Hi Jamie and all, We can rely on Jamie to bring up some thought-provoking ideas! The point at which Ca, K, and Mg compete, regardless of soil pH, is in the absorption receptors on the surface of the root hairs. These receptors are large proteins that extend through the cell membrane and have a pore in the center that leads from outside the cell to inside the cell. Here is where these three ions (ion = atom with electric charge) can interfere with each other, again because of similarities in size or electric charge. They have to pass through that pore to get from outside the plant to inside the plant. A point that is often made in regard to hydroponic culture -- and growing in soilless medium with continuous liquid feeding is much like hydroponics -- plants only absorb the nutrients they need and can use. The rest they ignore. That is why I said excess phosphate (PO4 with negative charge) is relatively harmless. Natural soils can contain a variety of constituents that interact chemically with plant nutrients. Clays can adsorb ions on their surfaces and can absorb ions internally. Add calcium to clay and the fine structure of the clay changes, leading to a gross change in the texture of the clay -- from gooey when loaded with K (potassium) to grainy and crumbly when loaded with Ca (calcium). Phosphate can, if I recall this correctly, adsorb onto the surface of clays. These adsorbed phosphate ions can then bind Fe (iron) and Ca (calcium) ionically (that is, as salts). Calcium and iron phosphates are quite insoluble at certain pH values. This in fact is the mechanism by which phosphate can deplete the available iron in a soil. An element must be soluble, as an ion in solution, for a plant to be able to take it up. Soils also contain lignins and lignic acids that can bind cations (positive ions like Ca, Mg, and K). These lignins are ion exchange agents, the cationic bindings are readily reversible, and lignic acid bound K (potassium) is a dynamic reservoir for K in soils. Excess rain can indeed leach the nutrients like Ca, K , and NO3 out of the topsoil and carry them away in the groundwater. In soilless media, as in true hydroponics, there is little in the way of commensal flora to assist the plants by processing nutrients from unavailable forms (e.g., ammonium, NH4 ions) to forms that the plants can physically absorb (i.e., nitrate, NO3 ions). We use soilless media because it also provides little support for pathogenic flora. My potting medium for bulbs is generally a mixture of Promix BX biofungicide (i.e., with commensal bacteria that eat fungi) + sand + granite chick starter grit in a ratio of about 2 : 1 : 1 by volume. This is my "gritty mix." I fertilize continuously from February through September (in the greenhouse) with Jack's Professional Peat Lite, a 20-10-20 with 2/3 N as nitrate (I'd prefer 100% as nitrate!) and most micronutrients, including some Fe. I inject it into the irrigation water to a final concentration of 100 ppm nitrogen (100 milligrams of N per liter). I'd also drop the P (as phosphate) to 5% if I had my 'druthers. Jack's Pro does contain a little iron, but not enough. I have to interrupt feeding once or twice a year and inject chelated iron for one cycle, to keep the plants from becoming a bit chlorotic. We grow arid land bulbs in greenhouses in the American Midwest. We are fighting climate, seasons, and latitude. It's about as unnatural a situation as you can get and want exotic bulbs to survive. Jim Shields in central Indiana (USA) where we are waiting for another winter storm to make up its mind At 12:06 PM 1/31/2008 +0100, you wrote: >Jim, > >this is an important point you mentioned with Ca, Mg and K competing for >free ions to combine. They do, but, as you wrote, the reaction is >largely governed by pH. Applying these elements at different times >would be the wisest solution, otherwise they may not be effective. > >The same can be said of phosphates, but they will precipitate with Ca+ >at higher pH, say around 7.2 or higher. I do not know at which pH it >combines with other elements. > >Again, with phosphor compounds, they have a wide range of 'users' in >bio-systems. All plants require them, which may mean that in a pot >there are more takers to consider than the main plant. Algaes are huge >absorbers of PO4. Perhaps we have a situation similar to nitrogen in >orchid bark? > >Just some more thoughts, > >-- >Jamie V. ************************************************* 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