ChileanJAR

The kinetics of phosphorus retention was studied in 16 surfaee solls: 8 alluvial (centralland) and 8 volcanic (south land). A shaking method for 72 hours, a soil/solution ratio of 1/20 and solutions containing 0.258 and 1.033 mmol P/L for the alluvial and volcanic soils, respeetively, were used. Two kinetics step were detected. In the alluvial soils, the lirst took from 8 to 16 h and the seeond showed an asymptotic behaviour, while, in the volcanic soils the first took from 2 to 4 h and the second showed a state of equilibrium. The data were adjusted to the modified Elovich equation obtaining high determination coellicients for the alluvial soils and Ultisols (R² = 0,946 to 0,997), and slightly lower for the Andepts (R² = 0,858 to 0,897). The α and β constants varied with the type of soil; for the volcanic soils α was 10 to 1000 times greater than lar the alluvial soils, and was correlated (P < 0.01) to pH in KCI, extractable Al at pH 4.8, AI-oxalate, Si-oxalate and water retention at 33 kPa. For the alluvial soils u was not correlated to the measured properties neither an ellect of CaCO₃ was detected. The β constant was 10 times higher for the alluvial soils. It was not correlated to the anlyzed properties. It is postulated that: 1) the dominant mechanism for the P retention would be one of chemical adsorption, 2) it would also occur a more physical adsorption (slow step) for the alluvial soils, and 3) the modified Elovich equation is adequate to interpret the kinetics of P retention fer the volcanic soils.