Measurement of Isotopic Exchangeable Soil Phosphorus and Interrelationship among Parameters of Quantity, Intensity, and Capacity Factors

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Abstract
The applicability of various techniques to measure isotopically exchangeable phosphorus for widely different soils was investigated. The E‐values obtained in the laboratory by carrier‐free (E‐value) and carrier‐methods (Ec value) were comparable to L‐values obtained in the greenhouse in low‐phosphorus fixing soils but were very much higher in high phosphorus fixing soils. In the latter group E‐values obtained by inverse‐dilution technique (EI.D. value) were more comparable to L‐values.The parameters for available phosphorus in terms of quantity, intensity and capacity factors were discussed in relation to soil properties. The E, Ec, and Q0 values were influenced primarily by total phosphorus content of soils whereas L value was significantly correlated with exchangeable Ca and Mg, soil pH, organic C and oxalate, and dithionite extractable Fe (Feo and Fed) of soils. EI.D. value was correlated with oxalate extractable Al and organic C of soils. Thus both L and EI.D. values were affected by soil characteristics that influence the availability of phosphate in soil. The intensity factor was mainly influenced by soil reaction. The parameters of capacity factor, both phosphate buffering capacity (PBC) and maximum buffering capacity (Mb) were significantly correlated with dithionate extractable Al (Ald) of soils. It is suggested that since parameters of quantity, intensity, and capacity factors were generally influenced by different soil characteristics, plant available phosphorus can be described better by consideration of more than one factor.

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