Comparing tests for soil fertility.11. the hydrogen peroxide/sulfuric acid treatment as an alternative to the copper/selenium catalyzed digestion process for routine determination of soil nitrogen-kjeldahl

A two-step digestion with hydrogen peroxide (H202)/boiling sulfuric acid (H2SO4) (“peroxy treatment”) in the absence of heavy metals catalysts is proposed as an alternative to the copper (Cu)/selenium (Se) catalyzed digestion process for the routine determination of soil Kjeldahl-nitrogen (N). Sixty-eight samples from pedotypes representative of the Mediterranean environment, with contrasting features and use, were chosen in order to ensure the largest variability of total N and organic carbon (C) contents. The average soil N content was 0.223 g/lOOg for the H2o2treatment, and 0.196 g/lOOg for the Cu/Se one, with significant difference between the means and with limits ranging from 0.042 to 0.678 and from 0.025 to 0.696 g/lOOg for H2o2-nand Cu/Se-N, respectively. The amounts of soil N determined by the contrasted procedures were strictly dependent on the respective organic C contents, and they correlated each other by the highly significant linear regression: Cu/Se-N = -0.017 + 0.955/H2O2-N, R2 = 0.910. The goodness of fit of this equation was not significantly improved when the organic C and the Al-Oxa contents were introduced as accessory explanatory variables in multiple regression models. This should allow an easy and quick data. © 1995, MCB UP Limited