STP stands for what in soil testing?

• A. Standard Test Procedure
• B. Soil Testing Protocol
• C. Soil Test Parameter
• D. Soil Test Phosphorus

Answer: (D)

In soil testing, STP refers to Soil Test Phosphorus. This term is used because the soil test reports how much plant-available phosphorus is in the soil, which directly guides phosphorus fertilizer recommendations. Phosphorus is essential for root development and energy transfer in plants, but soils vary in how much P they can supply. The STP value comes from a chemical extraction that estimates the phosphorus plants can actually use, and this value is compared to sufficiency ranges to decide whether to apply phosphorus fertilizer and, if so, how much. Other options describe procedures or generic parameters rather than the nutrient level reported, so STP is specifically about the phosphorus portion of the soil test results.