Lime rate (tons/acre) = (Target pH − Current soil pH) × soil buffer coefficient ÷ (lime CCE × fineness factor). For a silt loam soil with pH 5.4, target pH 6.5, buffer pH 6.2 (SMP), using agricultural lime with 90% CCE and 80% fineness: lime requirement = (6.5 − 5.4) × 4.5 ÷ (0.90 × 0.80) = 1.1 × 4.5 ÷ 0.72 = 6.9 tons/acre. This guide explains every variable and how to get it right.
Soil pH is the master variable of soil chemistry. It controls nutrient availability, microbial activity, aluminum and manganese toxicity, and herbicide efficacy. Most crops grow best at pH 6.0–7.0 (slightly acid to neutral). Below pH 5.5, aluminum and manganese become soluble and toxic to roots, stunting growth and reducing yield by 20–50%. Phosphorus, calcium, magnesium, and molybdenum become unavailable. Above pH 7.5, iron, manganese, zinc, and boron become unavailable, causing micronutrient deficiencies.
Liming is the application of calcium- and magnesium-rich materials (primarily ground limestone, CaCO₃/MgCO₃) to neutralize soil acidity. It is the single most cost-effective soil amendment for acidic soils: a $30–80/acre lime application can increase yield by 10–40% and improve fertilizer use efficiency by 20–30%. A soil test showing pH below the crop optimum is a clear signal to lime.
Soil pH measures the active acidity — the concentration of H⁺ ions in the soil solution. It is measured with a pH electrode in a 1:1 or 1:2 soil:water slurry. Soil pH tells you how acidic the soil is NOW, but it does NOT tell you how much lime is needed. Two soils can have the same pH (e.g., 5.5) but require very different lime rates because of differences in buffering capacity (reserve acidity).
Buffer pH measures the reserve acidity — the H⁺ and Al³⁺ ions held on soil cation exchange sites (clay and organic matter). When you add lime, it first neutralizes the active acidity (solution H⁺), then the reserve acidity (exchangeable H⁺ and Al³⁺) releases more H⁺ into solution. The buffer pH test simulates this by adding a buffering solution (SMP, Woodruff, or Mehlich buffer) to the soil and measuring the pH change. The lower the buffer pH, the more reserve acidity, and the more lime is needed.
Key relationship: A soil with pH 5.5 and buffer pH 6.0 (high reserve acidity, typical clay loam) may need 5–8 tons/acre of lime. A soil with pH 5.5 and buffer pH 6.6 (low reserve acidity, typical sand) may need only 1–2 tons/acre. Always use buffer pH, not soil pH alone, to calculate lime requirement.
Not all lime is equal. The neutralizing value and reaction speed depend on two key quality parameters.
CCE measures the neutralizing capacity of a liming material relative to pure calcium carbonate (CaCO₃), which is defined as 100%. CCE depends on the chemical composition: Pure calcite limestone (CaCO₃): 95–100% CCE. Dolomitic limestone (CaMg(CO₃)₂): 95–108% CCE (magnesium carbonate has slightly higher neutralizing value per unit weight). Hydrated lime (Ca(OH)₂): 120–135% CCE (faster reacting, more expensive, caustic). Burned lime (CaO): 150–175% CCE (very caustic, rarely used in agriculture). Industrial byproducts (slag, fly ash, paper mill lime): 50–90% CCE, variable quality — always request a lab analysis.
CCE is also called "effective calcium carbonate equivalent" (ECCE) when adjusted for fineness.
Lime reacts only at the surface of particles. Finer particles have more surface area per unit weight and react faster. Standard grading: Particles passing a 60-mesh sieve (<0.25 mm): react within 1–2 years, 100% effective. Particles passing 8–20 mesh but retained on 60 mesh (0.25–2.38 mm): react in 2–5 years, 50–80% effective. Particles retained on 8 mesh (>2.38 mm): react very slowly (>5 years), 0–30% effective in a typical 3–5 year planning horizon.
The fineness factor is calculated as: % passing 60 mesh × 1.0 + % passing 8 mesh but retained on 60 × 0.5 + % retained on 8 mesh × 0. Most agricultural lime has a fineness factor of 0.70–0.90. Always ask your lime supplier for a guaranteed analysis including CCE and fineness (many states require this by law).
The lime requirement from the buffer test is usually provided directly by the soil testing lab (in tons/acre of pure CaCO₃ needed to reach the target pH). If the lab does not provide it directly, you can estimate from buffer pH using university extension tables.
