>> gypsum. It is a great amendment to clay soil.
Actually, from what I read, that holds true only where the clay soil is also high in sodium ("sodic" soil, or saline).
I think the mechanism is that highly charged Calcium ions (Ca++) are able to displace Sodium ions (Na+) from the clay. (Clay has negatively charged surface sites that attract positively charged ions like Ca++, Mg++, Potassium (K+) and Iron FE++ or Fe+++).
When one clay particle becomes coated with enough Ca++ ions, there become extra + charges on the surface, and now that entire clay particle can form ionic bonds to the next clay particle.
Joy and Halleluiah, when enough clay particles clump together, you have FEWER clay particles, and more of something that acts like silt particles! A big improvement in soil structure.
But if there were very few Na+ ions to start with, added Ca++ doesn't help much or at all.
Assuming that Fresno California is in a big drought like the rest of CA, and that the soil has been at all fertilized in the past with barely enough irrigation, there is a fair chance that it has become desalinized (excess NaCl), and hence IS sodic.
