Surface adsorption of phosphorus and determination of its buffering indices in different soil orders along a climo-toposequence

Soil phosphorus (P) buffering capacity of the soil is very important in plant nutrition and it is the most important soil property for determination of P release potential. In this study, 23 soil samples from different horizons of five orders along a climo-toposequence were collected to determine P sorption characteristics, P buffering indices and their relationship with the soil properties. According to the results, the L-shaped adsorption isotherm obtained from all the soil samples showed a high relatively tendency of the soils for P adsorption at low concentrations, which was well-fitted by the Freundlich, Langmuir and Temkin equations. This study also indicated that the adsorption processes in P concentrations less than 20 mg/l and precipitation processes in P concentration greater than 20 mg/l control the soil phosphorous. Among the different soil characteristics, organic matter and cation exchange capacity have a significant positive correlation with most of the P sorption properties. The overall results of this study showed that the Histosols and Mollisols orders (located at the end of the studied area), Aridisols and Inceptisols (located at the beginning of the study area) and Alfisols (located in the middle of the study area) have had the high to the low values of P buffering capacity, respectively. Therefore, simultaneous low and frequent application of organic and chemical fertilizers are suggested to reduce the P adsorption and to increase the P release at the end of the study area due to high phosphorus adsorption capacity.