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Translate a soil test report into a fertilizer and lime recommendation using the standard university extension build-up / maintain / drawdown thresholds for P and K.
Enter soil test P (ppm), K (ppm), pH and target crop. The classifies P and K as low, medium or high, and returns P₂O₅, K₂O and lime recommendations.
Your soil test showed P at 30 ppm and K at 200 ppm. The local dealer still recommended 200 kg of 15-15-15 per hectare. At those levels you do not need any P or K fertilizer — applying it builds up residual nutrients and wastes money.
✓ Expert Reviewed: This calculator and its content have been reviewed by agricultural experts. Formulas are based on standard extension service recommendations. For site-specific advice, consult your local agronomist.
Enter soil test P and K in ppm, soil pH, target crop (which drives N demand) and field area. Click Calculate to see whether P and K need build-up, maintenance or drawdown, and how much P₂O₅, K₂O, N and lime to apply.
This calculator follows the University of Minnesota / North-Central US extension approach: P below 10 ppm Bray-1 requires build-up (~80 kg P₂O₅/ha), 10–20 ppm maintenance (~30 kg P₂O₅/ha), above 20 ppm no P needed. K thresholds are 100 and 160 ppm. N is driven by crop demand. Lime is estimated when pH is below 5.8.
P recommendation: <10 ppm → 80 kg P₂O₅/ha; 10–20 → 30 kg/ha; >20 → 0. K: <100 → 100 kg K₂O/ha; 100–160 → 40 kg/ha; >160 → 0. N follows crop (maize 100, wheat 60, soybean 30, vegetables 80 kg N/ha). Lime ≈ (5.8 − pH) × 2.5 t/ha when pH < 5.8.
Soil test P = 12 ppm (maintain → 30 kg P₂O₅/ha), K = 120 ppm (maintain → 40 kg K₂O/ha), pH = 6.0, maize. Recommendation: 30 kg P₂O₅/ha, 40 kg K₂O/ha, 100 kg N/ha, no lime.
Common mistakes: using Bray-1 thresholds on Olsen P soils (calcareous), ignoring texture differences in K thresholds, applying P and K every year regardless of test, and over-applying lime to the point of pH > 7. Soil test thresholds vary by region and lab method — always follow your local extension.
Build-up: soil test is below the critical level, so apply more nutrients than the crop removes to raise fertility over several years. Maintain: at the critical level, replace what the crop removes. Drawdown: above critical, apply no fertilizer and let the crop draw down the excess.
Soil test P and K are expressed in parts per million (mg per kg of soil). Critical levels vary by extraction method: Bray-1 P critical is ~15 ppm, Olsen P critical is ~20 ppm, Mehlich-3 P critical is ~25 ppm. K critical is commonly 100–140 ppm.
Most crops prefer pH 6.0–7.0. Apply lime when pH drops below 5.8, especially for legumes, brassicas and vegetables. Acid-tolerant crops (potatoes, blueberries, acid-loving ornamentals) can tolerate pH 5.0–5.5. Use buffer pH to determine exact lime rate, not just pH.
pH controls nutrient availability: below pH 5.5, aluminum and manganese become toxic, and P ties up with iron and aluminum. Above pH 7.0, P ties up with calcium, and micronutrients (iron, zinc, manganese) become less available. The optimal pH for most crops is 6.0–6.8.
Test every 2–4 years on most fields, annually on high-value vegetables or where recent manure or lime was applied. Sample at the same time each year (spring before planting, or late autumn) to track trends. Composite 10–15 cores per field into one sample.