THE INFLUENCE OF SOIL CONDITIONS AND FARMING PRACTICES ON THE AVAILABILITY OF HEAVY METALS TO GRASS AND THEIR DISTRIBUTION IN SOIL FOLLOWING THE APPLICATION OF SEWAGE SLUDGE
Report No FR0127
A D BUTTIGEIG, D A KLESSA & D A HALL
To evaluate the influence of soil conditions and farming practices on the availability of heavy metals to grass, and their distribution in soil following the application of sewage sludge.
Grassland is of considerable strategic importance for sludge disposal, and a clearer understanding of the effects of soil conditions and farming practices will provide a sound basis for managing and protecting the outlet.
Liming reduced the uptake of heavy metals by grass whereas fertiliser nitrogen application increased plant concentrations. Ploughing tended to increase plant uptake initially but had the important effect of reducing soil concentrations at the surface.
Liming and cultivation may be used to reduce potential risks to grazing animals and extend the availability of grassland sites for sludge application by reducing the soil concentrations of heavy metals in the surface 7.5 cm.
Application of anaerobically digested sludge, of mixed industrial and domestic origin, to permanent grassland close to Carbarns sewage treatment works for the past 50 years has led to elevated metal concentrations in the soil. Most of the metals applied have tended to persist in the surface layers (0-10 cm) although some movement to greater depth (30 cm) is also evident. This increase is observed not only in the total metal concentrations but also in the proportion of the total metals which was extractable by acetic acid and EDTA. Liming, fertiliser N application and soil cultivation all had significant effects on the metal content of the grass sward.
Liming of the sludged soil resulted in decreased Cd, Cu, Ni and Zn concentrations in the ryegrass sward. In contrast, increasing the rate of fertiliser N application over the range 0-500 kg N/ha per year led to increased herbage concentrations of Cd, Cu and Zn. Similar increases in the herbage concentrations of Cu and Zn as well as Ni were also recorded following soil cultivation and reseeding.
However, this latter effect was of a short term nature (over the first 2-3 cuts), unlike that of fertiliser N and lime which were persistent with time. Chromium and Pb concentrations in the herbage were found to be independent of any of these three management practices.
Cultivation had a second equally important effect, The inversion of the soil resulted in the soil having a lower metal concentration at the surface. This effect is of particular significance to the Pb intake of grazing animals for which soil ingestion is a major source of Pb. Any reduction in the metal load of ingested soil would therefore be desirable.
Irrespective of management practices, metal concentrations in the herbage were well below those thought to give rise to phytotoxic or zootoxic problems.
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