Cation exchange capacity is the total amount of exchangeable positive ions (cations) that a medium can hold on negatively charged sites on clay, organic matter and organic fibres. Held cations, such as Ca²⁺, Mg²⁺, K⁺, NH₄⁺ and Na⁺, are not washed straight out; they swap with others in the soil solution, giving a nutrient reserve and buffering against sudden change.
In practice
High-CEC media (clay soils, compost-rich soil, peat, vermiculite) are forgiving: they hold nutrients between feeds and resist pH swings. Low-CEC media (rockwool, perlite, clay pebbles, sand) hold almost nothing, so the plant sees exactly what is in the solution. Coco sits in between and has a characteristic preference: its sites load with potassium and sodium during processing and tend to grab calcium and magnesium from feed, which is why coco is buffered with calcium before use.
CEC is measured in centimoles of charge per kilogram (cmol(+)/kg), equivalent to milliequivalents per 100 g (meq/100 g). Sandy soils are often under 10; clay- and humus-rich soils 20-40 or more.
Don't confuse with
- Buffering (pH): resistance to pH change; related but not the same quantity.
- EC: dissolved salts in solution; CEC is nutrients held on solids.
Vermiculite is expanded mica-like mineral flakes that hold water and nutrients in a mix, used to increase moisture retention rather than drainage.pH measures how acidic or alkaline a solution is on a 0–14 scale; in growing it controls nutrient uptake, with soil best near 6.0–7.0 and hydro 5.5–6.5.Rockwool is an inert growing medium spun from melted basalt rock into fibres, sold as plugs, cubes and slabs, that holds water and air in fixed proportions.Perlite is a light, white, puffed volcanic glass added to growing mixes to increase drainage and air space; it is chemically inert and holds no nutrients.Clay pebbles (LECA) are fired, expanded clay balls used as an inert hydroponic medium that drains freely, holds a little water and anchors roots.Coco coir is a growing medium made from processed coconut husk, prized for holding water and air at once; it binds calcium and magnesium unless buffered.Sodium (Na) is not essential for cannabis; it enters through water, softeners and some inputs, and at high levels causes salt stress and crowds out potassium and calcium.EC (electrical conductivity) measures how well a solution conducts electricity, a proxy for total dissolved salts; growers read it in mS/cm to gauge nutrient strength.Buffering capacity is how strongly a water, solution or medium resists pH change when acid or base is added; high capacity means stable but hard-to-move pH.Peat is partially decomposed sphagnum moss from bogs, the base of most potting mixes; it holds water and nutrients well but is acidic and needs lime.In cannabis growing, soil means a living, organic-rich growing medium, either natural earth or a bagged mix, that supplies some nutrients and buffers pH and feeding mistakes.
