Application of Kinetic Models in Describing Soil Phosphorus Release and Relation with Soil Phosphorus Fractions across Three Soil Toposequences of Calcareous Soils

Abstract

Knowledge on P release characteristics and availability of different forms of soil phosphorus can be useful for crop production. Kinetics of P release and distribution of inorganic P from in calcareous soils at summit, shoulder, back slope, foot slope and toe slope of three soil toposequences (Fars province southern Iran) were determined and the relationship between P release and soil properties with phosphorus form was investigated. The kinetics of P release in the soils was determined by successive extraction with 0.01 M CaCl₂ over a period of 72 hours. The kinetics of P release followed Elovich, power function and first order kinetic models. The results demonstrated that the pattern of P desorption was similar in all topographic units of the three regions at all times. The highest amount of P release was observed in the lower part of the slope specially in Eghlid and Abadeh (xeric and aridic regimes). Generally, the average content of cumulative phosphorus release and most forms of phosphorous and physical and chemical characteristics in Eghlid soils were greater than other studied soils. Different P forms were determined by sequential extraction; the fractionated inorganic P forms namely apatite type and dicalcium phosphate possessed the highest and the lowest amounts of P reserve in the soils, respectively. As a result, P release and various P forms distribution within the soil profiles along toposequences varied in different topographic units and increased down the slope with an irregular trend. Due to such variability, P fertilizers are recommended by consideration of the physiographic units.