Introduction

There is much information on the microbiology of soils (Nedwell & Gray, 1987), especially the soil near roots which will mostly concern us. Soils are very variable on many different scales. There are the differences that occur between soil types, usually based on the parent material, the climate and the vegetation, which control the amount of clay, organic matter and so on. Secondly, there are differences within soil in relation to depth, which reflect the addition of organic matter to the surface and are the result of leaching down the profile: the soil may be divided into a number of layers (called horizons) which have very different physical and chemical characteristics. Thirdly, there are differences on a very small scale, the microhabitat, which reflect changes in nutrient status, substrate availability, aeration, etc. on different parts of a soil crumb or a sand grain: we may here be talking about distances of a few tens of micrometres making a significant difference in oxygen levels because of the very low solubility of this gas in water (Campbell, 1983; Bruehl, 1987). There is, therefore, great variation in microbial numbers and activity between and within soils. There may be several million bacteria and hundreds of thousands of fungi which can be cultured from a gram of soil, but many of these will be inactive in the soil because of the environmental limitations which most commonly are temperature, water availability, aeration and available substrates for metabolism and growth.