What conversion is needed if STP is reported in pounds per acre?

• A. Divide the pounds per acre by 4 to convert to PPM
• B. Multiply the pounds per acre by 2 to convert to PPM
• C. Divide the pounds per acre by 2 to convert to PPM
• D. Subtract 5 to convert to PPM

Answer: (C)

Dividing by 2 is the right move because, in soil, about 1 ppm (parts per million) corresponds to roughly 2 pounds per acre (lb/acre) of the nutrient in the topsoil. This comes from the typical mass of soil in the sampled layer and the conversion between units (mg/kg, lb/acre, etc.). So to convert from pounds per acre to ppm, you use ppm ≈ (lb/acre) ÷ 2.

For example, 8 lb/acre is about 4 ppm. This is an approximate rule of thumb used in practice; exact values can vary with soil depth and density. The other options don’t fit the common conversion: dividing by 4 would give a smaller number than actual, multiplying by 2 would give a larger number, and subtracting 5 has no basis in this measurement.