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The use of animal manure in organic farming has been significant in the sustainable agriculture movement. Manure is a great source of many crop nutrients, including both micronutrients and macronutrients. Nitrogen is typically the nutrient with the most value, as well as the greatest potential for soil and water pollution. Quality and potential for contamination are both factors when learning how to use manure and selecting a manure source. Similarly, there are concerns with food safety when applying manure, and specific application guidelines have been designed to reduce the risk of pathogen contamination.
Nutrients are essential to proper growth of all plants, and farmers carefully plan to provide them, including finding the best source of manure for their nitrogen needs. Different animals produce manure with variable nutrient content, and some manure sources are more readily available and cost effective than others.
Manure from layer poultry, for example, provides nearly four times the nitrogen per ton as that from lactating cows. It also contains upwards of 12 times the potassium and phosphorus content of dairy manure. However, poultry manure is more costly than dairy manure — sometimes running twice the price. Poultry manure can also burn plants because of the large quantity of nitrogen it contains, so it’s generally composted or aged before being applied to a garden or farm. Another option is to apply it to a fallow field months before planting, so the soil microorganisms can break down the nutrients and make them more available to the plant. Whatever method is used, it’s important to note that under the best conditions only about half to three-quarters of the nitrogen in the manure is available to the crop in the year it’s applied. The remaining nitrogen will become available over a period of years. That’s why it’s important to regularly sample the soil to determine nutrient needs for the year. It’s also key to monitor crop nitrogen needs so that manure isn’t over-applied, contributing to contamination of water by ammonia, organic matter, nutrients and bacteria.
Another factor to consider when selecting a manure source is potential contaminants. Some contaminants, such as heavy metals, can be avoided by requesting a laboratory analysis. Heavy metals are a concern in manure, since there can be high potential content and farmers may also use high application rates. Heavy metals present in manure may include cadmium, lead, zinc and arsenic. Poultry manure is particularly at high risk for arsenic contamination, because nonorganic chickens are often fed arsenic to promote growth and weight gain. For this reason, poultry manure from organic sources is popular.
Another way to avoid heavy metals in manure is to select an Organic Materials Review Institute (OMRI) Listed manure product. OMRI requires heavy metals to be below a certain threshold before listing the product for use in organic production. Although other contaminants are present in manure, heavy metals are the easiest to avoid. Contaminants such as hormones and antimicrobials are difficult to even identify because they are so pervasive in the conventional manure supply, and there are no guidelines in place to control this contamination. Finding an organic source of manure is therefore the best way to avoid many potential hazards.
The application of manure also has implications for food safety. Pathogens such as Salmonella and fecal coliform are the main concerns when applying manure to edible crops. The USDA organic regulations require that a harvest interval be followed after applying manure, where crops in contact with soil (carrots, potatoes, lettuce) may be harvested only after 120 days, and crops not in contact with soil (blueberries, apples, peppers) may be harvested after 90 days. The logic behind this harvest interval is that pathogens will likely be rendered unviable by soil microorganisms after a certain time period, and will no longer pose a threat to food safety.
Another way to avoid pathogens from manure is to compost it first. One can purchase raw manure and compost it, or it can be purchased already composted. Both methods are effective at reducing the risk of pathogen contamination when applying these materials to an organic farm. However, it is sometimes difficult to determine whether a purchased compost product may pose a contamination hazard. The organic standards require a specific time period and temperature for composting, in order to ensure that any pathogens are, in fact, eliminated. Compost that does not meet these requirements is considered to be the same as raw manure, which means that application rates must meet the 90/120-day harvest interval requirements listed above. OMRI Listed products may fall into either category, so it can be helpful to research compost products on the OMRI Products List. OMRI’s restriction text will indicate whether the harvest interval periods must be observed.
Anaerobic digestion is a new technology that has been used to process manure into a composted product. The use of anaerobic digestion is especially growing on conventional livestock farms, where large amounts of manure are produced and increasing regulations require proper disposal. Typically, manure is gathered in a lagoon or tank, where microbes break it down in an oxygen-free environment. Some anaerobic digesters have external heating systems to achieve pathogen reduction, similar to traditional composts. Although anaerobic digestion is similar to composting, it must achieve the same time and temperature requirements in order to be used without a harvest interval. Before using an anaerobic digestion product, one should verify whether it was heated to at least 131 degrees Fahrenheit for three or more days. If not, the harvest interval must be observed. The resulting components of the digestion process include a liquid effluent rich in nutrients, and dry matter that is great as a soil amendment or even as biodegradable planting pots. Methane is captured as a by-product and used as a renewable energy source, instead of being emitted into the environment as a greenhouse gas.
There’s no doubt that learning how to use manure and applying it amply is one of the best steps toward providing nutrients in a well-managed organic farm or garden. Animal manure has many positives that make it worth the trouble of seeking out the best source and applying it with care. Manure use also contributes to the recycling of resources, which further reduces the environmental impact of livestock production in general. So, the next time you bite into an organic tomato, pepper or ear of sweet corn, be sure to also thank the animals that provided the nutrients used to grow your delicious food.
Photo by Fotolia/Jack: An organic farmer works with manure in a field.
Thank you to OMRI Technical Director Lindsay Fernandez-Salvador for providing this guest blog post for MOTHER EARTH NEWS. She holds a B.S. from Oregon State University in Natural Resource Management and an M.S. from University of Florida in Geography. She has more than 10 years of work experience on both conventional and organic farms.