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Variable Charge of Ultisols due to Phosphate Application and Incubation Time

Author(s): Muhammad Mahbub

Journal: Jurnal Tanah Tropika
ISSN 0852-257X

Volume: 15;
Issue: 2;
Start page: 103;
Date: 2010;
Original page

Keywords: Negative variable-charge | pHo | phosphate | salt (CaCl2) concentration | Ultisols

The laboratoryexperiment was conducted to study the effect of phosphate (P) application and its incubation time on pHo (pH at thepoint of zero charge) and variable charge of ultisols . The determined parameters were pHo and variable surfacecharges.Soil samples were added by 0, 375 and 1,125 mg P kg-1 (or 0, 50 and 150% of the P sorption maximum,respectively). Then, they were incubated for 2, 4, 6 and 8 weeks. The variable surface-charges and pHo weredetermined by using the activity of potential determining ions (H+ and OH-) within two salt concentrations as counterions (0.1N and 0.001N CaCl2) through a potentiometric titration method. The results were indicated that the high Psorption and 766 mg P kg-1 in maximum sorption were due to high contents in clay fractions and aluminum as well aslow pH of experimental soil. Application of P and incubation time were able to decrease pHo and to increase negativesurface-charges. Additionals of 375 and 1.125 mg P kg-1 incubated for 8 weeks gave the value of pHo 2.86 and 2.69;as well as the magnitude of negative charges 14.48 and 16.76 cmol(-).kg-1, respectively (both for 0.001N CaCl2).For pH > pHo (the characteristic of variable charge soils), the higher the salt (CaCl2) concentration and pH solution,the higher the negative surface-charge.
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