PLANT‐INDUCED CHANGES IN THE RHIZOSPHERE OF RAPE ( BRASSICA NAPUS VAR. EMERALD) SEEDLINGS

Abstract

Additions of soluble phosphate to P‐deficient Begbroke sandy loam delayed or completely prevented the fall in rhizosphere pH observed when rape was grown at high root densities in this soil (Parts I to III, this series). Whereas the pH of the unamended rhizosphere soil fell from 6–6.5 to 5.1–5.3 by day 41, the rhizosphere pH of soil with an extra 2 μmol P g −1 remained steady (6 to 6.5) until day 35, and that of soil with an extra 20 μmol P g −1 increased slightly from pH 6 to 6.4 during 41 days of growth. The increase in rhizosphere phosphatase activity with increasing severity of P deficiency appeared to be a response to increasing root density and decreasing concentration of soluble inorganic P in the soil. No significant change in levels of soil organic P was detected. Plants that acidified their rhizosphere (low P status) depleted acid‐soluble forms of soil P and absorbed twice the amount of P which could be desorbed from the control soil in 10 −2 M Ca(NO 3 ) 2 at pH 6.1. Uptake of P by plants which did not acidify their rhizosphere (high P status) was never greater than the amount of P desorbable in 10 −2 m Ca(NO 3 ) 2 at pH 6.1, and was derived from alkali‐soluble and resin‐extractable forms of soil P. Cation uptake exceeded anion uptake in both low and high P plants, but the difference was greater in low P plants. These differences in cation and anion uptake (mEq) were approximately equal to the milliequivalents of H + (OH − ) required to produce the observed changes in rhizosphere pH. These results explain why models for P uptake based on in vitro measurements of physicochemical parameters governing phosphate diffusion in soil work well for P sufficient plants, but tend to underestimate P uptake by P deficient plants growing in soils in which soluble P levels are sensitive to pH change.