[USCC] Iron Contect in Compost

[Chaney, Rufus]

Dear Craig et al.:

When one is adding Fe, one usually adds a 10-fold molar excess to bind
the P. It is sorption, not precipitation that prevent runoff, although
inorganic Fe-P compounds can form over time and regenerate the sorption
surface.

The Fe you noted was some extractable Fe method, not total, or
amorphous. The best measure of Fe sorption potential is the amorphous Fe
method using oxalate in the dark. Crystalline Fe has little sorption
capacity. And agricultural extraction methods give results irrelevant to
P sorption.

There are anumber of papers in Journal of Environmental Quality on the
role of Fe in biosolids on prevention of P runoff. Some biosolids are
very rich in Fe from addition of Fe to the secondary treatment system to
improve P removal during treatment. Those biosolids commonly contain 4%
Fe and 2 or more % P. Interestingly, the high Fe does not interfere with
plants obtaining adequate Fe. As I noted earlier, roots change the
microenvironment in their rhizosphere to release P (and Fe) from the
soil matrix.

We have published papers on using drinking water treatment residue to
compost or manure to reduce P solubility and runoff. Such DWTR is rich
in Al and Fe depending on the nature of the water treated and the
chemicals added. You could add DWTR to your compost before composting to
obtain a uniform mixture of the DWTR chemicals with the compost to
reduce P runoff potential. 

Last, the P number you noted was probably a plant availability method
such as Olsons bicarbonate-P, or Mehlich or other method, not the total
P. It may help you to know that the plant available method is a poor
measure of P solubility. Rather, water soluble P is the method used to
assess whether a manure, compost, biosolids or soil is likely to release
P to runoff or leachate water.

Regards,

Rufus Chaney
USDA-ARS
Beltsville, MD

-----Original Message-----
From: compost-bounces at mailman.cloudnet.com
[mailto:compost-bounces at mailman.cloudnet.com] On Behalf Of Craig Coker
Sent: Thursday, October 08, 2009 9:00 PM
To: 'Compost Discussion List'
Subject: Re: [USCC] Iron Contect in Compost

Rufus, et.al.:

Is there a way to predict the extent of P-binding due to Fe content? 

I have a client who makes a 80% sand / 20% compost bioretention pond
media.
The media contains 113 ppm Total Phosphorus, 6 ppm Water-Soluble
Phosphorus
and 35 ppm Iron (lab test by A&L Great Lakes 8/17/09).  The project
engineer
is rejecting this as "out-of-spec" for a rain garden job as the spec
calls
for a maximum P content of "75 lbs/acre" (which, in agricultural
nutrient
management terms = 37.5 ppm).  

Is there a way I can calculate how much of the Total P will remain bound
in
the soil based on this iron content (I've already lost the argument that
"Only 6 ppm is soluble, the rest stays bound in the soil").

Thanks for any insight you (or anyone else) can provide.

Craig

Craig Coker | Coker Composting & Consulting
1213 Spradlin Rd.  Vinton Va 24179
Tel. (540) 890-1086; Fax. (540) 890-1087
Cell. (540) 874-5168; Email. cscoker at verizon.net
Web. www.cokercompost.com


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