Soil and Herbage Heavy Metal/ Trace Element Variability and Relationships at Farm and Regional Level.

Abstract

Variability of heavy metal trace/element amounts in soil and herbage from 3 farms encompassing 26.5, 33.6 and 21.6 ha and in a 230 km2 surrounding area has been estimated. Variability was least in the farm with the greatest soil physical uniformity. It increased in line with increasing sampling area. In soil, variability was least (CV 10%) for sesquioxide metals, iron and aluminium, and elements chromium and nickel. It was highest where there was evidence of geochemical contamination with cadmium and selenium. In general, variability was higher (>20% CV) for extractable components including copper, zinc and manganese. Regression analysis of elements in soil showed up a number of useful associations particularly for the volatiles selenium, sulphate sulphur, mercury, cobalt, vanadium and fluorine with soil organic matter. Associations were also found between fluorine, vanadium and chromium. • Elements in herbage exhibited two types of associations (i) those reckoned to be within the plant and confined mostly to major elements and (ii) those involving metals associated primarily with soil and indicative of the extent to which herbage was contaminated by soil. • Elements that showed lower variability in soil exhibited large variability in herbage. These included iron, aluminium, chromium, vanadium and fluorine, where the cause was attributed to contamination of herbage by soil. It was suggested that iron could be used almost equally as well as titanium as an indicator of the degree to which herbage has been contaminated by soil. • Long term monitoring of herbage for selenium content highlighted the unreliability of relying on a single determination to characterise the selenium status of pasture.