What are the standards for soil test analysis?
Soil-testing laboratories use standards and protocols to determine soil nutrient availability; however, standards, methods, and units of measure might vary from one lab to another. By knowing the various methods and units of measure used, you can verify that you are receiving the proper analysis.
Units of Measure
- For measuring levels of macronutrients in a sample, including Phosphorus, Potassium, Calcium, and Magnesium, there are two typical units of measure:
- Parts per million (ppm)
- Pounds per acre
- Laboratories typically generate results in ppm and convert to Pounds per Acre upon request.
- Assumes there are 2 million pounds of soil per acre to a 6.67-inch depth (plow layer)
- 1 ppm = 2 pounds per acre
- Results in pounds per acre are more often generated for soil samples taken in Illinois, Indiana, and southern states.
- Results in ppm are more common in other states and generally accepted.
- If results are reported in pounds per acre, you can convert these values to ppm by dividing the results by 2.
Phosphorus Extraction Methods
Various methods have been developed to determine the level of plant-available phosphorus in the sample. Each method generates different results that might equate to similar levels of fertility.
- Bray-P1: Developed in 1945, it uses a combination of HCl and NH4F to remove easily acid soluble P forms. It is commonly used in acidic and neutral soils.
- Olsen: Developed in 1954 to extract phosphorus in calcareous, alkaline, and neutral soils. Does not perform well in acidic soils.
- Mehlich 3: Created in 1984, it uses a combination of acids, salts, ammonium nitrate, and EDTA chelating agent. This method can be used in acidic, neutral, alkaline, and calcareous soils without concern.
- Mehlich 3-COL: The initial implementation of Mehlich 3 used a colorimeter to measure the color intensity and convert to P level. This is less common today.
- Mehlich 3-ICP: Later implementations used inductively coupled plasma mass spectroscopy to measure phosphorus levels. Most common today for Mehlich 3.
Comparison of Phosphorus Relative Levels by Extraction Method - Iowa
Potassium Prep and Extraction Methods
- Ammonium Acetate: Older method commonly used when Bray-P1 or Olsen phosphorus is used.
- Mehlich 3: This method is commonly used when Mehlich 3 (-COL or –ICP) is used for phosphorus.
Note: Ammonium Acetate and Mehlich 3 Potassium extractions generate nearly identical results.
- Soil preparation methods might vary between labs.
- Dried: Sample is dried in an oven for uniform analysis. This is the common method.
- Field Moist: Sample is analyzed as received.
Comparison of Potassium Relative Levels by Preparation Method – Iowa
When working with soil sample test results, notice the methods the laboratory uses to accurately understand the level of fertility in the sample.
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