Scientists at the USDA-ARS in Orono, Maine have discovered that dairy cows producing USDA-certified organic milk also produce different manure than cows fed in a commercial operation. The results showed that conventional and organic dairy manures from commercial dairy farms differed in concentrations of plant nutrients, including phosphorus, metals and minerals.
"The researchers found that the two types of manure had at least 17 different chemical forms of phosphorus that varied in concentrations. The organic dairy manure had higher levels of phosphorus, calcium, potassium, manganese, zinc and magnesium.
Organic dairy manure also contained more types of phosphorus found in association with calcium and magnesium. Such forms are comparatively slow to dissolve and would thus gradually release the nutrients. Slow-release fertilizers generally increase the likelihood that they eventually will be taken up by crops, rather than being washed out of fields into nearby surface or groundwater sources.
Because of this, slow-release fertilizers often can be applied at comparatively low rates. Manure produced by cows in organic production systems may show similar characteristics compared to manure from conventional systems."
Read more here-http://www.ars.usda.gov/IS/pr/2009/090422.htm
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