Rockwool chunks in the root zone from potted marijuana. (Image courtesy of Thomas Cacheski)

Scenic Roots: Our Guide To Soil, Coco Coir, Soilless Mix & Rockwool Root Zone Materials

The medium you use in your cannabis root zone has a big impact on your cultivation workload and ultimate success as a grower. There’s plenty of confusion and misinformation about marijuana root-zone materials, so we’re going to clear that up by giving you just the facts.

Soil Root Zone Medium

Of all the choices of what to use for your indoor cannabis root zone, soil is the least useful for indoor growers. This is because when you use soil, you lose the ability to control all nutrients that enter your root zone and plants.

Unless you build your own soil mix — which is difficult and expensive to do — you’ll likely buy prepared soil. Almost all potting soil sold at big-box gardening stores like Home Depot and Walmart is made by Scotts Miracle-Gro and its front company Hawthorne Gardening, or its affiliates EcoScraps and Black Magic.

The majority of commercial potting soil brands come loaded with coarse fertilizers and materials that make them unsuitable for marijuana. Worse yet, these soils are often poorly manufactured, containing sticks, stones, root aphids, fungus gnats, spider mites, heavy metals and other contaminants.

Soil from major brands sold in dedicated hydroponics grow shops isn’t much better and costs a lot more than generic soil.

The main problem with soil, other than pests and diseases that may hitch a ride in the bag, is it’s not a neutral, inert base. You can’t use soil in a scientific way to get the yield-boosting, quality-enhancing total nutrient control you get with neutral hydroponics mediums. (More on that shortly.)

So, unless you’re growing marijuana outdoors, it’s best to avoid soil. For outdoor growers, here’s a super soil recipe. Be sure you have the right ratios and ingredients before you attempt to build this grow recipe.

The most useful strategy is to place only a few seedlings or clones — or even a few non-cannabis plants — in your new soil and see how it works before you run an entire cannabis crop cycle. Perhaps the main problems with using soil, whether you blend your own or buy it, is that it either doesn’t have enough nutrients, it has too many nutrients, or it’s not aerated enough. Here are some quick fixes for soil with the wrong ration of nutrients:

  • Soil that has too much on-board nutrients: Dilute with soil that has no nutrients.
  • Soil with too few nutrients: Add nutrients via fertilizer.
  • Soil that’s too dense and not aerated enough: Add coarse perlite.

Bottom line, cannabis can grow in poor soil, amended soil, homemade soil, and soil with fertilizers added to it. Growing outdoor marijuana, you often have no choice but to use soil. However, most soil inhibits rather than enables maximum yield and bud potency, and can often contain damaging materials, root diseases or pests.

Growing Cannabis In Soilless Mix

If you want something that’s kind of like soil but has the proper aeration density, is inert, and has no contaminants and pests, we recommend professional soilless mix. It looks a lot like soil, but is a neutral, peat-based mix that maximizes hydroponics feeding. Soilless mix has long been a favorite for hydroponics growers, and a handful of manufacturers have honed it to be a very reliable hydroponics root-zone media.

Soilless mix comes in a variety of configurations to suit your grow needs. Some configurations have less or more aeration and drainage materials, such as perlite, depending on how and where you grow. My favorite soilless mix brand is Pro-Mix, which tends to be more cost-effective than other forms of neutral hydroponics root-zone media.

Growing Cannabis In Coco Coir

Made from the husks of coconut shells that are processed into a malleable neutral material easy for roots to navigate, coco coir has favorable water-holding, drainage and aeration traits, and is relatively rugged and cost-effective.

But coco coir is not without its problems. Coconut husks contain a high amount of sodium, which is bad for your plants as it can interfere with your feed program and ultimately fry your roots. Growers should always thoroughly rinse and flush coco coir with reverse osmosis water before use.

Quality coco coir is also difficult to source and what you do find costs more than an equivalent amount of soilless mix or rockwool. My favorite coco coir is from Green Texan Organic Farms. We advise getting it in the loose form rather than in brick form.

One thing to be aware of when using coco coir for your grow is that it does weird things to nutrient transfer and absorption. The Advanced Nutrients hydroponics feed program works well with coco coir, and is likely to ensure no problems with pH or with nutrient lockout.

Growing Cannabis In Rockwool

Rockwool is like inert cotton candy for your cannabis root zone, although it’s slightly denser than cotton candy and is made from spun basalt rather than spun sugar and food coloring.

Rockwool is the most inert and neutral of all hydroponics root-zone materials commonly used by growers. It comes in various configurations, including different-sized blocks useful for starting seeds and rooting clones. You’ll find rockwool formatted as slabs, cubes, blocks and chunks.

Rockwool must be pre-soaked to adjust its pH before you use it, and is best used in drip irrigation, flood and drain, and ebb and flow hydroponics systems — although, hand-watering can be made to work, if you’re willing to put in the effort.

The Dutch company Grodan manufactures the highest quality and most complete array of rockwool gardening products. It offers diverse gradients and types so you get the right size, format, drainage and density that best suits your growing needs.

Deep Water Culture, Aeroponics & Nutrient Film Technique

Root zones can also utilize the minimal grow methods of open space, mist, hydroponics nutrients and water. These are called deep water culture (DWC), aeroponics and the nutrients film technique (NFT).

Precisely managed and with 100 percent constant, reliable electricity supply for their crucial pump apparatus, these pure hydroponics systems produce much faster and healthier root growth, propelling stronger plant growth aboveground.

Pure hydroponics water culture systems have no material to buffer nutrients elements and pH, or to hold water around the roots. That’s why these systems are only for use when you have constant, assured electricity supply (i.e., a generator), and are a Master Grower. If electricity fails, then the pumps fail and water and nutrients aren’t delivered to roots. Subsequently, your cannabis plants die within a few hours.

Hydroponics systems utilizing soilless mix, rockwool, coco coir, DWC, aeroponics and NFT produce their best outcomes when you use premium hydroponics nutrients and supplements made specifically for cannabis. Hydroponics root zones with inert materials possess few if any on-board nutrients until you provide them, and the brand and type of nutrients you use is crucial.

Using a hydroponics system and one of the above inert, neutral root-zone materials or pure hydroponics systems, along with a customized feed program, will give you complete control over plant nutrition and consequently the fastest crop turnarounds and the heaviest harvests with the most cannabinoids and terpenoids.

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