Cathy and all, It's all a matter of flocculation - or deflocculation. I did my Ph.D. on the chemistry of clay minerals, so I will try to explain in simple, layman's terms. Clay minerals are very small (usually less than 2 microns - about one ten thousandth of an inch) flat, platy aluminium silicate particles. Because of their chemical structure, they have negative electrical charges on their surfaces and, in some cases, in layers within the mineral structure as well. To balance these charges, and become electrically neutral, they absorb positively charged metal ions. The amount of metal ions they are capable of absorbing is known as the "cation exchange capacity" (CEC). Generally, the finer the clay, the higher the CEC, as lots of finer particles have more surface for absorption than a few larger particles of the same weight. Typical surface areas for clay minerals are between 100 and 600 square metres per gram of mineral or, roughly translated 30,000 to 200,000 square feet per ounce! Depending on what metals are absorbed onto the clay surfaces, the properties of the clays are changed. If sodium or potassium are absorbed, the clays are deflocculated - the individual particles tend to repel one another, and water (usually) is absorbed into the spaces between the particles. You end up with a typical very sticky clay. If, however, you can replace the sodium or potassium with calcium or magnesium, the individual particles begin to attract one another, stick together into bigger particles, and squeeze out the absorbed water. You end up with something that, although it still is 100% clay mineral, has particles that are more silt sized. Gypsum is calcium sulphate, and it works by simply replacing absorbed sodium and potassium on the clay with its calcium. To know how much you would need to use to be effective, you would need to determine the CEC value of your soil; it will be (relatively) low for clays that are formed dominantly of kaolinite (china clay), moderate for clays made of illite and chlorite (typical clays in Europe) and very high for clays formed from montmorillonite (otherwise known as bentonite). Hope this is some use to you! Paul Dr Paul Chapman, Wallington, Surrey, UK - south London commuter belt suburbia - zone 9a, but the coldest winter for about 10 years at the moment ----- Original Message ----- From: "Cathy Craig" <batlette@cox.net> Subject: [pbs] garden soil prep > In much of Southern California we have clay soil. In certain parts of our > lot in San Clemente, most notably those areas closest to the house and > garage, most everything below about 6 inches is pure clay. Not clay-ey, but > pure grey clay (no soil). In the front yard the people who put in the sod > applied some gypsum, which did seem to help. I am now working on the beds > and they need a lot more work than was done to sod the lawn. > > I bought a bag of gypsum today. Customarily I just dig out all the old soil > and have it hauled away. I am going to try amending some of it but there > aren't very many instructions on the gypsum bag and no explanation at all on > how it works or why it works or whether it 'wears out' over time and I will > have to re-do the whole job again in future. > > Cathy Craig President PBS