The major source of negative charge on humus colloids is due to

Answer: Option D

Humus is the organic component of the soil. The charge on humus colloids is dependent on pH. It mostly exhibits a low negative charge. Under strongly acid conditions hydrogen is rigidly bound and not easily replaceable by other cations.
The major source of negative charge on humus colloids is due to carboxylic, enolic, and phenolic groups.