By Dorivar Ruiz Diaz, Nutrient Management Specialist; Doug Shoup, Southeast Crops and Soils Specialist; Peter Tomlinson, Environmental Quality Specialist
Poultry litter can provide a significant and important supply of nutrients for crop production and can serve as an excellent complement to commercial nitrogen (N) fertilizers. Phosphorus (P) content in poultry litter is usually high, and applications rates should be based on P levels to avoid potential surface water contamination.
Moisture content and nutrient concentration in poultry litter can be highly variable and depends mainly upon production conditions, storage and handling methods. Therefore, laboratory analysis is the best way to determine the level of N and P in the material to be applied.
The table presents average values for the different types of poultry manure collected over a period of time. The graph presents the actual laboratory analysis of 67 poultry manure samples from southeast Kansas. There is a large range in nutrient values, likely due to the source of the litter. However, a good sample average to expect would be a 55-55-47.
For maximum efficiency of manure use, it is essential to know the nutrient content of the manure. Using a manure lab analysis will help in determining the actual nutrient rates applied. A laboratory analysis should be done on the poultry litter before applying it to land. A laboratory analyses provides information regarding nutrient levels, as well as the chemical forms of these nutrients. This information is necessary for an adequate estimation of nutrient availability and application rates. For more information, see K State Extension publication MF-2562, “Estimating Manure Nutrient Availability.”
N and P crop availability shortly after application is a common question. In the case of N, it is important to consider that this nutrient is primarily in the organic form in poultry litter (up to 75-80% organic). Organic N needs to mineralize before becoming available to crops. A fraction of this organic N may become part of the soil organic matter pool and unavailable to crops in the short term.
Field and laboratory studies suggest the fraction of total N that becomes plant available the first year of application is approximately 45-55%, which includes both the inorganic N in the manure and a percentage of the organic N. This value varies depending upon components in the litter, and the method of handling and application.
For example, poultry litter that contains a large fraction of bedding material will tend to have lower N availability the year of application. Reduction in N availability may also occur when litter is aged, and has undergone some level of composting.
N lost from the volatile ammonium fraction at the time of application to the soil surface can also reduce plant-available N. Ammonium volatilization is typically higher during windy and warm days. Incorporation of litter immediately after application will reduce volatilization and potential nutrient loss by water runoff in case of a rainfall event, in addition to reducing the odor of the litter.
If the manure is applied to pastures, the percentage of N utilized by the forage the first year will depend on whether the pasture consists of cool-season or warm-season grasses. For cool-season grasses, such as fescue pasture, N utilization will likely be less than 50% the first year. Most of the growth in cool-season pasture occurs early in the year. Microbes will not mineralize as much N early in the spring as they will later in the summer. Fall applications may utilize more N for fescue than winter or spring applications. For warm-season grasses, such as bermudagrass pasture, N utilization from manure will likely be close to 50%. In both cases, growers should base application rates on the P and K needs of the grass, and supplement additional N fertilizer to meet the N needs of the grass.
Phosphorus and Potassium Availability
When manure is applied to the soil, what percentage of this P and potassium (K) is available to the crop during the first year?
A large fraction of the P in manure is considered to be plant available immediately after application. The fraction that is not plant available shortly after application will become available over time.
Estimated values of P availability are 50%-100%. This range accounts for variation in sampling and analysis, and for P requirements with different soil test levels. Use the lower end of the range of P availability values (50%) for soils testing “Very Low” and “Low” (below 20 ppm) in P. In these situations, large yield loss could occur if insufficient P is applied and soil phosphorus buildup is desirable.
On the other hand, use 100% availability when manure is applied to maintain soil test P in the Optimum soil test category, and when the probability of a yield response is small.
Several studies have shown that manure P is a valuable resource, comparable to inorganic fertilizer P for crop production. These two P sources are similarly effective when the manure P concentration is known and the manure is applied properly.
Nevertheless, excessive application of manure P (for example, applying manure at rates sufficient to meet the crop’s N needs) often results in excessive soil P buildup over time, resulting in higher risk of surface water contamination. This problem of excessive P buildup in the long-term can be minimized by:
- Applying manure to meet the P needs of the crop and using inorganic sources of fertilizer to complement N needs
- Constantly monitoring soil test P levels
- Using the P-index to assess potential impact of P buildup on water quality
Growers should think in terms of actual P application rates and not just tons per acre of manure being applied. Uniform application of manure at precise rates can also be difficult. Careful calibration of manure applicators is needed. If these aspects are not considered, the efficiency of manure P compared with inorganic fertilizer P may be reduced. Careful management pays off.
Availability of K is usually near 100% with proper application. Poultry litter can also provide significant amounts of secondary and micronutrients.