Increasing plant availability of legacy phosphorus in calcareous soils using some phosphorus activators

Legacy phosphorus (P) in soil, accumulated over several years of fertilizer application in excess of crop demand, represents a huge and largely untapped resource. P activators can increase the availability of this P to plants by accelerating its transformation into soluble P fractions. In this study, we evaluated the potentials of four P activators (oxalic acid, lignin, phytase and ascorbic acid) to increase plant available P in a laboratory incubation experiment with two P-deficient calcareous soils used for wheat production. Samples were analysed for Olsen P, phosphomonoesterase and with Hedley sequential P fractionation. All four treatments had significant effects on different soil P fractions. Oxalic acid mainly enhanced inorganic P (P-i) solubility from the HCl-extractable P pool. Lignin enhanced P lability from the NaOH-, HCl- and residual-P pools. Phytase and ascorbic acid principally affected the organic P fractions (P-o). Oxalic acid and lignin showed most potential to improve P (H2O-P, NaHCO3-P-i and NaHCO3-P-o) availability, which increased by 110-419% and 4.1-122%, respectively. These findings illustrated the potential mechanisms responsible for P release associated with different P activators and reinforced the case for their use in increasing legacy P availability for agriculture in calcareous soils.