Using the Iowa Phosphorus Index and Variable-Rate Technology for Effective Agronomic and Environmental Phosphorus Management

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Mallarino, Antonio
Wittry, David
Klatt, Jeremy
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Proceedings of the Integrated Crop Management Conference
Iowa State University Conferences and Symposia

The Iowa State University Integrated Crop Management Conference is Iowa's premier crop production education event. No other program in Iowa brings together the diverse range of topics, slate of expert presenters and results of the latest University research.

The ICM Conference offers workshops focusing on the latest in crop production technology. Experts from Iowa and surrounding states will provide research updates and results in soil fertility, soil and water management, crop production and pest management.


The phosphorus (P) index is a tool that was developed to assess the potential for P losses from fields to surface water bodies. In 1999, the United States Department of Agriculture (USDA) Natural Resources Conservation Service (NRCS) issued national policy and general guidelines on nutrient management to include risk assessments for P. States were required to revise these guidelines by April 2001. These guidelines apply to nutrient management where nutrients are applied to the land, including organic by-products and animal manure. All NRCS staff will use this guidance when providing financial or technical assistance to producers. Third party vendors and other non-NRCS employees will use these guidelines when providing assistance through federal conservation programs for which NRCS has technical responsibility. The national guidelines suggested the use of one of the following risk assessments: soil-test P values, threshold limits, or a P risk index. The Iowa State Technical Committee recommended the use of the P index approach based on the advice of an interdisciplinary group that involved scientists, technical personnel, and representatives from various groups. The P index tool provides a rating of the risk of P delivery from agricultural fields to surface water supplies that can be used to prioritize fields for nutrient and soil management practices. The purpose of this presentation is to provide a brief overview of how the P index and variable-rate technology can be used to do better agronomic and environmental P management within a field.