Rock phosphate combined with phosphate-solubilizing microorganisms and humic substance for reduction of plant phosphorus demands from single superphosphate

Phosphorus (P) plays an important role in agroecosystems as a limiting nutrient for crop production because of its low soil availability and high requirements by plants at an early stage. Soluble P-fertilizer amendments contrast with low P use efficiency in weathering tropical soils. The aim of this work was to use P-solubilizing microorganisms (PSMs) and humic substance (HS) to enhance P solubility of natural rock phosphate (RP) of Araxá for partial replacement of single superphosphate (SSP). Two pot experiments were designed under greenhouse conditions. First, defined proportions of SSP and RP were combined in the following six treatments (T1, 0/100%; T2, 20/80%; T3, 40/60%; T4, 60/40%; T5, 80/20% and T6, 100/0% SSP/RP), using two different P-placement methods (broadcast and deep placement). The sub-optimal P fertilizer combination of 40% SSP + 60% RP was selected. In addition, deep placement of the P-fertilizer combination of SSP + RP produced a better plant response for all P rates. Based on the selected proportion of SSP/RP, a second assay was performed using mixed strains of bacteria and fungi (PSM, previously selected for RP solubilization) combined with humic acid (HA). We showed that PSM + HA treatment positively stimulated root and shoot weight compared with non-inoculated plants by 17 and 22%, respectively. Despite this biomass increase, no difference was observed in P concentration, indicating an increased P use efficiency. Overall, our findings suggest that the application of both PSM and HS with RP may be a suitable method for reduction of soluble P fertilizer demands without compromising plant yields.