Making the most of manure and compost

By Desiree Jans, Ph.D.

Organic production strives to recycle materials and resources within the farm system. Manure and compost are great examples of recycling. But to make the most of these nutrient goldmines, we need to be familiar with what’s inside.

Not all manure and compost are created equal. Manure nutrient content varies considerably from farm to farm. Compost nutrient content varies with composting materials and processing conditions. Begin by having samples analyzed by a reputable lab. Analysis will tell you:

• Total Nitrogen: Total N is the sum of organic and inorganic (ammonium, nitrate) forms of nitrogen in the sample.
• Ammonium Nitrogen or Available Nitrogen: The portion of the total N that is available for plant uptake. Mainly the ammonium form of nitrogen but may also include nitrate.
• Phosphorus and Potassium: P and K are reported as P₂O₅ and K₂O as this is the standard method used in the fertilizer industry.
• % Carbon or C:N Ratio: The balance between carbon and nitrogen determines nitrogen dynamics in the soil. Amendments with high C:N ratios (>25) lead to nitrogen immobilization. This happens because the soil microbes responsible for decomposition must balance the large amount of carbon with some nitrogen. As a result, they incorporate into their cells (immobilize) not only the nitrogen from the amendment but also nitrogen from the surrounding soil. Immobilized nitrogen is not available to crops and may result in a short-term nitrogen deficiency. Over the growing season, the immobilized nitrogen becomes slowly available. Amendments with lower C:N ratios lead to nitrogen mineralization. This results in a quick release of plant-available nitrogen, but does not contribute to the build up of soil organic matter levels. Amendments with low C:N ratios are particularly useful for crops with high nitrogen demand.
• % Moisture or Dry Matter: The percentage of the sample which is water (your “fresh” sample contains water plus dry matter). % moisture is used when calculating application rates.

Not all of the nutrients in manure or compost will be readily available to the crop. Nutrient management planning protocols use various percentages to calculate availability but these percentages are only estimates and more research is needed in this area. Also, most estimates are based on experiments with manure which makes them even less accurate when applied to compost. However, some general guidelines are useful.

For example, ammonium-nitrogen is particularly sensitive to method of application because ammonium is subject to volatilization (loss as a gas). To best conserve nitrogen, apply your manure or compost in cool conditions and incorporate it into the soil immediately. This should make 75% of the ammonium-nitrogen available to the crop. If you wait two days between spreading and incorporating, only 65% will be available, four days, 40% and 7 days, 25%.

Organic nitrogen (calculated as Total N – Ammonium N) is mainly affected by climate and source of manure or compost. Your local extension agent or qualified nutrient management planner may have regional figures for organic nitrogen availability. You can expect roughly 35% of organic N to become plant-available with spring or early fall application of cattle manure. Under similar conditions, expect 60% availability from poultry manure. The rest of the organic N will be released slowly, possibly over many years. If a field has received manure 2-3 years out of the last 5, expect 10% of the average annual organic nitrogen application to become available this year.

Phosphorus and potassium availability are not affected by the timing and method of incorporation. In the first year, expect 40% of total P₂O₅ to become available and 90% of total K₂O to become available.

Although most organic farmers use composted manure, fresh manure can be useful for high nitrogen demanding crops. The key is to know how much plant-available nutrient the manure contains. Use a method and timing of application that will maximize crop uptake while minimizing losses to the environment. Be aware of potential nutrient imbalances (high levels of phosphorus and potassium), weed problems and contamination issues. Some organic standards have restrictions on the use of fresh manure.

Compost has relatively low plant-available nitrogen but it is a great source of slow-release nitrogen. If you calculate your compost application rate to satisfy your crop’s immediate nitrogen requirement, you may end up using a lot of compost. This can lead to excess application of phosphorus with risks of nutrient imbalances and phosphorus runoff. Rather than treating compost as an available nitrogen source, view it as a soil organic matter builder and source of slowly released nutrients.

Desiree Jans, Ph.D, is a Web Course Instructor for the Organic Agriculture Centre of Canada (OACC). For information about OACC courses or to send comments, contact us at 902-893-7256 or oacc@nsac.ca.

All coefficients are based on semi-solid cattle manure data from: Commonwealth of Virginia. 1995. Virginia nutrient management standards and criteria. Department of Conservation and Recreation, Division of Soil and Water Conservation, Richmond, VA.