About phosphorus water pollution from agricultural land: yes this is a serious
problem where soil reserves of P have been built up, especially where organic
fertility sources as animal manure and compost have been used. The main pathways
of P loss to waterways are through (1) P bound sediment (and later released from
the lake or river bottom) and (2) P DISSOLVED in the surface runoff water. The
concentration of dissolved P increases as the soil test (available) P content of
the soil increases and also as the organic matter content increases. The latter
effect occurs because both humus and active organic matter (through differing
mechanisms) act to reduce the capacity of the soil to bind P. In a low fertility
situation this is a benefit, but in a high P build-up situation this allows more
P to become dissolved in the surface runoff water. There are even a few areas
where soils are so sandy or so organic (as in the vegetable and sugar cane
growing areas north of the Everglades in Florida) that they do not hold on to P
strongly enough to prevent significant losses via leaching to the groundwater.
So P loss IS a significant water pollution problem mainly in areas where animal
production is concentrated. Phosphorus is, by the way, a much greater threat to
most freshwater (as opposed to estuarine) waters than is nitrogen. A more
detailed explanation is given in chapters 13 and 16 in the book I write with
Nyle Brady (The Nature and Properties of Soils-12th edition-1999- now out).
All this gets back to the earlier topics brought up by Joel Gruver (who by the
way just happens to be my graduate student at the moment), namely the need for
true nutrient balance and cycling. If a farm which raised only those animals
that it could support with feed and forage grown on that farm then the manure
from those animals should be able to be recycled on the land that produced the
feed with no subsequent nutrient build-up. In fact, some outside supplementation
would be needed in most cases because of the export of nutrients like P in the
milk, wool, meat, etc. sold off the farm (unless Joel's idea for sewage
recycling were also in effect!). By "farm", we could mean group of farms, so
long as the nutrients were rationally redistributed among them. [That is one
farmer could grow the feed and her neighbor could raise the animals and return
the manure to the feed farmer.]
The problem comes occurs when the animal producers, be she or he a family farmer
or a large corporation, import feed and/or fertilizer--whether organic or
synthetic--that more than makes up for the nutrients exported in the crops and
animal products. Relying mainly on animal manure (composted or raw) for a source
of Nitrogen (as do most dairy farmers and many organic farmers) will usually
result in a build up of excessive Phosphorus because the ratio of N to P in the
fresh animal manure (and even more so in the compost from which some N has been
lost) is lower by almost an order of magnitude than the ratio in the crop plants
being grown. In the case of swine and poultry manure this situation is often
aggravated by excessive animal feed supplementation with P as calcium phosphates
to make up for the fact that non-ruminants cannot digest most of the P in grain
(the phytin form). Hence the new push for low phytic acid grain varieties as a
band-aid measure to help the nutrient balance in our unbalanced animal
I hope this helps to explain why P has become a pollution problem despite the
great (but variable and finite) ability of most soils to bind it tightly.
Gary Matson wrote:
> In reference to the clip below, could someone explain what the
> implications really are regarding management of soil P? It has been my
> understanding that phosphorous essentially does not leach from soils,
> and that the way P gets into waterways to cause hypertrophic problems is
> from direct soil erosion into the water, thus physically carrying the p
> ions in their adsorbed state into the waterway. Have I missed
> something? Is there soil so sandy and lacking in clay that leaching of P
> into waterways is a real problem? Is this organic farmer actually
> losing soil to erosion that somehow would be alright if there were less
> P in it? Somehow I think there must be a lot more to this story. I
> admit to some parochialism, coming from red clay landscape that is
> perpetually low in Phosphorous, but never before have I heard of over
> phosphorous being a problem, nor have I ever heard a reference to
> groundwater pollution with Phosphorous as with Nitrogen or pesticides.
> Some background please? Thanks, Gary Matson
> joel b gruver wrote:
> > Hello to all...
> > I talked to a MD organic vegetable farmer today that in the past has
> > relied nearly entirely on heavy applications of compost to meet his
> > fertility needs.
> > As a result his soils test very high in P... As mandatory P based nutrient
> > management looms over his head, (mandatory in MD as of 2004) he is in
> > somewhat of a quandry as to how he will meet future crop N needs if he
> > greatly reduces his reliance on compost (which has a ~1:1 N:P ratio).
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Content-Type: text/x-vcard; charset=us-ascii; name="vcard.vcf"
Content-Description: Card for Ray R. Weil
Content-Disposition: attachment; filename="vcard.vcf"
fn: Ray R. Weil
n: Weil;Ray R.
org: University of Maryland, College Park, MD 20742
adr: Dept. of Natural Resource Sciences & L.A.;;1103b H.J. Patterson Hall;College Park;Maryland;20742;USA
title: Professor of Soil Science
tel;work: 301 405 1314
tel;fax: 301 314 9041
note: http://www.agnr.umd.edu/users/nrsl/faculty/weil.htm http://www.prenhall.com/books/ect_0138524440.html
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