Phosphorus sorption in soils of the Mekong Delta (Vietnam) as described by the binary Langmuir equation

Summary Phosphate adsorption isotherms were determined for 20, mostly very acidic, soils from the Mekong Delta. The experimental data were well described by a binary Langmuir equation which considers two groups of sorption sites that differ in their P bonding energies. The maximum P‐sorption capacities of these sites were related to the soil properties by simple linear correlation and by stepwise multiple regression. Results suggest that high energy sites are on Al‐oxihydroxides or small Al‐substituted Fe‐oxides and, to a lesser extent, on poorly ordered Fe‐oxihydroxides. On the other hand, the P‐sorption capacity of low energy sites is mainly related to clay content, and it increases as pH decreases. These sites are also positively correlated with organic carbon and poorly crystalline Fe‐oxihydroxides. However, as these two variables are closely correlated with each other, organic matter is likely to be considered as an indirect factor of P fixation through its association with Fe‐oxihydroxides and not as an important source of P‐sorption sites. The maximum sorption capacity, i.e. the sum of sorption capacities of the two groups of sites, is well described (r 2 = 0.88) by an equation that takes into account the four variables identified above: Al‐bearing oxihydroxides, poorly ordered Fe‐oxihydroxides, clay content and pH. Grouping the soils according to the orders of Soil Taxonomy, the P‐sorption capacity increases in the following sequence: ultisols < entisols < inceptisols. A P concentration often considered adequate for plant nutrition is 0.2 mg P 1 −1 solution, and only the high energy sites are involved in sorption at that concentration. Thus an equation including only Al‐ and Fe‐oxihydroxides could be used to fix P norms in these soils.