The effects were measured of varying the pH and the concentration of adsorbing ion on adsorption of phosphate, citrate and selenite by goethite and on the charge conveyed to the surface. The ability of the model of Bowden et al. to describe these effects was investigated. The observed effects were closely described by the model, provided it was modified to permit the adsorbed ions to reside in a plane between the surface and the diffuse layer. The model requires that individual ionic species be considered. For phosphate and selenite, the divalent ion appeared to be the only ion adsorbed whereas for citrate it was the trivalent ion. The model also requires that adsorption depends on the electrostatic potential in the plane of adsorption. This potential decreases with increasing pH. Thus the effects of pH on adsorption were explained by changes in this potential, together with changes in the proportion of the ionic species present. Because adsorption made the surface more negative, it also decreased the electrostatic potential in the plane of adsorption. This made further adsorption more difficult, and as a result, adsorption at a constant pH did not follow the Langmuir equation. The model showed that the increase in negative charge as a result of adsorption was partly balanced by an uptake of protons by the surface. This was most marked at near-neutral pH and as a result the net charge per adsorbed ion was least.
Phosphate which cannot be desorbed from kaolinite by chloride at the same pH and ionic strength at which adsorption took place is significantly desorbed by citrate and bicarbonate.In the presence of citrate or bicarbonate, phosphate adsorption by kaolinite is reduced. The extent of reduction depends on the way in which phosphate and citrate or bicarbonate are added to the clay. Maximum reduction in phosphate adsorption occurs when phosphate and citrate or bicarbonate are added together or when phosphate is introduced after citrate or bicarbonate adsorption.These effects are attributed to the fact that mole for mole exchange between phosphate and the other anion does not occur. The reduction in phosphate adsorption results not only from competition but also from a concomitant change in charge.
A sap test has been developed to measure petiole nitrate and potassium concentrations in Sultana grapevines. Sap extraction was carried out after freezing the petioles to cause cell rupture. The thawed samples were macerated along with water in a blender. Merck RQflex(r) test strips were used to measure nutrient levels and the test strip colour intensity was measured using the RQflex reflectometer. The accuracy of the sap test was checked against conventional analysis and found to be satisfactory. Considerable variation in nitrate and potassium concentrations were observed in petiole samples collected in a vineyard. More studies are needed to determine the number of petioles that must be collected for the sap test. In the meantime, it is tentatively suggested that at least 200 petioles be collected for the sap test. Nitrate and potassium concentrations showed slight non‐uniform diurnal variations between 0800 h and 1600 h (Eastern Summer Time). The effect of diurnal nutrient variations on the test results may be reduced by collecting petioles between 0800 h and 0930 h. The petiole dry weight percent increased from about 6 to 12% during the season. This change should be taken into account when sap test results are converted from fresh to dry weight basis.
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