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Competency Area 3: Soil Testing and Plant Tissue Analysis

PO 17. Indicate how the following may cause variability in soil test results.

  1. Time of sampling
  2. Depth of sampling
  3. Number of samples taken
  4. Sample handling
  5. Type of extraction method

In addition to affecting how soil samples are taken, various aspects of management can affect the results obtained from soil tests. These should be taken into account and procedures should be observed to maximize the accuracy of samples.

Nutrient content of the soil solution and soil matrix vary depending on the time of year. Temperature, moisture, pH, and aeration vary with the climate, and during or after the growing season, nutrient levels are affected by crop removal. To minimize variability and build the strongest historic records, take samples at the same time each year.

Samples should be taken over a constant depth to minimize variability and build strong historic records. The soil sample depth depends on the tillage type used (usually 6-8" deep), as lime and fertilizer are mixed in the tillage layer. For determining lime requirements in a no-till system, take two subsamples: 0-1" and 0-6 or 8".

Number of samples taken depends on the size of the field, field characteristics, and field management, as discussed in PO 16 and PO 20. One sample generally should not represent more than 10 acres, and should only represent one management unit, ideally a uniform soil-type area. Samples should be taken at least once in three years, or twice per rotation.

Samples should be taken with clean equipment (probe or auger and plastic bucket), taken consistently (2-3 subsamples per acre on a uniform field), and selected to avoid misrepresentation (avoid fence rows, manure, stones, wood; sample between rows away from fertilizer; avoid sampling when wet).

The type of extraction method varies with the laboratory and with which tests are being done. The laboratory you send youl soil sample to and the extraction type should remain the same for most reliable year to year records. Many different methods exist, and it is important to have soils analyzed for the same extraction method as was used to derived the recommendation system. Historical records cannot be developed with results from different labs.

Remember – your soil testing results and the fertility recommendation you make based on the soil test can never be more accurate than the soil sample you take!