How to Make the Best Soil For Cannabis

If you care about the quality of the thing you’re growing, you need to care about what it’s being grown in.

That’s especially true for something like cannabis that’s meant for human consumption, whether medicinal or recreational. Flavor, potency, robustness, and more can be optimized by growing your cannabis in a high-quality organic soil.

Now, I’m sure there are some of you that might question whether soil is the right medium for growing cannabis as opposed to going hydro. No doubt that hydro can let you achieve faster vegetative growth and bigger yields (assuming same conditions like temperature and lighting). However, as the late Ras Truth (RIP), creator of the infamous Royal Kush line, said:

I guarantee the stoniest, most medicinal, best tasting, smoothest smoking; cleanest burning herb you have ever smoked was organically grown in soil.”

And I happen to personally agree with that sentiment. As someone who has grown in hydro and who has sampled hydroponically grown cannabis from other growers, there’s just something missing taste wise that you seemingly can only get from growing in soil. Of course I’m also a fairly old geezer so maybe I’m just stuck in my ways as my hydro friends swear that their herbal medicine is the best tasting.

Anyway, if you’re reading this, I’m going to assume you agree with me that soil is the best way to go for growing cannabis. *High Five*

In this article, I’ll cover the following:

  • What is soil, technically?
  • Why soil is the best growth medium for cannabis
  • Factors to consider when trying to create quality organic soil that grows the best possible cannabis
  • The best cannabis soils to purchase in case you don’t want to make it yourself

What is Soil, Exactly?

The rich, robust nature of plants grown in soil might be due in part to the other critters that live there… soil, by definition, is “alive” with bacteria and other microbes. The microbes are responsible for releasing nutrients into the soil that are critical to healthy plants. Soilless growth media are instead inert, and nutrients have to be added to the mix.

Soil isn’t just microbes and their byproducts; it’s also inorganic (nonliving) sand, silt, and clay particles that result from the erosion of rocks and other nonliving things. 

Soil is Mother Nature’s chosen growth medium. Though it varies in composition, texture, chemical and physical properties for one place to another, it’s evolved over millennia to support local plant life with everything it needs to thrive. 

Often, cannabis growers can’t just go grow in native soil, or their native soil isn’t quite right for growing the highest quality cannabis plants. Luckily, there are ways to buy or make your own.

Why a Good Organic Soil is the Best Medium for Cannabis

A good organic soil, similar to natural, native soil, will ideally have all the nutrients, chemical, and physical qualities needed to optimize plant growth, without having to add chemicals.

Better soil grows better cannabis. A good organic soil can provide nutrients to the plant in the way food provides nutrients for us–so starting with a good soil provides the best possible diet for your plants, which lets them grow into their full glory.

There are many, many options when it comes to what growth medium to use for cannabis, from prebagged organic soils to fancy custom mixes to DIY recipes to full hydroponic, soilless media. And it also matters exactly what type of strain you’re growing, the growing conditions and environment, etc., so slightly different soils will work best in different situations.

If you choose to go with soil, you’ll generally either buy it, use native soil, build a soil yourself, or a combination of those. Either way, you need to know what to look for in a quality soil. So what are the defining characteristics that all good cannabis soils have in common?

The Building Blocks of a Quality Cannabis Soil

There are three factors to consider when building or purchasing a quality soil.

1. A balance between drainage and water retention, or its composition. You don’t want the water to drain too quickly from your soil, but you also don’t want roots sitting in a puddle for long, either. A soil with larger particles like sand, for example, has lots of big holes in it because the particles don’t pack tightly, and water can flow right through. A soil with tiny clay particles will be dense and tightly packed, and water will sit on the surface and soak in very slowly. 

You want neither of those scenarios–you want soil that retains the proper amount of water, draining well but not drying out too quickly.

The composition can depend on both the soil texture, discussed further in the next section, as well as what soil amendments you might choose to use or might be added to your soil.

2. Nutrients. Like all living things, plants need nutrients for survival. Many of these come directly from the soil. Staple nutrients that plants need a lot of are called macronutrients; they’re analogous to macronutrients in the human diet. 

Just as we need lots of carbohydrates, protein, and fats, plants need primarily nitrogen, phosphorous, sulfur, potassium, magnesium, and calcium. Since the plant can’t supply these themselves, they must come from the soil. A quality soil will already contain these macronutrients, making the addition of fertilizers unnecessary.

