What Is The Best Soil For Growing Marijuana Outdoors?

outdoor marijuana plant in plant pot with soil
July 24, 2020

Growing marijuana outdoors is straightforward, undemanding, and downright fun. To get started, all you need are marijuana seeds, pots, and soil. It is simple but it still requires some forethought to get the most out of your cannabis seeds. Soil mediums, for example, could be the most natural substrate, but there might also be some drawbacks to using them.

In this article, you will come to understand the different types of soils as well as their benefits to growing marijuana plants and any possible negatives to using them.

A Guide to the Best Soil for Growing Marijuana Outdoors

Cannabis plants essentially need five things to survive – air, light, water, nutrients, and a space to grow in. Soil – and any other growing media for that matter – constitute the final requirement. And to facilitate plant growth, this medium carries out several vital functions.

  • As a base and structural support, soil anchors the plants while providing the roots ample room to develop and expand.
  • It facilitates the exchange of gases between the root system and the external environment. In so doing, it also ensures that the root zone is always well-aerated.
  • Acts as a conduit, allowing the plants to absorb and hold onto the moisture and nutrients that they need to grow.
  • Soil, being naturally fertile, can also provide marijuana plants with essential minerals and nutrients.
  • It is home to millions of beneficial microorganisms that help fix nitrogen and break down organic matter. That eventually adds more nutrients to the soil – one that the roots can readily absorb and use.
  • Soil medium protects the plants from harmful elements, including extreme temperature changes, erosion, and contaminants.

The best soil for growing marijuana outdoors, in this case, is one that can successfully perform the above tasks. Naturally different strains of marijuana may be more suited to different soils, so always check the growth time, nutrient requirements and more for each weed strain before you start growing it so you can make the best decision.

The Composition of Soil

Cannabis plants need a specific type of soil to grow and flourish. You cannot just use whatever substrate you find. Even the standard garden soil sold in local stores will not suffice. Ideally, the chosen mix should feature an optimal blend of light and heavy materials. This facilitates adequate water and oxygen intake. On top of that, it should contain a host of organic matter, making it hospitable to the microbial populations.

Soil can be broken down into four core components.

  • 45% minerals (sand, silt, and clay)
  • 25% water
  • 25% air
  • 5% organic matter


Sand is light, coarse-grained, and fast-draining. It primarily contains tiny fragments of rocks and hard minerals, such as granite, quartz, and limestone. The high porosity helps enhance the drainage and aeration of the mix. On the flip side, sand has poor moisture retention and little to no nutrient content.

Note: It is called “sandy soil” if it contains at least 85 percent sand and up to 10 percent clay.


Silt feels powdery when dry but slippery and mud-like when wet. It mainly consists of loose sedimentary material and rock particles that are much smaller than a grain of sand. It is not well-draining but has fantastic water retention properties. More than that, silt is also rich in nutrients and other essential minerals.

Note: It is called “silt soil” if it contains 80% or more silt and less than 12 percent clay.


Of the three types, clay has the smallest particles. Each grain is tightly-packed, leaving barely any airspace in between. The result is a heavy material that could store water and nutrients well. It is also highly fertile and contains a range of essential minerals. At the same time, though, being compact also makes it hard for moisture and air to pass through it. If there is too much clay, for example, the soil mix will not drain properly and may even turn concrete-like when saturated.

Note: It is called “clay soil” if it contains over 25 percent clay.


Water comprises about 25% to 30% of soil. It is not just plain water, however. Instead, it is referred to as ‘soil solution’ – water containing dissolved gases, salts, minerals, and organic matter. More importantly, it holds ions – the form of nutrients that the roots can absorb.


Just like water, nearly 25% to 30% of soil exists in a gaseous state. These gases fill the pore spaces in the soil. Note that soil naturally contains high amounts of carbon dioxide – but low levels of oxygen. That is why it is important to boost aeration in the root zone. Remember, plants need oxygen to convert sugars into usable energy – a process known as respiration.

Organic Matter

Soil is made up of about 5% organic matter or carbon-based compounds. That refers to any living material – including plant and animal debris – at various stages of decay. Although the organic content is relatively small, it still plays a critical role in the quality and quantity of the yields.

The millions of microorganisms in the soil – such as bacteria, fungi, protozoa, arthropods, and nematodes – are responsible for breaking down the once-living biomass. In other words, organic matter serves as the food source of these microbes. Along the way, it produces and synthesizes nutrients, making them more available to the roots. At the end of the decomposition process, what you get is humus – a dark, spongy material teeming with nutrients.

The main benefit of organic matter is that it enhances soil tilth – the soil’s physical condition, especially its ability to support plant growth. In particular, it boosts aeration, water absorption and retention, and drainage, among others. It also serves as a nutrient reservoir.

The Qualities of Good Soil

Defining top-quality soil is easy. Essentially, it needs to have the right texture, drainage, ability, and water retention capacity. A favorable physical structure also helps maintain sufficient amounts of air, water, and organic matter. Thus, it should fulfill the soil composition requirements, as discussed earlier.

Loam soil easily fits the bill. Many growers consider it the ultimate substrate for cultivating marijuana plants. Some even go as far as to proclaim it the ‘Holy Grail of soils’.

What exactly is loam, and what is so special about it?

Technically speaking, loam refers to soil containing less than 52% sand, 28% to 50% silt, and 7% to 27% clay. For gardening purposes, the most popular sand-silt-clay ratio is 40:40:20. The proportions, though, may vary, resulting in different soil subtypes.

