Soil sampling requirements for manure management plans
by Jeremy Klatt, Iowa Department of Natural Resources
The DNR’s manure management plan (MMP) rules were recently revised to include the phosphorus (P) Index as required by state law. The P Index considers factors such as soil P, erosion and location of the field to estimate the risk of P delivery from fields to surface waters. This article explains the minimum requirements that must be met when taking soil samples for a P Index based MMP.
The soil sampling depth and analysis methods needed to run the P index are those that are recommended for crop production in Iowa. Therefore, the samples taken for the P Index can also be used for making nutrient and lime recommendations.
Minimum Sampling Requirements
Using Existing Soil Samples
For producers who must submit an original plan now, existing soil samples that do not meet the above requirements can be used for the original MMP, if they are four years old or less. In this case, soil samples that do meet the requirements must be taken no more than one year after the MMP is approved.
Use Phosphorus-Based Rates to Reduce Soil-
A P-based application rate replaces the P that is removed from the field with harvest or is based on a P soil test recommendation. Because up to four years of P removal can be applied in a single application (if the N requirement of the crop is not exceeded), P-based application rates may not be much different than N-based rates. When developing a P-based manure plan, not applying to the same fields every year is often more important than the actual application rate.
Reducing the P concentration of the manure (e.g. with phytase) will also make a P-based manure plan more achievable.
Soil Laboratory Analysis
The Bray P1 soil test method should not be used if the pH of the soil is greater than 7.4, as this test does not provide an accurate measurement of available soil P under these conditions. If the Bray P1 test is used and the pH is greater than 7.4 in one or two samples in a field, do not include these samples in the field average for the P Index. If the majority of samples have a pH greater than 7.4, use the Olsen or Mehlich-3 soil test when samples are taken again. For moreinformation, Iowa State University has a soil fertility Web site that includes information about soil sampling at extension.agron.iastate.edu/soilfertility/.
© 1997-2004, Iowa State University. All rights reserved.
Page last updated December 15, 2004
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