Thisbook isabout ions,about variable-chargesurfacesand about models. Jti~about ions because most of the substances in which soil scientists are interested occur in the soil solution as ions. This applies to both plant nutrients and pollutants. Thus the,reilction between soil and say phosphate does not involve a substance called "phosphate"; it involves phosphate ions. Ions are charged particles. When these charged particles react with charged surfaces, the outcome of the reaction is affected by both the charge on the particles and the charge on the surfaces. Hence we also need to understand the charge on the surfaces. If our understanding is adequate, we should be able to express our ideas precisely - ideally bywritingequations. Unfortunatelytheequationsturnoutto bequite complex and to interrelate with each other in a complex way. Ifwe want to envisage the effect of varying some of the conditions we have to include the equations in,computer programs. Because these programs describe physical systems, they are called models. Aword aboutthe roleand function ofmodels is in order. Acomputermodel is no more than a precisely expressed hypothesis. Like all hypotheses, it can be used to make predictions. The predictions from some hypotheses turn out to be better than those of others and we are to prefer the one that makes the best predictions. However, in practice, old ideasarenotabandoned readily. We clingtothem untilthe new ideas becomefamiliar. Forexample,someofthemodelsthat have been used to describe ionreaction with surfaces have a pleasing familiarity about them.
