Originally Posted by Smallaxe
That would be interesting to know... They've recently shifted the the idea that P doesn't leach,,, yet now they are highlighting the conditions in which there may be some leaching...
This was your quote the other day. As far as I know, P has never been thought to leach at all, so I don't think that anyone is highlighting conditions under which P may leach.
You are right to recognize that K is not as tightly held on cation exchange sites as P. But, soil K is a different animal.
K usually hangs out in four places in soils.:
1) Unexchangeable K is held in K-bearing minerals, like micas and
feldspars (90-98% of soil K). This is NOT leachable and NOT plant
2) Unexchangeable K can be held inisde layers of 2:1 clays (1-10% of soil
K). This is NOT leachable and NOT plant available.
3) Exchangeable K can be adsorbed to soil particles on CE sites and in soil
solution (1-2% of soil K). This is somewhat leachable and is totally plant
4) Exchangeable K can be bound in OM (<1% of soil K).
A study by Jerry Sartain on sandy soils at University of FL in 1998 showed K source to be an important factor in leaching loss from turf. K2SO4 required 50 inches of water to move any K below the rootzone, while K3PO4 required 100 inches of water to move any K below the rootzone. Maximum leaching depth was 10 inches.
So, K can be leached, but it doesn't move far.