Determination of Ca, K, Mg and P in composts

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. 

Caicuiation. If the measured concentration of trace element was y pg m - this becomes yx 10 mg ml-k A sample of 1.2 g soil was extracted into 60 ml solution, therefore 1 kg is equivalent to 10 (50/1.2) ml. Therefore the measured concentration is equivalent to yx 10 x 10 (50/1.2) mg kg- which becomes 50/ mg kg air-dry soil. 

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