What Is Coco Coir? Properties, the Cal-Mag Trap and How to Prep It

Coco coir is a 100% organic growing medium made from the ground-up husk of coconuts. It holds a lot of water without going soggy, stays airy around the roots and works in hydroponic setups as well as in classic container grows (yep, you can even mix it into soil).

Coco coir's physical and chemical properties

Standard coco coir is mostly made up of coco pith — the fine, dusty part of the husk — which is the main ingredient in most coco products. It's packed with millions of tiny capillary sponges that soak up and hold as much as nine times their own weight in water.

Coco is a well-aerated medium that barely compacts once it dries out. It has a natural pH of 5.7–6.5 and a high cation exchange capacity, or CEC, which makes it a quality growing medium.

So what exactly is CEC? It has to do with how well a medium can hold on to positively charged nutrients and hand them over to the roots when the plant needs them. Put simply, a medium's ability to store and release a given group of nutrients is what we call its cation exchange capacity, or CEC.

What is coco coir used for?

Coco coir is super spongy and light, with tons of air pockets in its structure. It soaks up a lot of water without losing that fluffiness or falling apart. It also wicks moisture really well, so when you wet one part, the water spreads evenly through the whole thing — and since it adds next to nothing on its own, it's treated as a nearly inert medium.

Good-quality coco can also be reused two or three times, after which you can compost it or use it in the garden or veggie patch. Just as important, coco supports a rich, healthy rhizosphere (the living zone around the roots), letting beneficial microbes colonize the medium and help the plant take up certain substances and nutrients more efficiently.

How coco coir helps your plants while they grow

Growing in coco boosts the development of roots, stems and flowers. Potting soil compacts easily; coco, on the other hand, keeps enough air pockets for a strong root system. That gives you a healthy aerobic rhizosphere, which is key for proper water and nutrient uptake.

Coco is naturally high in lignin, which encourages beneficial microbes in the root zone and keeps the medium from breaking down — that's what makes it ideal for reuse.

Many growers also believe that, thanks to those beneficial microbes, coco offers some protection against plant pathogens. Several in vitro experiments (done in the lab, not in a real garden) have shown coco can hold back the growth of soil-borne pathogens.

One of those experiments, for example, found that mycelium growth of the fungus Phytophthora capsici was strongly inhibited when the medium was treated with a suspension of unsterilized coco.

Growing in coco coir: what to watch out for

As we mentioned, coco has a few quirks that need the grower's attention. One of the biggest is that it has a strong affinity for positively charged ions, especially double-charged ones. What does that mean in practice? New coco will grab a lot of nutrient ions like calcium and magnesium, so grows where coco is the main medium can end up with calcium and magnesium deficiencies, because the coco basically “steals” them from your fertilizer.

What about hydroponic coco grows?

This is a classic issue in hydro grows. To get around it and enjoy everything coco has to offer in hydroponics, there's a simple fix: use a fertilizer formulated specifically for coco. So what are these fertilizers? The best ones carry extra calcium and magnesium, so there's enough left over to properly feed the plant even after the coco takes its cut. Another option is pre-buffered coco, which has already been treated so it won't lock up those nutrients.

Loose coco vs. compressed bricks: how to prep coco coir

When you go to buy coco coir, it comes in two formats. The first is loose coco in bags. The second is dehydrated coco compressed into blocks or bricks.

Bagged loose coco is ready to use. Coco bricks, on the other hand, need to be rehydrated by adding water little by little until they expand. Either way, your best move is to fluff the coco by hand, folding it over gently without squeezing, to aerate it and get it as spongy as possible.

Tips before you use coco coir for your plants

• Always use coco from a well-known, reputable brand (honestly, that goes for every product you put on your plants)
• We never recommend unbranded coco or bags that don't list their origin, because they usually aren't properly washed, haven't aged long enough and sometimes carry way more dust than fiber (which means the coco ends up dense and pretty much caked together)
• Don't use coco if you plan to water with hard water and regular fertilizers the way you would in a soil grow
• Use hydroponic fertilizers that supply every mineral the plant needs. If you're rehydrating a coco brick, use tap water, not softened water.