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Soil Test Recommendation Calculator
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.
Results
How to Use This Calculator
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.
About This Tool
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.
Applicability: Designed for temperate and subtropical soils, applicable to farming systems across North America and Europe.
How the Calculation Works
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.
Real-World Example
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 & Tips
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.
Reviewed by Agricalc Editorial Team — Agricultural engineers and agronomists with 10+ years of field experience. Our team verifies every calculator formula against peer-reviewed extension publications. For site-specific advice, consult your local agronomist.
References
- USDA NRCS — Soil Health Assessment
- University of Minnesota Extension — Soil Management & Health
- FAO — Soils Portal
Frequently Asked Questions
What are build-up, maintain and drawdown?
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.
What do ppm P and ppm K mean?
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.
When do I need to apply lime?
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.
Why does soil pH matter for fertilizer?
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.
How often should I soil test?
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.
