Application of phosphate solubilizing microbes to promote the effectiveness of rock phosphate on cacao seedling growth in acid soil

The effectiveness of rock phosphates (RP) on plant growth is often lower than that of chemically synthesized fertilizer SP-36. Phosphate solubilizing microbes (PSM) are potentially used to enhance RP dissolution and agronomic effectiveness for cacao plant. This study was carried out to assess the ability of several PSM to dissolve insoluble phosphate, produce phytohormones, and increase the effectiveness of RP in cacao seedling at acid soil. Five phosphate solubilizing bacteria (PSB) and one phosphate solubilizing fungi (PSF) were evaluated for their ability to solubilize insoluble phosphate in liquid medium (added Ca3[PO4]2, RP, and AlPO4) and to produce phytohormones. Six isolates were also evaluated in cacao nursery consisting of nine treatments (without P fertilizer; SP-36 fertilizer; only RP; RP with inoculation of five PSB and one PSF, respectively). The result showed that the highest P solubilization in liquid medium with Ca3(PO4)2, RP, and AlPO4 was obtained by Aspergillus niger F-E1, Burkholderia ambifaria B-SS1.2, and Pseudomonas fluorescens B-SS13.5, respectively. All isolates could produce the IAA, GA3, trans-zeatin, and kinetin. Whereas B. ambifaria B-SS1.2, B. cepacia B-SS19.7, and A. niger F-E1 increased shoot dry weight and relative agronomic effectiveness (RAE) of RP. Notably, the B. ambifaria B-SS1.2 increased N, P, and K uptake in shoot of cacao seedlings. The utilization of these microbes potentially promote the effectiveness of RP for cacao plant growth in acid soil.