Phosphorus acquisition in the soil-root system of Eucalyptus pilularis Sm. seedlings. I. Characteristics of the soil system

Abstract

An ancient sand-dune system located in the Myall Lakes National Park, N.S.W. (32°28'S., 152°30' E.) supports an open eucalypt forest with Eucalyptus pilularis as one of the co-dominant tree species. Profile development of the sand mass is typical of a light humic podzol. The relative pool sizes of available nutrients contained by the acidic podzol were such that phosphorus was the most limiting for the growth of E. pilularis seedlings. The bulk of seedling phosphorus was assimilated from the A, horizon of this podzol. Soil from the A, horizon was characterized by a low total level of phosphorus (= 30 µg P g-1 soil). Approximately 90% of this phosphorus was 0.1 M H2SO4 insoluble and probably organic in nature. In addition, 30% of the total phosphorus was associated with compounds containing aluminium and iron. The concentration of soluble phosphorus in the soil solution (CaCl2, extractable) was 0.36 µg P g-1 soil. Although this value is comparable to those reported for fertile soils, the amount of readily exchangeable phosphorus was found to be very low (1.43 µg P g-1 soil). Anion exchange resin studies indicated that after this pool was exhausted, release of phosphorus from slowly available sources was extremely limited (0.013 µg P g-1 soil h-1). Studies showed that transfer of phosphorus to seedling roots in A, horizon soil was predominantly a diffusive process. Moreover, the effective diffusion coefficient for phosphate (10.81 x 10-8 cm2 s-1) indicated that this process was comparatively fast.