When you choose to grow cannabis in soil rather than with hydroponics, it is important to ensure the potting soil mixture is optimal for plant growth and development. Running out to the nearest nursery and grabbing a bag of Miracle Gro—and most other premixed soils—simply will not get the job done. You cannot always be sure of what ingredients and additives are present in these potting soil mixtures. It is necessary to strictly control the acidity level and nutrient content of the soil. Though a basic potting soil will do the trick in the flower garden (which is mostly visual appeal), you want to know the exact composition of your soil when growing anything intended for human consumption. Quality, organic soil mixtures made specifically for cannabis are widely available in states that have legalized.
The materials and substances you choose to add to your soil mixture will determine its composition and quality. For those new to growing pot—or gardening in general—these factors can prove to be overwhelming. Beyond nutrient content and pH levels, the biggest concerns are soil texture, proper drainage, and water retention.
Soil Nutrient Content
When gardening (not just cannabis, most any plant) Nitrogen (N), Phosphorous, (P), and Potassium (K), are key factors in achieving optimal plant growth and development. Each of these nutrients can be found in some beneficial soil additives:
Worm castings (N/P)
This organic fertilizer, otherwise known as worm poop, is already loaded with many vitamins, nutrients, and healthy bacteria. Worm castings create larger pores in the soil to better retain water. They are great for repelling aphids, spider mites, and other plant-eating pests.
Bone Meal (P)
This organic soil nutrient is a powder made from ground up animal bones. You can buy it is most specialty gardening stores. This allows for a slow and steady release of Phosphorous and proteins into the soil. You always want to test your soil before adding bone meal. If the pH higher than 7, you’ll need to correct your soil’s pH first before adding bone meal.
Blood Meal (N/K)
Blood meal is similar to bone meal. It is the blood that is collected from animals who are slaughtered for food. The blood is then dried and turned into a powder. This nutrient rich additive promotes overall plant development. It is primarily a nitrogen booster, but also makes the soil more acidic. Use blood meal sparingly because its high nitrogen content can actually harm marijuana plants.This allows for a slow and steady release of Phosphorous and proteins into the soil.
Bat Guano (N/P)
Guano has been used historically in gardening to improve soil composition. High levels of Nitrogen and Phosphorous create a diverse microbiome and promote growth. In terms of fertilizers on the commercial market, guano is the most nutrient diverse. Because of the varieties of bat and their diets, the guanos very distinctly. Read the packaging to determine the best nutrient for your soil, and start slowly. You can burn your crop with too much.
Kelp Meal (K)
The organic remains of seaweed is often mixed with fish products, dried, and sold as a powder fertilizer. The brown powder, although smelly is incredibly rich in Potassium. Kelp also enhances the microbiome. The slow-release fertilizer can feed your plants for up to 4 months.
Limestone is rich in both Magnesium and Calcium and reduces acidity in soil. There are mixed reviews on whether or not Dolomite should be used on cannabis. Some claim they have used it for years and yield positive results. Other say to avoid it. Let us know your experience.
This patented fertilizer is natural product mined from special mineral deposits in Utah. Unlike other fertilizers, Azomite does not focus on macronutrients. Instead, it restores trace minerals which may have been lost during the watering process. Users claim it also helps to strengthen root systems.
Humic acid is the remains of broken down animal and plant matter from prehistoric deposits. Like Azomite, it is not a traditional fertilizer, but rather a soil conditioner for restoring trace minerals. This organic matter allows minerals to combine with compounds that are easier for the plant to absorb.
Soil pH Levels
A slightly acidic pH level of about 6 is ideal. pH levels up to 7.0 (neutral) are acceptable when growing cannabis in a soil. A soil with pH levels lower than 6.0 or higher than 7.0 will suffer from nutrient deficiencies. It is okay for pH levels to fluctuate as long as they remain in the 6.0-7.0 range.
Soil Texture/Water Drainage
Get your hands in and get a feel for the texture and consistency of your soil mixture. The texture should be light and loose, but the soil should be able to retain water well. There must be enough water to feed the plant and maintain this loose texture. Too much water will create a lack of oxygen.
Soil texture will depend mainly on how its ability to properly drain water. There should be no pools of water left behind after watering—or rain if grown outdoors. Test for drainage ability before growing. Saturate soil with water, then add more water. If the water does not drain within a few hours, the soil has poor drainage. Adding a compost tea of the ingredients listed above will create bigger bores in the soil, giving it better air and water circulation.
You do not want to over water your cannabis plants. After watering, your soil should hold water but should not be muddy. Too much water will allow for the growth of fungus, leading to root damage. Perlite and vermiculite can aid in aeration when added to the soil mixture.
Sticking to Soil
A number of benefits are found when choosing soil over hydroponic growing. Nature takes care of a lot of the work on its own when growing in soil, as sunlight is consistently available. Enriched soil introduces the plant to various microbes (bacteria, fungi, etc.). If you are not expecting massive yields in short periods of time, it can be fun and rewarding to grow cannabis in soil.
We want to know more about your soil. What do you use, how often, and what are the results? Tell us your combination of nutrients in the comments below.