When phosphate is added to soil, it slowly becomes more firmly bound and less available to plants. This change is accelerated by high soil temperatures. The present results show that this effect extends to at least 80°C. Incubation of soil and phosphate at high temperatures was therefore used to provide treatments with firmly bound phosphate equivalent to that achieved after a much longer period of contact in the field. Responses to freshly added and to firmly held phosphate were then compared in the presence and in the absence of vesicular-arbuscular (VA) mycorrhiza to test whether VA mycorrhiza improved the plant's access to firmly held phosphate.
Incubation of soil and phosphate for 12 days at 70°C reduced the availability of the phosphate to about one fifth of that of freshly added phosphate. Inoculation with VA endophytes more than doubled availability of the phosphate to both Trifolium subterraneum and Allium cepa but the improvement was no bigger for firmly held phosphate than for freshly added phosphate.