A chemical study of quick-test technics for potassium and calcium
1942
Melsted, S.W.
A study of the quantitative extraction of replaceable potassium and calcium from soils by several extracting solutions was made. Technics used in the determination of these bases in the extracts were evaluated. The following conclusions are based on the data presented: 1. The accuracy of any quick test for replaceable bases depends primarily on the quantitativeness with which the extracting solution removes these bases from the soil. 2. None of the extracting solutions recommended for the simultaneous extraction of all the nutrient ions has been found to extract all of the exchangeable bases quantitatively. 3. Any quick-test method that measures the aliquot of extract in drops is subject to errors. 4. A turbidimetric or colorimetric method that does not give accurate results with standard solutions cannot be expected to work satisfactorily for quick tests. 5. The determination of potassium in a filtrate by methods involving modifications of the cobaltinitrite method may show variations caused by (a) the order of adding the reagents, (b) the temperature at which the precipitation is made, (c) the mixing technic involved, (d) interference by other ions, (e) the ratio of alcohol to solution, and (f) the method of estimating the amount of precipitate formed. 6. The turbidimetric determination of calcium as the oxalate in the filtrate is subject to errors that may be caused by (a) order of adding reagents, (b) the time interval between the adding of reagents and shaking, (c) the amount of calcium present, (d) the use of stabilizing agents, (e) the pH at which the precipitation is made, (f) method of estimating the amount of precipitate formed, and (g) presence of interfering ions.
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