In this benefits of calcium pt 3 there is an explanation of a soil colloid and what it does, which is something I have always had trouble trying to understand.

Below is one of the best and more easily understood explanations of what a soil colloid does to effect the exchange of minerals from the soil to the plant. And how adding crushed limestone improves all soils with low calcium content.

It is from page 90 of the pdf Olena Farms, referred to in the previous post Calcium Part 2

Soil colloids may be either clay or organic matter. The clay colloids include minerals that are chemically active and possessing negative electrical charges, which will combine with calcium ions when limestone is applied to the soil. Since calcium has a weak affinity for water the saturated colloid is associated with low amounts of water resulting in very little swelling of the soil, and thus allowing very little shrinkage when the soil dries out.

Such soils show very little cracking in severe droughts. If calcium is low or deficient in the soil, the charges on the colloid will combine with other ions such as sodium, potassium, ammonia, or others. These ions will hold more water molecules which causes the colloid to swell considerably. Since these colloids form a film around fine sand, gravel, and other particles the swelling will tend to fill in the air spaces between larger soil particles making it difficult for water to penetrate readily so erosion allows most of the rainfall to run off and end up in the streams and lakes.
Thus, if we are going to build up our so-called “worn-out” soils to produce more food for the future and to insure better health, our first consideration must be to saturate the soil colloids to 85 per cent of their capacity with calcium before we can expect to make progress using commercial fertilizer or producing more food stuffs to feed hungry mouths.