Distribution constants for Cd and Pb were measured for 33 temperate soils. The distribution constants were related to soil organic matter and pH at three different ionic strengths of soil extractants. The soil extractants consisted of solutions of CaCl2, NaCl, and KCl and were taken to be representative of the composition of the soil solution phases. For each ionic strength a significant log‐log correlation was found between the distribution constants, related to organic matter content of the soil and the hydrogen ion concentrations of the soil extracts. Exchangeable or mobile fractions of Cd and Pb in the soil were calculated from the product of distribution constant and concentration in a soil extract. With the aid of adsorption data of previous work with other soils, assuming a similar relationship as found for Cd and Pb, comparable results were obtained for Cu and Zn. Exchangeable fractions were found to lie in a range of about 1 to 5% of total metal in the soils for Pb and about 10 to 50% for Cd, Zn, and Cu.

In five consecutive years lettuce, spinach, spring wheat, endive and maize were grown in pots and the effects of 'native' and soil-applied Zn and Cd on plant Zn and Cd concentrations were studied. The normal interactive pattern was antagonistic, Zn reducing plant Cd uptake, and conversely, but less so. Only in loam soil Zn and Cd were synergistic to some extent, plant Zn uptake increasing with applied Cd.When relating total soil Cd/Zn to plant Cd/Zn separate sets of data could be distinguished for loam and sandy soil, each fitting a straight line. The use of 0.1 M CaCI 2 instead of 'total' extractable soil Cd/Zn makes the two sets of data to coalesce around a single straight line. All crops were found to show a positive linear relationship between 0.1 M CaCl~-extractable soil Cd/Zn and plant Cd/Zn.

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Lolium multiflorum, spring wheat cv. Adonis, carrots and spinach were grown in the field and in a dust-free growth chamber in pots containing soil labelled with 109Cd and 210Pb. The contribution of atmospheric Cd and Pb deposition to plant concn was examined by isotope dilution. Atmospheric deposition contributed 73-95% to the Pb concn of leafy material of L. multiflorum, carrots, spinach, wheat grain and wheat straw. Cd deposition was only significant in wheat grain and straw (21-48%). (Abstract retrieved from CAB Abstracts by CABI’s permission)

Crop uptake of heavy metals from polluted river clay soils is shown to be reduced by covering the polluted soil with a layer of unpolluted clay soil. Plant experiments have been performed to determine the thickness of such a layer required either to comply with permissible levels for metal concentrations in foods and feeds, or to exclude any effect on plant metal levels. The experiments included cover layers up to 0.7 m and 1.6 m, respectively. Crops grown included cereals, potatoes, sugar beet, maize and various vegetables. Protection of all food crops tested against exceeding permissible levels for cadmium requires a clean topsoil of over 1.6 m; for individual crops ranging from zero (no cover layer required) for red cabbage, leek, onion, potato) to 1.2 m-1.6 m for celery tuber and leaf. Results for feed crops were variable: required topsoil depths for maize range from 0.25-l.2 m, and for wheat straw from 0.55 to 1.6 m. No-effect depths calculated for Cd, Cu and Zn demonstrate that i, many experiments the effect of the polluted soil may be observed at all topsoil depths tested. Heavy-metal concentrations in the soil profile, measured after completion of the experiments, showed no significant migration of metals from the polluted soil into the cover soil.