Livestock inputs make importing manure a concern, even for composting
Organic farmers are safe under NOP rules but need to know what they’re getting and how to manage it.

By Maggie Fry-Manross

Cadmium lurks in rock phosphates; varies by source

If toxic manure isn’t scary enough, consider the possibility of hazardous waste in fertilizer.

According to Joseph R. Heckman, specialist in soil fertility with Rutgers Cooperative Extension, commercial fertilizers can contain heavy metals and other hazardous materials. While fertilizers are regulated for nutrient content, the quality of their contents is not controlled.

Heckman publishes a newsletter called The Soil Profile. In volume 16, Heckman outlines problems with rock phosphate and other fertilizers. Heckman reports that the greatest concern is with phosphate fertilizers.

“Rock phosphate, the starting material from which most phosphate fertilizers are manufactured, contains varying amounts of cadmium, ranging from trace to greater than 100 ppm,” Heckman’s article states. “Rock phosphates from Florida contain 9-17 ppm cadmium and North Carolina rock phosphate contains about 47 ppm cadmium. Because there is a limited supply of low cadmium rock phosphates and a high cost for removing the cadmium during manufacture, there usually remains a significant presence of cadmium in phosphate fertilizers.”

Heckman also reports significant levels of lead and arsenic in commercial fertilizers. These substances have been found in both organic and chemical products. A complete copy of the article can be found at:


Tracking cadmium, evaluating risk

Cadmium, a known carcinogen with no health benefits to either plants or animals, is becoming more prevalent in agricultural soil and crops, according to a 2003 European Commission study.

Bioremediation of cadmium using alpine pennycress is enhanced when the pH is lowered from a normal reading of 7.0 to 4.7 using sulfur. For more details see:

Dr. Rufus Chaney (Agricultural Research Service, USDA Beltsville) says he has found no evidence that manure will increase human food chain bioavailable cadmium. He explains manure is much higher in zinc (Zn) than cadmium (Cd), so the competition between Zn and Cd limits Cd flow into plants, and into livestock tissues. For details on the complex links between soil Cd and crops see:

--Greg Bowman

Antibiotic resistance genes reduced
by composting, but it takes time

The extensive use of antibiotics in some livestock systems results in pharmaceutical residues in manure, as well as dozens of strains of antibiotic resistance genes (ARG) that develop within the animals fed. What happens to these genes in the environment is the focus of a study by Dr. Amy Pruden, assistant professor of civil engineering at Colorado State University.

Dr. Pruden is continuing to analyze research done in 2005 comparing the fate of ARGs from straw-pack manure that was either placed in a static (unturned) pile or composted. Both processes resulted in significant reductions of ARGs to nearly the point of non-detection.

Significant for composting, the results showed a spike upward in the number of microbes carrying the drug-resistant genes several days into composting that correlates with heating of the pile. Pruden said that following the full composting cycle is critical for ARG reduction. She hopes to complete her analysis this summer.


OMRI proposes dual limits for cadmium, arsenic, lead

The Organic Materials Review Institute (OMRI) has posted a proposal to establish limits on the amount of arsenic, cadmium, and lead that OMRI listed products may contain. The proposal is open for public comments through July 16. OMRI proposes to set two levels of these three heavy metals in fertilizer and soil amendment products, including manure and compost: a “caution” level where use would be closely monitored for possible degradation of the soil, and a higher “danger” level, above which the product would not be listed by OMRI.

To view the extensive research OMRI commissioned on this question and to comment, visit:


June 8, 2006: The ideal sustainable livestock farm is a closed system made up of crops grown to feed the livestock and humans who live there, and manure that is returned to the soil to ensure fertility. As with everything else in life, however, the ideal is rarely attained.

Vegetable farmers need manure to feed their crops. Dairy and meat producers usually have more manure than they can use. It would seem the perfect arrangement for vegetable growers to haul away the manure from a nearby farm (in some cases, your neighbors are happy to deliver it) to apply it to fields. But modern agricultural inputs—especially those used in confined, intensive settings—means that growers, especially those who are certified organic, need to ask a few questions before accepting that generous offer.

