1.5 Interactions Between Nutrients in Higher Plants A.D. ROBSON and M.G. PITMAN 1 Introduction Interactions between nutrients in higher plants occur when the supply of one nutrient affects the absorption, distribution or function of another nutrient. Thus, depending on nutrient supply, interactions between nutrients can either induce deficiencies or toxicities and can modify growth response. Interactions between nutrients can be assessed by examining both the relationship between nutrient supply and growth and the relationship between nutrient concentrations in plants and growth. Additionally where nutrient supply is neither deficient nor toxic for plant growth, interactions can be assessed by considering nutrient concentrations and contents within the plant. There are several ways in which nutrients can interact either within the soil or plant to affect nutrient absorption or utilization. However, these interac- tions can be classified in two major categories. Firstly there are interactions which occur between ions because the ions are able to form a chemical bond (either ionic or covalent). Hence interactions can occur due to the formation of precipitates or complexes. This form of interaction is generally most marked when the interacting ions have very different chemical properties [for example anions interacting with cations, a small cation with a high charge density (elec- tron acceptor or hard acid) with a small anion with a high charge density (electron donor or hard base), or a large cation with a low charge density (soft acid) with a large anion with a low charge density (soft base)]. Interactions between nutrients relating to the formation of precipitates or complexes (either in the soil or the plant) can be predicted from solubility products and binding or stability constants. However, to use this chemical data to define the role of these processes in interactions among nutrients, concentra- tions and forms of nutrients (oxidation state, complexed anion or cation) in each of the compartments (soil solution, cell wall of root, cytoplasm of leaf, vacuole in leaf and phloem) must be known. Our knowledge of the concentra- tions and chemical states of nutrients in many of these compartments is very limited. Many micronutrients do not exist as divalent cations in either the soil solution or the xylem (Table 1). The formation of complex anions between micronutrients and organic ligands may have several important consequences. For example, from a consideration of the solubility of eu and Zn compounds alone it would be predicted that liming soils would decrease the concentrations of both eu and Zn in soil solutions to a similar extent and hence decrease the uptake of eu and Zn (LINDSAY 1978). However, because eu is more com- plexed by soluble organic matter than Zn (HODGSON et al. 1966), effects of A. Läuchli et al. (eds.), Inorganic Plant Nutrition © Springer-Verlag Berlin-Heidelberg 1983