Dear Liz, et. al., CEC means Cation Exchange Capacity. That is, how much capacity does a given soil have to hold and exchange cations, which are positively charged ions? It's like measuring how big a larder one has. Some people have a 24' by 24' root cellar with 8' ceilings. Others may only have a 500 gallon drain tile sunk in the ground.
In general it is clay that provides the negative charges that hold onto cations, which are positively charged. Clays are aluminum silicates which can come in a wide variety of combinations with various admixtures. As well, phosphates as well as halides (e.g. chlorine, bromine, iodine) provide some stable negative charges. However, nitrates and nitrites, while negatively charged, are so soluble they cannot provide stable negative charge as they quickly enough wander off into the water table, as, indeed, positively charged ammonia volitalizes off into the atmosphere. Where you get some stability with nitrogen is when it is in the form of amino acids and complex humates. Carbon compounds can go either way, so organic matter acts as a buffer; and thus in sandy soils where there is little or no hope of applying any significant amount of clay to ammend the low cation exchange capacity, building organic matter commonly is the answer to raising the CEC. (I have found that with up-and-running BD corn can do this admirably with little or no nitrogen fertilizer as long as calcium is adequate.) The CEC can be filled with almost any mixture of cations. It could be 70% Mg, 15% Ca, 3%K, 1% Na and 10.8% H, with some traces thrown in, for example. But that would hardly be desirable. Ideally one hopes for about a ratio of betwseen 4 parts Ca to one part Mg to 7 or 8 parts Ca to one part Mg. Magnesium, being lower in the periodic table than its sister, calcium, works more strongly. It draws the light into the carbon framework in the leaf. Whereas calcium, working more moderately, draws nitrogen into the soil at the root. Available calcium levels must be fairly robust before significant nitrogen fixation can occur, especially free fixing of nitrogen by azotobacters, in the root zone. BD 500 is rich in azotobacters and creates the conditions for them to thrive--but there must be adequate calcium for this to occur, which is why both the chamomile and the oak bark remedies involve calcium. As you say, aluminum, though it is a plus three (trivalent) cation, is so tightly bound to silica that it does not become available in significant quantity until the pH gets rather low. Hope this helps. Best, Hugh Lovel >----- Original Message ----- >From: Liz Davis <[EMAIL PROTECTED]> >To: <[EMAIL PROTECTED]> >Sent: Friday, August 30, 2002 5:28 AM >Subject: Rain/CEC > > >> We also experienced our first rain yesterday and last night. Heralding in >> the Spring. The drought seems to have made it a mild winter, with my >> dreaded willows only loosing their leaves for 5 weeks. >> >> I'm trying to get my head around CEC, if you add Ca2+, Mg2+, Na+, K+, you >> get the CEC. The remainder is what, hydrogen? Is this assumed, what >about >> aluminium? >> Any comments would be greatly appreciated. >> >> Hope many went to sleep to the sound of rain on the tin roof last night. >> >> L&L >> Liz >> >Hi Liz > No rain in the Riverina - we'll probably get a wet summer I suppose. > Re CEC - first -who did the test? - most fertiliser company tests dont >count hydrogen in the exchange complex, and in an acid soil this can >artificially inflate your calcium % by 20 or 30 % easy. > The Perry, Brookside, and Swep tests that I have all show hydrogen in >the raw numbers from the lab but if you're working with the jazzy graphs >that consultants provide for these same labs, most dont show it. You would >then assume most of whats missing is hydrogen but leave a little for >assorted other bases (usually up to 2% but sometimes as high as 6). A rule >of thumb is if your pH (in water) is in the ideal 6 to 6.2 then exchangeable >hydrogen is close around 12% of CEC - we need this little bit of acidity to >keep some chemical activity happening in the soil > Aluminium is everywhere in soil but its locked up as alumino-silicates >and exchangeable Al only becomes available when soils get very acid - >increasing rapidly as pH goes into low 5's (water) or under 4.5 (calcium >chloride test) - this seems to happen a little quicker and worse in grey >gravelly soils than in red soil - these low pH soils grow a diverse range of >aluminium tolerant plants - blakeleys red gum (snappy gum - brittle gum) - >ironbark - black cypress pine etc - but when we try to farm them its a >disaster - serradella and lupins do ok but most normal crop and pasture >species just can't hack it without serious help. There is usually also very >low organic carbon so holding added calcium in the profile is a problem too. >Hope maybe this helps >Cheers >Lloyd Charles Visit our website at: www.unionag.org
