Interpret soil test results for pH, phosphorus, potassium, organic matter, and CEC. Classify nutrient levels and get recommendations.
The Soil Test Interpretation Calculator helps you understand what your soil test numbers mean by classifying key nutrients and properties into standard fertility categories: Very Low, Low, Medium, High, and Very High. Soil testing is the foundation of sound nutrient management, but the raw numbers from the lab report can be confusing without context.
This calculator interprets results for soil pH, Mehlich-3 phosphorus, exchangeable potassium, organic matter percentage, and cation exchange capacity (CEC). Each parameter is classified based on widely used agronomic thresholds from Midwest land-grant university guidelines.
Keep in mind that optimal ranges vary by crop, soil type, and region. Always consult your local extension service for region-specific interpretations, especially for specialty crops or unusual soil conditions. Whether you are a beginner or experienced professional, this free online tool provides instant, reliable results without manual computation. By automating the calculation, you save time and reduce the risk of costly errors in your planning and decision-making process.
A soil test is only useful if you know how to read it. This interpreter turns lab numbers into actionable categories, helping you decide whether you need to build, maintain, or draw down nutrient levels before the next growing season. Having a precise figure at your fingertips empowers better planning and more confident decisions.
Classification is based on agronomic thresholds: pH: <5.5 = Very Low, 5.5–5.9 = Low, 6.0–6.5 = Optimum, 6.6–7.0 = High, >7.0 = Very High P (ppm): <10 = Very Low, 10–19 = Low, 20–40 = Optimum, 41‐80 = High, >80 = Very High K (ppm): <80 = Very Low, 80–119 = Low, 120–200 = Optimum, 201–300 = High, >300 = Very High OM%: <1.5 = Very Low, 1.5–2.4 = Low, 2.5–4.0 = Medium, 4.1–6.0 = High, >6.0 = Very High CEC: <5 = Very Low (sandy), 5–10 = Low, 10–20 = Medium, 20–30 = High, >30 = Very High (clay)
Result: pH: Low | P: Optimum | K: Optimum | OM: Medium | CEC: Medium
pH 5.8 is slightly acidic (Low) — consider lime. P at 25 ppm is in the Optimum range — maintain with crop removal rates. K at 150 ppm is Optimum. OM at 3.2% is Medium. CEC of 12 is Medium, indicating moderate nutrient-holding capacity.
A soil test report typically includes pH, buffer pH (for lime recommendation), macronutrients (P, K, Ca, Mg), CEC, base saturation percentages, and sometimes micronutrients. Each value needs context — a phosphorus level of 40 ppm might be optimum for corn but excessive for a watershed management plan.
When soil test levels are below optimum, you need to both replace what the crop removes AND add extra to build levels. Once at optimum, you only need to replace crop removal (maintenance). Above optimum, you can skip fertilization for that nutrient and let the crop draw down the surplus.
A single soil test is a snapshot. Trends over 3–5 tests reveal whether your nutrient management is building, maintaining, or depleting soil fertility. Keep all your soil test reports in order and track trends by field.
Test every 2–3 years for row crops and every 1–2 years for high-value crops. Annual testing is warranted during corrective programs (liming, P building) to track progress.
Mehlich-3 is a multi-element soil extractant used by most Midwest and Eastern U.S. labs. It extracts plant-available phosphorus, potassium, calcium, magnesium, and micronutrients in a single analysis. Results are reported in ppm (mg/kg).
Different extractants (Mehlich-3 vs. Bray P1 vs. Olsen) dissolve different amounts of nutrient. Make sure you compare results from the same extraction method. Also stick with one lab for consistency.
Cation exchange capacity is the soil’s ability to hold positively charged nutrients (Ca, Mg, K, Na, H). Higher CEC = more nutrient-holding capacity. Sandy soils have low CEC (2–5), clay soils have high CEC (20–40). CEC affects fertilizer management strategies.
Humus is the stable fraction of organic matter that resists further decomposition. Total organic matter includes both active (decomposing residues) and stable (humus) fractions. The soil test measures total organic matter.
Test for micronutrients (Zn, Mn, Fe, Cu, B) when: soil pH is above 7.0 (reduces availability), you grow sensitive crops (corn zinc, alfalfa boron), or visual deficiency symptoms appear. Standard tests often include micronutrients.