----- Original Message ----- From: Jose Luiz M Garcia
Hi Liz : Jose > "Im 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." > --------------------------------- > > The CEC is calculated taking into consideration the bases mentioned > above plus > Hydrogen and Aluminum which constitutes the acidity part of the CEC. I guess I am being picky on this but - exchangeable aluminium is a result of excess acidity brought about by too much hydrogen in the soil solution (for whatever reason). What Jose has said is not wrong but I think we should see this as a chain of events - the end result of which is an acid, aluminium toxic, low organic matter soil, the pH of which will be somewhere under 4.5 in calcium chloride solution test. Available aluminium will increase rapidly from 1 to 5% CEC at 4.5 pH to probably 15 to 25% CEC at pH 4.2 to probably in excess of 40%CEC at 4pH, add enough time and a moderate rainfall and we end up with a washed out sand with a layer of bauxite (aluminium ore) underneath as we find in Western Australia. (probably some other chemistry involved in this last bit) First comes the run down in base exchange (depleted calcium usually) and organic matter. Accompanied by increasing acidity (hydrogen) Then comes excess aluminium increasing eventually to toxic levels. Plant available aluminium is not a problem so long as we keep the soil pH up above that 4.5pH (calcium chloride) level! Lloyd Charles > Some > micro nutrients like Fe, Mn, Zn and Cu also should be considered but > their quantity > is so small as compared to the others that thay are not generally > considered in > the CEC > > > jose > > >
