Influence of soil grinding degree on the determination of total nitrogen, total carbon and organic carbon concentrations

Abstract

According to ISO Standard 11464 (Soil quality -Pretreatment of samples for physico­chemical analyses) for test samples below 2 g, the air dried soil sample must be ground to a particle diameter ≤ 0.25 mm, in order to have a homogeneous test sample and, there­fore, minimize the variability of results. This is quite a time consuming task. The aim of this study was to compare the influence of two different degrees of grinding (<2mm and < 0.15 mm) on the determination of total ni­trogen, total carbon and organic carbon, with test samples of 0,5 g. These elements were analyzed in 7 different soils from Continental Portugal, derived from different parent mate­rial. Five representative samples were taken from each soil and manually crushed to pass a 2 mm plastic sieve. Each sample was then split in two portions: in one of them, nitrogen and carbon were analyzed directly on the fine earth fraction (< 2 mm); on the other portion, the analyses were carried out only after grind­ing the soil sample on a agate mill till it just passed a plastic sieve of < 0.15 mm. Total ni­trogen and carbon were determined by dry combustion on a CNS elemental analyzer, according to ISO Standards 13878 and 10694, respectively. Organic carbon was de­termined indirectly after correction of the to­tal carbon content for the carbonates present in the soil sample. Results suggest that the soil grinding degree had no significant (p>0.05) effect on C and N concentrations. Grinding the soil to 0.15 mm didnt increase significantly (p>0.05) the precision of results suggesting that, for the determination of total nitrogen or total and organic carbon, it is not necessary to grind the soil to a fraction less than 2 mm.