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No. 12
Human Health and the Use of Animal Manure in Crop Production
Animal manure (and certain other organic materials) is a good source of essential plant nutrients when properly used. It improves soil fertility and biological and physical properties. However, manure has several disadvantages when compared to commercially produced fertilizers, including low nutrient content and high volume, making it uneconomical to transport far from its production source. Manure continues to release nutrients during periods when crops aren’t actively growing to take them up. This can result in unused nutrients being left in the soil, to potentially be washed into surface water through erosion and runoff or leached into groundwater. Careful timing of application and incorporation can offset these concerns. Because nutrient contents of manure vary with animal species and management, it is critical to analyze its nutrient content and factor that into nutrient management planning.
There are other disadvantages to the use of animal manure as well. Its somewhat fixed nutrient ratios have led to widespread concern of excessive buildup of phosphorus (P) in heavily manured soils because crops require much less P than nutrients such as nitrogen (N) and potassium (K). So, when manure is applied to the soil in quantities necessary to meet crop need for N, soil P levels can be built to excessive levels. This leads to potential degradation of water quality. Animal manure may also carry pathogens that can present dangers to human health.
The U.S. Environmental Protection Agency has cited pathogenic bacteria as a leading cause of water quality impairment in streams, rivers, and estuaries. There are more than 150 organisms that can spread infection from animals to humans. Many of those can be found in animal feces and urine. They include bacteria—such as E. coli, Salmonella, and Campylobacter— viruses, and protozoa. The U.S. Centers for Disease Control (CDC) estimates there are more than 70,000 cases of infection in humans each year from E. coli alone. Food-borne illnesses are considered the most serious food safety problem in the U.S. The application of raw manure is one way pathogens are spread in the environment. Manure is not applied directly to crops intended for human consumption. Composting kills many harmful pathogens, reducing the human disease risks. Composting also reduces the volume of manure, making it more practical to transport.
 
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