Graduate Thesis Or Dissertation

Phosphorus characterization in Sri Lankan Alfisols : Soprition, fractions, plant availability and soil test suitablitiy

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  • Twelve Sri Lankan Alfisols were characterized for P. Soils were incubated for three weeks with sufficient P applied (P2) to elevate soil solution P to 0.2 ug P/g solution as determined by sorption curves, with 75 percent of P2 (P1), and without P (P0). For PO treatments, (a) organic P, (b) 0.5 M NaHCO₃ extractable "labile" inorganic P, (c) 0.1 M NaOH extractable "moderately labile" inorganic P, and (d) concentrated HC1 extractable residual P fractions contained 44, 10, 13, and 22 % of total P respectively. Fifty five percent of applied P (P1 and P2) was recovered in the inorganic 0.5 M NaHCO₃ fraction and 25 percent in the inorganic 0.1 M NaOH fraction. Significant amounts of applied P were recovered in the organic fractions of some soils. Factor analysis of PO treatment fractionation data suggested that inorganic and organic fractions were separate equilibrium systems. Both inorganic and organic systems, however, contributed significantly to P uptake in a Neubauer experiment. Moreover, applied fertilizer P appeared to increase contributions of organic P to P uptake. Correlation of Neubauer plant uptake data to Olsen, Bray 1, Bray 2, CAL, and Double acid extractable soil P showed that P uptake was most highly correlated to Double acid extractable P. Regression analysis suggested that the higher correlation was due to the Double acid extractant's ability to measure the organic P contribution to P uptake. Phosphorus sorption by the soils ranged from low to medium and was described satisfactorily by either the Langmuir two surface or the Freundlich equation but not by the Langmuir one surface equation. Correlation analysis indicated that oxalate extractable Fe was the soil component most active in P sorption. Although organic matter and pH were correlated to sorption capacity and may have been directly involved in the P sorption mechanisms they could also have covaried with other components responsible for sorption. Although citrate-dithionite extractable Fe did not appear to be involved in P sorption its high correlation with residual P fractions suggests an involvement with "slow" sorption. Phosphorous sorption had no apparent influence on P uptake by foxtail millet from the 12 soils in a Neubauer experiment.
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