Comprehensive Nutrient Management Plans

by Robert C. Schiffner, Resource Conservationist
Natural Resources Conservation Service
Hays, Kansas

Livestock manure has been considered an asset to crop production since the beginning of organized agriculture. Because manure contains several essential plant nutrients, it contributes to increased crop yields when properly applied to soils. Therefore, manure can represent a valuable nutrient resource for crop producers.

However, producers need to be aware of potential environmental risks from manure’s nitrogen (N) or phosphorus (P). A Comprehensive Nutrient Management Plan (CNMP) needs to be developed and implemented as part of an operation’s overall nutrient management plan.

Manure can benefit a farm fertilization program. Besides providing valuable nutrients to the soil, including all micronutrients, manure supplies organic matter to soils. This improves soil tilth, aids in the retention of water and nutrients, lessens wind and water erosion, and promotes growth of beneficial organisms.

P, N, and K Availability

The total amount of nutrients in manure is not as important as the availability of these nutrients. Nutrients, such as N and P, that can be in organic portions of the manure, are not available until they are transformed into inorganic nutrients. Generalized statements for P and potassium (K) are straightforward: 80 percent of the P and 90 percent of the K in animal manures are available the first year. Therefore, when using a manure analysis report, multiply the P and K values by these constants to determine the first year’s available nutrients.

Nitrogen availability is more complex to estimate. The two main forms of N in manure are the organic N, which is designated unavailable N, and ammonium N, which is the available N. Although manure also contains nitrate N, nitrite N, and ammonia N, these forms comprise a very small amount of the inorganic N.

When manure is applied to soil, the organic N begins to break down to inorganic N, which is available to plants. This process is termed mineralization. This process is affected by soil temperature and moisture, as well as time. Warm, moist soils will have more organic N converted to plant available N with time than the cool, dry soils. Between 25-30 percent of the organic N will be converted to ammonium N each year after the manure is applied. This value is affected by the method of application and climatic factors of the year.

The other portion of manure N is ammonium. Ammonium N, which is available to plants, is relatively immobile in the soil and is not subject to loss. The process of nitrification eventually converts all ammonium N to nitrate N. While nitrate N is also available to plants, it is also susceptible to denitrification on fine-textured soils and to leaching on medium- or coarse-textured soils.

Application Methods and N Losses

Broadcasting manure onto a field is the oldest method of spreading. It is possible to broadcast during any season (manure should not be applied to frozen soils); however, there are some limitations to the method. From a nutrient standpoint, substantial amounts of the inorganic N are lost via volatilization that happens when manure is exposed to air, and the ammonium N can chemically convert to ammonia N and be lost.

This loss occurs within several days after application. Broadcasting and incorporating manure into the soil within a few days reduces volatilization losses. Incorporating mixes the manure into the soil and promotes conditions for the mineralization of organic N.

Injection is a method of application for liquid manure handlers. The concern of volatilization losses, as well as odor problems, created the demand for injecting manure with chisel-type knives. Volatilization losses were minimized using this method.

Sweep knife injection systems were developed that reduced the concentrated zones of manure beneath the soil surface. Instead of creating vertical bands of manure through the soil, a broad horizontal band is created. This method effectively eliminates volatilization potential, reduces denitrification potential, and encourages rapid breakdown in the soil.

Other factors also influence the N availability from manure applications. These include rainfall amounts and soil characteristics such as texture and organic matter levels. The manure application method is the most influential factor that producers can control.

Fall applications of manure, injected or broadcast, allows more time for the manure to break down before the plant needs the nutrients. Fall applications also provide more time for potential loss of N. Fall applications of manure on coarser-textured soils, where leaching can be a concern, should be avoided. Fall application should be done in late fall when soil temperatures are below 50º F.

Manure applied in the spring has the least amount of time for loss potential to occur. However, the rapid breakdown of organic material in the spring is more likely to temporarily tie up available N in the soil, creating some short-term N imbalances when applying manure with high levels of organic N (dairy and beef, or manure with bedding). Applying manure in the spring may also slow down other spring field operations to the point of delaying crop planting.

Rate of Application

Manure application rates need to be calculated based on available N and P, and how the availability is affected by the method of application. Crop phosphorus and potassium needs are often exceeded when manure is applied at rates which meet crop N needs. The following steps should be followed in determining application rates:

Step 1. Determine nutrients available in the soil.

The first step is to determine the available N, P, and K on a per-acre basis. This is best accomplished by taking a soil test. Soil test on fields where manure has been applied should not exceed 40 acres.

Step 2. Determine nutrient content of manure.

Next, the total nutrient concentration of the manure must be determined, regardless of its form. The ideal method would be to get a manure analysis. Following good manure sampling procedures is a key to getting a representative sample and having confidence in the resulting analysis.

Step 3. Determine nutrient needs for the next crop.

Using the fertilizer recommendations found in pamphlet MF-2586, Soil Test Interpretations and Fertilizer Recommendations, determine the recommended nutrient needs for the next scheduled crop.

Step 4. Calculate rate of application

In the last step, knowing what is needed and what should be available for the growing crop makes it possible to calculate application rates.

Selecting manure rate to use will depend on nutrient management and environmental stewardship goals of your operation. If you select the highest rate, which is normally based on crop N needs, over application of P and K will take place. This may have negative environmental consequences if continued over years, so one strategy is to reduce the frequency in which manure is applied on any one field. If you choose a lower manure application rate, there may be a deficit between the nutrients the manure supplies and what the crop needs, and supplemental fertilizer may be needed to make up the difference.

For more information about nutrient management plans, please contact your local Natural Resources Conservation Service (NRCS) office or conservation district office located at your local county USDA Service Center.

For more information about NRCS programs, visit the Kansas NRCS Web site at www.ks.nrcs.usda.gov.

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Comprehensive Nutrient Management Plans (DOC; 61 KB)

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Last Modified: 08/05/2008