Abstract
The root/soil interface is a zone of intense microbial and biochemical activity due to the exudation of organic and inorganic nutrients by plants. The 'rhizosphere effect' is usually quantified by comparing the total counts, number of individual species, or some biochemical function, in the soil associated with the root with that in soil not occupied by roots. These R:S ratios may range from >1000:1 (denitrifiers) to 100:1 (total bacteria). R:S values vary according to plant age and general well-being, indicative of changes in root exudates and a microbial succession as substrates change. Stimulated root zone microorganisms contribute to the success of the plant by supplying nutrients and other growth substances as well as preventing attack by pathogens. Current ideas concerning the role of the 'normal' rhizosphere microflora in increasing the plant's resistance to pathogens are discussed, especially the role of siderophores and cyanogenic microorganisms.-from Author