Roadside cultivation - a heavy metal health hazard?

Nearly three-quarters of Ugandan vehicles are found in Kampala. Most run on leaded petrol and at peak times the city's main arteries bear a steady stream of traffic. In such an environment, when Kampala City Council took the decision in 2003 to legalise urban agriculture, they faced a hostile media reaction. How could crops grown in such polluted air be safe to eat?

Research in other cities has found that urban soils often contain high concentrations of heavy metals, including lead, cadmium and zinc. Lead is particularly dangerous, since even small concentrations in the blood can seriously affect children's mental development. A variety of sources can contribute to lead contamination but fuel exhausts are the most serious source. A
direct link has been proven between distance from roads and heavy metal concentrations in soil.

A study conducted in Kampala revealed that media concerns were not misplaced. Soil, plants and flat surfaces at 11 roadside sites around the city were tested. In three sites - those with heaviest traffic flows - heavy metal concentration in the soil was found to exceed safe limits for agriculture. In addition, leafy vegetables grown near the road at all sites exhibited dangerously high concentrations of lead.

Tests across a range of plants revealed heavy metal concentrations to be highest in the plant leaves, followed by roots, fruit, and finally the seed. The research team established that in leafy vegetables, such as Amaranthus, contamination was primarily via atmospheric deposition, rather than uptake through plant roots. Some of the contamination was in the form of a surface film which could be washed off, but most of the contaminants were found in the leaf tissue.

The research findings have helped to develop new ordinances for urban agriculture in Kampala. Crop choice and site selection have been key issues for discussion. Maize, for example, is recommended as safe for roadside cultivation. The grain is protected from exhaust fumes by the outer leaves, and only insignificant amounts of heavy metals reach the grain via the plant roots. Legume crops such as peas and beans are also recommended, although the outer pods should not be eaten. Similarly, tubers such as cassava and sweet potato should be peeled prior to cooking if they are grown on roadside land.

The new ordinances, which include a provision that crops should not be grown along busy highways, have now been formally adopted by the City Council. The research team has made a number of other recommendations to further reduce risks. These include a zoning exercise to delineate areas suitable for cultivation, roadside planting of hedges and trees to minimise the spread of airborne pollution, and that leafy vegetables should not be grown within 30 metres of a road. However, such steps will require considerable planning, research and funding to become a reality.

New Agriculturalist