THE EFFECTS OF ROOT-INDUCED PH CHANGES ON THE DEPLETION OF INORGANIC AND ORGANIC PHOSPHORUS IN THE RHIZOSPHERE

A new method allowing control of rhizosphere pH and mineral nutrition was applied to study depletion of various organic and inorganic phosphorus fractions extractable sequentially with 0.5M KHCO3 (pH 8.5), 0:1 M NaOH and residual P extractable with 6M H2SO4 from thc rhizosphere soil. Soil pH was affected about 2 mm from the root mat. Depletion zones of inorganic P (KHCO3-P(i)) extractable with 0.5M KHCO3 extended up to about 4 mm but the depletion zones of all other P fractions were about 1 mm only. The root-induced decrease of soil pH from 6.7 to 5.5 increased the depletion of total P from all fractions by 20% and depletion of KHCO3-P(i) and residual P by 34% and 43%, respectively. Depletion of organic P (KHCO3-P(o)) extractable with 0.5M KHCO3 was not affected by a change in rhizosphere pH. With constant or increased pH, depletion of inorganic P (NaOH-P(i)) was 17% and organic P (NaOH-P(o)) was 22% higher than with decreased pH. Only 54-60% of total P withdrawn from all fractions was from KHCO3-P(i). Substantial amounts of KHCO3-P(o) and NaOH-P(o) were mineralized and withdrawn from the rhizosphere within 1 mm from the root mat, as 11-15% of total P withdrawn originated from the organic P fractions. A remaining 11-16% was derived from NaOH-P(i), and 15-18% from residual P fractions likely to be rather immobile. Thus, 40-46% of the P withdrawn near the root mat of rape originated from non-mobile P fractions normally not included in 0.5M NaHCO3 extraction used to obtain an index of plant-available soil P.