Model for Long-term Phosphate Reaction Kinetics in Soil
- Sjoerd E. A. T. M. van der Zee * and
- Willem H. van Riemsdijk
A simplified reaction kinetics model for long-term P-sorption in soils receiving large quantities of P-rich animal manure is presented. Results from soil analyses for samples taken in 84 fields in nine different soil types indicate that the oxalate extractable P provides a good estimation of total P. Sorption experimental data for reaction periods up to 249 d show that measured sorption is proportional to In(t). Total sorption, F, (at long reaction periods) that includes P already present (Pox) appears to be proportional to the sum,M, of oxalate-extractable Fe and Al. These results are described with a model where soil reactivity is related linearly to M, and the reaction kinetics are expressed in terms of a composite concentration time variable, I. This leads to the relationship F = kM In(I), where k is a characteristic of the soil. Comparisons with results in literature indicate that the concentration dependence of the reaction is accurately described by using I instead of time as a variable. Differences in reactivity (k) for different soil horizons or soil types are apparent from the different k values for topsoils and subsoils. Sorption measured during 40 h (Fr) may be extrapolated to long-term (1–2 yr) sorption (Fm) for each soil separately without correction of the value of I for the reaction period in the field by simply setting Fm = Pox + 1.8 Fr. For a P concentration of 5 mmol L−1 this leads to a value of αm = Fm/M = 0.63 ± 0.14 for topsoils. Extrapolation using the average k value found for short-term sorption experiments results in the same value of αm.
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