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07-03-2003, 07:44 PM

In article ,
(Victor M. Martinez) wrote:

Yes, but it while it may be toxic levels, it is not toxically available
unless
it is met with other elements.

It is not available, period! While there might be an excess of phosphorous,
that doesn't mean plants don't need bio-available phosphorous to thrive.
Our plants respond great to bone meal. I'm sure if we test our soil it would
be high in phosphorous.

--
Victor M. Martinez

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.

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.

Another interesting point raised in this book is how difficult it is to
raise soil organic content. The example they run through is the
following: the top 6 inches of 1000 sq ft of soil weighs 46000 pounds!!
To raise the organic content by 1% you need to add 460 pounds of stable
gums and humates and this will take 4600 pounds (dry weight) organic
matter because the stable gums and humates are about 10% of the original
dry weight. In practical terms this would require a layer of 2 feet of
Oak leaves. Similarly a cover crop only yields about 230 pounds of dry
weight/100 sq ft . I know compost by wet weight is about 1000 pounds a
yard. Its dry weight is ? I would guess in the range of 100-200
pounds/yard. So for each yard of compost you add to 1000 sq ft you are
adding about 10-20 pounds of stable gums/humates and raising the stable
organic matter by .04-.08 % !!! The only way to raise organic content is
slowly over time.

Roland