The roots of plants are involved in the uptake of mineral nutrients and water for plant growth, but they also release a wide range of organic compounds in the surrounding soil. Consequently, the area of soil in contact with the plant root, termed the rhizosphere, is a site of intense microbial activity. Not surprisingly, many microorganisms are present at higher numbers on the surface of plant roots and in the rhizosphere than in soil not influenced by the presence of roots.

The rhizosphere is a key soil habitat, where the numerous interactions taking place between plant root and soil microorganisms will determine growth conditions for both the plant and the microorganisms in the rhizosphere. Indeed, the rhizosphere microbiota exerts an important influence on roots and on growth of the plant. Consequently, the rhizosphere represents an important soil habitat where introduction of beneficial microorganisms (e.g. biofertilizers, phytostimulators, and biopesticides) as inoculants can result in significant improvements in crop yield and/or crop quality.

Modern agricultural practices largely rely on high inputs of mineral fertilizers to achieve high yields and involve applications of chemical pesticides to protect crops against pathogens and pests. These practices are now being reevaluated and are coming under increased scrutiny as our awareness of potential health and environmental consequences of excessive mineral fertilizer and chemical pesticide usage improves. It is widely recognised that applications of mineral fertilizers (especially nitrogen) can result in groundwater contamination by nitrates leaching through the soil profile. Under certain soil conditions, denitrification of applied nitrogen fertilizer can give rise to gaseous nitrogenous compounds that volatilise the from soil into the atmosphere. Some of them (e.g. nitrous oxide) are thought to contribute to the greenhouse effect and/or the alteration of the ozone layer. Similarly, use of chemical pesticides has raised concern about their possible presence and/or that of their residues in the food chain and in the environment.

Concerns about the possible health and environmental consequences of using increasing amounts of mineral fertilizers and chemical pesticides have led to strong interest in alternative strategies to ensure competitive yields and protection of crops. This new approach to farming, often referred to as sustainable agriculture, seeks to introduce agricultural practices that are more friendly to the environment and that maintain the long-term ecological balance of the soil ecosystem. In this context, use of microbial inoculants in agriculture (biofertilizers, phytostimulators and biopesticides) represents an attractive environmentally-friendly alternative to further applications of mineral fertilizers and chemical pesticides. A continued exploration of the natural biodiversity of soil microorganisms and the optimisation/manipulation of microbial interactions in the rhizosphere of crops represents a prerequisite step to develop more efficient microbial inoculants.