Many quantitative studies on root exudation have expressed the concentrations of compounds released as per plant or per gram of root rather than a percentage of assimilated carbon released. Newman (1985) reviewed the data on exudation and estimated that soluble exudates are released in the range 10-100 mg g-1 root and insoluble root derived material in the rhizosphere of still functioning roots is in the range 100-250 mg g-l. However, exudation ex- pressed as a dry weight of root are misleading because only a small area of the root may be exuding a particular compound and therefore the concentra- tions of the compounds in certain areas may actually be much higher.