Manure is a complex organic material used as a fertilizer and soil amendment in agriculture. It is a byproduct of animal digestion, composed of feces and urine, often mixed with bedding materials like straw or wood shavings. This mixture is a rich source of organic carbon, which improves soil structure, water retention, and overall soil health. The exact physical and chemical makeup of manure is highly variable, depending on the animal species, diet, and post-excretion management.
Core Physical and Structural Components
The bulk of fresh manure’s physical composition is water, typically accounting for 60 to 85% of its total mass. This moisture content dictates handling and storage; solid manures have over 20% solids, while liquid slurries have less than 15%. The solid portion consists mainly of organic matter, including undigested feed residue like cellulose and proteins. These complex compounds eventually contribute to soil humus.
Bedding material, such as straw or sawdust, adds to the organic matter and affects the physical structure. Carbon-rich bedding changes the carbon-to-nitrogen ratio, which is important for decomposition. Inorganic ash and mineral salts also contribute to the mass. These salts, which were not fully absorbed by the animal, provide a source of secondary and micronutrients.
Essential Plant Nutrient Content
Manure’s value as a fertilizer comes from its concentration of primary plant nutrients: nitrogen (N), phosphorus (P), and potassium (K). Nitrogen exists in both organic and inorganic forms. The inorganic component starts as urea in urine, rapidly converting to volatile ammonium. This ammonium is readily available for plant uptake but can be lost as ammonia gas if the manure is not promptly incorporated into the soil.
Phosphorus and potassium are also abundant, as a large percentage consumed in the animal’s feed is excreted. Phosphorus is mostly found in the solid, organic form, making it less mobile in the soil than nitrogen. Potassium is largely inorganic and water-soluble, meaning it is quickly available for plant processes. Manure also supplies secondary nutrients like calcium, magnesium, and sulfur, along with micronutrients such as zinc, copper, iron, and manganese.
Composition Differences by Source Animal
The animal species is the largest factor determining manure’s nutrient content and physical characteristics. Ruminants, such as cows, extensively break down fiber, resulting in a uniform, “cold” manure that releases nutrients slowly. Non-ruminants like horses, pigs, or poultry pass more undigested fiber and higher nitrogen concentrations, leading to “hot” manure.
Chicken manure is notably high in nitrogen, largely as uric acid, which can damage plants if applied fresh due to its rapid breakdown. This high-nitrogen content classifies it as a “hot” manure with fast decomposition speed. The animal’s diet also impacts nutrient concentration; for example, high-protein diets result in higher nutrient content. This significant nutrient variability means relying on average values can lead to inaccurate application rates.
Biological Components and Microbial Activity
Manure is teeming with biological components, primarily a vast microbial community of bacteria and fungi that drive decomposition. This microbial load is essential because these organisms break down complex organic matter into simpler, plant-available nutrient forms. Applying manure significantly increases the overall microbial biomass and activity in the soil.
The biological makeup also includes potential health risks, specifically pathogens like Salmonella and E. coli. These microorganisms necessitate proper management, such as composting or aging, to reduce disease transfer. Manure also contains viable weed seeds that pass through the animal’s digestive tract undigested, which can sprout when applied to a field.