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Soil Phosphorus

  • Subject: Soil Phosphorus
  • From: Ted.Held@hstna.com
  • Date: Thu, 9 May 2002 13:32:46 -0500 (CDT)

Here is a shot of what I know. Phosphorus is probably present as some form
of phosphate, probably calcium phosphate. If the phosphate is added in the
soluble forms, like potassium phosphate or ammonium phosphate, it will tend
to react with available iron and calcium in the soil. So you end up with
those minerals anyway. While these phosphates are largely insoluble
minerals, they can be solublized on both sides of the pH scale. For our
purposes as horticulturists that will mean low pH. In my work I am involved
with deposition of phosphate crystals on metal to provide a good base for
painting of automobiles, etc. We deposit the phosphate as salts of zinc,
iron, and manganese, mostly. These all begin to dissolve at a pH of about
5.5 and lower. While your soil or soil mix is probably higher in pH than
that, I think that plant roots exude juices that can be locally more acidic
than your overall soil. This then dissolves the phosphate (and this scheme
also works for iron), which can subsequently be soaked up by the roots in
this now-soluble form.

If you have high levels of phosphorus present you are probably stuck with
it. It is not very volatile (at least in its common forms), so you can't
convert it into a gas as you might nitrogen. You could acidify the soil and
then leach it out with water followed by neutralizing the soil again. But
that sounds really tedious and icky. You could dilute its concentration by
adding phosphorus-poor materials. Finally, since most plants will contain
at least some measurable phosphorus on a dry weight basis. So repeated
cropping and removal will deplete small amounts. Again tedious.

Or you could adapt your species mix to favor phosphate lovers.

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