PGPRs increase the availability of nutrients through the solubilization of unavailable forms of nutrients and by the production of siderophores which aids in the facilitating of nutrient transport. Phosphorus, a limiting nutrient for plant growth, can be plentiful in soil, but is most commonly found in insoluble forms. Organic acids and phosphotases released by rhizobacteria found in plant rhizospheres facilitate the conversion of insoluble forms of phosphorus to plant-available forms such as H2PO4−. PGPR bacteria include "Pseudomonas putida", "Azospirillum fluorescens", and "Azospirillum lipoferum" and notable nitrogen-fixing bacteria associated with legumes includes "Allorhizobium", "Azorhizobium", "Bradyrhizobium", and "Rhizobium".