By Michelle Mukonyora
12 February 2016
Farming is a large part of the African livelihood, and it is not only limited to subsistence farmers in rural areas. Urban farming is a major feature of the suburban landscape. Maize, rape and covo (kale) are just a few of the food crops that one may come across on tracts of available land in our cities and towns. With ever increasing rural-urban migration rates accompanied by limited economic growth, urban farming has become necessary as a means of poor families ensuring food and financial security for their families. Urban farming is however not only for the poor. It is becoming increasingly popular amongst those who consume organic food as a lifestyle choice. Urban farming unfortunately comes with the risk of crops absorbing toxic trace metals (TTMs) from activities such as mining, incorrect waste disposal, and exhaust fumes from vehicles.
In biology, there are certain trace metals that living organisms require in minute quantities for normal growth and function. These include zinc, manganese and silicon. TTMs on the other hand are non-essential metals that have found their way into the food chain, and accumulated through human industrial activities. Examples of TTMs are mercury, cadmium and lead. Studies have shown that we consume a significant percentage of these trace metals through agricultural produce. The World Health Organization (WHO) reports that 98% of mercury, 80% of cadmium and 40% of lead detected in human bodies were acquired through food and water consumption.
Foetuses and babies are especially vulnerable to accumulating TTMs from their mothers via the placenta and breast milk respectively. The harmful effects of TTMs on developing babies include lead encephalopathy, which is a dysfunction of the brain. This may be associated in the long term with mental retardation, epilepsy and blindness. Anaemia, kidney dysfunction, high blood pressure, cancer, sterility and abortion are among the consequences of heavy metal poisoning in adults. The full WHO report may be found here.
Plants take up TTMs via polluted soil and air. Air pollutants have been shown to accumulate on stomata openings and in the waxy cuticles of leaves. Thorough washing of vegetable leaves may reduce the amounts of TTMs but this is dependent on the type of vegetable. Those TTMs taken up via the roots of some vegetables are capable of being translocated to and accumulated in the leaf tissues. Vegetable species belonging to the genus Brassica are among over 400 plant species capable of accumulating TTMs in their leaves. Examples commonly grown in Zimbabwean urban farming include cabbage, rape, and covo (kale). A popular leafy vegetable in South Africa, morogo (wild spinach), which belongs to the genus Amaranthus, is another example.
A peer reviewed review article published in the South African Journal of Science, “Urban farming as a possible source of trace metals in human diets” by Olowoyo and Lion (2015) may be found here.