For centuries, animal manure has been recognized as a soil “builder” because of its contributions to improving soil quality. Environmental benefits are possible from manure application if manure and manure nutrients are applied and timing and placement follows best management practices. When compared to more conventional fertilizer, manure properly applied to land has the potential to provide environmental benefits including:
Manure contains most elements required for plant growth including N, P, potassium, and micronutrients (Manure as a Source of Crop Nutrients and Soil Amendment). However, it is manure’s organic carbon that provides its potential environmental value. Soil organic matter is considered nature’s signature of a productive soil. Organic carbon from manure provides the energy source for the active, healthy soil microbial environment that both stabilizes nutrient sources and makes those nutrients available to crops. More…
Several long-term manure application studies have illustrated its ability to slow or reverse declining soil organic levels of cropland:
The ability of manure to maintain or build soil organic matter levels has a direct impact on enhancing the amount of carbon sequestration in cropped soils.
Manure organic matter contributes to improved soil structure, resulting in improved water infiltration and greater water-holding capacity leading to decreased crop water stress, soil erosion, and increased nutrient retention. An extensive literature review of historical soil conservation experiment station data from 70 plot years at 7 locations around the United States suggested that manure produced substantial reductions in soil erosion (13%-77%) and runoff (1%-68%). Increased manure application rates produced greater reductions in soil erosion and runoff. More… Additional studies during years following manure application suggest a residual benefit of past manure application. More…
In addition, surface application of manure behaves similarly to crop residue. Crop residue significantly decreases soil erosion by reducing raindrop impact which detaches soil particles and allows them to move offsite with water runoff. Data has been published showing how manure can coat the soil surface and reduce raindrop impact in the same way as crop residue. More… Therefore, in the short-term, surface manure applications have the ability to decrease soil erosion leading to a positive impact on environmental protection.
In addition, organic N (manure N tied to organic compounds) is more stable than N applied as commercial fertilizer. A significant fraction of manure N is stored in an organic form that is slowly released as soils warm and as crops require N. Commercial fertilizer N is applied as either nitrate or an ammonium (easily converted to nitrate). Nitrate-N is soluble in water and mobile. These forms contribute to leaching during excess precipitation (e.g., spring rains prior to or early in growing season) or irrigation. Manure N’s slow transformation to nitrate is better timed to crop N needs, resulting in less leaching potential. In fact, manure N is a natural slow-release form of N.
Recycling of manure nutrients in a cropping system as opposed to manufacturing or mining of a new nutrient resource also provides energy benefits. Commercial nitrogen fertilizers consume significant energy as a feedstock and for processing resulting in greenhouse gas emissions. More… Anhydrous ammonia requires the equivalent of 3300 cubic feet of natural gas to supply the nitrogen requirements of an acre of corn (assuming 200 lb of N application). Phosphorus and potassium fertilizers also have energy requirements for mining and processing. Substituting manure for commercial fertilizers significantly reduces crop production energy costs
It is important to remember that the environmental benefits of manure outlined in this article are only beneficial when best management practices for reducing soil erosion are implemented in concert with proper levels of manure nutrient application and use.
Authors: Rick Koelsch, University of Nebraska, and Ron Wiederholt, North Dakota State University Reviewers: Charles Wortmann, University of Nebraska, and Steve Brinkman, Iowa NRCS