Recent studies have shown that manure can contain residues of antibiotics and other medications routinely given to livestock, as well as pesticides and heavy metals such as copper, zinc, arsenic, cadmium and lead. Appropriate composting may take care of some of these substances, but some it will not.

Manure allowable

Organic certification standards allow raw manure to be spread on fields at least 90 days before harvesting crops where the edible portion does not touch the soil (i.e. sweet corn) and 120 days where the edible portion does touch the soil (lettuce, carrots, onions, etc.). All other animal manure must be composted, and standards require that manure be shaped into a windrow, turned a minimum of five times in 15 days and achieve a recorded temperature between 133°F and 170°F in order to be considered finished compost. Anything else is termed “raw manure,” regardless of how long it has been piled up behind the barn.

Organic standards prohibit the use of sewage sludge, sometimes called biosolids, because of the possibility of heavy metal contamination. Municipal yardwaste, such as grass clippings and leaves, fall under the same category as manure—not prohibited, but demanding scrutiny as to possible contamination from pesticides, herbicides and synthetic fertilizers.

"Residues of antibiotics and other drugs, such as Ivermectin, a popular worming medication, have been found in animal manures, and traces of these drugs can be found in plants grown in soil where residue-laden manure was applied."

Antibiotics, such as tetracycline, are routinely fed to swine, poultry and dairy cattle. Residues of antibiotics and other drugs, such as Ivermectin, a popular worming medication, have been found in animal manures, and traces of these drugs can be found in plants grown in soil where residue-laden manure was applied. In a study published in the Journal of Environmental Quality in October, 2005, Kumar found that antibiotic residues were detected in green onions, corn and cabbages grown with the addition of manure containing antibiotic residues. In another article in the same publication, Chandler found that antibiotic residues in manure still retained their bacteria-killing properties and could be contributing to the formation of new strains of antibiotic-resistant bacteria. These manures were not composted.

Other substances that can contaminate manure are metals such as copper and zinc, which are part of the recommended minerals added to livestock feed, and cadmium and lead, which can enter the chain through crops grown in contaminated soil or air pollution in industrial areas (see sidebar). In a paper published in The Scientific World in 2002, researchers Allan Barker and Gretchen Bryson found that composting can significantly reduce pesticide residues and can bind heavy metals and reduce their uptake by plants.

One substance of particular concern is arsenic. Recent news stories reported that arsenic was found in several commercial brands of chicken and samples from ten fast food restaurants. The study was conducted by Dr. David Wallinga, Director of Food and Health for the Institute for Agriculture and Trade Policy (, a Minnesota-based, nonprofit advocacy group promoting sustainability and family farms.

Arsenic is included in Roxarsone, a medication sometimes fed to broilers raised in confinement to protect against outbreaks from coccidia. The fact that arsenic is detectable in the meat of chickens begs the question: How much is ending up in the manure, and where is it going then?

Arsenic: common additive, complex agricultural contaminant

Brian Baker, research director of the Organic Materials Review Institute (, located in Eugene, Oregon, is trying to answer that question. “We're looking at ways to reduce the accumulation of heavy metals, and arsenic is of particular concern,” Baker said. “Arsenic toxicity is problematic in a number of ways. The arsenic substitutes for phosphates causing a deficiency in plants. Beans, peas and other nitrogen fixing plants are very susceptible to this phosphate deficiency.”

Arsenic was one of the first-generation herbicides and was used as a pesticide in apple orchards. Soil scientists noticed that orchards with high arsenic content in the soil couldn’t grow clover (another nitrogen fixer) as an under crop.

“Organic farmers don’t want arsenic in the soil,” Baker pointed out. “It will accumulate in crop tissue and can pose a human health hazard. Everything goes somewhere. We've been looking at [arsenic] levels in the 30-40s ppm range. We’re not sure yet what the levels should be, but it definitely should be a concern.”

