SIMULATION MODELING AS A METHOD TO STUDY LAND QUALITIES AND CROP PRODUCTIVITY RELATED TO SOIL-STRUCTURE DIFFERENCES

Research on soil biota under natural conditions in the field is complicated because large fluctuations in populations of certain species may occur as a result of natural or man-induced fluctuations in soil physical and chemical conditions. As a result, many experiments have therefore been made in the laboratory under controlled conditions. Though quite valuable, such data may be difficult to extrapolate to field conditions. To better relate biotic data to physical and chemical conditions in the field, expensive long-term monitoring programs can be initiated to collect both types of data. Physical, and chemical processes in soils can also be characterized by simulation techniques. Simulation requires one-time measurement of some basic and characteristic soil parameters, such as hydraulic conductivity and moisture retention, which are used in a model that calculates water contents in the soil at any given depth and time as a function of precipitation, evaporation and water table levels. Simulation modelling has been applied successfully to calculate water regimes in the soil, for periods of many years. The technique would appear to be particularly appropriate for use in a dynamic physical characterization of different types of soil structure, formed by various soil biota. Results are presented of a study in a sandy loam soil in the Netherlands with biogenic structures in grassland and a more compact physicogenic structure in arable land. Measured hydraulic conductivity and moisture retention data, and their variability were used in a simulation model to calculate important land qualities for a thirty year period. Land qualities were expressed in stochastic terms by focusing on the probability that certain water and associated air contents and trafficabilities would occur at any given data. Results also reflected the effects of variability of input data. Exploratory modelling was used in this study to estimate effects of an increase of the hydraulic conductivity (which might result from an increase of biological activity), on water and air contents in the soil during the year.