Trace Elements in Soils

In soils, a certain number of elements are predominant or exist in important quantities: oxygen, hydrogen, silicon, aluminium, iron, carbon, nitrogen, calcium and magnesium, sodium and potassium, phosphorus and sulphur, their contents varying widely from one soil to another, Soil element contents vary widely, for example, in a ferrallitic soil of the lower Ivory Coast, rich in kaolinite, and the same, type of soil, essentially composed of iron oxide, in the centre of New Caledonia, or aluminium oxide in some nearby islands. Content also varies from one horizon to another in the same soil profile. Other elements, such as copper, lead, nickel, cobalt, molybdenum, etc, are found only in traces in soils. Nevertheless, the majority of these elements, if not all, play a primary role in plant nutrition and development. Even if it is not usually possible to determine with exactitude the effect of small variations of these elements on plants, at least it is possible, most of the time, to observe the consequences of their deficiency, or their presence in excess, in forms which have a stronger effect on plants or are more available to them. Some of these elements can, because of the nature of the parent rock, or climatic or topographic conditions (pedogenic factors) become very abundant in some soils. This phenomenon can reach a point where the soils, or the horizons of particular soils, are transformed into veritable ores, leading to an industrial exploitation (manganese, nickel, cobalt, etc.). In other cases lesser trace element concentrations can play an importantpart in soil evolution. Therefore, the knowledge of their presence and, also, of their dynamics is very important to pedologists. Although these facts have been known for a long time, trace elements in soils have not been the object of much investigation until recent decades, in spite of the noteworthy work of Gabriel Bertrand, in France, dating from the beginning of the century. This situation may have been due to the difficulties involved in the analyses of some of the elements. In the last thirty years, however, with the development of the use of physical methods in soil analysis, such as spectrography and atomic absorption, trace element research has increased markedly.