The 2 g samples test portion were dried and then incinerated in a
muffle furnace at 460 ◦C for 15 h. The ash was bleached after cooling
by adding 2 ml of 2 N nitric acid, drying it on a thermostatic hotplate
and maintaining it in a muffle furnace at 460 ◦C for 1 h. Ash recovery
was performed with 5 ml of 2 N suprapure nitric acid, making up
to 15 ml with 0.1 N suprapure nitric acid. The determination of the
P was carried out by the 991.25 colorimetric method indicated by
the AOAC (1990) in solutions of samples resulting from the drying
treatment previously described. The determinations were carried
out by flame atomic absorption spectrophotometry (Perkin-Elmer
model 23801, Waltham, MA, USA), except for K, which was analyzed
by flame photometry. For the determination of Fe and Zn, a
1/100 dilution was necessary.