Spring 2006

Sample soil and plan now for fall manure management that includes the P index


By Jeremy Klatt, Iowa Department of Natural Resources

soil samplingProducers looking forward to having the phosphorus (P) index included in their manure management plans (MMP) may want to conduct soil tests this spring. For those who submitted an original (new) MMP to the DNR between April 1, 2002, and Oct. 25, 2004, the P index will be required as part of their first annual MMP update after Aug. 25 this year. If an MMP was submitted prior to April 1, 2002, the P Index will be due with the first update after Aug. 25, 2008.

Testing soil now will allow producers to consider factors such as soil P, erosion and location of the field to estimate the risk of P delivery to surface waters. Producers can control some of these factors by using conservation practices that reduce the risk or find additional manure application fields, if necessary.

Either way, its to the producer’s advantage to take soil tests now that meet DNR requirements and plan ahead. The samples taken for the P Index also can be used for making nutrient and lime recommendations. The following minimum requirements for soil testing must be met to meet state law requirements:

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 Take a good soil sample to help make good decisions (PM 287)available on-line and NCR-13 Report 348 Soil Sampling for Variable-Rate Fertilizer and Lime Application available on-line.

Minimum Sampling Requirements

Regardless of which sampling method is used, there are minimum requirements that must be met.

  • Samples must be analyzed for P and pH once every four years (the P Index and the MMP must be updated with the new soil samples every four 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 four 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 MMP, 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

Initial samples must be taken at one sample per 10 acres for all fields. 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 one sample per 20 acres for fields with a Very Low, Low or Medium P Index.

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) also will make a P-based manure plan more achievable.

Soil Laboratory Analysis

questions Soil samples must be analyzed by a lab enrolled in the Iowa Department of Agriculture and Land Stewardship soil testing certification program. For a list of soil sampling labs enrolled in this program please call (515) 725-1478.

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. If you know from past sampling that areas of a field have a pH greater than 7.4, or are unsure of the field's pH, request the Olsen or Mehlich-3 P soil test method for all soil samples.

For more information, Iowa State University has a soil fertility Web site that includes information about soil sampling at http://extension.agron.iastate.edu/soilfertility/.


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