The total phosphorus content in manure varies depending on the animal species, age, diet and how the manure has been stored. Concentration of phosphorus in some manures may be up to 80-90 pounds P2O5 per ton (some poultry manures, for example), whereas other manure may contain as little as 4 pounds P2O5 per ton. It will require a laboratory analysis to know for sure.
When manure is applied to the soil, what percentage of this phosphorus is available to the crop during the first year?
A large fraction of the phosphorus in manure is considered to be plant available immediately after application. The fraction that is not plant available shortly after application will become potentially available over time.
Estimated values of phosphorus availability are from 50%-100%. This range accounts for variation in sampling and analysis, and for phosphorus requirements with different soil test levels. Use the lower end of the range of phosphorus availability values (50%) for soils testing “Very Low” and “Low” (below 20 ppm) in phosphorus. In these situations, large yield loss could occur if insufficient phosphorus is applied and soil phosphorus buildup is desirable.
On the other hand, use 100% availability when manure is applied to maintain soil test phosphorus 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. However, one factor that can affect the efficiency of manure as a P source in the field is the variability associated with application, and the typical variability associated with manure in general.
Nevertheless, excessive application of manure P (for example, applying manure at rates sufficient to meet the crop’s nitrogen 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 cover 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
For maximum efficiency of manure use, it's essential to know the nutrient content of the manure. Using a manure lab analysis will help in determining the actual nutrient rates applied. Producers should think in terms of actual P application rates and not just gallons or 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.
For more information, see K State Extension publication MF-2562, Estimating Manure Nutrient Availability at: http://www.ksre.ksu.edu/