Comparison of the Eppawala Rock Phosphate (ERP) solubilizing efficiencies by fungi in selected agricultural soils

Abstract

Phosphorus (P) is a macro nutrient, which mainly presents as fixed forms in soils. The fixation limits the P availability for plant uptake. Conventionally, P fertilizers are applied excessively for upland vegetables and paddy fields in dry zones in Sri Lanka though these crops do not give expected yields with increased P applications due to high fixation. The excessive usage of P fertilizers not only adversely affects the root rhizosphere but also accelerates the diminishing of the natural rock phosphate deposits used as the major source for P fertilizer production. Microbial application is one of the sustainable and ecofriendly strategies to improve P solubility and, thereby, to facilitate efficient use of the limited P resources. The laboratory experiment was conducted for qualitative comparison of the ERP solubilizing capacity by the randomly isolated microorganisms (PSM) in soils at the horticulture farm, Gannoruwa. Microorganisms extracted from the diluted soil series were introduced to Pikovskaya agar media (PKV) containing ERP less than 125 μm particle size in petridishes. Halo zones were developed by the P solubilizing microorganisms, and these microorganisms were identified as Aspergillus flavus (PSF1), Aspergillus niger (PSF2), Penicillum sp. (PSF3), and Aspergillus terreus (PS4) by morphological studies. These PSMs were single spot inoculated for creating pure culture using 20 ml of PKV media with ERP as prepared previously. Measurements of P solubilizing index (PSI) [(PSI=Colony diameter + Halo zone diameter) / Colony diameter] and P solubilizing efficiency (PSE) [(PSE=Colony diameter/Halo zone diameter) × 100%)] were taken in three, six, nine, twelve and fifteen days intervals. Four replicates were done for each treatment. The PSI and PSE values were recorded with descending order as PSF1(4.33)>PSF2(2.75)>PSF4(2.56)>PSF3(2.056) and PSF1(335.06%)>PSF4(172%)>PSF2 (157.39% )>PSF3(110%), respectively. According to the scale introduced by Silva Filho and Vidor, the P solubilizing capacity is low, medium, and high when the PSI is lower than two, in-between two and three, and higher than three, respectively. Accordingly, the PSF1 shows high P solubilizing capacity and other fungi show medium P solubilizing capacities. Therefore, the PSF1 with ERP is identified as a potential P-biofertilizer for improving the plant availability P in agricultural soils.