Chapter 9

Manure Management on Organic Farms

Manure Application Rates

Manure application rates are based on the soil analyses and fertilizer recommendations, manure analyses, timing and method of application and weather conditions. Traditionally, manure application rates have been based on nitrogen. Applications of 30 tons per acre per year often are recommended to increase the organic matter content of soils. However, growers should be cautioned that the nitrogen supplied above the agronomic rate could have unfavorable effects on plant growth and quality and on the environment. Many certifiers specify that manure application must not exceed “agronomic application rates,” which means the amount applied must be less than or equal to the requirements of the crop. Applications in the range of 40 to 90 tons per acre per year should be avoided because of the chances of environmental pollution. Applications of manures at or below the rates required for crop nutrition in the long term will have favorable effects on increasing organic matter in the soil. Since little economic benefit is received from accelerated building of soil organic matter, in most practices, growers should limit applications to about 20 tons per acre per year (Barker, 2010).

Calculating Plant-available Nitrogen

Nutrient management plans for crops require estimates of plant-available nitrogen (PAN) provided by application of manure or compost. Plant-available nitrogen (PAN) of manure or compost is defined as inorganic nitrogen (ammonium and nitrate) plus the portion of the organic nitrogen that will be mineralized during the season or year following application. There are seven steps to estimating nitrogen availability in manure:

Step 1. Submit Manure or Compost Samples for Analysis

At a minimum, all manure samples should be analyzed for total nitrogen, ammonium-N, nitrate-N, total phosphorus (P), total potassium (K), and moisture content (or dry matter). Total nitrogen is a measure of all nitrogen contained in the sample and represents both organic and inorganic nitrogen fractions. Because organic nitrogen 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. Laboratory results will typically include measurements ammonium-N because it is primarily found in manure but not in compost. Generally, there is little nitrate-N in raw manure; therefore, there is no value to requesting this analysis. However, nitrate-N is present in composted manure and therefore a nitrate analysis should be requested.

Step 2. Convert Manure or Compost Test Results to an “As-Is” Basis

If the manure/compost test results are reported on a dry matter basis and not reported on an “as-is” basis (also referred to as “wet” basis) use the equations in Table 9.2 to convert nutrient concentrations from your manure test results to an as-is basis.

Step 3. Convert Manure or Compost Test Results to Desired Units

If the test results provided by the lab are not in the desired units, use Table 9.3 to convert the results to the desired units, such as “pounds per 1000 gallons” for liquid samples or “pounds per ton” for solid samples.

Step 4: Estimate the Amount of Available Organic Nitrogen

The forth step is to estimate the amount of organic-N that will mineralize during the first year. This is calculated by multiplying your value for organic-N by a mineralization factor. Table 9.4 can be used to obtain a mineralization factor that matches a particular manure type.

Step 5. Estimate the Amount of Available Ammonium Nitrogen

The fifth step is to estimate the amount of ammonium-N that will be available following land application. This can be estimated using the following equation and the volatilization factors from Table 9.5.

Step 6. Calculate Plant-Available Nitrogen (PAN)

Plant-available nitrogen (PAN) is the sum of the organic nitrogen and inorganic nitrogen in the manure/compost that is available for crop use in the year of application using the following equation:

Step 7. Calculate Manure or Compost Application Rates with PAN

Examples 5 and 6 show how to determine the manure application rate based on a target nitrogen need for a crop, the amounts of phosphate and potash applied and the amount of nitrogen that will be released in the second year after manure application.

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