Plant-Availability Coefficients

Some of the nutrients in manures are primarily in organic forms and must undergo microbial mineralization before those nutrients are available to plants. Once nutrients are in a plant-available form, some nutrients can potentially be lost in gaseous forms. For example, nitrogen is susceptible to gaseous losses due to volatilization of ammonia. As a result, the availability of nitrogen is dependent on the mineralization potential of the manure as well as the application method of the manure. Subsurface placement of manure tends to reduce volatile losses whereas surface applications tend to increase volatile losses.

Neither phosphorus nor potassium are subject to volatile losses, meaning manure application method has minimal effect on the availability of these two nutrients. In fact, current data reviewed by the NC Interagency Nutrient Management Committee shows that phosphorus and potassium availability from manure sources is no different than the availability from commercial-inorganic fertilizer sources. For this reason, effective 1 July 2013, the availability of phosphorus and potassium in manure sources will be assumed to be 100% (i.e., plant-availability coefficient of 1.0).