Agricultural soil testing is critical to determining soil’s nutrient concentration and variability across your fields. It is ideal to test your fields’ soil at least every two to three years to inform critical decisions like fertilizer recommendations and how much liming material to use. But which sampling methods should you choose, and what benefits does each method offer? What are the best practices for soil sampling? Read on to learn more.
Agricultural soil testing methods
Your soil sampling approach should be based on the field’s specific needs, the level of data precision desired and cost. There are three standard agriculture soil sampling methods:
- Field composite sampling: This method takes samples throughout the field and compiles them into a single sample, assuming soil properties are consistent throughout. Typically more cost-effective than more precise methods, field composite sampling applies the same management techniques uniformly across the entire field. This is the least accurate method if there is any field variability.
- Zone-based sampling: This method groups areas with similar characteristics (historic yield, elevation, electrical conductivity, etc.), providing testing efficiencies and a more targeted approach than composite sampling. By grouping into zones (usually three to five zones per field), you can match fertilizer requirements across multiple areas, streamlining intervention tactics.
Grid sampling: Consider grid sampling when you have a variety of soil types in a field. Different soil types retain and supply nutrients differently, and grid sampling can be helpful to determine distinct soil conditions across a field due to topography, climate and historical land management.
This method divides a field into a grid system with equal-sized cells to provide a high level of detail. Grid size can vary depending on field size and variability, but grid cells are commonly between 0.25 and 2.5 acres. Grid sampling should be conducted every four years, unless something changes drastically, as annual grid testing would be costly.
How to take soil samples for agriculture
Regardless of the agriculture soil testing method you use, once you get into the field, soil analysis generally follows a consistent routine:
- Determine the ideal sample depth and density: The density of agricultural soil sampling depends on the level of detail you’re seeking and variability within the field. In areas with high variability, collect more samples per test plot to adequately capture the range of soil conditions. Every sample should contain many cores (at least 14 cores are recommended).
- Sample with consistency and care: Use a soil probe or auger to collect soil cores at a consistent depth within each test plot. Collect samples uniformly and avoid preferential sampling spots, (e.g., areas that clearly show variation, such as a mound or depression in the soil).
- Collect GPS data for mapping: Use GPS technology to accurately record the location of each soil sample, enabling the development of precise soil fertility maps to guide management decisions in specific areas of the field.
- Submit samples for analysis: When you send the soil samples to the lab, be sure to label them to match their respective locations within the field.
Factors that impact soil testing sampling depth
As with all soil testing variables, collecting samples from a consistent soil depth year after year is critical. Generally, you’ll want to test to a depth of 6", but if fertilizer has been frequently placed below 6", an 8" soil test depth is recommended.
Key factors that impact sampling depth include:
- Conventional till: Conventionally tilling fields at 0 to 6" incorporates nutrients relatively uniformly throughout the soil profile.
- Reduced- or no-till: Nutrients are usually broadcast on the surface or banded near the soil surface of a no-till field. Test a 6" core, but break that sample into two increments to test the top 0 to 2" separately from the bottom 2 to 6", and monitor differences.
- Testing for mobile nutrients. Taking some deeper samples (up to 24") can help test for nutrients that are more mobile in soil, like nitrogen, sulfur or chloride. Deeper soil testing for mobile nutrients is particularly important following a crop failure or lower-than-expected yields. Testing deeper reveals residual levels of various nutrients more accurately, informing your fertilizer decisions to avoid excessive nutrient leaching.
The majority of the samples are done on a 6" sample. While sampling deeper is fine, just remember to stay consistent and record your testing depth over the years.
How to interpret soil test results
Soil tests don’t measure total nutrients in the soil but instead are used to measure soil nutrients that are plant-available or expected to become plant-available during the growing season.
Soil test results and analysis can differ depending on the provider. However, most soil test results fall in one of three categories:
- Low or yes: a fertilizer addition will likely increase growth and yield
- High or no: fertilizer will not likely increase growth or yield
- Intermediate or maybe: fertilizer may increase growth or yield
Soil test values typically don’t vary greatly from year to year, but you must sample soil correctly, at the same time each year, to obtain meaningful records of change over time.
Some soil and environmental conditions such as pH and nitrate-nitrogen levels can cause fluctuations in soil test measurements. However, if you detect drastic changes in test values year to year, this may indicate an unrepresentative soil sample or laboratory error. If you receive unexpected results, submit a new sample or ask the lab to repeat the analysis.¹
Experts are available to help you make your decisions. Reach out to your seed retailer, a nearby extension office agent, or a seed company professional like your regional BASF representative.
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Endnotes
- Horneck, D. A., et al. “Soil Test Interpretation Guide.” College of Agriculture & Natural Resources, July 2011, www.canr.msu.edu/foodsystems/uploads/files/soil_test_interpretation.pdf. Accessed 2 Oct. 2024.
