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Manure: Headache or Opportunity?


March 14, 2008
By Trevor Wallace

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Economic pressures, like recent
spikes in fuel and fertilizer prices, are forcing many producers to
consider ways to reduce costs while maintaining productivity.

Economic pressures, like recent spikes in fuel and fertilizer prices, are forcing many producers to consider ways to reduce costs while maintaining productivity. One possible answer is to make better use of available on-farm resources, specifically livestock or poultry manure. By optimizing manure utilization, producers can go a long way toward making a dent in their fertilizer bill.

In addition to the tangible fertilizer cost savings, manure provides several long-term soil benefits that are more difficult to quantify. For instance, manure is an excellent source of rapidly cycling organic matter, which acts as an important “gradual release” reservoir of nutrients for growing crops.

Over time the addition of manure helps to build soil organic matter levels, in the process improving the soil’s ability to hold both water and nutrients in the rooting zone to support crop growth. Organic matter added to the soil through manure application (particularly solid manure) also helps to improve soil structure and root infiltration, while reducing draft requirements for tillage equipment, as well as a soil’s susceptibility to erosion.

When discussing the economic value of manure, we should first acknowledge the economic cost associated with manure handling and transport. These costs are variable and depend on the moisture and nutrient content of the manure, application method, distance to application field and equipment value. The moderate to high moisture content of most manure reduces the nutrient density in the manure, which in turn makes the cost per pound of nutrient transported higher compared to fertilizer. Manure handling costs are a fact of business for the majority of livestock operations so the focus should always be on offsetting these costs through optimizing the utilization and therefore the economic benefit of the manure.

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When comparing manure to fertilizer it is important to remember that the nutrients in manure are in different forms than those in fertilizer. Fertilizer nutrients are generally in plant-available, inorganic forms. In contrast, a significant proportion of nutrients in manure exists in organic forms, which only become available after conversion to available forms by soil micro-organisms.
Another important difference between fertilizer and manure is that phosphorus (P) and potassium (K) in manure is often expressed as percent total P and total K, whereas in fertilizer these nutrients are expressed as percent phosphate (P2O5) and percent potash (K2O). Prior to calculating manure application rates, based on soil test recommendations, it is necessary to convert manure P and K values to the fertilizer equivalents, using the equations below:

•P2O5 equivalent = Total P in manure × 2.291
•K2O equivalent = Total K in manure x 1.2

Only a portion of the nitrogen (N) in manure is available to plants in the first year of application (i.e., 25 to 30 percent of the total N in solid manure, 40 to 45 percent of the total N in liquid manure). The remainder of the N becomes available to plants in subsequent years after application. In contrast, much of the P , K and sulphur (S) in manure are almost immediately plant-available.

It is also worth highlighting some of the differences between solid and liquid manures. Liquid manure has a greater proportion of N in plant available form (i.e., ammonium, NH4+) than solid manure, meaning a greater amount of total N is available in the year of application. Solid manure has a higher percentage of organic matter, per unit weight, which will supply nutrients over a longer period of time than liquid manure, as this organic matter breaks down. In addition, the higher organic matter content (per unit weight) in solid manures has a greater benefit to soil quality, building soil organic matter reserves, than does liquid manure, which is actually very low in solid material (i.e., less than 10 percent).

As with fertilizer, manure N can be lost either through gassing off (volatilization of ammonia-N) or through water movement (leaching of nitrate-N). Timing and method of manure application is therefore key to reducing N losses and capturing more of the economic benefits of the manure applied.

Depending on the source of the manure, approximately 50 to 80 percent of the plant available N, in the first year of application, is in the form of ammonium-N, which can be quickly converted to ammonia-N gas (NH3), which is subject to volatilization losses. Surface application of manure, without incorporation, has the highest risk of N loss through volatilization.

Environmental conditions will also influence N loss, with warm dry conditions promoting greater losses than cool wet conditions. Liquid manure contains a higher percentage of ammonium-N than does solid manure and is therefore more susceptible to N losses if manure is not incorporated immediately after application. Once in the soil, ammonium-N is quickly converted to nitrate-N, which is also plant available but is not susceptible to volatilization losses.

The remaining 50 to 20 percent of the nitrogen that becomes available in year one, is applied in the organic form and is converted to plant available forms (i.e., nitrate-N or nitrite-N) by soil microbes over the course of the growing season. Generally nitrogen will not be lost from the soil system as ammonia-N gas. Nitrate-N is susceptible to loss from the soil system through leaching, run-off and denitrification (i.e., production and emission of nitrogen or nitrous oxide gas).

It is important to realize that manure handling and application methods can affect nutrient retention. We’ll look at that, and some specific approaches to attaching a value to manure, in the next column in Manure Manager Magazine.

Trevor Wallace is a Nutrient Management Specialist. Environmental Practices and Livestock Welfare, Alberta Agriculture, Food and Rural Development
Detailed information on the methods described above is available at the department’s website at www1.agric.gov.ab.ca.


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