TRACE elements, micro-nutrients and minor elements are terms applied to a number of chemical elements which are essential for the lives of plants and animals, but which are required in extremely small quantity. The development of our knowledge of the part played by the trace elements in the life of plants and animals is very recent, nearly all our present information on the subject having been acquired during little more than the last twenty years. Nevertheless, in the course of that time a great many observations have been made in the field, and much experimental work carried out in both field and laboratory, while many hundreds of publications, some of slight value, others of considerable scientific and economic importance, dealing with trace elements in living plants and animals have appeared in the scientific press. The time thus seems ripe for the presentation of a digest of this information, so that the salient facts may be available in a convenient form, and the present position of our knowledge of trace elements in living organisms made plain. It is with this intention that the present book has been written.

The role of the trace elements in organisms is, in the first place, a matter for the plant or animal physiologist, but as deficiency or excess in the supply of the various trace elements may lead to a diseased condition of the plant or animal with serious economic consequences, the trace elements are also of interest to the plant pathologist, the veterinary surgeon, the agriculturist and horticulturist. Indeed, as regards plants at any rate, much more is definitely known of the plant pathological aspect of the trace elements than of their physiological functions.

I would take this opportunity of expressing my thanks to Mr W. Morley Davis of Harper Adams Agricultural College, who first introduced to me the effects of trace-element deficiencies in the field, and who has generously and constantly put at my disposal both his knowledge of the pathological effects of trace-element deficiency, and the pathological material itself.