AGRI-BRIEFS   AGRONOMIC NEWS ITEMS

From Agronomists of the Potash & Phosphate Institute 655 Engineering Drive, Suite 110 Norcross, Georgia 30092-2837 Phone (770) 447-0335

Fall 2000, No. 2

Manure can be a cost effective means for livestock producers to build soil nutrient levels. It can supply nitrogen, phosphorus, potassium, and a wide array of other essential macro and micronutrients. However, unlike the guaranteed analysis found on commercial fertilizer, manure nutrient content is both highly variable and less predictable in its release to plant-available forms. For example, almost half of the nitrogen in fresh manure is in the ammonium form and ready for plant uptake. The remainder is in organic forms requiring microbial decomposition and conversion over a period of two to three years before becoming plant-available. The availability of phosphorus and potassium in freshly applied manure ranges from 50 to 100 percent.

Getting the greatest value from manure applications requires careful management. Balancing the application of manure with crop uptake can become somewhat of a juggling act for the farmer due to the challenge of ensuring a sufficient nutrient supply for a given crop from the manure and fertilizer applied. Over-application of nutrients can result in crop losses due to lodging and delays in crop maturity with excess nitrogen. There is an increased risk of surface water pollution from phosphorus lost in soil eroded with the runoff of rain and snowmelt water. Balancing the phosphorus applied in manure, while avoiding negative environmental impacts, becomes a site-specific management issue.

Use of an environmental phosphorus index helps the farmer to identify areas at risk and rank them for vulnerability to nutrient loss. The phosphorus index uses transport and source factors to help rate a site for manure application. The transport factors are those that will influence runoff and erosion, such as length and slope of the landscape and distance to any local watercourse. The source factors include soil residual phosphorus levels and rate and method of manure and fertilizer phosphorus application. Using this method of assessment, it is possible for farm operators to identify those specific areas within a field where the majority of phosphorus loss is likely to occur and avoid them during manure application. Using the index also allows the identification of low-risk areas where manure applications can help to build a more productive soil.

Making the most of manure and fertilizer application requires the management attention of the farmer and the support of useful tools like the phosphorus index. Details on the use of the phosphorus index can be found by checking under the “Site-Specific Nutrient Management Guidelines” on the PPI web site, www.ppi-far.org.