By-products from animal production operations have been applied to enhance soil productivity for many years. Depending on the livestock management system, animal manure may include: animal excrement (including urine), wastewater, spilled feed, open feedlot runoff and bedding. This manure contains considerable amounts of nitrogen (N), phosphorus (P), potassium (K) and other minerals. These minerals are available for recycling through the soil as plant nutrients. In addition, manure applications improve soil structure, tilth and other soil physical properties.
Proper management is the key to the efficient utilization of animal manure as a nutrient resource in a comprehensive cropping program. If mishandled, animal manure may contaminate water supplies with nitrogen, phosphorus, inorganic salts, organic solids and microorganisms. This bulletin provides general guidelines on how to maximize manure nutrient utilization while protecting Indiana's water quality.
Animal production generates manure and wastewater year round. Due to storage or other limitations, manure is often applied more than once during the year. As a result, a growing crop may not be present to utilize the nutrients from the manure. Improper management practices, such as high disposal rates, further increase the risk of ground water and surface water contamination. Nutrients and other compounds present in manure, that are not retained in the soil or taken up by plants, may contaminate surface water and ground water through runoff and leaching.
Runoff occurs when soil, manure solids, nutrients, dissolved chemicals and microorganisms present in the soil solution move across the soil surface into drainage ditches, lakes, streams and other water bodies. If present in sufficient quantities, these contaminants can cause considerable problems.
Phosphorus is one of the most common and serious surface water contaminants. The natural filling of lakes and ponds over time is called eutrophication. Excessive phosphorus inputs accelerate this process. Phosphorus causes increased algae blooms and excessive weed growth followed by die-off. The cycling of plant growth and die-off creates wide fluctuations of oxygen in the water that can cause fish kills. In addition, the dead plant remains and soil carried in runoff water form bottom sediments. Sediment fills in lakes, reduces water clarity and covers the breeding grounds of many aquatic species.
Controlling manure runoff minimizes the potential of surface water degradation. Incorporation of manure greatly reduces the potential for runoff and minimizes odors. Incorporation, by direct injection or some type of tillage, also maximizes crop utilization of manure nutrients. If incorporation is not possible, apply manure to fields with growing crops or substantial crop residues. The surface cover will help slow overland flow of water and minimize runoff. A growing crop extracts manure nutrients while crop residue "traps" some nitrogen otherwise lost to leaching or volatilization. Avoid spreading manure on steeply sloping land or land close to surface waters.
Spreading manure in the winter significantly increases the potential of nutrient loss through runoff. If the ground is frozen, manure applications should be made only on relatively flat land and as early in winter as possible. Runoff potential decreases when ice sheeting forms on top of manure rather than under it. Fields with substantial crop residue are best suited for winter manure applications.
Leaching is the process in which water, dissolved salts and soluble organic compounds move down through the soil. This "solution" may drain to surface waters through drainage tiles or move below the root zone and eventually reach ground water. Phosphorus generally does not move down through the soil profile except for some sandy soils and soils with long histories of excessive phosphorus applications. Because soil particles attract and hold phosphorus, levels found in drainage tiles are usually very low and pose little threat to the environment.
The major concern with manure nutrient leaching is the movement of nitrate (NO3-) nitrogen to ground water. Growing plants can take up both nitrate and ammonium (NH4+) nitrogen from the soil, but manure contains mostly organic and ammonium nitrogen. These forms of nitrogen do not move through soils. However, some soil microorganisms convert these compounds to nitrate, the mobile form of nitrogen in soils. Under some conditions, nitrate may leach below the root zone and eventually reach ground water.
Unlike phosphorus, nitrogen can pose a direct threat to human health. When ingested, nitrates present in food and water can be converted to nitrite (NO2-). Nitrite can interfere with the oxygen-carrying capacity of red blood cells and produce a condition known as methemoglobinemia. Because this condition primarily affects newborns and infants less than six months old, it is commonly known as infant cyanosis or blue baby syndrome. In 1962, the Environmental Protection Agency (EPA) established an upper limit of 10 milligrams per liter (ppm) nitrate (as NO3-N) for public drinking water.
Several practices minimize nitrate leaching from manure applications. Most importantly, manure application rates should not exceed the nitrogen requirement of the crop. As a crop grows, its nutrient requirements changes. Applying manure when the crop needs nutrients reduces the risk to water supplies. Fall manure applications should be made to a cover crop which will take up the nutrients in the manure. The cover crop also reduces runoff and erosion. If fall applications must be made to fields without cover crops, delay the application until the soil temperature drops below 50F or consider using a nitrification inhibitor to reduce nitrate leaching during the winter months.
