SL28ave wrote:I'm still curious what the "ppb" test is; I am correct to assume it's a different test, right? Maybe it's a measure of P in the "soil solution", which is the liquid phase of soil? Maybe it's a measure of immediately available phosphate ions?
It is a solution test and finds the immediately available Orthophosphate (PO4). I can't track down the name of the test but I emailed a friend who can provide more details.
P can be divided into three forms in soil: Solution, active and fixed.
Orthophospate is the solution segment and represents the smallest amount, generally a fraction of a pound per acre, and is the only group that can have plant uptake and the only group that has mobility within the soil.
The active phase represents the potential contribution. This can range from a few pounds to a few hundred pounds per acre. This group requires water and microorganism mineralization before plant uptake. This P has reacted with calcium to become semi-soluble and requires just a little mineralization to complete the transition.
Finally we have fixed P which can range from a few pounds to hundreds of pounds per acre. This is locked up and entirely unavailable to plants. This can represent unused P that was applied inefficiently and then turned into crystalline variscite and strengite as they bond with Al, Mg and Fe in the soil.
If all three are measured we may see very high levels but it is entirely possible that available P will be very limited.
Reading some studies that were done 20-40 years ago I noticed some things that will undermine soil health. One in particular referred to the use of paraquat, other fungicides and pesticides. Use of microbe-killing chemicals will alter the microorganism make-up in the soil, adversely altering the transfer of active-phase P to PO4. Since most fungicides and pesticides kill indiscriminately it is a very valid concern when we discuss plant health and P metabolism at the soil level.
P is one of the most over-applied fertilizers because very little effort is made to address overall soil health. Relying on Bray-1 is a bit like flying without instruments in fog, while it does measure active and solution P and PO4 it also finds P bonded with Al and Fe. The latter two are rarely made available to plants.