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

Abstract

A two‐step digestion with hydrogen peroxide (H₂O₂)/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/100g for the H₂O₂ treatment, and 0.196 g/100g 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/100g for H₂O₂‐N and 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 conversion with no need of additional laboratory analyses. Altogether, the achieved results clearly show that the tested methods work with homogeneous performance under a range of soil properties, providing evidence that the current digestion procedure by heavy metals catalysts could be conveniently replaced by the less hazardous “peroxy”; treatment. This really represents an attractive innovative procedure for routine determination of soil Kjeldahl‐N with reference to the environment safety concerns.

Notes

Associate Professor. Project leader, correspondent author.

Ph.D. Student. Assistant for nitrogen analyses.

Researcher, Dip.to di Produzione Vegetate, Universitá della Basilicata, Italy. Assistant for soil survey and laboratory analyses.