Soil Phosphorus

From: Ted.Held at hstna.com on 2002.05.09 at 18:32:46(8741) 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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