Micronutrients include things needed in much smaller quantities, though still essential, much like vitamins and supplements for people. These include boron, iron, zinc, manganese, copper, molybdenum, and chlorine. These can be native to the soil, but the best way to ensure a good balance of micronutrients is to add organic material, i.e. compost.

3. pH. The acidity level of the soil is an important component that can affect the availability of various nutrients. Different plants will have different nutrient needs, and vary in their ability to tolerate different soil acidity levels.

pH is measured on a scale from 0-14. It ranges from very low, which is extremely acidic (think battery acid) on one end of the scale, to very high, which is extremely alkaline, like soap or baking soda. 

Most plants prefer a specific soil pH range, and outside of that range, the plant may not be able to absorb certain nutrients. That means that even if your soil contains the right nutrients, they won’t be available to the plant if the soil isn’t within the right pH range.

For cannabis, that range is about 6.0 – 7.0. 7.0 is considered neutral, so this puts the ideals oil for cannabis in the slightly acidic category. 

If you use a soil that incorporates compost or other organic matter, pH isn’t usually something you need to worry about, as in this case the biology going on takes care of the chemistry needed. But if you choose a soil-free growth medium, you’ll likely need to adjust the pH by adding chemical fertilizers.

Getting the Texture Right for Your Cannabis Soil

The soil texture essentially refers to the sizes of the particles making up the soil. There are three broad categories, based on relative size:

  • Sand, the largest size particles
  • Silt, medium-sized particles, and
  • Clay, the finest particles.

Most native soils are a mix of these three in various ratios. A very sandy soil is generally very porous, and water will flow through it quickly, leading it to dry out quickly due to poor water retention. Sandy soils also don’t hold nutrients as well. 

Clay soils, conversely, can be very dense and drain very slowly since it takes time for water to work its way around tightly packed particles. However, they hold on to nutrients well.

Different plants may prefer different soil textures, but many prefer loam soil. Loam soil has a roughly equal balance of all three particle sizes, which allows it to take advantage of the clay particles’ ability to hold on to nutrients, as well the aeration and drainage provided by the larger silt and sand particles.

Easy tests you can do yourself

If you want to get an idea of the composition and texture of your soil, there are a few simple tests that will give you a pretty good idea without calling in the pros.

Testing composition

There are a few methods for doing this. The main thing you’re looking for is a good balance of drainage and water retention. Here are two easy tests:

  1. Check your soil drainage by digging a hole that’s about 12 inches deep and 12 inches across. Fill it completely full of water, then time how long it takes to drain completely. Soil with good drainage should drain within 10 to 30 minutes. If it takes hours, you might have soil that’s on the clay-ish side. If it drains in a few minutes, it’s probably on the sandier side.
  2. Grab a handful of wet soil and squeeze it into a ball. Soil with the right level of water retention should stay clumped in a loose ball. If it’s squishing through your fingers, it’s too clay. If it just falls apart as soon as you open your hand, it’s too sandy.

Luckily, this isn’t a big problem if you use a quality soil that incorporates organic matter such as compost. Organic matter will loosen up a dense, clay soil, improving drainage, whereas adding it to a sandy soil will act like a sponge, holding on to water and nutrients for longer.

Testing soil texture

The mason jar soil test can tell you pretty accurately what the breakdown is between clay, silt, and sand in your soil.

You’ll need:

  • A clear jar like a pint or quart mason jar with a tight-fitting lid
  • Water
  • Dish soap
  • The soil you want to test

To perform this test:

  1. Fill the jar about 1/2 of the way full with soil taken from the top 12 inches of soil from each area of interest (make one jar for each area). 
  2. Fill the jar almost to the top with water.
  3. Add ~1 tablespoon of soap to each jar and seal.
  4. Shake the mixture vigorously for about 45 seconds.
  5. Let the mixture sit and settle for several hours.

In this test, the heaviest particles (sand, followed by silt) will settle first, followed by the smaller, lighter particles. Clay, having the smallest particle size, can remain suspended in the water for many hours. Once settled, you should be able to see the stratification of different components: sand at the bottom, topped by silt, then clay. You may also see organic matter floating on the top, but this layer isn’t considered for the texture.

