Method 514 Determination of Ca K Mg and P in composts

The above authors did not specify the reagents, however, a suitable acid extractant for Ca, K, Mg and P would be the Mehlich 1 Extracting Solution (Mehlich, 1953), also known as Dilute Double Acid or the North Carolina Extractant. This is a mixed reagent of 0.0125 M H2SO4 and 0.05 M HCl, where relatively greater amounts of phosphate are dissolved using the mixed acids than by using HCl alone. This reagent would also dissolve any free carbonates. If necessary, activated carbon (NoritTM SG Extra, or equivalent), which has been purified by washing with extractant, is used to provide colourless extract solutions for the subsequent colorimetric determination of phosphate. Otherwise, the absorbance of solutions, without added reducing agent to develop the colour, are measured and subtracted from the normal absorbance readings (see Method 5.9a).

• Mehlich 1 Extracting Solution, (0.0125 M H2SO4 + 0.05 M HCl) - using a measuring cylinder, add 12 ml H2SO4 (approximately 98% m/m) and 73 ml HCl (approximately 36% m/m) to 15 l of water in a 20-l polypropylene bottle, dilute to 18 l and mix.

Procedure. Carry out the extraction and then determine phosphorus as in Method 5.9a, except that working standard solutions should be made up in the Mehlich 1 extractant. Determine potassium and magnesium as in Method 5.10, except that standards are made up in Mehlich 1 extractant. Determine calcium as in Method 5.2, except that standards are made up in Mehlich 1 extractant. The composition of composts is so variable that appropriate dilutions of extracts may be required, and calculations adjusted accordingly.


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