Why is humic acid obtained from leonardite is insoluble in water while sodium humate and potassium humate are water-soluble?

Humic acid got from leonardite is a kind of organic matter with a large molecular weight.
With a weak acidity, the hydrogen ions in it are difficult to ionize (the stronger the ability to ionize hydrogen ions, the stronger the acidity), which makes humic acid basically non-polar.
Since water a kind of polar solvent, and according to the principle of “Like dissolves like”, it is natural that humic acid cannot be dissolved in water.
on the other hand, sodium humate and potassium humate are salts of strong alkali weak acid.

When they encounter water, they will immediately dissociate into humic acid anions and potassium or sodium cations.
Under alkaline conditions, humic acid anions formed after the hydrogen ions are taken up by hydroxide ions are of a strong polarity and thus can be dissolved in water.
When the pH value is adjusted to acidity with acid, humic acid anions restore to the original humic acid molecules and become water-insoluble again.
This principle is employed to determine the potassium oxide content in potassium humate.