How Are Rockwool and Coir (AKA Coco Fiber) Nutrient Formulas Different?
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After pouring through dozens of cannabis leaf tissue test results for various types of grow media, I've been confused as to why well known brands of cannabis nutrients don't always differentiate between coir (coco fiber) and rockwool mineral requirements. There are a tiny handful of well known brands that do offer formulas for both, but my lab analysis of those formulas doesn't reflect the differences that I believe they should.
The main difference between rockwool and coco fiber in terms of unique nutrient behaviors is that coco fiber has what's known as a cation exchange capacity (CEC). This basically means that some percentage of the minerals you water into the root zone won't be available to the plant and get "stuck" to the media. This is especially the case with Calcium, but also notably so with all of the micronutrients. To put another way, if we apply a well balanced rockwool formula to a plant that's growing in coco fiber, the plant will develop these deficiencies in the leaf tissue.
There are a couple of complications that come along with using coco, however, in addition to the CEC. One is that every brand of coco fiber has a starkly different nutrient profile and pH to start with. They can also vary from batch to batch within one brand. This makes coir challenging to dial in for growers who really want consistency in their product quality. The other complication is that those exchange sites (the "grabby" particles that make up total CEC) behave differently based on how and if the coco fiber has been buffered (more on that below) and how far into plants are into their growth cycles. At the beginning of the growth cycle, for example, it might take a lot of Calcium to get much of it into the plants because the exchange sites are holding a lot of it hostage. However, at some point, those sites get filled and much more of the Calcium applied in the nutrient mixture gets sent straight to the plants.
Coco fiber naturally has significant amounts of Potassium, Sodium and Chloride in it. This comes from their ocean plant origins where these minerals are plentiful. Good coconut fiber producers use advanced freshwater washing techniques and presses to get most of these minerals out of the media, and some companies (but not all) re-buffer the newly emptied exchange sites with Calcium Nitrate, and I've also seen one company that buffers with bat guano (for organic growers). They add minerals back in (that attach to the exchange sites) after washing the natural coir so that growers can use typical nutrient formulas without getting immediate deficiencies.
There is actually an upside to having a CEC- it creates what you could think of as a "cushion" between the nutrients being applied and how the plant reacts. For newbie growers, this can be really helpful. If exchange sites are holding back some of the nutrients being watered in, this can keep plants from getting sick if too much fertilizer is being applied. I'm bringing this up because it's pretty common for growers to over-fertilize with salt based nutrients, so it's nice to have an insurance policy against that.
In comparison to coco fiber, rockwool is much less involved in the process of nutrients getting the plants. It has no CEC and is pH neutral. Roots don't really love it as a substrate, but they like it well enough. The nutrient recipe that works well with it needs lower Calcium and micronutrient quantities than the ideal coco fiber recipe would have. As an R&D grower and consultant, I appreciate rockwool for its simplicity and how its use removes a variable from the already complicated equation of how to keep plants perfectly healthy from clone to harvest.