When you think of soil, a pre-mixed plastic bag often comes to mind. Admittedly, it’s pretty easy to just dish out $20-$30 for a bag to start your cannabis grow. Many of these pre-mixed super soils are, in fact, organic and are a terrific starting point for novice gardeners.
However, building your own soil at home can be just as easy, cost-effective, and accessible as buying a pre-mixed bag at the store. Creating your own super soil will also teach you a lot about soil science and how the components of soil will effect the outcome of your cannabis grow.
What is an organic super soil?
The key factor in organics is soil life. A proper organic soil, in the words of famed organic soil scientist Jeff Lowenfels, is “teaming with life.” To put this into perspective, a shot glass filled with a proper organic soil will contain more individual living organisms than humans who inhabit the planet. That’s a lot of life!
Each organism within this habitat serves it purpose in the web of life (sometimes also referred to as a soil food web). This web is not only virtually self-sustaining, it also has the capabilities to both convert and provide readily available nutrients to your plants. Maintaining an organic cannabis garden grown in super soil may require little to nothing more than adding water. Take that, synthetic nutrients!
The term “super soil” dates back to when organic cultivation practices were just beginning to make their way into the canon of cannabis grow methodology. Perhaps the undisputed king of organic super soils is Subcool of TGA Genetics, who a many years back posted a super soil recipe in a High Times article.
Since then, his recipe has served as the de facto totem for the organic cannabis cultivation community. Virtually every subsequent article on organic super soils references SubCool’s soil recipe, and for good reason: It works.
How to build organic soil for cannabis
Growing cannabis organically allows growers to produce a high-quality product without the interference of chemical nutrients that can burn or kill the plants. It’s important to truly understand what you want in your soil so you can produce your own custom blend that costs less and contains all essential nutrients.
Step 1: Know what’s in your soil
To build a quality soil, you first need to know what’s in it to begin with. When buying soil, you’ll see a list of ingredients and information on what nutrients are readily available in it. If you’re hoping to use soil that is already in your garden or taken from another source, it’s impossible to know what is actually in that soil.
To remedy this problem, you can submit a soil test to get a base understanding of what nutrients it contains.
Additionally, you can work your hands through the soil and get to know it: Is it compact or fluffy? Sandy or claylike? Does it hold moisture or is it dry? Are there worms and insects present? These are all things you can consider by simply using sight and touch.
Step 2: Find the right soil amendments
Once you’ve gotten to know your soil’s nutrient levels and texture, it’s time to add any necessary soil amendments. The basic building blocks for plants are based around N-P-K—these stand for Nitrogen (N), Phosphorus (P), and Potassium (K). To build a strong, healthy soil, it should have these nutrients readily available for your plants to absorb.
- Worm castings: Provide a quick-release source of nitrogen for your plants while also introducing healthy bacteria; contain many micronutrients depending on where they are sourced from.
- Crustacean meal: A little slower to release than worm castings, crustacean meal adds nitrogen, phosphorus, calcium, and chitin to your soil. Chitin-eating microbes will help keep nematodes at bay.
- Bat guano: Bat guano provides the highest levels of nitrogen and phosphorus of all three; it does wonders for sustained plant growth while diversifying the soil’s bacteria and microbes.
- Bone meal: Bone meal generally comes from cattle bones, and it helps to keep phosphorus levels up. Keep in mind that your soil needs to be at a pH below 7 for bone meal to be most effective.
- Chicken manure: Chicken manure is a great way to introduce both phosphorus and nitrogen. Choose a high-quality manure that is fully processed and make sure to amend the manure into your soil with enough time to let hot manure cool off.
- Rock dust: Rock dust is a very slow-releasing phosphorus source that can be effective in soil for years, but it does not perform well in soils with a pH above 7.
- Kelp meal: Kelp meal is a great source of potassium that promotes microbial diversity in the soil. A water-soluble amendment, kelp can be applied with water or by hand directly into the soil.
- Wood ash: Wood ash can be used to increase the potassium levels in your soil, but be aware that it generally raises the pH so make sure to test your soil’s pH levels regularly.
- Compost: Your compost bin can be an excellent source of potassium for your garden, especially if it contains fruit rinds and banana peels.
The three basic N-P-K amendments are most important, but there are other micronutrients that will help your plants grow. Products like gypsum and azomite can add micronutrients to strengthen your soil. A wide diversity of nutrients can introduce nuanced flavors, aromas, and effects into your end product.
