Total of 25 fungal isolates were isolated from 6 ferralsols samples of tithonia at two provinces (DakNong and DakLak) of western highland of Vietnam. All of them had the ability of phosphate solubilization however there were 18 isolates made a halo and 7 isolate did not make a halo around colonies, these isolates were evaluated on insoluble phosphate (Ca₃(PO₄)2) medium and 4 fungal isolates were chosen to br evaluated on thermophosphate (phosphate fertilizer) medium and all of them dissolved a vast amount of soluble phosphate (>100 mg/L after 4 to 6 days after inoculation). These four isolates were identified based on nucleotide sequence from internal transcribed spacer (ITS) rDNA. The sequences from selected phosphate-solubilizing fungi showed high degrees of similarity to those of the GenBank references strains (99%), all of them were identified as various species of Penicillium. A strong negative correlation between content of soluble phosphorus and pH was observed in this study. All of four fungal strains synthesed oxalic acid and released into medium, organic acid reduced pH of medium and enhanced quantity of soluble phosphate among strain FMT1.2 synthesed a huge amount of oxalic acid, dissolved high soluble phosphate from substrate (33% thermophosphate and 67% dolomite), improved yield component and grain yield of soybean cultivated on ferrasols in pots experiment, improvement of soil fertility through increasing available phosphorus in ferralsols after soybean cultivation. The study showed that the potential of phosphate-solubilizing Penicillium in rhizospheric soil of tithonia and their application on crop growth and improvement of soil fertility.

Key words: Ferrasols, Oxalic acid, Penicillium, phosphate solubilization, tithonia, tricalcium phosphate, thermophosphate