Dear All, It's not often that I organise myself to respond to emails on a reasonable timescale to make sense to those following a thread, but I've been at home for the last 2 weeks (very unusual with my job), and am up to date on receipts. Jim may be a biochemist, but he has a good memory... my Ph. D. (1986) was on the breakdown of clay mineral structures as rocks weather into soils. Clay structures are exactly what Jim describes, and do exactly what he says. The only question is - how much? This depends on the surface area of the clay, NOT the surface area just on the outside of the clay particles, but the surface area of the "regular cavities", as Jim describes them, within the particles. Kaolins don't have many of these empty layers, so have a low surface area, and low cation exchange capacity. Illites and chlorites, which are the most common clay minerals in temperate northern hemisphere soils, except where something special like volcanic activity has gone on, have moderate surface areas. Montmorillonites and bentonites, which are the clay minerals used in drilling muds and cat litter, have large surface areas, and very large cation exchange capacity. Typical surface areas (sorry about the metric units) vary from 10-20 square metres per gram for kaolintes, through 50-100 for illites, up to 600-700 square metres per gram (my guess 200,000 square feet per ounce?) for montmorillonites. If you happen to garden on monmorillonite clays, don't waste your money on potassium fertiliser - you will have spent many thousands of dollars before the exchange capacity of these clays is taken up, and there is some free potassium for your plants. However, it is, of course, released as the clay minerals break down over thousands of years... Paul Dr Paul Chapman, Wallington, Surrey, UK South London commuter belt suburbia - zone 9a, where today I have got the first flowerings of Crocus angustifolius, paschei and candidus, all from seed exchange seeds and, I'm delighted to say, all true to name - many thanks to the donors, whoever they were ----- Original Message ----- From: "J.E. Shields" <jshields@indy.net> Well, I changed it back and forth a couple of times. I settled on "absorb" because the kaolins can do an ion exchange, switching Ca++ for K+ in the internal matrix of the clay. On the other hand, ion exchange phenomena are probably best thought of as "adsorptions" as well, so (this morning) I tend to agree with you. I have heard about the "different" clays of the North American West, but I've not had any personal experience gardening in them. I think you have it pretty much correct. Clay is an inorganic polymer, mainly silicate and aluminate forming the extensive covalent polymeric matrix. This matrix is anionic (i.e., negatively charged) and does reversibly bind cations (i.e., positively charged ions). There are regular cavities throughout the polymer, and these are where the cations are located. Jim Shields