Manure Management on Organic Farms
Managing Nutrients in Manure
Livestock manure can supply nutrients required by crops and replenish nutrients removed from soil by crop harvest. Since manure contains multiple nutrients, applications should consider not only what is needed for the crop to be grown but also how the ratio of nutrients in manure could affect soil test levels. This ensures adequate nutrient supply and reduces potential for over- or under-application and subsequent buildup or depletion in the soil. Applying too little can lead to inadequate crop growth because of lack of nutrients. Overapplication can reduce crop quality, increase the risk of plant diseases, and increase the risk of contaminating surface or groundwater. Balancing crop nutritional needs with manures is an ongoing challenge.
Nutrient Composition of Manure
Manure has characteristics that make nutrient management different and sometimes more complicated than fertilizer. These include a mix of organic and inorganic nutrient forms; variation in nutrient concentration and forms; variation in dry matter and resultant handling as a liquid or solid; and relatively low nutrient concentration requiring large application volumes. Unlike fertilizer, manure form and composition, and therefore nutrient analysis, can vary widely (See Table 9.1).
Manure Nutrient Availability for Crops
Manure nutrient management planning has moved into the spotlight in recent years. While organic farmers have traditionally considered nutrients supplied by manure, there has been increasing attention on ways of improving overall crop production efficiency and profitability with manure. The analysis of manure provides total nutrient content, but availability of the nutrients for plant growth will depend on their breakdown and release from the organic components. A significant portion of nutrients in manure are in organic forms and not available for plant uptake until undergoing mineralization.
The nitrogen content of livestock manure is highly variable. Nitrogen in manure includes inorganic nitrogen [ammonium, (NH4?) and nitrate (NO3¯] and organic-N. Ammonium-N is the predominant inorganic form of nitrogen in manure and it is immediately available to the crop following application.
Organic-N: When manure is applied to soil, the organic-N begins to break down to inorganic nitrogen, which is available to plants. This process is termed mineralization. The actual rate of mineralization is determined by several factors, including the properties of the manure (such as C:N ratio, lignin content), soil temperature and moisture content, and placement of the material. Mineralization is more rapid in warm, moist soils and slower in soils that are cold or dry. The C:N ratio is an important factor affecting the rate of mineralization and release of available nitrogen from manures.
Ammonium-N: Besides the organic-N fraction of manure, the other major portion of manure nitrogen is ammonium-N. Organic-N converts to ammonium-N in the soil, which is now available to plants. Ammonium-N is relatively immobile in the soil and is not subject to loss.
Nitrification is the process by which microorganisms convert ammonium to nitrate to obtain energy. Nitrate is the most plant available form of N, but is also highly susceptible to leaching losses on medium- or coarse-textured soils.
Phosphorous and Potassium
Phosphorus and potassium in manure are mostly present in the inorganic form. This means that phosphorous and potassium are similar to commercial fertilizer in that they are readily available for plant uptake. Generally, 70 to 80 percent of the phosphorus and 80 to 90 percent of the potassium will be available from manure the first year after application (Rosen et al., 2005). In many areas of intensive livestock and poultry production, manure normally is applied at rates designed to meet crop nitrogen requirements.
Manure also contains micronutrients including copper, manganese, zinc, and boron. There is less information on the forms and availability of micronutrients in manure than macronutrients, as micronutrient chemistry in manures and soils is complex.
Manure Sampling and Analyses
Use of average or “book” nutrient values can be useful but not very helpful in determining specific manure nutrient supply or application rates due to wide variation in nutrient concentrations between production facilities. Therefore, collecting multiple manure samples and maintaining a history of analysis results will improve use of manure nutrients. It is important to use a laboratory that routinely tests livestock manure, as they will know the correct type of analysis to use.
Manure composition can vary widely from one pile to the next due to animal type, bedding type, management practices, age of manure, manure stockpiling practices, and other factors. The nutrient content of solid manure can also vary from one part of the pile to another. This variation depends on the distribution of bedding materials and the depth of the dried surface layer. The nutrient content of liquid manure can be variable due to solids settling with time, referred to as nutrient stratification. If variability is not addressed, manure analyses will not be representative of the nutrient content of the manure being applied. This could result in management decisions that lead to over or under nutrient application for crops, and potential loss of revenue
Analyses typically include total nitrogen, ammonium-N, total phosphorus, total potassium, total carbon, electrical conductivity, and moisture content (or dry matter). Total nitrogen, often reported as TN or TKN, is Total Kjeldahl Nitrogen. Kjeldahl refers to a specific analytical method. Total nitrogen is a measure of all nitrogen contained in the sample and represents both organic and inorganic nitrogen fractions. Because organic-N is not immediately available to plants, the total nitrogen value does not, necessarily, represent plant available nitrogen, nor does it represent any losses that may occur due to volatilization, denitrification, or leaching after application.
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