Manure is a natural and organic source of nutrients that has been used in agriculture since the beginnings of farming. Historical evidence suggests Neolithic farmers were using animal dung to fertilize crops as early as 6,000 BC. Manure improves soil by adding organic matter and nutrients, which support the overall health and productivity of the soil ecosystem.
The Dual Role of Manure in Soil Health
Manure functions through a dual mechanism, providing nutritional content and soil conditioning benefits. It contains the primary macronutrients—Nitrogen (N), Phosphorus (P), and Potassium (K)—along with various essential micronutrients. These nutrients are bound within the organic matter and released slowly over time through mineralization.
This slow, sustained release provides a long-term feeding strategy for crops, unlike the rapid availability of synthetic fertilizers. The bulky organic matter also improves soil structure by promoting aggregation. Better soil structure enhances water-holding capacity, increases aeration for root growth, and reduces soil erosion potential. Manure application also stimulates microbial activity, increasing beneficial bacteria and fungi necessary for nutrient cycling.
Comparing Different Types of Manure
The nutrient profile and characteristics of manure vary depending on the animal source, diet, and bedding materials used. Cow manure is considered a milder or “cooler” manure, containing lower concentrations of nitrogen compared to other types. This makes it safer to use in larger quantities.
Horse manure is slightly richer in nitrogen than cow manure, but often contains a higher concentration of undigested weed seeds, making composting important. Chicken manure is known as a “hot” manure because its high nitrogen content can burn plants if applied fresh. Rabbit manure contains high percentages of both nitrogen and phosphorus, often having four times the nutrients of cow or horse manure. The exact nutrient analysis for any manure is highly variable and depends on the animal’s age, feed, and storage method.
Preparation and Safe Application Guidelines
Raw or fresh manure poses risks, including the potential transmission of human pathogens like E. coli and Salmonella. It also contains high ammonia levels that can chemically burn plants. Composting is the recommended preparation step because the high-temperature process destroys most pathogens and weed seeds.
Composting also stabilizes the nitrogen content. To be considered properly composted, the material must reach a minimum temperature of 131°F (55°C) for a specific duration, typically 15 days when using a windrow system. If using uncomposted or aged raw manure on food crops, specific application timing rules must be followed to minimize food safety concerns.
Application Timing Rules
The “90-day rule” and “120-day rule” dictate the minimum waiting period between the final application of raw manure and the crop harvest. For crops where the edible portion does not contact the soil, such as tree fruits, the raw manure must be incorporated at least 90 days before harvest. For crops that do contact the soil, such as lettuce or carrots, the waiting period extends to 120 days. When applying aged manure, it should be thoroughly tilled into the soil rather than left as a top dressing to reduce the risk of nutrient runoff and surface contamination.