The Mineral Nutrition of Single Plants in Soil

Earlier chapters in this book have dealt with the various components of the soil –root system. In this chapter we aim to synthesize them into a unified treatment of a single whole plant growing in soil. Solute movement and root system uptake models are still the central subject, but we must also deal with the growth of the whole plant, which provides the growing sink for the absorbed solutes, and the expanding root system through which they enter. Here we deal only with homogeneous soils and constant growing conditions, usually in pot culture, and call this ‘simplified conditions’. This is necessary in dealing with such complicated systems, so that essential principles shall not be obscured. In chapter 11 we apply these ideas, so far as it is possible, to crops and natural vegetation. Models are often referred to in this book, because the ideas and concepts are most easily and precisely formulated in this way (Nye 1992a). Here, we outline the different types of models that will be dealt with, and their relationships with each other. Readers may consult Rengel (1993) and Silberbush (1996) for recent reviews of the modelling of nutrient uptake, and Penning de Vries & Rabbinge (1995) for general crop modelling concepts. There are three basic situations: (1) Models of single or few plants growing in pots under simplified conditions in greenhouse or growth chambers, in homogeneous soils, with ample supplies of water, constant temperature, etc. (2) Models of monoculture crops. If a unit cell can be defined, only the vertical dimension need be considered, except possibly for light interception, and for radial transport around roots. These models are normally used for field situations. (3) Vegetation models with mixed species. Separation of the uptakes by the different species can be extremely difficult. If the geometrical arrangement of the species is regular, it is possible to determine a recurring unit cell, which simplifies treatment. Within each situation there is a hierarchy of complexity in the number of processes covered. All models may include water uptake as well as nutrient uptake.