STUDIES OF SOIL ORGANIC MATTER USING FORMIC ACID SOLVENTS: FACTORS AFFECTING COMPOSITION AND DEGREE OF FORMYLATION OF ORGANIC ISOLATES

Two soils were extracted under reflux with formic acid/H₂O for 0.25 h, followed by formic acid/HF for 0.25 h. In one set of experiments ¹⁴C‐labelled formic acid was used in the first extraction and unlabelled formic acid in the second: in a second set this labelling sequence was reversed. After separation from the soil residues, the soil extracts were further refluxed for up to 3.75 h. Organic matter was then precipitated from the various extracts by addition of di‐isopropyl ether. Prolonged boiling of the extracts reduced the recovery of organic matter and nitrogen, and increased the inorganic contamination of the organic isolates. The degree of formylation of the isolates from the first extraction with formic acid/H₂O increased with reflux time to 0.50 h and thereafter decreased, but overall it was appreciably greater than that of the isolates from the second extraction with formic acid/HF. Formylation of the latter increased slowly with reflux time up to 4.00 h. In both solvents hydrolysis and formylation of organic solutes occur concurrently during refluxing. Treatment of formic acid/H₂O extracts with a TiCl₄/HCl/propan‐2‐ol reagent to precipitate organic matter and washing the precipitate in a hydrous HF medium resulted in lower yields than precipitation with di‐isopropyl ether, but the final washed precipitate contained much less formyl groups and inorganic material.