How to Use Manure in Organic Growing

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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.

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