Geochemical aspects of the soil-plant-animal relationship in the development of trace element deficiency and excess.

I t is well recognized that the trace element content of soils and associated grassland or forage may, under certain conditions of farming practice, influence the health of livestock and the economics of agricultural production. There is also evidence, though mainly empirical, that disease in man may at times be related to the trace metal status of the environment in which he lives. The prime source of trace elements in soils is the parent material from which they are derived. In the UK this is largely weathcred bedrock or transported overburden of relatively local origin. The soils developed from these materials tend to reflect their chemical composition. Thus soils developed from acid igneous rocks, such as rhyolites and granites, and from coarse-grained arenaceous rocks, usually contain smaller amounts of the nutritionally essential trace elements than those developed from basic igneous rocks and fine-grained sediments. Trace element deficiencies in grazing livestock, particularly of copper and cobalt, are frequently reported on the former soils, while problcms due to excess are sometimes found on the latter. This relationship between animal health and bedrock geochemistry does not apply in those parts of the world where appreciable areas of soils have been developed from wind-borne, water-borne or laterized materials or from old weathered surfaces bearing little compositional relationship to the underlying rock. Similarly, in parts of the more advanced countries, the trace metal content of soils and plants may be considerably enhanced by contamination from urban (Purvis, 1972) and industrial sources, including past and present-day metalliferous mining (Alloway & Davies, 1971 ; Thornton, 1974~) . Examples of trace element problems in animals related to the composition of soils or plants or both are numerous and worldwide (Russell & Duncan, 1956; Underwood, 1971) and it is not the purpose of this paper to review them. A typical example is the problem of molybdenosis or ‘teartness’ in cattle initially recognized in an area of some 8100 ha in south-west England and attributed to excess molybdenum in soils and pastures, with concentrations of 15->100 pg Mo/g in ‘teart’ compared with 3-5 pg/g in healthy pastures (Ferguson, Lewis & Watson, 1943; Lewis, 1943). The source of the Mo was ascribed to the clay component of the Lower Lias formation, and calcareous soils derived from this material containing 20 pg Mo/g or more were described as potentially ‘teart’ and similar soils with only 2-3 pg/g as ‘non-teart’ (Lewis, 1943). Later geological studies showed