A soil test reports an STP value of 50 PPM. This value reflects

• A. The total phosphorus in soil
• B. The amount of phosphorus bound to iron
• C. The amount of phosphorus readily available to plants
• D. The phosphorus added to soil this season

Answer: (C)

The main idea is that soil test phosphorus (STP) reflects the portion of phosphorus in the soil that plants can readily take up under normal growing conditions. A reading of 50 PPM means there is a moderate amount of plant-available phosphorus present in the root zone, measured as milligrams of phosphorus per kilogram of soil. This value represents the pool available to plants now, not the total phosphorus in the soil, nor phosphorus that is tightly bound and unavailable (such as some forms bound to iron). It also does not indicate how much phosphorus was added during this growing season; it only reflects the current, accessible pool.