Example: Lab recommends 4.5 tons/acre pure CaCO₃ to raise pH from 5.4 to 6.5. Your lime has 90% CCE and 0.80 fineness factor. Actual lime needed = 4.5 ÷ (0.90 × 0.80) = 4.5 ÷ 0.72 = 6.25 tons/acre of your specific lime material.
Target pH by crop: Corn, soybeans, wheat: 6.0–6.5. Alfalfa: 6.5–7.0 (higher need for calcium and molybdenum). Vegetables: 6.0–6.8 (varies by crop — potatoes prefer 5.0–5.5 to suppress scab). Blueberries, azaleas: 4.5–5.5 (acid-loving, do NOT lime). Most field crops: 6.0–6.5 is optimal.
Application method: Agricultural lime is typically applied with a bulk spreader (lime spreader) at rates of 1–10 tons/acre. For rates above 4–5 tons/acre, split into two applications (half now, half in 1–2 years) to avoid over-raising pH in the surface layer while subsoil remains acidic. Incorporate lime into the top 6–8 inches with tillage (disk, chisel, or moldboard plow) for fastest reaction. No-till systems: lime is applied to the surface and reacts slowly with the top 1–2 inches; may need lighter, more frequent applications (every 2–3 years) to maintain surface pH. Some no-till farmers use 1/3 rate annually.
Timing: Apply lime 3–6 months before planting the most pH-sensitive crop (alfalfa, vegetables) to allow time for reaction. Fall application is ideal — winter freeze-thaw and moisture help incorporate and react the lime. Spring application is acceptable but may not fully react before planting. Never apply lime and urea/ammonium fertilizer in the same pass (ammonia volatilization risk from high pH). Wait 1–2 weeks between lime and N fertilizer applications.
Lime lasts 3–5 years in most soils (longer in heavy clays with high CEC, shorter in sandy soils with low CEC and high rainfall). Retest soil pH every 2–3 years to monitor.
Calcitic lime (CaCO₃): Primarily calcium carbonate. Use when soil test shows adequate magnesium (Mg > 10% of CEC, or >50 ppm Mg). Most common and usually cheapest.
Dolomitic lime (CaMg(CO₃)₂): Contains both calcium and magnesium carbonate (typically 10–20% Mg). Use when soil test shows low magnesium (Mg < 10% of CEC, or <50 ppm), especially for crops with high Mg demand (alfalfa, corn, vegetables). Dolomitic lime may cost $2–5/ton more but provides essential Mg.
How to choose: Look at your soil test report for magnesium level and base saturation. If Mg saturation is <10% of CEC, use dolomitic lime. If Mg is adequate (>12–15%), calcitic lime is fine. On very low-Mg sandy soils, dolomitic lime is strongly recommended. Do NOT use dolomitic lime on high-Mg soils (can cause Ca:Mg imbalance and reduce potassium availability).
An 80-acre field in Ohio with the following soil test (0–8 inch): pH 5.2, buffer pH (SMP) 6.1, organic matter 2.8%, CEC 14 meq/100g, Mg 8% saturation (low). Previous crop: soybeans. Planned rotation: corn → soybeans → wheat. Target pH: 6.3 for corn/soybeans/wheat.
Step 1 — Determine lime requirement from buffer pH: Using Ohio State University extension table for SMP buffer pH 6.1 and target pH 6.3: lime requirement = 4.0 tons/acre pure CaCO₃. (The table accounts for soil CEC and organic matter implicitly through the buffer test.)
Step 2 — Select lime type: Mg saturation is 8% (below 10% threshold), so use dolomitic lime. Local quarry offers dolomitic lime with: CCE = 92%, fineness: 75% passing 60 mesh, 20% passing 8 but retained on 60, 5% retained on 8. Fineness factor = (0.75 × 1.0) + (0.20 × 0.5) + (0.05 × 0) = 0.75 + 0.10 = 0.85.
Step 3 — Calculate actual lime rate: Actual rate = 4.0 ÷ (0.92 × 0.85) = 4.0 ÷ 0.782 = 5.12 tons/acre. Round to 5.0 tons/acre (practical spreading rate).
Step 4 — Total lime needed and cost: 80 acres × 5.0 tons/acre = 400 tons total. At $28/ton delivered and spread: 400 × $28 = $11,200 total = $140/acre. This is a 3–5 year investment = $28–47/acre/year. Expected yield response: corn yield increase from pH 5.2 to 6.3 = 15–30 bu/acre (at $4/bu = $60–120/acre/year). The lime pays for itself in the first year.
