SAIFUL MD. ISLAM, A. M. MOAWAD
Georg-August-Universität Göttingen, Institute of Agronomy and Animal Production in the Tropics and Subtropics, Germany
In a calcareous soil, phosphorus (P) nutrition appears to be a limiting factor for successful crop production. To make use of a strategic P nutrition to grow maize in Bangladesh soil (initial carbon 2.2%), a mixture of organic and inorganic P sources (kgha P) as F0, F25 and F50 (100% inorganic + 0% organic, 75% inorganic +25% organic and 50% inorganic +50% organic P, respectively from oil cake, bone meal and triple super phosphate) were used. In addition biological agents Glomus manihot (Gm) a (V)AM mycorrhizal fungus, Bacillus megaterium var. phosphaticum (Bm) a phosphorus-solubilising bacteria and Rhizobium leguminosarum (Rh) a nitrogen fixing bacteria were applied in a factorial design. A modified sequential phosphorus fractionation scheme by TIESSEN and MOIR (1993) was followed to determine the labile (0.5 M NaHCO3 extractable), Al and Fe associated P (0.1 M NaOH extractable), Ca associated P (1 M HCl extractable) and residual P (Conc. H2SO4 and H2O2 digestible). pH of the initial soil was 7.6, which remained unaffected by the treatments. Available P (Olsen P) was found to increase from initial 6.0 upto [13.4]mgkg soil after the treatment. All the treatments showed an increase in available P with the increase in organic sources except Rh, which showed a reverse trend. Sequential fractionation of phosphorus showed a large quantity of Ca-associated (HCl-extractable P) phosphorus between 450 to mgkg out of 793 to mgkg of total P. Among the treatments Rh was found to cause a decrease in NaOH and HCl extractable inorganic P with increased organic P source. Labile inorganic P (Pip) and organic P (Porg) were varying most within the treatments. It ranged between 24.5 to 30.2 mg, 22.0 to 34.7 mg 2.7 to 12.9 mg and 0.8 to 10.9 mg per kg soil in NaHCO3-Pip, NaOH- Pip, NaHCO3-Porg and NaOH-Porg, respectively. Glomus manihot and Bacillus megaterium var. phosphaticum were able to enrich organic and inorganic P pools in comparison to Rhizobium. Organic sources of P fertiliser appeared to improve P availability to the crop and biological agents contributed to drive P among the organic and inorganic pools.
Keywords: Organic P source, P availability, P nutrition, phosphorus fractionation