Manure and Compost
Manure
Manure is both a natural by-product of livestock production and an excellent source of nutrients for crop production. The use of manure as a fertilizer is a beneficial way to recycle manure nutrients within an agricultural system. Most livestock operations are on, or surrounded by, large areas of productive agricultural land where manure can be applied in a sustainable manner. In many instances, manure can be a substitute for commercial, inorganic fertilizers. Manure not only acts as a source of plant nutrients but, through the addition of organic matter, it also helps to improve soil tilth, structure, aeration, and water holding capacity. In some cases, prohibitive transportation costs and food safety concerns often discourage producers from relying on manure fertilizer to meet the nutritional needs of the crop.
Manure Nutrient Content
Nutrients in manure are valuable resources, but not all manures are created equal. Manure nutrient content depends on many factors, including: animal species, livestock diet, livestock housing and bedding, manure storage and handling system, and dilution from water (wash water or rain water).
Nitrogen
Manure application rates are usually nitrogen-based because nitrogen is usually the nutrient needed in the largest quantity for crop growth. The nitrogen content of manure is highly variable. Manure is not like commercial fertilizer in that it does not come with a guaranteed nitrogen content. There are primarily two forms of nitrogen in manure: inorganic (ammonium) and organic nitrogen. Ammonium is the predominant inorganic form of nitrogen in manure and it is immediately available to the crop growth following application.
Phosphorus
The phosphorus content of the manure is also highly variable. Phosphorus in manures is present in both inorganic and organic forms. For most animal species, the inorganic phosphorus forms are dominant. Although phosphorus is less dynamic in soil than nitrogen, its management in manure is challenging. Crops growing on low phosphorus soils require fertilization to reach optimal yields and will benefit from a buildup in soil phosphorus until agronomic requirements are met. However, repeated applications of manure can result in soil phosphorus levels well above agronomic requirements.
Potassium
Manure is an excellent source of potassium. Potassium in manures is in the inorganic form and is readily available to plants. The potassium content of manure varies depending on the source of manure and the way the manure is handled. For example, liquid pumped from lagoons can be high in potassium. Continued pumping of lagoon effluent on the same area of land can lead to excessive potassium buildup relative to other cations. The high K:Ca ratio can alter the physical and chemical properties of the soil.
Manure Analysis
Testing manure for each livestock operation and each application event is the best way to estimate the nutrient content of manure. A manure analysis should be based on a well-mixed, representative sample. Manure is very heterogeneous and obtaining a representative sample can be very difficult. Sometimes, more than one sample is required to estimate the nutrient concentration because the characteristics of the manure change (for example, during pump-out of a liquid manure storage structure).
Rate of Manure Application
Manure application rates to match crop nitrogen needs can be calculated from information about nitrogen availability, and how the availability is affected by the method of application, incorporation practices, and type of manure. Crop phosphorus and potassium needs are often exceeded when manure is applied at rates which meet crop nitrogen needs. Sometimes there are economic and environmental benefits of applying manure at rates lower than crop nitrogen needs.
Time of Manure Application
Timing manure application can help reduce ammonia volatilization and retain nitrogen. Nitrogen loss can be minimized by avoiding application on warm, windy days, or when weather conditions are forecast to be sunny and dry. Fall applications of manure, either injected or broadcast, allow more time for the organic portions of the manure to break down before crop uptake compared to spring application. In contrast, fall applications also provide more time for potential loss of nitrogen if not incorporated. If fall application is necessary, it should be done in late fall when soil temperatures are below 50 degrees F (10°C).
120 Day Pre-Harvest Interval
Food safety impacts of the timing of manure applications. Food safety requires a waiting period between manure application and crop harvest. Crops intended for human consumption and whose edible portion has direct contact with the soil surface or soil particles require a 120-day pre-harvest interval (PHI). A 90-day PHI is required for those crops whose edible portion does not come in contact with soil particles (i.e., orchard fruit).
Environmental Guidelines
deally, manure should be incorporated as soon as possible into soil if it hasn’t been injected directly. Incorporation within two days of application is necessary to significantly reduce nitrogen volatilization and runoff. In cases where incorporation is not possible or practical, manure should be applied immediately before a non-runoff-producing rain event. Do not apply manure within 100 feet of streams or ponds unless there is a 35-foot permanent vegetative buffer.
Manure Application Methods
Manure incorporation represents a compromise between best management practices for soil erosion control and manure management. Manure should be incorporated into the soil for odor control, increased availability of nutrients, and control of potential manure runoff; however, disturbing the soil and crop residue may increase soil erosion and water runoff. There are several basic methods used to apply manure to cropland. Solid manure, like that from poultry, beef feedlots or other bedded systems, are generally broadcast onto fields. This may be incorporated into the soil using a tillage operation such as disking or harrowing. Liquid or slurry manure, like that from swine or dairy operations, has several application options including broadcasting on the surface with or without incorporation and direct injection.
Dragline System
Farms where manure slurry transfer pipe can be laid to all application fields have the option of using a dragline injection system for manure application (Figure 13.2). Dragline systems pump manure slurry through a transfer pipe to an umbilical line directly attached to a manure injection toolbar on a tractor. The tractor drags the umbilical delivery hose across the field as it injects manure into the soil.
Broadcasting
Broadcasting manure (e.g., solid, slurry, or liquid) onto the surface of a field is the oldest application method (Figure 13.3). It is easy, inexpensive, and can be done during almost any season. There are some significant disadvantages. From a nutrient standpoint, a substantial amount of nitrogen can be lost within a few days of application. This happens when ammonium converts to ammonia gas, a process called volatilization. Because significant ammonia nitrogen losses can occur in as little as one or two days after application, the way to derive the full nitrogen benefit from surface-applied manure (or urea for that matter) is to incorporate it (e.g., moldboard and disk plow, cultivator, disk harrow, etc.) after broadcasting. In addition, a significant amount of the manure’s nitrogen may also be lost due to leaching or runoff when application happens a long time before crop uptake occurs.
Direct Injection
Compared to surface application of slurry manure, incorporation by injection can reduce nitrogen losses from ammonia volatilization, reduces odor emissions, decreases the risk of manure runoff from the field, and allows for greater manure application uniformity. Slurry manure is typically generated in systems where little or no bedding is added to the excreted manure/urine. Slurry manure is typically between 4 and 10 percent solids.
Click on the following topics for more information on manure and compost.
