Understanding Soil Test Reports for Phosphorus Values
Individual labs may report phospohrous in a variety of ways using different reporting standards. Farmers should understand and look for three things on a soil test report so they are correctly interpreting the values.
The three items to identify are:
- Is P reported as elemental P or some other form?
Fortunately the majority of soil test (99%) will be reported as “P” or “Phosphorus” which indicates the elemental form.
If you see a reported value as “P2O5” do not use this value with recommendation charts. Stop and call the lab for a corrected copy expressed as Elemental P.
- What Units are used with the P value?
Common report units with a soil test are:
Pound per acre expressed as “lb/A”, “lbs/A”, “pounds/A” or other expression.
Parts per million expressed as “ppm”, “PPM” or “mg/kg”
To convert pounds per acre to parts per million:
60 pounds per acre ÷ 2 = 30 parts per million
To convert parts per million to pounds per acre:
30 parts per million X 2 = 60 pounds per acre
- What Extractant is being used to express the P value?
There are two common extractants which are solutions mixed with the soil to determine the phosphorus content. While both are good test, they do not result in equal soil test values.
- Bray P1 expressed as “P1”, “P-P1” or “Bray P1”.
- Mehlich III expressed as “P-M3”, “M III”, “Mehlich 3” or “Mehlich III”.
- If other terms are used that are not Bray P1 or Mehlich III call the lab.
For Ohio the conversion from Mehlich-3-ICP to Bray P1 is
-8.08 + 0.832(Mehlich-3-ICP value)=Bray P1-colormetric value
-8.08 + 0.832 (22) = 10
Several resources are available for those who are a bit unsure how to look at soil test reports for phosphorus.