The reaction of anions and cations with soil

Abstract

One of our strongest urges is to classify things. As a result, an article on the chemistry of a particular nutrient often begins with a statement on the forms thought to be present in soil. For phosphate, for example, the forms may be stated as: calcium phosphates, adsorbed phosphates, occluded phosphates and organic phosphates. I want to avoid this approach. There are two reasons for this. One is that I have doubts about the importance of some of the postulated forms. For example, calcium phosphates may be rare indeed in many soils of the world (see later). The other is that the erection of sharp boundaries between postulated categories has a restricting effect on our thinking about the problem. It leads us to think in terms of these categories — for example to think in terms of “available” and “unavailable” nutrients. However, as we shall see, a better picture is that there is a continuum between these two ends of the spectrum. Thus, rather than start with a very simple model of the forms of the nutrients present in soil, I will start with a description of the observed behaviour. We will then be in a better position to consider a model to describe all of this behaviour.