CEC Estimate & How Much Nutrient Soil Holds
Holds calcium
Cation exchange capacity is the soil's ability to hold nutrients — estimate it from your clay and organic-matter content, because higher CEC holds more and resists leaching.
CEC estimate
Next: with 19 cmol (medium) the soil banks nutrients well — you can apply less often, but keep base cations balanced (Ca:Mg:K).
This is a quick estimate (clay% × 0.5 + OM% × 2); true CEC depends on clay mineralogy and pH. A lab CEC test is the gold standard.
CEC estimate — key facts
- CEC
- nutrient-holding ability, cmol/kg
- Clay
- ≈ 0.5 cmol/kg per %
- Organic matter
- ≈ 2 cmol/kg per %
- Estimate
- clay part + OM part
- Low CEC
- < 10 cmol/kg, leaches
- High CEC
- > 25 cmol/kg, holds
- Raise it
- build organic matter
- Privacy
- Runs in your browser; nothing uploaded
The soil's nutrient bank account
Cation exchange capacity (CEC) is the soil's ability to hold nutrients on the charged surfaces of clay and organic matter. It can be estimated from those two components — clay contributing about 0.5 cmol/kg per percent and organic matter about 2 cmol/kg per percent. Think of it as the soil's nutrient bank: a high CEC stores more calcium, potassium, magnesium and ammonium and hands them to the crop, while a low CEC holds little and lets nutrients leach away with the next rain.
This tool gives the estimated CEC, the clay contribution, the organic-matter contribution and a rating from your clay and organic-matter percentages. Use it to understand why a sandy soil leaches, to see how much building organic matter would help, and to set a fertiliser strategy. Pair it with the CEC & Base Saturation, Cation Ratio and Soil Organic Carbon tools.
Gauge nutrient holding
See how much your soil can store.
Understand leaching
Low CEC explains why nutrients wash out.
Value organic matter
See how much OM lifts your CEC.
Plan fertiliser
Split doses on low-CEC soils to cut loss.
Frequently Asked Questions
What is cation exchange capacity (CEC)?+
CEC is the soil's ability to hold positively charged nutrients — calcium, magnesium, potassium, ammonium and others — on the surfaces of clay and organic matter. It's measured in cmol(+)/kg. A higher CEC means the soil can store more nutrients and release them to plants, while a low CEC holds little and lets nutrients leach away quickly.
How is CEC estimated from clay and organic matter?+
Clay particles and organic matter carry the negative charges that hold cations, so CEC can be estimated from their content. A common rule contributes roughly 0.5 cmol/kg per percent of clay and about 2 cmol/kg per percent of organic matter. This calculator sums those two contributions to give an estimated CEC and a rating.
Why do clay and organic matter drive CEC?+
Both have large surface areas covered in negatively charged sites that attract and hold positively charged nutrient ions. Sand grains are large and chargeless, so they add almost nothing. That's why sandy, low-organic-matter soils have low CEC and leach readily, while clay-rich, humus-rich soils hold nutrients tightly.
What counts as a low or high CEC?+
As a rough guide, below about 10 cmol/kg is low (sandy soils, low storage), 10–25 is moderate, and above 25 is high (heavy clay or very organic soils). The rating in the result places your estimate on that scale so you can judge how well your soil will hold added nutrients.
Why does higher CEC resist leaching?+
Nutrients held on exchange sites are not free in the soil water, so drainage washes away far fewer of them. A high-CEC soil therefore acts as a reservoir — it buffers against leaching and lets you fertilise less often. A low-CEC soil needs smaller, more frequent applications to avoid losses.
How can I raise my soil's CEC?+
Clay content is fixed, but you can lift CEC over time by building organic matter — adding compost, manure, cover crops and crop residues, and reducing tillage. Because each percent of organic matter adds several times more CEC than a percent of clay, organic-matter buildup is the practical lever on most farms.
Does CEC affect how I should fertilise?+
Yes — low-CEC soils hold little, so split nitrogen and potassium into smaller, more frequent doses to cut leaching. High-CEC soils can take larger, less frequent applications. CEC also underpins base saturation and the Ca:Mg:K balance, so pair this with the CEC & Base Saturation and Cation Ratio tools.
Is this estimate as good as a lab test?+
It's a useful planning estimate, not a replacement for measurement. Real CEC depends on clay mineralogy and on soil pH (charge on organic matter and some clays is pH-dependent). Use the estimate to understand your soil and plan, then confirm with a laboratory test for precise fertiliser recommendations.
What inputs do I need?+
Just the clay percentage and organic-matter percentage of your soil, from a soil test or texture analysis. The calculator returns the estimated CEC, the contribution from clay, the contribution from organic matter, and a rating so you can see which component is doing the work.