Soil is arguably one of the top factors that influences plant growth and development. A well chosen or prepared soil, or soilless growing media will alleviate many issues throughout the growing cycle. Soil is composed of: mineral solids (sand, silt, clay), water, air, organic matter and organisms (bacteria, fungi, earthworms, arthropods). Soilless growing media is composed of organic matter (peat, coco, compost) and aeration/drainage (perlite, rice hulls, sand). An ideal soil for plant production would consist of 25% water, 25-50% air, 45% mineral and 5-20% organic matter. A second description of an ideal soil would be about 30% sand, 30% silt, 30% clay and 5-10% organic matter.
The relative proportions of these components describes soil texture, or structure. Loamy soil with a granular structure is generally preferred plant production. The second important factor of soil, or soilless growing media is soil chemistry. Soil pH, indicates the amount of hydrogen ions in the soil, the more hydrogen that a soil contains the more acidic the soil is. Soil pH affects nutrient availability. The ideal pH for cannabis is 5.5 to 6.8. Generally field soils have higher pH’s and soilless growing media tends to be slightly more acidic. If the pH is too low (acidic) or high (alkaline) nutrients become unavailable, or locked out, or overabundant, leading to toxicity. Alkalinity causes some minerals, such as copper, iron and manganese to become less available. Counteract this by adding sulfur, which is converted to sulfuric acid by bacteria, or by adding nitrogenous fertilizers. Acidity inhibits growth of nitrogen-fixing bacteria. Counteract this by adding calcium or magnesium compounds (lime). The best way to know both soil texture and chemistry is by submitting a soil sample to a lab for analysis.
The third important component of soil is the organisms living in the soil. Bacteria, fungi, earthworms and arthropods (insects and spiders) are just some of the organisms that are critical to soil health. Many important species are naturally present in soils, or may need to be introduced. Soilless growing media generally has much lower biological activity at the start of the growing cycle due to it being an engineered soil, designed for container growing.
There are many recipes for preparing your own soil, soilless growing media and enriched/super soils. A simple mix contains equal parts of peat (or coco), perlite (or rice hulls), and sand (or bark). A small amount of lime is often needed to raise the pH, but depending on the mix you may need to add sulfur. Recently, I have seen one pre-made growing mix test at 4.4 and another at 8.2, so knowing your pH is very important for ensuring healthy plant growth. Again, many recipes are online so it’s relatively easy to find one to meet your preferences. If you are interested in super soils I suggest checking out
Subcool’s Super Soil Recipe
- 8 large bags of a high-quality organic potting soil with coco fiber and mycorrhizae (i.e., your base soil)
- 25 to 50 lbs of organic worm castings
- 5 lbs steamed bone meal
- 5 lbs Bloom bat guano
- 5 lbs blood meal
- 3 lbs rock phosphate
- ¾ cup Epson salts
- ½ cup sweet lime (dolomite)
- ½ cup azomite (trace elements)
- 2 tbsp powdered humic acid
The type of soil you choose to grow in will depend on how you plan to fertilize the plant. If you want strict control over nutrients at each watering, you will probably choose a simple soil, so that you can add products as needed. If you want to use only water and no additional fertilizers, you should look into enriched or super soils. Build-A-Soil is a great resource for anyone interested in making their own soil.
For many individuals and growing operations preparing your own soil is not a feasible option. There are countless choices of pre-made soils and soilless growing media. Pro-Mix has long been the grower standard for those looking for a simple soil where you want to have control over feeding the plant and is widely available. Soils containing wood fiber, in order to reduce the amount of peat being used, are now available, one example is Berger BM5 Super HP. Fox Farm has a whole line of enriched soils tailored for growing cannabis, Ocean Forest and Bush Doctor Coco Loco are two examples. Roots Organics has a large number of offerings, with Formula 707 and Emerald Mountain being examples of enriched growing media. These are just several examples, with so many growing styles and preferences, there is a mix for everyone!