Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Oceanography and Coastal Sciences

First Advisor

William H. Patrick, Jr


The study of P-sorption by soils has practical interest but little is known on the P-sorption characteristics of acid sulfate soils of Thailand, particularly under reduced soil conditions. The effects of pH, redox potential, and various forms of Fe, Al, and Mn on P-sorption of the actual acid sulfate soils (Sulfic Tropaquept) and the para-acid sulfate soil (Typic Tropaquept) were studied in laboratory microcosms. Soil suspensions with a soil to 0.01 M CaCl$\sb2$ solution ratio of 1:7 were incubated under various pH and redox potential conditions. After the incubation period, the soil suspensions were equilibrated with KH$\sb2$PO$\sb4$ ranging from 0 to 500 mg P kg$\sp{-1}$ soil. Some sets of soil suspensions were extracted by 1 N NH$\sb4$OAc (pH 4.0) and sodium-citrate dithionite solution (20%), intended to remove Fe, Al, and probably Mn in the forms of exchangeable and free oxides, before P addition. Three sorption equations were used to describe the P-sorption data. The classical Langmuir equation provided the best fit of the P-sorption data. More native insoluble P was released under reduced than oxidized conditions. Para-acid sulfate soil released more native insoluble P than did actual acid sulfate soils under both conditions. Para-acid sulfate soil also sorbed less added P than did actual acid sulfate soils. The P-sorption of both actual and para-acid sulfate soils was significantly affected by pH, redox potential, the extracted treatment before P addition, and their interactions. The P-sorption increased significantly with increasing pH and decreasing redox potential. Treating soils with NH$\sb4$OAc and sodium-citrate dithionite solutions reduced P-sorption, and decreased the differences in P-sorption due to the effects of pH and redox potential. The treatments did not remove the difference in P-sorption between the actual and para-acid sulfate soils. Closely significant correlation between P-sorption parameters and iron-oxides indicated the primary role of iron-oxides in P-sorption of the acid sulfate soils of Thailand. Aluminum-oxides seemed to play a secondary role in P-sorption of these soils. Manganese also showed a significant effect on P-sorption.