Soil Testing

The Sufficiency (or Crop Yield Response) Approach provides the link between soil tests and crop nutrient guidelines, but consider soil testing, itself, for a moment. There are a number of different soil test methods.

The amount of nutrient extracted from the soil sample for a particular soil test depends on:

  1. Nature of the extraction solution used (e.g. Morgan, Mehlich-III, Modified Morgan, Bray-I).
  2. Shaking time.
  3. Solution to soil ratio.
  4. Analytical procedure and instrumentation.
  5. The way the lab reports results (e.g. lbs/acre or ppm).

There are four main nutrient extraction methods used by soil testing labs:

  1. Morgan solution: the Cornell Nutrient Analysis Laboratory (CNAL) method
    Sodium acetate, pH 4.8. Developed for Northeastern US soils.
  2. Modified Morgan
    Ammonium acetate, pH 4.8. Used in Vermont.
  3. Mehlich-III
    Acetic acid, nitric acid, ammonium fluoride and EDTA. Unbuffered.
  4. Bray-1
    Ammonium fluoride and HCl. For Midwest soils with rock phosphate.

The different methods will extract different amounts of nutrients from the same soil sample. For an example, consider the lab method comparison, below, in which a single soil sample was divided into 4 sub-samples and analyzed with 4 different methods. The results show the difference in extracted soil P among the soil test methods.

Comparison of Soil Test P Methods
Soil Extraction Solution
Reported Soil Test P
Morgan P
6 lbs/acre
Modified Morgan P
7 lbs/acre
Mehlich-III P
70 lbs/acre
57 lbs/acre

Soil test results from one soil laboratory cannot be compared with those from another unless the soil analyses were done exactly the same way or an accurate conversion equation exists.

The link between soil test values and Cornell crop nutrient guidelines is based on soil analyses from CNAL. Therefore, if a sample is from another lab, then a conversion to CNAL equivalents is necessary in order to calculate a nutrient guideline. Currently, the following conversions are available:

In order to make the conversion from Mehlich-III to CNAL equivalents, the following soil test info in needed:

When making the conversion, be aware of units. Some labs use parts per million (PPM) and others use lbs/acre (1 PPM = 2 lbs/acre).

Click to the next page for the crop nutrient guidelines portion of the Sufficiency (Crop Yield Response) Approach.

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