Also, consider amendments that change the soil’s density, airflow, and ability to hold water. For example, perlite and peat moss can improve drainage and water retention. Introducing a healthy worm population and fungal colonies will also help fluff up the soil, thus improving its structure.
A healthy soil maintains itself, and living organisms both reap and share the benefits of a healthy soil with your cannabis plants.
Step 3: Mix your soil
To till your soil, apply the amendments and start digging or use a rototiller. It takes time, but make sure everything is adequately mixed, including all corners of a pot or bed.
Once you’ve mixed everything in, water your soil to help cool any added manure. This process should be repeated every few days until the soil is cool to the touch when you bury your hand. At this point, the soil is ready for your seeds or clones.
Though tilling soil is necessary for your first crop, it’s up for debate whether or not tilling annually is beneficial. The argument against tilling is that it breaks down a complex network of mycelium, worm tunnels, and cavities created by beneficial organisms in the soil.
However, by tilling every year, you can easily add amendments and break down cover crops, making sure all nutrients in the soil are mixed and readily available.
Still not sure whether to till or not to till? Try analyzing soil samples at the beginning and end of the season to determine whether or not annual tilling benefitted your garden in the end.
Step 4: Prepare your soil for the next season
After harvest, and to protect your soil for the next season, remove the stalks and roots of harvested cannabis plants. You can compost and re-apply them to your soil once they’ve decomposed.
Next, introduce a cover crop to your soil to help maintain it during winter. Common cover crops include legumes or radishes: Legumes sequester and preserve nitrogen while radishes have long taproots that help aerate and free up deep, compacted soils. These cover crops need to be destroyed before they go to seed. This can be accomplished by tilling or cover mowing.
In the offseason, you can add nutrients back to your garden by top dressing the soil. Adding compost, manure, and other amendments and allowing rainfall to bring the nutrients into the soil is a great way to improve soil quality in the offseason without having to do much work.
Building your own soil is a rewarding investment that will only strengthen your understanding of gardening and how plants grow and thrive. With quality soil, you the need for liquid nutrient feeding will be greatly reduced, saving you time and money. You’ll also find that your soil will become a sanctuary for other living things as you improve the environment surrounding your garden.
Simple super soil recipe for your cannabis
1. Begin with a base mix
Combine equal parts compost, organic material, and aeration. How much you begin with will depend on your needs. Consider that this base will account for approximately 20% of the total volume of your mix.
One popular formulation of this recipe that works well is:
- 1 part compost of your choice
- 1 part coco coir and/or peat moss
- 1 part perlite
2. Establish a fungal population
A diverse microbiology in your soil will be contingent on many factors, and establishing food to grow a healthy fungal population is one of the most important.
Fungi love to develop mycorrhizal relationships with root systems (the rhizosphere), and providing food for them to thrive will help facilitate these tiny symbiotic relationships.
The result? Friends always like to bring food to the party. Picture your cannabis as the partygoers—they like food! Here are some terrific amendments to build fungal life:
- Kelp meal: 1/4 cup per 5 gallons
- Humic acid: 1/4 cup per 5 gallons
- Mycorrhizal inoculant (endomycorrhizae powder): 1/4- 1/2 tsp per gallon
3. Establish a bacterial population
Just as important as fungi, bacterial populations can help break down complex sugars and convert them to readily available macro and micronutrients for your cannabis. They also help to establish and maintain the immune system of your plants, making them more adept to fight off pests, diseases, and inclement environmental conditions.
These ingredients help bring the bacteria:
- Vermicompost (worm castings): up to 20% total substrate
- Bat Guano: up to 5% total substrate
4. Fortify with micro and macronutrient sources
Cannabis, like any other plant, requires a regiment of micro and macronutrients to thrive. In many cases, these nutrients are often provided via plant fertilizers. However, with organic super soils they’re amended into the base mix to be released and/or broken down into food for your plants over time.
Building a successful organic soil is, in many cases, a trial and error endeavor for the homegrower. Subcool himself admits to changing his recipe (albeit very minutely) each year to adapt to the needs of his plants.
Be sure to experiment to your plant’s liking when mixing soil, and never be afraid to change things up in the process. Your crop will thank you for it in abundant yields and immaculate flavor!
Also check out how to amend your super soil mix, and our recipe on a compost tea soil.
This post was originally published on June 19, 2017. It was most recently updated on June 29, 2020.
Trevor Hennings and Patrick Bennett contributed to this article.