Clay particles normally lay together flat, but are repelled by the negative charges across their face. Salt (Na+) is present in minor amounts.
Soils with high clay content can become so dense and compact that they may resist plant rooting. This may happen for one of two reasons:
First, the salt in the soil has neutralized the negative electrical charges which normally cause clay particles to repel each other.
Second, the percentage of clay in the soil is so high that the positive charge on the edge of a clay particle combines with the negative charge on the flat surface of another, forming a tight three-dimensional structure.
Humic acid causes the clay particles to stand on end, allowing water penetration. It does this two ways.
First, it segregates salts and removes them from the surface of the clay particle. The net negative charge resulting causes the clay particles to repel each other, loosening the soil structure.
Second, a carbon group on the humic acid molecule (carboxyl group) bonds with the edge of the positively charged particles. This breaks the attractive force between the positive charge at the edge of a particle and the negative charge or the flat surface of another.
This action, called protective colloidal action, loosens soil, letting roots penetrate more easily. Humic acid’s effect on clay soil is more evident as time passes. In heavy clay soils, six months or more may be needed before you will see a noticeable improvement in the soil’s density.