5 Tips for Selecting Your Substrate

Columns - Hort How-To

What to consider when selecting growing media, and how those decisions can impact your crop.

November 22, 2022

Potting soils are often targeted for plant growth stages, with lighter, airier mixes intended for seedlings or younger plants that require good aeration, and heavier, more nutrient-dense mediums designed for larger plants and outdoor growing.
Photo by Christine DeJesus

With so many options available on the market, choosing a potting soil for your cannabis cultivation operation is a challenging decision. Though there are a few essential ingredients most potting soils contain, input proportions and quality can make an enormous difference in growing results, so it is important to research well before committing your high-dollar crop.

Here are five tips for choosing a growing medium that directly aligns with the goals and mission of your business.

1. Consider growth stage and environment when selecting substrates.

Potting soils are crafted growing mediums that range from one ingredient to a complex recipe of mineral and organic additives, but all are designed to meet the performance demands of container gardening. Potting mixes are composed of one or more bulking materials that provide moisture and nutrient retention, such as peat moss, coco coir or composted pine bark, along with something that adds aeration—typically perlite. Additionally, nutrients essential for plant growth may be incorporated in the form of synthetic or organic fertilizers or amendments.

Potting soils are often targeted for plant growth stages, with lighter, airier mixes intended for seedlings or younger plants that require good aeration, and heavier, more nutrient-dense mediums designed for larger plants and outdoor growing. Some growers prefer a light mix during vegetative growth as well.

When deciding between a lighter or heavier growing medium, it is important to consider not only the growth phase of your crop, but also your cultivation model. For example, outdoor-grown photoperiod plants require large containers to support sustained growth from late spring through early fall as they build substantial vegetative and flower biomass. In turn, outdoor plants require larger nutrient inputs and have higher water demands than indoor-grown plants, so light and highly aerated soils will dry out too fast and require regular additional feedings.

However, a lighter density can be desirable for indoor grows, as most commercial cultivators run short vegetative cycles—often one to three weeks long—and flower cycles are generally between seven and nine weeks long.

2. Plan around your watering style.

The method and frequency of your irrigation can make a major difference in which potting soil will work best.

When watering from the top of the pot by hand or with an automated drip irrigation system, the water runs down through the pot via gravity, and if there is excess water, it will drain away. Therefore, the soil needs to be able to hold enough moisture long enough to allow the plants time to absorb it and the dissolved nutrients, yet still have adequate drainage to achieve the desired dry back.

With a sub-irrigation system, roots draw up water through the bottom of the pot, so potting soil should have a good wicking action to move it. Bottom-watered plants usually require more porosity for a higher amount of aeration to keep lower roots from becoming oversaturated and oxygen-deficient. To help with this, a 1- to 2-inch layer of perlite or horticultural clay pebbles on the bottom of the pot can enhance oxygen levels in the root zone. Additional perlite or another aerating material in the potting mix helps, too, as these systems tend to stay moister.

3. Consider the risks and benefits of a biologically active option.

Cannabis cultivators often use the terms “living soil” or “super soil” in reference to potting soils composed of organic inputs and biologically active components found in decomposed organic matter, such as thermodynamic compost or vermicompost. These soils often contain prominent levels of plant nutrition as well as a diverse range of microorganisms, some of which may promote direct growth in many plant species.

One category of beneficial bacteria often found in these soils, referred to as plant growth-promoting rhizobacteria (PGPR), for example, has been observed to increase nutrient uptake, provide systemic resistance in plants against pathogens, reduce root pathogen populations in the rhizosphere, increase stress tolerance, and decrease the requirements of fertilizers and pesticides in various crops.

Agricultural inoculants, or soil amendments that contain beneficial microbes, are widely available and can be effective when used correctly. High-quality composted organic materials can also provide a diverse range of beneficial microorganisms. These biologically active potting soils are more commonly used in large containers or raised beds, but they can be used in smaller containers if the right environmental conditions and appropriate growing methods are applied. These high-end mixes also usually come with a bigger price tag, but the investment may be repaid in the form of healthier, more resilient plants, which may bring higher yields and potentially increased cannabinoid or terpene levels. Although more research on cannabis is needed, a 2018 study published in the journal “Industrial Crops and Products” showed increased cannabinoid levels in hemp with inoculations of certain rhizobacteria.

