CEC Calculator — Cation Exchange Capacity
Enter Ca, Mg, K (and optional Na, H) from your soil test. Use ppm or meq/100g.
ppm → meq/100g: Ca ÷ 200, Mg ÷ 122, K ÷ 391, Na ÷ 230
What is CEC and why does it matter?
CEC (cation exchange capacity) is your soil’s nutrient-holding bank account. It measures how many positively charged nutrient ions — calcium, magnesium, potassium, sodium, and hydrogen — your soil can hold at one time.
| CEC (meq/100g) | Typical interpretation |
|---|---|
| < 5 | Low — sandy, low OM; leaches quickly |
| 5–15 | Medium — typical row crop |
| 15–25 | High — heavy clay or high OM |
| > 25 | Very high — heavy clay |
Higher CEC means more room to store cations and more forgiveness on fertilizer timing. Low-CEC soils need split applications, lighter rates per pass, and more attention to sandy spots that leach.
What affects CEC?
Clay content — montmorillonite clays have much higher CEC than kaolinite. You cannot change clay type, but you can manage around it.
Organic matter — highest CEC per unit weight of any soil fraction. Building OM improves CEC over years through cover crops, reduced tillage, and manure.
pH — CEC is slightly pH-dependent. Limed soils hold more cations than very acid soils.
Reading your base saturation numbers
Base saturation shows what percentage of CEC is occupied by each cation:
| Cation | Ideal range |
|---|---|
| Calcium | 65–75% |
| Magnesium | 10–15% |
| Potassium | 2–5% |
| Sodium | < 2% |
If K% is high but Mg% is low, cattle on lush spring pasture may show grass tetany symptoms — even when absolute soil Mg looks adequate, because high K suppresses Mg uptake.
The calculator flags ratios outside ideal ranges when your test data suggests a problem.
CEC and fertilizer timing
Low CEC: split N applications, avoid heavy single passes on sand, watch K leaching.
High CEC: single preplant or sidedress applications often work; nutrients stay in the rooting zone longer.
Convert your fertility plan to product rates with the fertilizer rate calculator. Understand your full test report in the soil test guide.
ppm to meq/100g conversion
| Cation | Divide ppm by |
|---|---|
| Ca | 200 |
| Mg | 122 |
| K | 391 |
| Na | 230 |
CEC Calculator FAQ
What is a good CEC for farmland?
5–15 meq/100g is typical for row crops. Management matters more than chasing a single number.
Does CEC change over time?
Slowly — organic matter building increases CEC over years.
What is the difference between CEC and base saturation?
CEC is total capacity; base saturation is how it is filled.
How do I increase my soil’s CEC?
Build organic matter — cover crops, reduced tillage, manure.
Is CEC the same as soil organic matter?
No — clay also contributes significantly.