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Extraction of Organic Nitrogen and Ammonium From Soil with Hydrofluoric Acid¹

ABSTRACT

From about one-fifth to one-half of the organic N in some surface soils, and over two-thirds of that in some subsurface soils, occurred in compounds capable of being extracted with HF. The NH₄⁺-N/organic N ratios in HF extracts were generally highest in soils where the predominant clay minerals were illitic, intermediate in soils where the predominant clay minerals were montmorillonitic, and lowest in soils where the clay fraction contained high amounts of amorphous (allophanic) material. The presence of organic N compounds within the lattice structures of clay minerals was indicated. Analyses of the Morrow Plot soils showed that a considerably higher fraction of the organic N in the heavily cropped soils was extractable with HF than in soils where organic matter had been maintained at high levels by periodic application of manure or through growth of legumes. Evidence was obtained in support of the view that many soils contain significant amounts of naturally occurring fixed NH₄⁺.

¹ Contribution from the Department of Agronomy, University of Illinois, Urbana. Supported by Regional Research Funds (NC-55) and by a grant from the National Science Foundation (GP-1938).

² Professor of Soil Chemistry and Research Assistants, respectively. S. N. Tilo is now Assistant Professor of Soils, University of the Philippines, Laguna, Philippines.

Received for publication June 6, 1966. Accepted for publication September 21, 1966.