Food and AgriCultural Transformation — August 23, 2017

Healthy soils and healthy humans: Soil Micronutrients drive health in poor, agricultural countries

One Health is more than a trending abstraction for grants and NGO programs; micronutrient-deficient soils grow lower-nutrient food and drive micronutrient malnutrition in populations reliant on them for food.

In marking 2015 The International Year of Soils, and establishing December 5 as World Soil Day, the UN Secretary-General Ban Ki-moon noted that, “A healthy life is not possible without healthy soils.”

This statement is not merely an empty platitude, just as “One Health” – a catchphrase for multidisciplinary efforts that seek to simultaneously address environmental, human and human health – is more than a trending abstraction for grants and NGO programs. In one specific and measurable dimension at least, the health of soils very truly and directly impacts the health of the human beings living on them: micronutrient-deficient soils grow lower-nutrient food and drive micronutrient malnutrition in populations reliant on them.

This observation is not entirely new. In the 1960s, scientists recognized that spontaneous illness in New Zealand and Australian livestock occurred while grazing in low-selenium pastures. Keshan disease, a life-threatening human illness marked by cardiac enlargement and other cardiac abnormalities, is also driven by selenium deficiency, and occurs only in a belt of severely selenium-deficient soils in China. In the 1960s, Dr. Ananda Prasad hypothesized that a mysterious syndrome in Iran marked by dwarfism and reduced cognitive capacity was driven by zinc (Zn) deficiency, a product of low Zn content in local soils and crops. By the 1990s, further evidence had proved him correct.

Yet these examples are admittedly rare and extreme, effecting only tiny pockets of humanity. In contrast, when public health experts and economists lament the health and development implications of micronutrient deficiencies, they are referencing pervasive malnutrition --- deficiencies that impact entire regions. For instance, we think that 60-70 percent of Asia and sub-Saharan Africa are at risk of Zn deficiency, and 4 percent of global morbidity and mortality in children under 5 is due to Zn deficiency.

These widespread deficiencies, collectively known as “Hidden Hunger,” are often mild and all but invisible – yet in children they increase rates of diarrhea and respiratory infection, and retard physical growth and cognitive development. In adults they drive up the risk of cancer, autoimmune system diseases and reproductive problems, and they reduce educational achievement and earnings. On a regional level, micronutrient deficiencies almost certainly hamper economic growth and economic development.

Recent evidence suggests that soil micronutrient deficiencies contribute to this type of regionally pervasive, low-level micronutrient malnutrition too, not just the extreme cases. And this means that we should all --- economists, politicians, doctors, public health experts, and farmers --- be paying attention.

For instance, soil Zn deficiency is widespread in India --- 48 percent of soil samples and 44 percent of plants samples collected in states across India were Zn deficient, with soil deficiency rates much higher in particular districts, as shown in the map below. Wheat grain Zn content averages 22.3 mg/kg in low soil Zn areas and 53.2 mg/kg in high Zn areas of India. Meanwhile, 25 percent of the Indian population is at risk of Zn deficiency. Nearly half of Indian children are stunted, a condition that often indicates Zn deficiency.

Singh (2008) notes that in Andhra Pradesh, Zn status is higher in areas with Zn-sufficient soils and lower in areas with soil Zn deficiency. Singh discusses agricultural management strategies designed to increase Zn availability in soils, and argues that more research is needed on the interaction between soil micronutrients and micronutrient malnutrition in India. Cakmak (2009) follows by arguing that immediate Zn enrichment of Indian fertilizers will greatly benefit public health in the country.

In Africa, too, evidence is building that Zn deficiency in soils drives human zinc deficiency. Chilimba et al. (2011, 2012) show that maize grown in low-Zn soils puts semi-subsistence Malawian farming families at risk of Zn deficiency. Manzeke et al. (2012, 2014) find Zimbabwean soils low in available Zn, and show that manure application or Zn-enriched fertilizer increase the Zn content of cultivated crops. In East Africa, researchers at the One Acre Fund find staple crops to be highly responsive to micronutrient-enriched fertilizers, indicating widespread soil deficiency in multiple micronutrients. And according to a recent soil fertility mapping exercise by the Ethiopian Agricultural Transformation Agency, over 50 percent of Ethiopian soils are zinc deficient. A growing number of experts call for Zn enrichment of fertilizers in Africa, as a solution to widespread human Zn deficiency.

But let's be honest: we do not truly know the extent to which soil zinc deficiency or other soil mineral deficiencies impact human health in poor, agricultural societies. And this is largely because the measurement of both soil micronutrients and human micronutrient status is difficult, time-consuming, and expensive. However, new technologies like Near-Infrared Spectroscopy and improved geospatial data on soilsmay soon provide much better global data on soil mineral content. If we turn our mind to it, we might be able to manifest a second, oft-empty platitude: a “win-win” solution, where soil-level interventions simultaneously raise the mineral content of soils to increase agricultural output, and improve the nutritional status of poor, agricultural populations worldwide.