A. M. A.

Abstract

Exudation of low molecular weight carboxylates is an important phosphorus (P) acquisition strategy for plants in strongly P-fixing soils. The effect of citrate exudation on P availability was studied in five acid soils with a large organic matter content and a low available P pool. The amount of molybdate reactive P extracted by citric acid 10 mM (0.1–2 mg kg) was larger than the molybdate reactive P (MRP) extracted by CaCl 2 10 M (0.007–0.07 mg kg). Both extractants were adjusted to soil pH prior to extraction. Also, the total P content of citric acid extracts (2–44 mg kg) far exceeded the total P content of CaCl 2 (0.2–1.5 mg kg). However, ratios of MRP to total dissolved P (TDP) remained constant. When citric acid was added to the CaCl 2 -extracts (the pH of citric acid being adjusted to the pH of CaCl 2 , and thus eliminating release of P through acidification) a significant pH increase was noted. Simultaneously, MRP increased significantly in all but one soil. This suggests that the P availability enhancing properties of citric acid are not only due to acidification of the plant rhizosphere, but also to its Al and Fe complexing capacity.