Plant‐induced Changes in the Rhizosphere of Rape (Brassica Napus Var. Emerald) Seedlings

SUMMARY Rape ( Brassica napus , var. Emerald) plants were grown in thin layers of a phosphate‐deficient soil supplied with a P‐free nutrient solution containing N solely as NO 3 − (3.5 × 10 −3 M). Inter‐root competition (excluding root hairs) for soil P began on the 14th day of growth, after which all soil in the thin layer was considered to be rhizosphere soil. During subsequent plant growth the pH in the rhizosphere decreased as much as 2.4 units (from pH 6.5 to 4.1) before rising slightly towards the end of the experiment (35th day). The pH decrease was associated with an increase in solution P concentration in the depleted rhizosphere soils, compared to the P concentration attained when similar amounts of P were desorbed from unplanted control soils by shaking them in 10 −2 M Ca(NO 3 ) 2 at the original soil pH of 6.1. P desorption in 10 −2 M Ca(NO 3 ) 2 at pH values between 6.2 and 4.5 indicated that a decrease in the soil pH from 6.5 to 4.1 could result in at least a 10‐fold increase in the P released into solution, which would be available for plant uptake. These results may explain reports in the literature that rape is efficient in absorbing P from P‐deficient soils.