"If a grower has to choose between manure that may contain antibiotic residue and manure that may contain arsenic, Baker recommends staying away from the arsenic. Antibiotics break down quickly and the composting process should take care of them. Not so with arsenic."

If a grower has to choose between manure that may contain antibiotic residue and manure that may contain arsenic, Baker recommends staying away from the arsenic. Antibiotics break down quickly, and the composting process should take care of them. Not so with arsenic. “Heavy metals are more of a concern,” Baker said. “Organic growers are better off with dairy or layer manure.”

So perhaps organic growers should avoid importing manure altogether and stick to buying compost and fertilizers? That isn’t a fool-proof solution either. Prepared fertilizers are expensive, and Baker recommends staying away from commercial compost unless growers can be absolutely sure what is in it. According to Baker, during the debate on whether to allow sewage sludge in organic farming, the question of manure from factory farms was raised. Unlike the European Union, The U.S. has no definition of Concentrated Animal Feeding Operation (CAFO) or “factory farms” and does not prohibit this manure from being used by certified-organic growers.

Many scientists and sustainable farming activists are of the opinion that manure from factory farms contains as many heavy metals as sewage sludge and should be prohibited in organic agriculture. The EPA limits the amount of heavy metals in sewage sludge, but manure from factory farms is unregulated. “There are operations that will take sludge, CAFO manure and urban green waste and make it into commercial compost,” Baker said.

Always know your source and their ingredients

The best insurance with imported manure is to know your source. An organic grower for almost 20 years, Darrell Frey of Three Sisters Farm in northwestern Pennsylvania has imported many tons of manure to his farm. He had a long-standing agreement with a race-horse stable where he parked his dump truck at the stables and the owners filled it up for him every week.

The manure was then driven back to the farm and composted. Some of it was formed into windrows to be used in the vegetable gardens outside and some was loaded into bins in the bioshelter, or solar greenhouse, to be used in the indoor planting beds and in potting soil. The composting process also provides bottom heat for starting seedlings on top of the compost chambers, and fans can be used to circulate heat through the indoor growing beds.

Like many vegetable growers, Frey doesn’t have the time or space for farm animals. The bioshelter is home to a flock of laying hens that are fed organic feed from which they produce enough manure in a year to fill one of the compost chambers. To meet the rest of his compost needs, Frey must either import manure or use approved fertilizers. Fertilizer can be expensive, but manure has its drawbacks, too. “My biggest problem with imported manure was weeds,” Frey said. “Every year we seemed to find new varieties.”

"As with most aspects of sustainable farming, education is your best bet. Talk to neighbors about what they are feeding their animals and the possible effects it could have."

Frey is currently getting his manure from a nearby farmer who owns horses and other livestock. Before making the agreement, Frey went to the farm and checked out the agricultural practices to ensure that he wouldn’t be bringing anything onto the farm that he didn’t want.

According to Baker, this is a good idea. “No matter what nutrient source you use, it's not going to be perfectly clean,” Baker said. “Feed sources need to be monitored as well. Choose manure from feed sources that are relatively uncontaminated.” Baker also recommends layering suspect poultry or dairy manure with high-carbon organic matter during composting. This should take care of antibiotics and pesticide residues.

Where, what and how

The most important considerations when importing manure are these:

  • Where is it coming from?
  • What did the animals eat?
  • How will it ultimately impact my crops?

A grower’s best bet is horse manure, because antibiotics are usually not a concern, followed by dairy and layer manure. Baker recommends staying away from manure from factory farms, particularly hog and broiler operations which may rely heavily on drugs. Another consideration is that manure from confined hogs often contains high levels of copper.

As with most aspects of sustainable farming, education is your best bet. Talk to neighbors about what they are feeding their animals and the possible effects it could have. Chances are they will be just as concerned about what is going into the soil as you are—especially if they are buying your potatoes and chard.