Fall 2003
Use of the Phosphorus Index in Manure Management Planning
by Jeremy Klatt, Iowa Department of Natural Resources
Current manure management plan (MMP) requirements for confinement feeding operations base manure application rates on the amount of nitrogen (N) needed to obtain an optimum crop yield.Due to the typical N to phosphorus (P) ratio of manure, this type of manure management often leads to over application of P, which over time can increase the risk of P loss from fields to surface waters. In accordance with Senate File 2293, the Department of Natural Resources is developing rules that incorporate the use of the Iowa P-index to determine acceptable manure application rates in MMPs.
The P-index is a management tool that assesses the potential for P loss from a field to surface waters. It provides a comprehensive estimate of P loss that considers not only soil P concentrations, but also other factors that affect P loss from fields, such as soil properties and land management. Once the new rules become effective, all fields that receive manure from operations requiring an MMP will need a P-index risk assessment.The results of the P-index will be used to determine, on a field-by-field basis, if manure application rates may be based on N (current guidelines) or must be based on P.
If an application rate must be based on P, manure application rates cannot supply more P than will be removed from the field with crop harvest(s). As a result of the field-by-field determination of application rates, it is possible that some operations will apply N-based rates to all fields, other operations will apply P-based rates to all of their fields, and still others will apply N-based rates to some fields and P-based rates to others.The P-index provides flexibility for making management decisions to reduce P losses, therefore allowing reduction of a field’s P-index rating through the use of conservation practices.
The DNR will review the proposed rules with technical and stakeholder committees in September. The rule-making process will begin in October and DNR anticipates that the rules will become effective in March or April of 2004. The rules will be phased in over four years, starting 60 days after the rule becomes effective. Depending on when a particular operation’s original or first-time MMP was submitted, the P index must be used to determine application rates according to the following schedule:
In order to conform to U.S. Environmental Protection Agency (EPA) rules, this timeline may change. The EPA is requiring that large, concentrated animal feeding operations (CAFOs) implement plans that consider potential P loss when applying manure by December 31, 2006. Large CAFOs are open feedlots or confinement sites with more than 1000 animal units.
The Iowa P-index was developed by a team of scientists from Iowa State University, Iowa USDA-National Resources Conservation Service, and the USDA-Agricultural Research Service National Soil Tilth Laboratory. The P-index calculator and support documents can be accessed at this url.
