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The PZNC was observed at pH 6.96, 6.89, and 5.98 for the KW, PJ, and WG samples, respectively.
Soils dominated by Fe and A1 oxides and hydrous oxides typically exhibit a PZNC within the range 2.7-6.5, depending on the organic matter content (Theng 1980).
The PZNC of BRS was within the range of pH 6-7, which is consistent with values reported for the individual reactive mineral components of BRS.
For example, when pH was higher than the PZNC of the Oxisols, the soils possessed net negative charge, and the surface potential and the potential in the adsorption plane were negative.
When the pH was lower than the PZNC of the soils, they possessed a net positive charge, and the surface potential and the potential at the adsorption plane were also positive.
3), although the IEP was lower than PZNC and PZSE of the soil due to the use of soil clay fraction in zeta potential determination.
Point of zero charge of variable components (p[H.sub.0]) and point of zero net charge of the whole sample (PZNC) for laboratory-synthesised goethite and Ti-substituted goethite Ca[Cl.sub.2] KCl p[H.sub.0] PZNC p[H.sub.0] PZNC Control goethite 6.8 6.7 7.9 8-9 (A) 10% Ti-substituted goethite 5.7 5.5 6.3 6.3 20% Ti-substituted goethite 5.6 5.7 6.4 6.4 (A) Low values of Cl adsorption render PZNC value indeterminate.
The PZNC was calculated from the [K.sup.+] and [Cl.sup.-] adsorption data using:
[q.sub.K]- [q.sub.Cl] = 0) to allow estimation of the PZNC (Table 3).
1a) through deprotonation of edge sites on aluminosilicate minerals such as smectite (see above--Ion adsorption and PZNC).
The very low, essentially undefined value of PZNC is consistent with the explanation that permanent negative charge increased PZSE but not PZC (Gillman and Uehara 1980).
Values of point of zero net charge (PZNC) and permanent charge for surface and subsurface soil PZNC (A) PZSE Permanent charge ([cmol.sub.c]/kg) Surface soil, 0-15 cm depth Irrigated Plot 1 <1.0 5.0 -12.4 Irrigated Plot 2 <1.0 5.0 -9.0 Control Plot 3 <1.0 4.8 -10.3 Control Plot 4 <1.0 4.3 -4.0 Subsurface soil, 30-45 cm depth Irrigated Plot 1 <1.0 3.6 -5.8 Irrigated Plot 2 <1.0 4.0 -9.3 Control Plot 3 <1.0 4.2 -7.8 Control Plot 4 <1.0 3.6 -4.3 (A) Determined by ion retention.
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