Research

Paul Alexander and Brian J. Alloway

Areas in the UK are known to have elevated soil metal concentrations, particularly some urban areas. With the gradual urbanisation of the country and the need to build on land that has been used previously for industrial sites, householders and professionals are becoming more aware of low-level contamination of soils.

These metals can derive naturally, but are often augmented by, for example, atmospheric deposition, previous site use, use of contaminated soil to level a site, metal garden containers and furniture, bonfires, paint, composts, pesticides and fertilizers. The burning of fossil fuels, smelting and other processing techniques release these chemicals into the atmosphere, which are later deposited on soil and vegetation. Widely used soil additives, superphospate and lime, contain small quantities of cadmium, copper and zinc as well as other metals. Cadmium is also used in plating metals and in the manufacture of batteries. Lead comes largely from automobile exhaust fumes deposited mainly within 10 m of major roadways and near urban centres. The concentrations of some of these metals is increasing, while others such as lead is decreasing due to stricter legislation.

The concentration of heavy metals/metalloids in the soil is an important issue with regards to human health. Ingestion of vegetables grown in contaminated soil may pose health issues. The accumulation of metals varies greatly both between species and cultivars. Soil applied heavy metals are not readily absorbed by plants. Generally plants translocate larger quantities of metals to their leaves than to their fruits or seeds.

There is evidence to suggest that vegetable cultivars vary in the uptake of pollutants. Thus investigating which vegetables and cultivars accumulate higher and lower concentrations, under controlled conditions, could present useful information, in order to be able to make recommendations as to the suitability of the use of various cultivars on slightly contaminated land.

Since 1998 the RHS, in conjunction with Reading University, has grown five cultivars of spinach, pea, lettuce, French bean, onion and carrot in spiked soil. Salts of four heavy metals; cadmium, lead, zinc and copper were mixed with soil and the vegetables planted into this or into uncontaminated soil from the same source, in pots, to avoid contaminating the surrounding soil. The edible portion of the plant was sampled, dried and analysed.

The results of the screening so far have clearly indicated differences between the tested vegetables. In general, lettuce accumulated elevated concentrations of cadmium, lead and copper; spinach of cadmium, copper and zinc; carrot of lead and onion of cadmium and lead. Pea and French bean generally accumulated much lower concentrations of all four metals.

Apparent differences between cultivars of some vegetables were also shown. Significant differences between cultivars were found in carrot for cadmium, copper and zinc; lettuce for lead; spinach for cadmium and copper and pea for cadmium and copper.

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