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The chapter discusses the potential and limitations of modeling soil-plant systems. An important issue that must be considered by model developers is the problem of treating consistently the hierarchy of land units to be found in any area of land. Models should be applicable at all scales or some means of assessing the scales at which particular models are appropriate. It is very important to evaluate or validate a model if it is to be used for decision-making.

The chapter deals with the computer models used for simulating the growth, development and yield of crops within the soil-plant system. The models vary in complexity and detail, ranging from simple heat unit and chilling hour equations to calculate development, to comprehensive models that predict crop yield. The generic approach integrates growth, development and yield with crop and cultivar genetics and the interactions with weather, soil and other environmental variables.

The chapter discusses modeling crop responses to plant growth regulators. Plant hormones mediate a whole array of growth and developmental processes in plants, many of which involve interactions with environmental factors. An important role is played by the plant growth status at the time of the chemical application and the environmental conditions immediately before or following their application in determining plant responses to the chemical and the economics of its use.

The chapter presents information on agrochemicals. It refers to a large variety of synthetic compounds used to increase the production of agricultural crops. Sorption and degradation of soil control the environmental fate of agrochemicals. Contamination of the groundwater is a major problem caused by agrochemicals. Agrochemical sorption and desorption are slow.

The chapter deals with the models developed to describe the trace element reactions and processes in soil. The models help provide evidence for the mechanisms by which trace elements interact with soils. Iterative procedures determine the parameters required to run the models. The iterative model is the most simple, empirical or semiempirical models that show the greatest possibilities for practical application.

The chapter presents a brief description of the inputs and reaction in soils and plant and animal requirements of sulphur, calcium and magnesium in the agroecosystems. Sulphur contents occur in soils is estimated to be 7 to 53 percent. Sulphur-containing amino acids and sulfonates are some of the main forms of organic sulphur deposited in soils. To prevent calcium deficiency in soil, calcium containing compounds must be added. Magnesium is present in minerals and in organic matter as exchangeable cation and also in soil solution.

The chapter presents a conceptual comprehensive phosphorus-dynamics (P-dynamics) model in soil and discusses its possible implementation in terms of both available and incomplete knowledge. The dynamics are the result of multiple simultaneous reactions that determine the temporal P distribution in the soil-plant-microbial mass system. The processes are divided into three categories to include the processes that have been satisfactorily investigated and consequently incorporated in P-soil and P-soil crop models.

The chapter presents the different modelling concepts used for the description of potassium (K) dynamics in soil. The dynamics may be characterized by the stationary state with steady contents of various forms of soil K. Soil and plant potassium dynamics during vegetation may be predicted in multiparameter mechanistic models. Plant demand and K fixation and release in soil influence the transport of K to roots.

The chapter describes the theories behind various nitrogen-cycle (N-cycle) processes, approaches to model these processes and the extent of their occurrence in different soil-plant systems. The modeling approaches used for various N-cycle processes range from simple empirical to highly complex, process-based mechanistic approaches. The N mineralization is described fairly well by kinetic models with two pools of mineralizable N.

The chapter discusses soil organic matter (SOM) dynamics. The factors and processes that affect SOM pool and depletion include climate, soil, aggregation and erosion. Anthropogenic perturbations like land use change can easily disturb the soil equilibrium. It is estimated that 54 Pg is lost in the soil organic C pool as a consequence of cropping between 1700 and 2003. There are principal strategies for C sequestration which include reducing losses of C from soils, increasing C concentration in soils and improving C yields through agricultural intensification.

The chapter reviews the developments in the modeling of the exchange between land surfaces and the atmosphere of greenhouse gases including carbon dioxide, methane and nitrous oxide and the tropospheric ozone precursor, nitric oxide. Soil emissions of carbon dioxide generally take place within a two-way exchange between the land the atmosphere in all terrestrial ecosystems, except those with bare land resulting from agricultural or forestry operations. A comparison between simulated and measured hourly and daily average of carbon dioxide efflux in a slash pine plantation in Florida is illustrated in this article.

The article discusses the solute-water interactions in soil. The interactions depend on the type of solute and the rate of and pathway of water movement. The simplest model describing the interactions is piston displacement, wherein the transport volume occupies all the water-filled pore space. Another model is based on the water flow concept at variable velocities through a network of parallel, noninteracting stream tubes. For a successful solute-water interactions, correct parameterization is a major requirement.

The chapter discusses the models used in soil water dynamics. Models are used for demonstrating factors that influence soil water dynamics including evaporation and transpiration. The Soil Water Evapotranspiration (SOWATET) model is used for illustrating the impacts of different conditions on soil water dynamics. The limitations of models with homogenous soil water properties include soil structure, spatial variability and hysteresis.