Heavy metals such as copper, zinc and molybdenum are essential trace elements for plants and animals but excessive concentrations can damage overall soil fertility. Metals such as cadmium and lead can have adverse effects on human and animal health if they are allowed to accumulate in the food chain. The occurrence of high concentrations of heavy metals in agricultural top soils is generally very localized.  The residence time of most heavy metals is long and they tend to be unavailable for plant uptake.

Sources of heavy metals in agricultural land:

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Parent material (underlying geology)

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Mine wastes

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Atmospheric deposition

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Farm manures

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Sewage sludge

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Fertilizers and lime

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Agrochemicals

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Industrial wastes

30 atmospheric deposition monitoring stations throughout the UK found annual deposition rates of:

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Zinc and copper at 250g/ha/year  (1996) compared to:

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Zinc at 1200g/ha/year and copper at 200g/year (Wadsworth & Webb, 1977) representing a large reduction

Maximum permitted total heavy metal concentrations in agricultural soils receiving sewage sludge are laid down in the Sludge Use in Agriculture Regulations 1991:

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Zinc and copper concentrations in sludge decreased by about one third in the decade 1983 to 1993

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Lead concentrations decreased by half over the same period 

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Cadmium decreased by two thirds

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The recommended Nitrogen loading rate (MAFF, 1991) of 250 kg/ha/year, will in most circumstances, set a working limit on nutrient, and therefore heavy metal inputs from sludge

In addition to the heavy metals naturally occurring in UK soils, as a result of their presence on geological materials, conventional farming practices apply metals to soils via:

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Fertilizer additions (mainly cadmium)

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The contribution of animal feedstuffs to manures

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Pig and poultry manures are a valuable source of zinc and copper

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Copper can be of benefit to plants on certain types of free draining chalk soil

Current estimates of Zinc inputs:

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Atmospheric deposition - 50%

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Farm manures - 35%

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Sewage sludge - 8%

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Other sources (fertilizers, agro chemicals, limes and industrial wastes) - 7%

For Copper:

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Atmospheric deposition - 25%

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Farm manures - 50%

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Sewage sludge - 15%

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Other sources (fertilizers, agro chemicals, limes and industrial wastes) - 10%

 

Source: SSLRC, National soil inventory              

MAFF project, University of Reading   

Broad national trends are difficult to monitor, owing to the heterogeneous nature of soils and spatially variable metal sources. However, data collected by the Soil Survey & Land Research Centre from 904 arable and arable/ley farms  suggests the following changes between 1980 and 1995:

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Total top soil zinc had decreased, possibly by dilution, due to deeper ploughing

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Copper had increased on some lighter soils

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Cobalt, cadmium and nickel had remained fairly stable

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Chromium concentrations had only shown significant change in organic soils, where they have increased. This may be due to carbon mineralizing causing concentrations to rise in the remaining soil or by ploughing up chromium rich subsoils  

Heavy metal inputs to UK soils, will result, in the long term, to a very gradual increase in overall levels. As fertilizers (organic and inorganic) are the largest source of contamination it is essential that farmers and growers are aware of all metal inputs to soils and the current levels, and take account of these when planning crop nutrition.

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Pause for thought..........State 2 methods used to dispose of sewage sludge other than on the land and what implications would this have in terms of pollution?

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