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.

Inhalt
A1 The ionic species present in soil solutions.- Dissociation constants.- Values for dissociation constants.- The behaviour of some typical anions.- The behaviour of some cations.- A2 Variable charge oxides as soil constituents and as models of soil constituents.- The oxides present in soils.- Synthetic oxides as models of soil oxides.- The reaction of cations with oxides.- The reaction of anions with oxides.- Changes in charge resulting from adsorption.- Summary.- A3 Describing and explaining the adsorption behaviour of oxides.- The differences between describing and explaining.- The single-plane model.- The three-plane model.- The four-plane model.- Summary.- A4 The rate of reaction with oxides.- The rate of the adsorption reaction.- The rate of the second reaction.- Description of date using the model for rate of reaction.- Consequences of the model of the rate of reaction.- Summary.- A5 The four-plane model and how it works.- Varying the charge parameters.- Varying the parameters that describe adsorption.- Varying the dissociation characteristics.- Varying the background electrolyte.- Varying the amount of adsorption.- Summary.- A6 Fixed charge and variable charge components in soil.- The definition of surface charge.- The effects of adsorbed ions.- Separating permanent and fixed charge.- Is it important to distinguish permanent and variable charge?.- A7 The reaction of anions and cations with soil.- The effects of concentration on sorption.- Effects of time on sorption.- Describing the rate of the reaction.- The effect of level of application.- Effects of temperature.- Repeated additions of a nutrient and desorption.- Effects of pH and of electrolyte concentration 71 Summary.- A8 Modelling the reaction of anions and cations with soil.- The three assumptions of the model.- The formal equations of the model.- Modelling the characteristics of sorption.- The individual effects of the parameters of the model.- Summary.- B1 Speciation in solution.- Contains BASIC programs MOLLY and ZNOHP to calculate ion species.- B2 Solving simultaneous equations the four-plane model.- Contains BASIC program BOWDEN which solves the simultaneous equations of the model.- B3 Deriving equations to describe adsorption and rate of adsorption.- Presents algebra to show the solutions of rate equations in some practical cases.- B4 Modifications to the four-plane model.- Contains BASIC program ZNCL2 and RATEOX which apply the four-plane model to metal adsorption and to the rate of the reaction.- B5 Fitting models to data.- Considers the problems of finding the best value of parameters and presents BASIC program FITPINIT which fits data to the four-plane model.- B6 Applying models to soil.- Presents BASIC programs THETA and DIFPLUS which simulate the behaviour of soil and FITTHETA and FITRATE which fit date to models.- B7 Describing reaction of ions with soil.- Considers the problems which arise where sorption is calculated from the change in solution concentration. Program PREDX solves simultaneous equations to predict sorption. FITFUNC fits various functions to the observed variable.