You should follow the AOAC (Association of Official Analytical Chemists) procedure.  For a food it is;

1.  Dry the the sample overnight at 105 Celsius.

2.  Finely grind the dried sample.

3.  Ash ~1 g (accurately weighed) of sample above at 600 Celsius for 2 hours.

4.  Reconstitute the ash with 10 mL of 1/3 HCl solution.  Heat if necessary.

5.  Dilute to 100 mL with distilled water, mix, and present to AA or ICP.

I think dry ashing the samples at 550 celsius for 5 hours would be appropriate. Then dissolving the ash into 5ml of 2N HCl and topping up with distilled water to 50 ml. Then analyse for K, Ca, Mg, Fe, Cu, Zn and Mn using atomic absorption spectrophotometer. For mineral P, ammonium vanadate method using absorption spectrophotometer at 420 nm. For details, check,'The Soil-Plant Analysis Laboratory by  ICARDA'

Check this article. 

Please see the steps provided by Bruce.

Kind regards

Thanx Bruce

Thank you all