On Fri, 07 Mar 2003 13:26:54 -0600, (Joe Doe)
wrote:

You are partially correct (& practically correct). Phosphorous is
generally found as a precipitate because it precipitates with Aluminium
and Iron at acidic pH ranges and with Calcium in Alkaline pH ranges. So
under many common soil conditions it will not be mobile or available.
Because our soils are so alkaline it is generally found as a calcium
precipitate. If you do suceed in moving the pH to neutrality with sulfur
more phosphate becomes available but it is unlikely to ever really
runoff.

Phosphorous is generally associated with surface runoff rather than
through percolation.

OTOH a relatively fast moving, dilute P solution formed during a storm
event over porous soil offers a higher likelihood of phosphorous
runoff than that posed by normal irrigation rates. Since phosphorous
is rate-limiting for O2-depleting algal blooms even a few percent
runoff is significant,

The fact that phosphate levels are of the scale is widespread: for
example in the book Gardening In the Humid South (written by two retired
Louisiana State Horticulture researchers one of whom has expertise in soil
science) they say this is also true for all measurements made in
Louisiana. They make the same point that you do: even though the
Phosphate is off the charts, that does not mean it is available. They
recommend side dressing. They approach this reluctantly because the
phosphate sources are not renewable yet do come down on the side of adding
some.

It strikes me as rather like maintaining an addiction that you know
one day you must kick or it will get you in the end.

Do they propose a turning point - some method by which the overuse of
phosphorous fertilizers will be reversed and soils restored?