Odor and Nutrient Management Newsletter

Winter 2004

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.

Sampling Method
Soil samples can be taken according to any credible sampling method. For instance, grid sampling, sampling by soil type or elevation, or sampling by designated management zones within a field. Two good sources of soil sampling information are: Iowa State University Extension publication PM 287 Take a good soil sample to help make good decisions available on-line at: www.extension.iastate.edu/Publications/PM287.pdf
NCR-13 Report 348 Soil Sampling for Variable-Rate Fertilizer and Lime Application available on-line at: www.extension.umn.edu/distribution/cropsystems/DC7647.html

Collecting a soil core for soil sampling requirements
Collecting a soil core for soil sampling requirements.

Minimum Sampling Requirements
Regardless of what sampling method is used to take the samples, there are minimum requirements that must be met.

  • Samples must be analyzed for P and pH once every 4 years (the P Index and MMP must be updated with the new soil samples every 4 years)
  • Each soil sample can represent no more than 10 acres. For fields 15 acres or less, only one sample is necessary.
  • Each sample must be a composite of 10 cores taken at the depth of 0-6 inches.

Using Existing Soil Samples
Soil samples can be used for the plan if they are 4 years old or less and meet the above requirements. Therefore, producers who need to submit a P Index MMP update in two or four years could plan ahead and begin thinking about soil sampling.

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- Sampling Requirements
One way to reduce the soil sampling requirements associated with the MMP is to apply manure at P-based rates. If P-based application rates are used between soil sampling periods, soil sampling can be reduced to 1 sample per 20 acres for fields with a Very Low, Low, or Medium P Index. So while samples must initially be taken at one sample per 10 acres for all fields, if P-based application rates are adopted after the soil sampling, the next soil samples taken can be reduced to one sample per 20 acres.

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
Soil samples must be analyzed by a lab enrolled in the Iowa Department of Agriculture and Land Stewardship soil testing certification program. A list of certified soil testing labs can be found at http://www.agriculture.state.ia.us/certlabs.htm

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/.

Who needs an MMP? Confinement feeding operations (totally roofed) with an animal unit capacity of more than 500 animal units, i.e., 1250 finishing swine.

When submitting an “original” MMP for a new or expanding operation, a P Index-based plan is needed when the plan is submitted.

For those who submitted an MMP prior to April 1, 2002, the P Index will be due with the first update after Aug. 25, 2008.

For those who submitted their original MMP between April 1, 2002 and Oct, 25, 2004, the P Index will be due with the first update after Aug. 25, 2006.

If you have questions about the requirements contact your regional DNR field office.

Manchester (563) 927-2640
Mason City (641) 424-4073
Spencer (712) 262-4177
Atlantic (712) 243-1934
Des Moines (515) 725-0268
Washington (319) 653-2135

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Iowa Manure Matters: Odor and Nutrient Management is published by Iowa State University Extension, with funding support from the USDA Natural Resource Conservation Service through Cooperative Agreement No. 74-6114-8-22. To subscribe or change the address of a current subscription, write to Angela Rieck-Hinz, 2104 Agronomy Hall, Iowa State University, Ames, Iowa, 50011-1010 or call 515-294-9590, fax 515-294-9985 or email: amrieck@iastate.edu. Please indicate you are inquiring about the Odor and Nutrient Management Newsletter. The newsletter's coordinators are Angela Rieck-Hinz, extension program specialist, Department of Agronomy, Wendy Powers, environmental extension specialist, Department of Animal Science, and Robert Burns, Department of Agricultural and Biosystems Engineering; the editor is Jean McGuire, the subscription manager is Rachel Klein, the production designer is Beth Kroeschell, and the web page designer is Liisa Jarvinen.

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