Avoiding Soil Sampling and Testing Pitfalls in Qualifying for Government Programs Requiring a Nutrient Management Plan

Larry Bundy, Dick Wolkowski, John Peters, and Carrie Laboski, Department of Soil Science

Participation in some government programs including the Environmental Quality Incentive Program (EQIP), the Conservation Security Program (CSP), and certain county manure storage ordinances require a nutrient management plan that meets the requirements of the Wisconsin UDSA-NRCS nutrient management standard (590).  This standard and associated documents can be viewed at http://www.wi.nrcs.usda.gov/technical/cnmp.html .  Soil sampling, testing, and use of nutrient application rates based on these soil tests are required for a valid nutrient management plan.  Some plans prepared for use in these programs have been rejected because the required soil testing and fertilizer recommendation procedures were not used. The following comments are intended to provide information on acceptable soil sampling, testing, and nutrient recommendation procedures that will avoid the pitfalls some have encountered.  Farmers should confirm with their consultant, agronomist, or nutrient management planner that their plan will be prepared using the required procedures.

Soil Sampling

A soil sample must represent the field or portion of a field where the sample was taken to be useful in predicting the need for nutrient or lime applications.  Guidelines for soil sampling are summarized in UW Extension Pub. A2100 http://s142412519.onlinehome.us/uw/pdfs/A2100.PDF , and use of these sampling procedures is required in the 590 standard.  The basic requirements for taking a good soil sample are:

• Use a sampling probe or auger
• Sample to a uniform depth (usually 6 to 7 inches)
• Avoid unusual areas in the field that do not represent the majority of the area being tested.
• Collect a minimum of 10 cores per sample
• Take at least one composite sample per 5 acres and,  preferably, at least two composite samples for every field (See Table 1 below for sampling intensity suggestions for fields that have been tested previously)

Before collecting the soil samples, the overall approach of the nutrient management program should be determined.  This will affect the number of samples needed and method by which samples will be taken.  Specifically, will nutrient and lime applications be made at a single uniform rate for the whole field being tested or will applications be made at variable rates to field areas that have been mapped to identify different soil test levels?  The sampling strategy selected must also be appropriate for the field size and topography.  For example, on contour strip fields, sample each strip separately if it is approximately five acres or more in size.  Also, two or more small strip fields that have identical cropping and management histories may be combined following these recommended sampling guidelines.  However, superimposing a 5-acre grid point sampling approach on contour strips or small fields is not appropriate. 

Table 1.  Recommended sample intensity for “uniform” fields.*

Field characteristics	Field size (acres)	Suggested sample number**
Fields tested more than 4 yrs. ago and fields testing in the responsive range	All fields	1 sample/5 acres
Non-responsive fields tested within past 4 yrs	5-10 11-25 26-40 41-60 61-80 81-100	2 3 4 5 6 7
* From UW Extension Pub. A2100. ** 10 cores/sample minimum.

 Where a single uniform rate of fertilizer or lime will be applied to the field being tested, the field should be subdivided into sampling areas of approximately five acres each. The sampling areas do not need to be square, and the shape can be adjusted to fit field dimensions. Individual samples should consist of at least 10 cores taken in a zig-zag or “W” pattern over each of the 5-acre sampling areas (see Fig. 1 in A2100).   This sampling method is sometimes referred to as “cell sampling”.   Soil cores are collected in a systematic pattern throughout a “cell” representing the area from which soil cores are collected to make up each sample (usually five acres).  The number of samples needed per field (and thus, the cell size) depends on when the field was last tested (see Table 1 in A2100) and whether the P and K soil test levels found were in the non-responsive or responsive range the last time the field was tested.  A field is considered responsive if either the P or K test is in the high (H) category or lower. 

Where users intend to apply P, K, or lime using variable rate technology, the number of samples needed per field is also based on the previous soil test history and results, but more composite samples are usually needed than where a single application rate will be used for the whole field.  A grid-point sampling technique is suggested for fields where variable rate applications will be made.  In this method, the field to be tested is divided into grid cells of approximately equal size, and 10 soil cores are obtained from within a 10-ft radius of a geo-referenced (GPS) point within each grid cell.  For grid-sampled fields, an unaligned systematic grid (Figure 2 in A2100) is suggested.  Soil test data should be subjected to a statistical analysis to confirm that there is an identifiable pattern to the spatial variation in soil test values rather than a random distribution of these values.  Data that have a defined pattern to the spatial variation in soil test values are then interpolated using various mathematic procedures to produce a map containing areas of similar soil test category levels.

The size of the individual grid cells greatly influences the cost and accuracy of using a variable rate application approach.  A careful economic evaluation of intensive sampling options should be done by the user before proceeding.  To reliably identify areas within a field that may have different fertilizer or lime application needs, a relatively small grid size (1 to 2.5 acres) is needed.  Fields with P and K soil test levels in the non-responsive range should be grid-point sampled using a 300-ft grid.  This is equivalent to one soil sample for every 2 to 2.5 acres.  Where there is no information about the P or K status of the field or where previous tests were in the responsive range, a 200-ft grid size should be used.  This is equivalent to approximately one soil sample per acre.  Obviously, the expected benefits from using variable rate applications would need to be substantial to cover the added sampling and analysis costs. 

Grid point sampling using a 5-acre grid cell size (467-ft grid) technically meets the requirements of the 590 nutrient management standard where field size and topography allow this method to be used (larger, uniformly managed fields), but it has limited value for guiding variable rate fertilizer applications due to the inability to identify areas within fields that may have different nutrient or lime needs.  Therefore the best use of these results may be to calculate a single uniform rate for a field.  The grid-point sampling approach may have advantages for tracking changes in soil test levels over time, since soil samples are collected near the same geo-referenced (GPS) point each time the field is sampled.

Soil Testing and Interpretation

Soil samples for nutrient management planning must be tested in a Wisconsin Certified Laboratory using soil test methods specified in UW Extension Pub. A2809.  This includes soil tests for pH, available P (Bray P1), exchangeable K, and soil organic matter content.  Farmers should specify that soil samples to be used for their nutrient management plans be tested in a Wisconsin Certified Laboratory using the required UW test methods and recommendations.  A current list of Wisconsin Certified Laboratories can be found at http://uwlab.soils.wisc.edu/madison/  .  Soil test results from other laboratories or those using different test methods or recommendations are not acceptable for nutrient management planning purposes.  For example, soil tests for cation exchange capacity (CEC) or percent base saturation are not a part of Wisconsin recommendations and nutrient application rates based on these tests are not valid for nutrient management planning in Wisconsin.