The use of nitrification inhibitors with animal manure requires a high level of management to achieve desired results. The currently available nitrification inhibitors are volatile compounds and require the manure and inhibitor mixture be injected or immediately incorporated to assure a beneficial response. Recently developed nonvolatile inhibitor products show promise for easier handling.
Many producers apply enough manure to meet crop nutrient needs, then unnecessarily add commercial fertilizers to their fields. These excessive nutrient applications waste much of the manure's fertilizer value and increase potential environmental contamination problems. Repeated applications of manure, based only on nitrogen requirements, result in a buildup of phosphorus and potassium in soils.
Livestock producers should develop a nutrient management plan to maximize the value of manure nutrients. On fields receiving manure, use commercial fertilizers only as needed, based on soil test levels and realistic crop yield goals. The following steps will help maximize manure nutrient-use efficiency and minimize potential impacts on water quality:
- Rotate fields receiving manure to avoid nutrient buildup and maximize nutrient utilization.
- Supplement inorganic fertilizers only when manure nutrients do not meet crop yield goals.
- Incorporate manure for maximum nutrient-use efficiency.
- Calibrate application equipment.
- Avoid applying manure on wet soils to minimize compaction, runoff and leaching/denitrification.
- Avoid surface application of manure on sloping land, frozen soil or near surface waters.
- Use grass filter strips along ditches and waterways to reduce soil erosion, runoff and nutrient losses.
- Apply manure as close to the time of crop utilization as possible.
- Utilize fall cover crops to minimize soil erosion and runoff and to maximize nutrient utilization from manure applications.
- Delay fall application on bare soil until soil temperatures drop below 50F or consider using nitrification inhibitors to retain nitrogen and minimize nitrate leaching.
Livestock and poultry excrete the majority of the nutrients supplied in their feed ration. Proper land application of animal manure allows for the reuse of these valuable fertilizer nutrients to produce crops, while at the same time, minimizes potential environmental contamination.
This bulletin provides a general guide for proper land application of animal manure. For specific details on developing optimum manure management strategies on your farm, contact your local county Cooperative Extension Service. In addition, the following bulletins may be helpful:
- CES-227 "How and Where to Get a Livestock Manure Analysis"
- ID-122 "Solid Waste Handling for Dairy Operations"
- ILL-737 "Aerobic Treatment of Livestock Wastes"
- MWPS-18 "Livestock Waste Facilities Handbook"
- NCH-2 "The Philosophy of Soil Testing"
- NCH-12 "Managing Animal Manure as a Source of Plant Nutrients"
- PIH-25 "Fertilizer Value of Swine Manure"
- PIH-35 "Legal Guidelines for Swine Waste Management"
- PIH-67 "Swine Waste Management Alternatives"
- SB-342 "Improving the Fertilizer Value of Liquid Swine Manure with a Nitrification Inhibitor
- WQ 1 "Water Testing Laboratories"
- WQ 2 "What Is Ground Water?"
- WQ 3 "How to Take a Water Sample"
- WQ 4 "Why Test Your Water?"
- WQ 5 "Interpreting Water Test Results Part One: Inorganic Materials"
- WQ 6 "Buying Home Water Equipment"
- WQ 7 "Animal Agriculture's Effect on Water Quality Pastures and Feedlots"
- WQ 8 "Animal Agriculture's Effect on Water Quality Waste Storage"
- WQ 9 "Water Quality for Animals"
- WQ 10 "Wetlands and Water Quality"
- WQ 11 "Sulphur Water Control"
- WQ 12 "Distillation For Home Water Treatment"
- WQ 13 "Home Water Treatment Using Activated Carbon"
- WQ 14 "Reverse Osmosis for Home Treatment of Drinking Water"
- WQ 15 "Bacterial Contamination of Household Water"
Cooperative Extension work in Agriculture and Home Economics, state of Indiana, Purdue University, and U.S. Department of Agriculture cooperating; H.A. Wadsworth, Director, West Lafayette, IN. Issued in furtherance of the acts of May 8 and June 30, 1914. The Cooperative Extension Service of Purdue University is an affirmative action/equal opportunity institution.