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Phosphate Adsorption by Two Highly-weathered Soils

Dep. of Plant and Soil Science, P.O. Box 1208, Alabama, A & M, Normal, AL 35762

*Corresponding author.

ABSTRACT

Phosphorus adsorption by two highly-weathered soils, Decatur clay loam (clayey, kaolinitic, thermic, Rhodic Paleudult) and Hartsells sandy loam [fine-loamy, siliceous, thermic, Typic Hapludult] from northern Alabama was studied. Virgin and cultivated soils were used and adsorption curves developed at 15 and 25 °C.

From these data the isosteric heat of adsorption (
), the Langmuir adsorption maximum (b), and binding constant (K) were calculated. The virgin soils which had lower pHs and higher exchangeable Al and free Fe oxide, adsorbed more P than the cultivated soils. At the higher temperature, P adsorption (x/m) and K increased, indicating that P adsorption was an endothermic process and suggesting that P was held more tightly to the surface. Generally virgin soils had higher
s than the corresponding cultivated soils in agreement with the calculated K for the soils.
decreased with an increase in surface coverage, finally levelling-off for all soils which gave curvilinear (two slope) Langmuir plots. For the Hartsells cultivated soil, however,
was almost constant and a near perfect fit of the linear form of the Langmuir equation was obtained. This suggested that, in this soil, P was adsorbed basically with one energy of adsorption and may explain the good fit to the linear Langmuir equation. The curvilinear lines gave straight lines when plotted according to the Brunauer-Emmett-Teller (BET) equation. We speculate that P adsorpation by the soils may be described by at least two previously hypothesized mechanisms of adsorption.

Contribution from the Dep. of Plant and Soil Science, Alabama A & M Univ.

Received for publication July 28, 1987.