Cation Exchange Capacity: How a Substrate Holds Nutrients and Gives Them Back

Cation exchange capacity, CEC for short, describes how many nutrient particles a substrate can hold on its surface and swap for others. It refers to positively charged particles such as calcium, magnesium, potassium, and sodium. A substrate with a high CEC changes the nutrient solution poured into it: it takes up some nutrients and releases others. A substrate without CEC leaves the solution as it is.

What it means in the pot

Rockwool has practically no CEC. Whatever is in the feed either reaches the root or drains away, and you steer the plant directly through the solution. Peat and coco, by contrast, bind nutrients. In them the solution at the root can look different from the one you mixed.

How much CEC coco has

How much CEC coco has exactly is disputed; the often-quoted 10 to 30 meq per 100 g do not say whether they refer to weight or volume, and 103.3 cmol/kg was measured on washed coco, without the two figures being directly comparable. For practice it is enough to know that coco changes the nutrient solution, and what it releases depends on origin, washing, and pre-treatment. That is why in coco you measure the leachate and not only the feed.

Why coco is buffered

Unwashed coco can carry a lot of potassium and sodium on its sites. When a nutrient solution with calcium and magnesium meets it, the coco takes up those two and releases potassium and sodium. On the plant this can look like calcium deficiency, even though the fertiliser contains enough. Buffering means letting this exchange happen beforehand: soak the coco in a calcium nitrate solution and rinse it afterwards; in the trial that was up to 150 g of calcium nitrate per 1.5 kg of coco in 15 litres of water, for 12 to 72 hours. The bound calcium rose sixteenfold as a result. Against high sodium levels, the authors of a measurement on coco recommend thorough washing.

What to make of it

Watering coco with soft water, that is reverse osmosis water or rainwater, makes the problem worse, because the water brings no calcium or magnesium of its own. In that case, blend in tap water. A research report from a fertiliser manufacturer considers a straight calcium-magnesium product less effective, because it does not buffer the pH. If cannabis grows more weakly in coco than in peat, CEC is not necessarily the reason; in one comparison the authors saw the cause in coco’s water relations.

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