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Mehlich-3 Values for Relative Level Categories

Macronutrients Units Very Low Low Medium High Very High
Phosphorus pounds/acre 0-24 25-45 46-71 72-137 >137
Potassium pounds/acre 0-40 41-81 82-145 146-277 >277
Magnesium pounds/acre 0-45 46-83 84-143 144-295 >295
Calcium pounds/acre 0-615 616-1007 1008-1400 1401-1790 >1790


Values for soil test categories are specific to the extractant used, in this case Mehlich-3.

The "high" category is considered optimum fertility. Categories "very low", "low", and "medium" are below optimum; "very high" is above optimum (excessive).

Micronutrients Units Critical Level High
Zinc ppm soil 1.0 50
Copper ppm soil 0.5 20
Boron ppm soil 0.5 20
Iron ppm soil 50 100
Manganese pH dependent: calculate an activity index
MnAI = 101.7 + 3.75Mn - 15.2pH
25 100


Micronutrient categories are less well defined than macronutrient categories. Values below the "critical level" should be considered deficient; values above "high" should be considered a warning. Certain micronutrients can be toxic to plants at excessive levels.

In addition to its effect on manganese availability, soil pH also affects availability of other micronutrients. Aeration can also be a factor. See Micronutrient Statements or specified fact sheets for more information.