Hi Gene and all, My knowledge is even more antiquated than yours, I'm afraid. I graduated from college in 1956 and have not dealt with inorganic chemistry much since then. Many of the chemistry books I have on the shelf are almost that old, and none of them deals with inorganic chemistry. 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. As I understand it, phosphate can bind to the surface of the matrix of silicate and aluminate, and I surmise this is probably by forming covalent binds to them the same way they bind to each other. Phosphate would be too large to diffuse into the interior of the polymeric matrix, while cations do so readily. The negatively charged phosphate would not be attracted to the negatively charge interior of the matrix, but with cations present could bind to the surface. My vague recollection is that ammonium ions (NH4+) are bound loosely, and mainly by lignic acids in soils. It is also an ion exchange process. I'd like to hear what an inorganic or soil chemist has to say on this subject! We could see how far of the target I have wandered. Jim Shields in central Indiana (USA) At 10:41 PM 2/2/2008 -0500, you wrote: >Please bear with me -- I'm trying to remember things from my college days, >and that information has had 30+ years to degrade. (Corrections are >appreciated!) >But, if I remember correctly, soil clay particles have a net negative >charge. Certain plant nutrients, such as calcium, potassium, magnesium, >and ammonium, have a positive charge. Thus, these materials would be held >in the soil, bound to clay particles. Nitrate and phosphate, on the other >hand, have a negative charge. If not taken up by plants, they would pass >through the soil, to contaminate groundwater, or cause algal population >explosions in nearby ponds, lakes, and streams. Perhaps this is why high >levels of phosphate are recommended for crops. Ammonium bound in the soil >may not be readily available to plants, but it could be converted to >nitrate by bacterial action, which plants could then use -- a "long term" >source of nitrogen. >I wonder if fertilizer rates recommended for plants grown in regular soil >have much relevance to those of us who grow/garden in "soilless" media. > >Gene > >Eugene Zielinski >Augusta, GA > ************************************************* Jim Shields USDA Zone 5 Shields Gardens, Ltd. P.O. Box 92 WWW: http://www.shieldsgardens.com/ Westfield, Indiana 46074, USA Tel. ++1-317-867-3344 or toll-free 1-866-449-3344 in USA