Sodium bicarbonate as an extractant for soil phosphate,II. Effect of varying the conditions of extraction on the amount of phosphate initially displaced and on the secondary adsorption

The effects of varying the period of shaking, the concentration of bicarbonate, the pH, and the dryness of the soil on the phosphate extracted by sodium bicarbonate were investigated. A regression procedure in which the soil: solution ratio was the independent variable was used to separate the effects on the amount of phosphate initially displaced from those on the secondary adsorption.Both the amount initially displaced and the amount of secondary adsorption increased with period of extraction. At small soil:solution ratios the effect on displacement was dominant and the net effect was a continuing increase in the amount of phosphate in the extract. At large soil:solution ratios, secondary adsorption eventually predominated and caused a decrease in the amount of phosphate in the extract.Increasing the bicarbonate concentration from 0.01 M to 1.0 M decreased secondary adsorption but had no significant effect on the initial displacement. The effects of changing the concentration were most marked at low concentrations.Increasing the pH of 0.5 M bicarbonate solutions increased the amount of phosphate initially displaced but had no significant effect on secondary adsorption. In the absence of bicarbonate, the effects of pH differed: increasing the pH decreased secondary adsorption. Secondary adsorption was important when the ratio of soil to solution was large and in these cases the net effect of pH on bicarbonate - as measured by difference - was greatest near the pK₂ of carbonic acid.Drying the soil increased the amount of phosphate displaced from the soil after 30 min extraction but the effect was eliminated by extending the period of extraction to 16 h.