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Old 09-13-2012, 02:26 PM
Skipster Skipster is offline
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Join Date: Dec 2010
Location: Billings, MT
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Originally Posted by Kiril View Post
Solution P is an anion. It is incorrect to speak of P being bound by the CEC.

Further, P can and does leach. A review of available published literature on P mobility in soils will reveal this.
Kiril, you are kind of correct, but you're forgetting your soil knowledge here.

1) In acidic soils, mineral surfaces have a net positive charge (both + and - exist, but the +s outnumber the -s), so some phosphate ions are adsorbed electrochemically.

2) Phosphate ions don't exist as free H2PO4- and HPO4-- very long. They react very quickly and readily with Fe, Al, Mn, and Ca ions on CE sites and in solution. In acidic soils, Fe and Al are the main bonds. B/c the Al-O-P and Fe-O-P bonds are very strong, desorption is very difficult.

3) In alkaline soils, Ca and Mn bond more readily than Fe or Al and sequester phosphates on cations attached to CE sites.

So, while phosphates don't directly adsorb to CE sites, they bond with cations that are directly adsorb to CE sites and they adsorb to AE sites.
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