Soil pH and Mycorrhiza Development in Pinus

Abstract

MYCORRHIZAS of Pinus will not develop profusely on trees growing in neutral or alkaline soils, and it has usually been assumed that this is so because the mycorrhizal fungi are acidophilic. During investigations of the fertility status of a nodular podzolic soil from the coastal lowlands of southern Queensland, I have found that formation of mycorrhiza in pine seedlings is not always inhibited by a rise in soil pH, and when it is, the effect can usually be explained in terms of host nutrition. The direct effect of alkalinity in restricting the growth of the fungus may be less important than was formerly thought. These results are derived from a series of pot experiments designed primarily to test for nutrient deficiencies, but also to study the effect of nutrient supply on the degree of mycorrhizal infection of seedlings. Comparison of relative mycorrhiza development was based on the number of mycorrhizas expressed as a percentage of the total number of short roots per seedling (‘mycorrhiza per cent’). The criterion of infection used was the presence of dichotomous branching or forking of the short roots. Since dichotomy of short roots of Pinus is not always associated with mycorrhizal infection, some care is necessary in accepting forked short roots as mycorrhizas. Experience has shown that it is possible to recognize mycorrhizas of Pinus on the basis of their external morphology. In the work reported here, counting was carried out initially under a stereo-microscope (× 12.5), and after experience had been gained in this way a binocular loupe was used.