THE EFFECT OF SAMPLE GRINDING ON EXTRACTABLE IRON AND ALUMINUM IN SOILS

Pyrophosphate-, dithionite- and acid ammonium oxalate-extractable Fe a nd Al are often measured on a finely ground aliquot of soil. Grinding provides a homogeneous sample from which a representative subsample ca n be obtained at sampling aliquots of less than 1 g. When a significan t portion of the unground soil is greater than the sieve mesh size use d when grinding, abrasion of primary minerals may cause increased leve ls of extractable Fe and Al. This study examined the distribution of e xtractable Fe and Al in some sandy soils and compared their levels in samples ground to < 500 mum and < 150 mum with those in unground < 2-m m soil. Samples ground to < 150 mum yielded the highest amounts of ext ractable Fe and Al. This effect was most prevalent when a motorized mo rtar and pestle was used. Samples ground to < 500 mum, which were freq uently re-sieved to avoid over-grinding, contained similar amounts of extractable Fe and Al as the unground < 2-mm sample. If less than 1 g of soil is used for analysis, a sample manually ground to < 500 mum (3 5 mesh) is advisable and a protocol for sample grinding is recommended .