Step 5 — Application plan: Apply 5.0 tons/acre in fall after soybean harvest, then incorporate with chisel plow to 8 inches. Wait 2 weeks before applying any N fertilizer. Plant corn the following spring. Retest soil pH in 3 years. If pH drops below 6.0, apply maintenance lime at 1–2 tons/acre.
Step 6 — Verify with secondary check: Alternative calculation using the Shoemaker, McLean, and Pratt (SMP) method: Lime requirement = (target pH − current pH) × (buffer pH coefficient). For SMP buffer 6.1, coefficient ≈ 3.6 tons/acre per pH unit. (6.3 − 5.2) × 3.6 = 3.96 ≈ 4.0 tons/acre pure CaCO₃. Confirms the extension table value. Adjusted for lime quality: 4.0 ÷ 0.782 = 5.1 tons/acre. ✓
Agricultural liming is a foundational soil management practice. The key steps are: test soil pH and buffer pH, determine target pH for your crop, calculate lime requirement from the buffer test, select the right lime type (calcitic vs. dolomitic based on Mg status), adjust for lime quality (CCE × fineness), apply at the right rate and time, and monitor with follow-up soil tests.
Lime is one of the highest-return investments in agriculture — a $30–80/acre application can generate $60–120/acre/year in increased yield for 3–5 years. The biggest mistakes are using soil pH alone (instead of buffer pH), buying on price without checking quality, and applying lime with N fertilizer. Avoid these and your liming program will be both agronomically effective and economically sound.
Typically 3–5 years, depending on soil type and rainfall. Sandy soils with low CEC and high rainfall (leaching) may need re-liming every 2–3 years. Clay soils with high CEC and moderate rainfall may go 5–7 years between applications. The best indicator is a follow-up soil test — when pH drops below your target range, it is time to re-apply. Nitrogen fertilization (especially ammonium-based) accelerates soil acidification, so fields receiving high N rates may need more frequent liming.
Yes. Over-liming raises soil pH above the optimal range (6.0–7.0 for most crops), which can induce micronutrient deficiencies (iron, manganese, zinc, boron). This is most common on soils with naturally low buffering capacity (sands) where a heavy lime application can overshoot the target. Always calculate the rate using buffer pH and apply no more than 4–5 tons/acre in a single application — split higher rates into two applications 1–2 years apart.
Agricultural lime (ground limestone, CaCO₃ or CaMg(CO₃)₂) is the standard material: slow-reacting (3–12 months), safe to handle, low cost ($20–40/ton), CCE 90–100%. Hydrated lime (Ca(OH)₂, also called slaked lime or builder's lime) is produced by burning limestone then adding water: fast-reacting (weeks), caustic (can burn skin and eyes, damage equipment), more expensive ($80–150/ton), CCE 120–135%. Hydrated lime is sometimes used for rapid pH correction in vegetable production or greenhouse media, but agricultural lime is preferred for field crops due to cost, safety, and longer residual effect. Burned lime (CaO, quicklime) is even more caustic and rarely used in agriculture.
Yes, absolutely. Alfalfa is one of the most pH-sensitive field crops and requires pH 6.5–7.0 for optimal growth and nitrogen fixation. Alfalfa also has high calcium and magnesium requirements. Apply and incorporate lime 6–12 months before seeding alfalfa to allow full reaction. If pH is below 6.0 at seeding, alfalfa establishment will be poor, stands will be thin, and yield will be 20–50% below potential. Dolomitic lime is preferred for alfalfa if Mg is low. A well-limed alfalfa stand can persist 4–6 years; an acid-soil stand may fail in 1–2 years.
Request a "lime analysis report" or "guaranteed analysis" from your supplier. It should include: (1) Calcium Carbonate Equivalent (CCE) — the neutralizing power, should be 85% minimum for agricultural lime, 90%+ is good. (2) Particle size / fineness — percentage passing 8-mesh, 20-mesh, and 60-mesh sieves. At least 50% should pass 60-mesh for reasonable reaction speed. (3) Magnesium content — if dolomitic, should state % MgO (typically 15–25% for dolomitic, <5% for calcitic). (4) Moisture content — should be <5%. Many states (e.g., Ohio, Indiana, Illinois) have lime laws requiring dealers to provide this information. If your supplier cannot provide an analysis, buy from a different source.
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