Some growers also may choose to make their own “living soil” blends or start with a premade mix and reamend and reuse the same soil over multiple production runs with additions of fresh compost or individual spent nutrients. This can be a more affordable and environmentally sustainable option versus buying new potting soil for each production run, but it is important to have the infrastructure and support space to accomplish these goals. This can include machinery to store large quantities of soil, mix nutrients into the soil, and move the soil. However, these processes can be complicated to execute and often yield less consistent results.

Additionally, using a biologically active mix may bring unwanted pathogens or pests, so a grower may need more skill and be well-versed in plant health analytics to identify and tackle these issues should they arise.

4. Vet potting soil manufacturers.

It is important to examine soil manufacturers and request testing data. Check to see if any red flags come up in reviews or research and seek commercial references. Examples of what to look for can include soil that was not delivered on time; soil that wasn’t quality-controlled by the producer; or undisclosed heavy metal or pesticide levels, which can result in flower testing failures for growers. Suppliers should offer soil tests, which provide information on nutrient levels, heavy metal levels and pH levels. Suppliers can also offer a microbiological analysis for biologically active mediums (on specific batches when possible) and provide proof of certifications if defined claims are made.

At Galenas, we use a third-party lab to do additional nutrient and heavy metal testing of our potting soil and any fertilizers or inputs the plants receive for further analysis. Surprising amounts of heavy metals can be found in many common potting mixes and fertilizers, and cannabis can pull these contaminants from the medium and incorporate them into leaf and flower material. Elevated levels can cause testing failures in many states. Compost-based living soils and super soils may have elevated levels dependent upon ingredients, but more sterile mixes can test high as well. (Editor’s note: For more tips on avoiding heavy metal contamination in soils, read online at bit.ly/10-tips-avoid-heavy-metals)

5. Cut costs and hone inputs with regular testing.

Once you choose your growing medium, secondary testing of soil and plant tissue during a production cycle can be a valuable resource for evaluating your decision and dialing in nutrition. These tests are exceptionally useful in determining whether the plant has adequate nutrition, is running lean, or has an overabundance (or even toxicity due to elevated nutrient levels). “Test, don’t guess,” is the mantra of Bryant Mason, who owns Colorado-based consulting business, Soil Doctor. “Many growers believe [testing] is the highest ROI [return on investment] thing they can do. Spend $50 on a test and save thousands on inputs.”

Saving is especially important now, at a time when fertilizer and potting soil prices have increased. In June, the U.S. Department of Agriculture released a trade report detailing the surges in fertilizer prices, which are “reminiscent of the Great Recession period, when prices nearly doubled across all major fertilizer groups at the end of 2007,” the report says. The report attributes the high prices to a surge in natural gas prices, especially in Europe, along with coal price increases in China. It concludes with a forecast warning: “As the Russia-Ukraine war continues and the supply of fertilizer remains limited, high prices are likely to have a more profound impact on 2023 planting decisions.”

Choosing a potting mix with a gratuitous amount of nutrients can lessen or offset the need for supplemental fertilizers, thereby saving growers on overall fertilizer costs. Many “super soils” are nutrient-dense and can be beneficial to growing cannabis from start to finish, depending on pot size. Testing your medium can help you determine nutrient starting points and add only what is needed. Soil tests at the end of a cycle can also give you a glimpse into nutrient use by plants, which can provide valuable data for your next production cycle, particularly if you are reusing soil or dialing in a mix. Also, nutrient demands vary across cannabis cultivars, and this data helps to adjust feeding inputs for specific cultivars.

In addition, tissue testing performed at the end of the vegetative cycle and midway through flower can provide great insight into whether your potting soil and additional feeds have the required amount of nutrients for maximum plant health, yield and quality. Keep in mind that nutrients may not be utilized by plants if growing conditions are off, as environmental conditions combined with watering techniques are major contributing factors in nutrient uptake. (Editor’s note: For more insights on optimizing uptake and avoiding nutrient lockout, read more from CBT online here.)

Christine DeJesus is the director of cultivation at Galenas in Akron, Ohio.