How is the STP value commonly reported?

• A. In parts per million (PPM)
• B. In kilograms per hectare
• C. In percent
• D. In pounds per acre

Answer: (A)

The value from a soil test for phosphorus is a concentration. Labs report it as parts per million, which means milligrams of phosphorus per kilogram of soil. This unit is used because it directly compares how much phosphorus is present in the soil itself, regardless of field size or depth. Using pounds per acre or kilograms per hectare would mix in area and depth to turn the measurement into an application rate, not the soil’s inherent concentration. A percentage would imply a portion of the soil by weight, which isn’t how STP was designed to be expressed. So, the STP value is commonly reported in parts per million.