The total acidity of soil humic acid typically ranges between 580 and 890 cmol(p+) kg-1.

• 150-250: Range for carboxylic functional groups alone in low-acidity humic fractions.
• 250-350: Corresponds to phenolic hydroxyl functional group acidity alone.
• 640-1050: Standard total acidity range for fulvic acid, which contains significantly higher carboxylic density.
• 580-890: Standard total titratable acidity range (carboxyl plus phenolic-OH groups) for purified soil humic acid fractions.

• In heavy montmorillonitic clay soils (Vertisols), organic matter is tightly sequestered inside interlayer spaces and bound into resilient organo-mineral microaggregates; consequently, Humin (the alkali- and acid-insoluble fraction tenaciously bonded to smectite surfaces) constitutes the dominant and largest fraction of total humus (> 50-60%).
• Humic acid is alkali-soluble and precipitated by acid, remaining subordinate to humin in heavy clay soils.
• Gray fulvic acid is not a recognized primary humic fraction.
• Fulvic acid predominates in highly leached acidic soils (Spodosols).

• In the classical fractionation of humus (Sven Odén), hymatomelanic acid is defined as the ethanol-soluble fraction of humic acid; it is extracted with dilute alkali (NaOH) and dissolves readily in alcohol (ethanol), but precipitates in dilute mineral acids.
• Fulvic acid is the fraction soluble in both acid and alkali.
• Hymatomelanic acid does not dissolve in hot water alone.
• Neither hot nor cold water dissolves hymatomelanic acid.