SOIL FUMIGATION AND PHOSPHORUS SUPPLY AFFECT THE FORMATION OF PISOLITHUS EUCALYPTUS-UROPHYLLA ECTOMYCORRHIZAS IN 2 ACID PHILIPPINE SOILS

To examine the effects of microbial populations and external phosphoru s supply of two Philippine soils on mycorrhizal formation, Eucalyptus urophylla seedlings were inoculated with two Pisolithus isolates and g rown in fumigated, reinfested and unfumigated soil fertilized with fou r rates of phosphorus. The Pisolithus isolates used were collected fro m under eucalypts in Australia and in the Philippines, Soils were infe rtile acid silty loams collected from field sites in Pangasinan, Luzon and Surigao, Mindanao. Significant interaction was observed between i noculation, soil fumigation and phosphorus supply on mycorrhizal forma tion by the Australian isolate in Surigao soil but not in Pangasinan s oil. Soil fumigation enhanced mycorrhizal formation by the Australian isolate but did not affect root colonization by the Philippine isolate . Root colonization by the Australian isolate was highest in the reinf ested soil while for the Philippine isolate it was highest in the unfu migated soil. The Australian isolate was more effective than the Phili ppine isolate in promoting growth and P uptake of E. urophylla seedlin gs in both soils. Total dry weight and P uptake of E. urophylla seedli ngs inoculated with the Australian isolate were maximum in fumigated a nd in the reinfested Pangasinan and Surigao soils supplied with 8 mg P kg(-1) soil. In the unfumigated soil, growth of seedlings inoculated with the Australian isolate was significantly reduced. Seedlings inocu lated with the Philippine isolate had the largest dry weights and P co ntents in unfumigated Pangasinan and Surigao soils supplied with 8 mg P kg(-1) soil. These results indicate that the performance of the Aust ralian Pisolithus isolate was markedly affected by biological factors in unfumigated soil. Thus, its potential use in the Philippines needs to be thoroughly tested in a variety of unfumigated soils before its w idespread use in any inoculation programme.