Molecular Sizes and Functional Groups of Humic Substances Extracted by 0.1m Pyrophosphate From Soil Aggregates of Different Stability ¹

Summary Aggregates from non‐cultivated soil and from cultivated soil enriched with mineral fertilizers or farmyard manure were separated into fractions of different diameter and stability. The humic substances extracted from each type of aggregate were subjected to gel‐filtration on Sephadex. The more stable aggregates had a higher content of Na 4 P 2 O 7 ‐extractable humus and free iron hydroxides, and a predominance of humic substances of apparent molecular weight >100 000, whereas the less stable aggregates were richer in compounds of low molecular weight. Structure stability was lower in the cultivated soil than in the non‐cultivated, and in the soil with mineral fertilizers in comparison to that with farmyard manure. Humus of m.w. >100 000 had a COOH content twice as great and a phenolic OH content five times greater per unit of molecular weight than humus with m.w. <100 000. The former is the more suitable stabilizer of soil aggregates.