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Classify each nutrient as Very Low, Low, Optimal or High from soil test ppm values, using standard university extension critical thresholds.
Enter soil test values for N, P, K, Ca, Mg, Zn, Fe, Mn and B. The calculator rates each nutrient and flags which ones need correction.
Your corn showed interveinal yellowing on new leaves. You assumed nitrogen and side-dressed 100 kg urea/ha with no effect. The soil test showed Zn at 0.6 ppm — zinc deficiency in high-pH soil, not nitrogen. The wrong diagnosis wasted both fertilizer and time.
✓ 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 each soil test nutrient in ppm (use your lab's extraction method — thresholds are approximate for Bray-1 / Mehlich-3). Click Calculate to see the rating for each nutrient and which ones need attention.
This diagnostic uses approximate university extension critical thresholds for mineral soils: N 20/40/80 ppm, P 10/20/30 ppm, K 75/120/160 ppm, Ca 500/1000/2000 ppm, Mg 50/100/200 ppm, Zn 1/2/5 ppm, Fe 10/20/40 ppm, Mn 5/10/20 ppm, B 0.3/0.5/1.0 ppm. Ratings are Very Low, Low, Optimal, High.
For each nutrient v: v < low = Very Low; low ≤ v < medium = Low; medium ≤ v < high = Optimal; v ≥ high = High. Thresholds reflect North-Central US extension values; your local lab's agronomist should interpret results for your region.
N = 40 ppm (low-optimal boundary), P = 15 ppm (Low), K = 130 ppm (Optimal), Ca = 1500 ppm (Optimal), Mg = 150 ppm (Optimal), Zn = 2 ppm (Optimal), Fe = 20 ppm (Low), Mn = 10 ppm (Optimal), B = 0.5 ppm (Optimal). P and Fe need attention.
Common mistakes: ignoring that N in soil tests is a snapshot (not a recommendation), using one lab's thresholds on another lab's numbers, diagnosing by leaf color alone without soil or tissue tests, and applying micronutrients without confirming the critical deficiency. Always combine soil tests with tissue analysis.
Each nutrient has a critical threshold: below it the nutrient limits yield; above it the nutrient is sufficient or excessive. Low = likely to limit yield; Optimal = meets crop needs; High = excess that may tie up other nutrients or leach.
Zinc deficiency shows as broad white bands on either side of the midrib on new leaves (the 'white bud' or 'striped' symptom), most common in high-pH, calcareous or cold wet soils. Soil test Zn below 1 ppm confirms the diagnosis. Apply 5–10 kg ZnSO₄/ha as starter.
Iron is insoluble above pH 7.0 even when total soil Fe is high. Symptoms are interveinal chlorosis on new leaves. Correction: acidify soil with elemental sulfur, apply iron chelates (Fe-EDDHA) or improve drainage. Foliar Fe sprays give short-term relief.
Yes. Boron is required at 0.5–1.0 ppm in soil but becomes toxic above 2.0 ppm, especially to sensitive crops (beans, stone fruit, grapes). Excess B is common in arid soils or after repeated borax applications. Leaching with irrigation is the only correction.
They answer different questions. Soil tests tell you what is in the soil; tissue tests tell you what the plant actually took up. When symptoms are unclear, take both at the same time. Tissue tests are especially useful for micronutrients, where soil thresholds are unreliable.