In this work four stable bio-waste composts (C1, C2, C3, C4) were selected on the basis of their increasing water soluble P (H2O-P). The P speciation was assessed via sequential chemical extraction (SCE) on the same products. Moreover, the plant-available P was assessed via apparent recovery fraction approach (ARF) in a pot test on ryegrass over 21 weeks at 15 mg P kg-1 of soil. An inorganic P source (P-chem) was added as a reference at the same P rate in addition to a non-fertilized control (Control). SCE showed that the sparingly soluble P (HCl-P) was the most important fraction in all composts: C1 (HCl 65% > NaHCO3 17% = NaOH 17% > H2O 1%); C2: (HCl 51% > NaOH 23% > NaHCO3 18% > H2O 7%); C3: (HCl 58% > NaOH 21% > NaHCO3 12% > H2O 9%); C4: (HCl 39% > NaOH 23% > NaHCO3 22% > H2O 16%). The plant test showed that the different treatments had a different ARF (%) at the first harvest: P-chem (14.7)> C4 (14.4)> C3 (14.1)> C2 (3.4)>C1 (3.1), compared to the cumulated ARF (%) of the six harvests: C4 (50.1)> C3 (35.0)> C1 (21.1)> C2 (18.3)> P-chem (17.4). Data showed a good correlation of H2O-P vs. plant ARF at the first harvest and a good correlation of labile P (H2O-P + NaHCO3-P) vs. total plant ARF over 21 weeks. The free and labile P forms from SCE can be a valuable tool in the assessment of fast and middle term plant-available P from stable bio-waste composts in calcareous soils.
Keywords: Inorganic phosphorous; Organic phosphorus; Sequential extraction; Water extractable P; X-ray diffraction.
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