Publisher Summary This chapter reviews the work done with soils and with hydrous oxides of iron and aluminum, with particular reference to mechanisms and structural aspects of anion adsorption. Anion adsorption on clay minerals and calcium carbonate is also discussed. The determination of adsorption isotherms is one of the most useful experimental procedures in the study of the interaction of anions with hydrous oxides or soils. Anion adsorption involves an electrostatic interaction and some chemical interaction between the surface and the ion. The Langmuir model is incomplete in describing this adsorption because it takes no account of charge. The adsorption of anions is dependent on pH, with maximum adsorption usually occurring for fully dissociated ions at low pH where the surface becomes positively charged, owing to protonation of surface M·OH groups. Maximum adsorption of anions of incompletely dissociated acids occurs at a pH value close to the pK of the acid. Anion adsorption reactions in soil are complicated by competition for adsorption sites from anions, including carboxylates, and also by the presence of cations, such as calcium and aluminum.

Anion Adsorption by Soils and Soil Materials

Phosphorus fixation is generally understood as the transformation of soluble forms of P into less soluble ones after they react with the soil. Although this is a common occurrence in soils, it becomes a major management consideration in certain soils because of the large amounts of fertilizer P needed to meet crop requirements. Phosphorus fixation is most frequently a problem in acid soils high in finely divided sesquioxides, less frequently a problem in calcareous soils, and rarely a problem in soils that are neither high in free sesquioxides nor calcareous. A P-deficient soil which can be corrected by an application of 20 to 50 kg of P Iha is not a problem soil, but a soil which requires 300 to over 1,000 kg of P Iha needs a different management for profitable crop production. A considerable proportion of the world's potentially arable but presently unused land has this limitation. The purpose of this chapter is to examine the occurrence and magnitude of high P fixation and the management practices designed to cope with this situation. No attempt is made to give a full literature review on P fixation, rather we attempted to illustrate management concepts with specific examples. Our discussion is confined to acid soils where the problem is most severe. For example, Olsen and Watanabe (1957) reported that representative acid soils of the United States fixed more than twice the amount of added P per unit of surface area than a similar group of neutral or calcareous soils. They also found that the P fixed was held with five times more bonding energy in acid soils as compared to calcareous soils.

Management Considerations for Acid Soils with High Phosphorus Fixation Capacity

Soil pH and Lime Requirement Determination

A survey of recently published textbooks and journal papers in the field of soil fertility and plant nutrition indicates that the authors seldom see fit to devote much attention to the interplay between nutrients in relation to yield enhancement in field cropping systems. This is a reflection of the widely adopted approach of varying only one or possibly two nutrients at a time in field experimentation. Without factorial designs involving a number of nutrient factors at varying levels, serious study of interactions is not possible. Until field research in soil fertility pays greater attention to the multifactor approach, there will be little prospect of verifying the large number of nutrient interactions previously studied under greenhouse or solution culture conditions.

Phosphorus Interactions with Other Nutrients and Lime in Field Cropping Systems

Aluminum is a phytotoxic element in many soils and occurs in a variety of chemical species. In order to determine whether AlSO+4 is toxic, seedlings of wheat (Triticum aestivum L. cv. Tyler) and red clover (Trifolium pratense L. cv. Kenland) were transferred to solutions containing controlled activities of Al3+, AlSO+4, Na+ and Ca2+. Root elongation was inhibited by Al3+ (or mononuclear hydroxy-Al species that are in equilibrium with Al3+), but not by AlSO+4. We assumed a formation constant (KAlSO+4= {AlSO+4}/[{Al3+} {SO2-4}]; braces indicate activities} of 103.2 for AlSO+4 in the computation of ionic activities, but use of KAlSO+4 values ranging from 102.8 to 103.6 had very little effect on the computed toxicities of Al3+ and AlSO+4. Sulfate did not promote the formation of polynuclear Al complexes in our experiments. A practice in studies of Al phytotoxicity has been to attribute toxicity to mononuclear Al, but now it would seem advisable to exclude AlSO+4. That AlSO+4 is non-toxic, or is at least 10-fold less toxic than Al3+, has implications for the physiology of Al toxicity and for the use of sulfate salts in experimental work and in agriculture.

M. E. Sumner

Papers

Effects of Aluminum Toxicity and Phosphorus Fixation on Crop Growth on Oxisols in Natal1