the ground, where it could not reach plant roots. 

It was the condition of the soil that turned out to be the deciding factor in the increased sickness, said the researchers. Soils with higher pH, more organic matter and enough phosphorus released more selenium for crops to absorb. But iron had the opposite effect, trapping the nutrient in forms that plants could not use. 

The study found big differences between different farming communities in this. In some places, crops absorbed far more selenium than in others. For example, maize grown in Ruiri contained 0.047 milligrams of selenium per kilogram, compared with just 0.005 milligrams per kilogram in Mbuyu. Beans, potatoes and amaranth leaves showed the same pattern, showing how soil health can affect the amount of selenium that ends up in crops.

For many families, these crops make up most of their daily meals. Maize, beans, potatoes, rice, bananas, bread and milk are among the foods eaten most often by both women and children. Foods naturally rich in selenium, such as fish, meat and eggs, are eaten much less often, leaving many households with few other sources of the nutrient.

The differences in crops were reflected in people’s diets. Children’s daily selenium intake ranged from 7.6 micrograms in Mbuyu to 23.4 micrograms in Kibirichia. Among women, intake ranged from 4.4 micrograms in Mbuyu to 52.6 micrograms in Kiguchwa (about 55 micrograms per day is considered adequate for adults). Overall, 87% of children and 97% of women consumed less selenium than the amount considered necessary for good health.

Hair samples confirmed that the shortage was not temporary. More than nine out of every ten participants had selenium levels below the recommended reference value, showing that the deficiency had continued over time. About 41% of the children were stunted. While stunting has many causes, inadequate intake of essential nutrients such as selenium may contribute to poor growth and development

The study showed that healthy soils do more than produce bigger harvests. By building up organic matter, maintaining good soil conditions and improving soil fertility, farmers could help crops absorb more selenium and increase the amount that reaches family meals. For farmers in Kenya’s Central Highlands, healthy soil is about more than producing bigger harvests. It also determines how much nutrition people get for good health.

FarmBizAfrica has selected this study as one of its Frontier Science series as an exceptional contribution by Meru University to the health and lives of African farmers and to knowledge in African agriculture.