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SUMMARYRape plants that were grown in thin layers of ^^P-labelled Begbroke sandy loam (initial pH 6-1) attained such high root densities (> 90 cm cm^^) that after c. 14 days, all the soil volume became rhizosphere soil. At this time, the rhizosphere pH began to decrease and the quantity of plant available P in the rhizosphere soil, as measured by the L value, exceeded the isotopically exchangeable soil P (E value), determined on samples of incubated but unplanted soil. The P concentration of the rhizosphere soil solution dropped to a minimum of 0-6 fiM at 14 days, and remained low, but the phosphatase activity of the rhizosphere soil increased to 10 times that of the control soil after 35 days.In an identical experiment, after 20 and 41 days of growth, organic P (P^) and inorganic P (Pj) in the control and rhizosphere soils were fractionated into the following forms: that extractable by an anion-exchange resin, 0-5 M NaHCO, (pH 8-5), 01 M NaOH, 1 M NaOH, 1 M HCl and residual P. Fractionation of the soil P indicated that during the first 21 days, when the rhizosphei-e pH did not decrease, only resin Pj, 0-5 M NaHCO, Pj and 0-1 M NaOH Pj were significantly depleted by plant uptake. After 41 days, however, when the rhizosphere pH had fallen to 5-3, in addition to depletion of resin Pj, 0-5 M NaHCOg Pj and 01 M NaOH Pj, there was substantial depletion of acid-soluble forms of soil Pj and residual P. It is suggested that acidification ofthe rape rhizosphere (Parts I and II) led to the dissolution of acid-soluble forms of Pi that were not exchangeable with ^^P.