Entomology, Plant Pathology and Weed Science, New Mexico State University, Las Cruces, New Mexico 88003 USA

Background and objectives
The soil environment is both highly buffered and heterogeneous in space and time. It consists of a continuum of a largely microscopic matrix of solid particles, air, and water and forms an important part of most terrestrial ecosystems. All soil-borne organisms must live in the void space between soil particles must operate within specific limits of temperature, water activity, pH and other factors. Therefore, the objective of this presentation is to understand how the soil mediates variation in various abiotic factors and how the soil, as a habitat, differs from other terrestrial habitats in the interaction between these factors and the soil biota.

Results and conclusions
The major effects of soil abiotic factors on soil organisms can be classified into three broad categories, based on time and spatial scales. Abiotic soil factors may vary over short, diurnal periods, or longer, seasonal time scales or over pedogenic time scales, and may display a high spatial variability but little temporal variation. The most important soil factor that varies on both a diurnal and seasonal basis is temperature and this factor probably controls the activity of the soil biota more than any other single factor. Moisture potential is another important soil factor that varies seasonally but also may vary on short time scales (e.g. after rainstorms). Factors such as pH, texture, cation exchange capacity (CEC), organic matter and free Ca levels and soil nutritional status tend to vary over long time scales and are examples of spatially heterogeneous soil properties.

In the interactions between microbial biological control agents and their target soil-borne plant pathogens and the soil, the most important abiotic soil factors are temperature, water potential, pH and the equilibrium concentration of free cations, such as Ca. Temperature and water potential control the activity of virtually all enzyme systems in living cells, leading to many elaborate adaptations to alleviate cellular temperature and water stress. Temperature and water potential also affect diffusion rates of nutrients, signals and waste products in the soil and through membranes and limit the proper functioning of cells to a relatively narrow range of conditions. It is now clear that many biological control agents and pathogens exploit differences between their own adaptations and the adaptations of the host or other target species, to specific abiotic soil conditions, to cause disease or effect biological control. The role of pH and free Ca is significant in the growth and development of soil organisms through many transduction pathways and may be the most important abiotic soil factors affecting interactions between the soil microflora and soilborne plant pathogens.