Types of Loam Soils Mineral Composition
Sandy loam Sand: 43-85% Silt: 0-50% Clay: 0-20%
Silt loam Sand: 0-50% Silt: 50-88% Clay: 0-27%
Clay loam Sand: 20-45% Silt: 15-53% Clay: 27-40%
Sandy clay loam Sand: 45-80% Silt: 0-28% Clay: 20-35%
Silty clay loam Sand: 0-20% Silt: 40-73% Clay: 27-40%

The unique mineral composition of loam soil significantly impacts its texture, drainage, and water retention – in a good way.


As a combination of sand, silt, and clay, loam soil contains multi-sized particles. If you run your hands through the material, it should feel dry, soft, and friable. More importantly, it should maintain this consistency when dry and wet, making it easy to handle and work.

When compressed into a loose ball, loam should retain its shape for a moment before breaking up into chunks. That is a sign that it has an optimal texture – neither too compact nor too loose. Having enough airspace in the soil is critical as it allows the roots to grow through it. At the same time, it also boosts oxygenation while promoting drainage.

Drainage Ability

Having sufficient drainage helps prevent overwatering, which could drown and suffocate the roots. And without enough oxygen, the roots will starve and begin to decay. Eventually, this leads to nutrient deficiencies.

Loam soil has enough sand to make it well-draining. That is because the gap between the particles allows water to percolate through it. After watering, the pool on top of the medium should drain within a few seconds. Neither should it take too long to trickle out of the bottom holes.

Water Retention Capacity

The main concern with sandy soil is that it is fast-draining. It runs the risk of drying out, but it could also flush the essential minerals. That means that the plants may not have the moisture and nutrients they need to grow and prosper. If left unaddressed, this could stunt growth and cause other damage, compromising the yields.

Loam soil does not have drainage issues due to the addition of silt and clay – both of which are water-retentive and can negate the high drainage capacity of sand. Ideally, it should absorb and hold onto moisture without getting too muddy.

The Ideal Soil Amendments

Loam soil – regardless of the type – is by no means perfect. Depending on your needs, preferences, or situation, you might find it lacking in some aspects. Or, maybe you did not achieve the optimal sand-silt-clay concentration. At any rate, you will most likely want to improve its physical, biological, and chemical makeup.

Soil amendments – which include both organic and inorganic materials – could help you achieve precisely that. Ultimately, they help maximize plant growth and productivity.

Organic amendments come from materials that were once alive. These include compost, earthworm castings, and animal manure. They increase the organic matter content in the soil, dramatically improving tilth in the long run. Most of these materials have nutritional value, which is why they double as organic fertilizers.

Inorganic amendments, on the other hand, are usually mined minerals, such as perlite, vermiculite, pumice, and limestone. Generally, they improve the structure of the soil, particularly aeration, drainage, and water retention. Others help stabilize the pH or even add minerals.

Here are some of the most popular soil amendments and their specific purpose:

Primary Functions Soil Amendments
Improve Texture Gypsum Coco coir Biochar Perlite Vermiculite Pumice Compost Earthworm castings
Boost Drainage Ability Gypsum Coco coir Biochar Perlite Pumice Compost Earthworm castings
Increase Water Retention Capacity Coco coir Biochar Vermiculite Pumice Compost Earthworm castings
Add Organic Matter Biochar Compost Earthworm castings Chicken manure Blood meal Bone meal Bat guano Crustacean meal Fish emulsion Kelp meal Alfalfa meal Mycorrhizal inoculants Humic acid and fulvic acid
Adjust or Stabilize the pH Level Biochar Garden lime Compost Bat guano Blood meal Humic acid

Note: You have the option of starting with loam soil and amending it with the preferred ingredients. Alternatively, you can also buy organic potting soil from trusted brands. If the product contains any of the above amendments, you can expect that it has a decent texture and composition. Despite having nutrient-rich materials, though, you would still need to use separate nutrients across the different growth stages.

Bonus: Try Out Super Soil

If you want to take things to a wholly different level, consider super soil. This highly amended soil contains enough nutrients to feed cannabis plants throughout their life cycle. You do not have to use additional fertilizers. The only thing left to do is irrigate the plants with balanced water.

Using super soil not only streamlines the cultivation process but It also enriches the soil microbiota, as well as improves nutrient intake and availability. This results in bigger, smoother, and more flavorful buds.

If you are only growing a few plants, it might be better to buy pre-made super soil. That is because you could only prepare it in large batches – which is both pricey and wasteful.

But if you have a large garden and prefer to make it from scratch, it requires time and effort. You would need to haul and combine several bags of ingredients, for one. There is also the composting period, which could last for a couple of months or so. Nevertheless, rest assured that the rewards are sweet and worth the wait.

Top-Quality Soil for Healthy Marijuana Plants

Outdoor marijuana cultivation is simple enough. But, there are a few things you could do to guarantee and elevate the success rate. Providing optimal growing conditions is a given, and that includes using on the best soil for marijuana plants.

For most, this title is synonymous with loam soil. But if you cannot get your hands on it – or if you cannot meet the desired 40:40:20 ratio – that is okay. What is important is that you understand the soil requirements of marijuana plants. Remember, it needs to be friable, well-draining, moisture-retentive, and rich in organic matter. Afterward, it is just a matter of amending the available soil to achieve these qualities.