To determine the percentage of each component, measure the height of each layer and divide it by the total height of the settled soil. So for example if the height of all the layers is 2”, and the bottom layer of sand measures 1”, divide 1”/2” = 0.5, x100 = 50%, so in this example you know your soil is 50% sand.

The Best Cannabis Soil Requires the Right Amendments

Besides the actual dirt or soil, most good soils have added amendments to improve the composition. These include living, organic things like compost, as well as inorganic things such as sand or vermiculite. Amendments can also add nutrients and affect pH depending on the type of amendment.

What is a soil amendment, technically? According to the Colorado State Extension, is “any material added to a soil to improve its physical properties, such as water retention, permeability, water infiltration, drainage, aeration, and structure.” They note that the goal of adding soil amendments is to “provide a better environment for roots.”

In general, roots need a loose, aerated soil that allows for air, water, and nutrients to be available and lets the roots grow deep and strong. 

The list below includes soil amendment options primarily for the purpose of improving soil composition, texture, and water retention. However, a few do double-duty and provide nutrients, too. Here’s what cannabis growers have to say about these amendments:

  • Worm castings. Basically worm poop, worm castings are an excellent source of nutrients and humus, or decomposed organic material.
  • Coco coir. This is the fibrous material on the outer surface of coconuts. Many growers like it because it’s an agricultural byproduct that would otherwise go to waste.
  • Rice hulls. These are a good option because unlike many amendments, it doesn’t need to be mined. It’s an agricultural byproduct that would normally go to waste, but many growers use this to provide aeration and improve drainage.
  • Wood products. Wood pellets, sawdust, and other wood products can be a good thing to add to your compost, but don’t add this directly to your soil. Wood can tie up nitrogen, making it unavailable to plants, and takes a long time to decompose.
  • Biochar. Somewhat like charcoal, it’s defined as biomass that’s heated at a high temperature without oxygen. It can help maintain moisture in the soil, but it also raises the pH slightly. Since cannabis prefers neutral or slightly acidic soil, keep an eye on pH if you use this one.

Your soil also needs a source of nutrients, such as the macro- and micro-nutrients listed above. You want a good balance of macronutrients, which is sometimes called the NPK ratio. It stands for nitrogen-phosphorous-potassium. 

There’s quite a bit of debate about the “right” NPK ratio for cannabis, and some argue that every phase of the growing cycle require different soil conditions and nutrients. You’ll want to find out what’s best for your situation, but this resource may provide more detail.

The following soil amendments are popular choices for cannabis growers, and have varying levels of nutrients. The NPK ratio is noted where known:

  • Kelp or kelp meal (1-0-4) Made from dried seaweed, this is a superfood for plants. According to royalseeds, “It’s thought to increase sweetness, boost flavour, and enhance colours” of cannabis plants.
  • Alfalfa. (2.5-1-1) Alfalfa meal is a great way to add nitrogen to your soil. It also contains many of the micronutrients needed in trace but essential amounts.
  • Fish bone meal (varies, usually high in P). This is many growers’ favorite source of phosphorous.
  • Gypsum. Contains calcium and sulfur, and also supplies trace elements.
  • Blood meal (13-1-1). Very high in nitrogen, blood meal is often used for the growth phase.

Super Soil

If you grow organic cannabis, you’ve probably heard of super soil. It was popularized by cannabis breeder, the late Dave Bowman, AKA Subcool. 

The idea of a super soil is to build a supercharged organic, composted soil that contains all the ingredients needed by the plant over the course of its lifecycle. In theory, using this soil eliminates the need for any chemical intervention.

Super soil is the closest you can probably get to growing in the wild, where Mother Nature takes care of the soil, water, and fertilizer. For many growers, it’s an easy way to make sure your plants are getting the best diet and nutrients possible, to grow the highest quality, most robust cannabis plants.

Super soil is available for purchase online through a number of outlets, but there are also many recipes for those who would rather DIY. 

The Bottom Line

If you want the highest quality cannabis plants, you can’t go wrong with a quality organic soil as your growing medium. Since cannabis thrives under optimal growing conditions, it’s worth taking the time to figure out how to buy or build the best possible organic soil for your plants. Remember that this is a general guide that covers the most common strains and growing situations, so you might need to dig to find the best recipe for your soil. Hopefully, this guide helps you better understand what it takes to build the best quality organic soil for your cannabis.

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