Implementing the phosphorus index for manure application
by Jeremy Klatt, Iowa Department of Natural Resources
The Iowa Department of Natural Resources (DNR) has proposed rules that would use the phosphorus (P) index to determine manure application rates in manure management plans for confinement feeding operations. Public hearings are scheduled for the week of March 22-26 at five different locations around the state. Once effective, the rules will be phased in over four years, starting with the original plans submitted to the DNR 60 days after the rule becomes effective.
Schedule for Public Hearings
The Iowa P index was developed by scientists from Iowa State University, the USDA-National Soil Tilth Laboratory, and the USDA-Natural Resources Conservation Service (NRCS). The index estimates the risk of P loss from a field, based on several factors including erosion, soil P tests, management practices, and the location of the field. Using the P index results in a site-vulnerability ranking, which categorizes the risk of P loss as Very Low, Low, Medium, High, or Very High.
Although an important component of the P index, the soil P test does not alone indicate the risk of P loss. A soil P test is used to make P recommendations, which are based on the probability of a yield response from a nutrient application. Because of the impact of other factors such as erosion, a high soil test does not indicate a high risk of P loss nor does a low soil test indicate that there is a low risk of P loss.
Proposed Application Rates. Based on the P index risk categories mentioned above, the DNR has proposed the following for application rates:
Application rates for the Low and Very Low risk categories. Although manure applications can be based on nitrogen (N) crop usage rates in the Low and Very Low risk categories, producers should consider the effect of manure application rates on the soil P content when planning manure applications. Continual application based on N can increase soil P, causing an increase in the P index over time. In most circumstances, applying N-based rates to the same fields every year rapidly builds the P in the soil.
Application rates for the Medium risk category. Many fields in the Medium risk category will have the option of N-based or P-based rates. For instance, when a field P-index ranking is just above the Low category, the N-based management would still be appropriate. However, for a field that is just below the High risk category, the P-based management may be needed to avoid increasing the P index to the High risk category.
Application rates for the High and Very High risk categories. While the proposed rule indicates no application of manure if a field is in the High or Very High risk categories, soil conservation practices can be used to reduce the P index to the Medium risk category. For example, increasing residue cover, adding filter strips, and installing grassed waterways will reduce erosion or sediment loss and will therefore reduce the P index.
The phased-in schedule allows most producers four years to file a manure management plan using the P index. This schedule allows producers to determine the P index for each field and, if necessary, it gives them time to adjust management practices to allow for continued manure application or to identify the additional acreage needed to comply with the rule.
Applying manure on a P-based rate. A P-based manure application rate replaces the P removed from the field by the harvested crop(s) or applies the amount of P recommended by soil test results. The proposed rule provides for application of up to four years of P in a single manure application. No additional P (manure or fertilizer) can be applied during the time covered by the application. Table 1 illustrates P-based rates for a corn and soybean rotation when using Iowa State University standard table values for manure and yield goals of 160 and 50 bushels per acre, respectively.
When applying a P-based
rate, both the P removal and the recommendation for crop rotation, and
the N needs of the crop receiving the manure should be considered. With
yields of 160 and 50 bushels per acre for corn and soybeans, respectively,
the two-year rotation removes approximately 100 pounds of P2O5 per acre
and the maximum N rate for the corn crop is 142 pounds of N per acre (assuming
1.2 pounds of N per bushel). The P-based liquid swine manure application
rates in Table 1 range from 2,400 to 5,000 gallons per acre. In many cases,
in addition to meeting the P needs of the two-year rotation, a P-based
rate also provides most or all, of the N needed for the corn. By not applying
manure to the soybean crop, the P-based manure application rates are similar
to typical N-based application rates for a corn crop.
Although all manure management plans for confinement will eventually be based on the P index, the effect of the proposed rule on manure application rates will be highly site-specific. Fields that have received P in excess of crop removal for long periods of time and fields with relatively high erosion rates are more likely to have a higher P index and greater limitations on the amount of manure that can be applied. Conversely, fields with soil P levels near the optimum for crop production and fields that have relatively low erosion rates are more likely to have a lower P index and can probably continue to receive N-based manure applications. Therefore, while some operations may be substantially affected by the proposed rule for manure management, other operations will not.
The proposed rule and a fact sheet on the use of the P index in manure management plans are available at the Iowa DNR animal feeding operations Web site at http://www.state.ia.us/epd/wastewtr/feedlot/feedlt.htm
For more information
about the P index, visit the Iowa NRCS Web site at
© 1997-2004, Iowa State University. All rights reserved.
Page last updated October 5, 2004
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