In this chapter we deal with vegetation growing in the field. This introduces new and challenging questions of scale and heterogeneity, in time and space, of the environment in which plants grow. It builds on the concepts and methods explained in earlier chapters, especially the movement of water and solutes (chapters 2, 3 and 4) and the distribution of roots (chapter 9) in field soils. In some cases, it requires changes and simplifications in the methods that we have used earlier. The problems of dealing with water and nutrient movement and uptake at the field scale are discussed first. The modelling approach that we developed in the earlier chapters of this book, up to the end of chapter 10, logically resumes at section 11.3. This covers both uptake models and the more complex combined crop growth and uptake models that simulate the main interactions with the environment. This chapter considers increasingly complex systems: first, uniform monocultures, including models of a ‘green leaf crop’, a root crop, a cereal, and a tree crop. At this level, the presence of weeds or groundcover is deliberately ignored. Interspecies competition is included later, with vegetation composed of more or less regularly spaced plants of more than one species. This occurs in many agricultural systems, such as mixtures of forage species and agroforestry systems. The competition processes become even more complicated where there is no spatial symmetry, and models of crop/weed mixtures, grass/legume mixtures, and planted woodlands are used as examples. Progress with crops has been more rapid because of their more regular structure, so we deal mainly with these, but we believe that similar ideas will be applied to natural vegetation also, and this is discussed in section 11.5. Most of these models have a water submodel, or, if not, one could be added. As the physical basis is normally rather similar for all water models, one model for water uptake is explained in some detail (section 11.1.2), but elsewhere water uptake is dealt with very briefly. For each model, the preferred order of discussion is water; growth, including economic yield; nitrogen; potassium; phosphorus; and other nutrients, unless the logic of the subject demands a different order.
An Agent-Based Crop Model Framework for Heterogeneous Soils