Soil testing
Amber Anderson
- Outline steps for taking a reliable soil sample
- Match soil test output levels to recommended management
- Predict expected relative test values given site characteristics
Farmers utilize soil sampling to determine soil fertility, pH, nutrient variability, organic matter, and texture. Soil testing might be recommended for several reasons, including unexplained nutrient deficiencies. When compared to a tissue or sap test, soil sampling is typically a less expensive option but may not be the most appropriate for all situations. The test results are meant to guide your management plan for the current or following year. Several factors should be considered when determining when and where to sample.
When to soil sample
- At a time when application can occur before next cropping season or harvest. Typically late summer or early fall in Midwestern US annual systems.
- Consistent time of year each time you soil sample. Commonly every 3 to 4 years following the same crop in the same environmental conditions.
Places to avoid when sampling:
- Field edges
- Areas near roads
- Near livestock
- Near buildings or former buildings
- Uneven areas or unusual areas, unless they are given their own sample
Factors to remember during sampling:
- History, different past land use can result in different test levels. Sampling the area so one sample does not represent both areas will result in more accurate results.
- Consistent depth, timing, and lab procedures with clean equipment to maintain uniformity and accurate lab results.
- More samples will likely eliminate potential inconsistencies and bias.
- Avoid sampling in poor conditions, like excessively wet or frozen soil conditions.
- Samples should represent a small enough area to be representative, typically no more than 10 acres in annual row crop systems or an acre in more intensively managed situations.
Best practices for sampling
Follow the specific recommendations of your testing laboratory, but generally:
- Use clean equipment
- Subsamples should be used, mixed, and submitted sample used
- Sampling depth as indicated by your test/laboratory
- Differing areas should have their own soil sample, as combining them will not provide reliable results for either area
Strategies used for sampling
Grid
This method takes samples within regular divisions or zones of equal size, 1, 2.5, 5, or 10 acres. This regular sampling is meant to capture viability across the landscape. The method may be more helpful if the management history is unknown or variable rate application technology will be used. Drawbacks could include the additional cost resulting from more samples and samples less representative of the larger area.
Zone
This method is used when there is some historical knowledge of the area and to assess changes over time. Areas of similar soils, topography, and management are grouped for testing – samples are taken within the “zones”. This method can provide some basic information for management decisions at a decreased cost when compared to grid sampling, but may be less reliable if there is a history of manure application or specific concerns across the field.
Assessing your soil sample results
Reports received from testing laboratories generally include both a value and a rating, like ‘high’ or ‘very low’ to indicate relative status. Fertilizer application, when a soil test level is very high or high, is unlikely to provide economical return on investment and may be environmentally irresponsible. Fertilizer applications with low or very low values are likely to result in yield increases and should generally be applied at rates higher than anticipated crop removal rates.
Considerations
High pH values may be associated with roads in some areas due to limestone used in road material or the potential transport of liming materials.
Many standard soil tests don’t include heavy metals or other potentially harmful contaminants. A test may be warranted if you are producing food in an area with potential contamination (particularly urban areas or former industrial sites).
Previous livestock on the land can contribute to high or very high phosphorus levels long after animals are present. Erosion and/or further application in these areas can negatively impact water quality.