If you’re growing cannabis outdoors and want to learn more about soil, you’ve come to the right place!
In this article, we’ll guide you through the basics of soil properties, such as composition, structure, and drainage capacity.
We’ll also delve into the organic part of the soil, and talk about the complex relationships between plants, microbes, insects, worms, and everything weird and wonderful that lives in soil!
Why Understand Soil?
If you know your soil, it opens the doors to deeper understanding of exactly how your cannabis plants receive the nutrients they need, and the importance of things like soil microbes in creating an ecosystem that your plants fit right into.
With a little dedication, you can even make your own precisely-tailored soils to give your plants exactly what they need.
What’s more, if you follow certain approaches to soil management, such as permaculture, organic or biodynamic techniques, you can create a living, thriving soil that will continue to improve for years to come – and with very little effort!
So What is Soil, Anyway?
Soil is fascinating and complex stuff.
You may think of it as simply the solid particles you can feel if you pick up a lump of earth and rub it between your fingers – but it’s much more than that.
In fact, pedologists (people who study soil – the “pedolith”, literally meaning ground-up rock!) see soil as a three-state system:
- Solid organic matter and minerals that make up the physical structure and texture.
- Liquid water and dissolved compounds (together known as the soil solution).
- Gases such as oxygen, nitrogen and various byproducts of biological reactions that occur in soil (collectively known as soil atmosphere).
A typical soil is made up of around 50% solid matter (45% minerals and 5% organic matter), 20-30% liquid, and 20-30% gas.
Of course, this is very approximate and depends on lots of different variables – for example, how long ago it last rained!
In reality, soil is incredibly complex, and the thousands of different types of solids, liquids, and gases that make it up, work together to affect its overall quality in countless different ways.
So… let’s dive headfirst into the (subject of) soil, and find out how to choose what’s right for you – from the ground up!
Soil Structure
Soil structure refers to the size of the particles that make up a sample of soil, the way that the particles are arranged, how the particles stick together to form clumps (a process delightfully known as flocculation), and the spaces between the particles.
Soil particles are divided into categories according to their size.
- The smallest particles are termed “clay“, with diameters of less than 0.002mm.
- The next biggest are “silt” particles, with diameters of 0.002-0.5mm.
- Then, “sand” particles have diameters of 0.5mm-2mm.
- Everything bigger than that is a piece of gravel or a rock!
In healthy soil, clay, silt and sand particles (along with small stones, gravel and organic matter) flocculate into clumps, which provide stability and structure to the soil.
When the soil is watered, the clumps don’t dissolve or move, so the overall structure of the soil remains intact.
The spaces between the clumps allow water and air to move freely and efficiently throughout the soil, and the moisture-retaining elements of the soil hold on to enough nutrient-rich soil solution to gradually supply it to local plants.
Soil Texture
The ratio of clay, silt, and sand in soil determines its texture.
Usually, a well-balanced soil will consist of around 40% sand, 40% silt and 20% clay. This type of well-balanced soil, rich in organic matter, is typically termed “loam“.
The ratio can vary somewhat, giving us clay loam, silty loam, and sandy loam – but if it varies too much it can lead to unhealthy, unbalanced soils. This can lead to the following problems:
- Over 30% clay in soil causes poor drainage and aeration. Soils can become impacted and/or waterlogged easily.
- Over 50% silt in soil also causes a risk of impaction, as their particles are almost as fine as clay. When impacted, silt soils will exhibit poor drainage and aeration.
- Over 50% sand in soil causes excessively-rapid drainage, and poor moisture retention and structure. Soils fall apart easily, as their large particles do not clump together well.
To improve poor-textured soils, the key is in the addition of organic matter. Sandy soils will gain structure and ability to retain moisture, while silty and clay soils will gain structure and improved aeration.
Don’t make the mistake of simply adding sand to heavy clay soils, as without added organic matter, it will form something more like cement than good soil!
Another point – in very hot, humid climates, add charcoal to sandy soils instead of “fresh” organic matter. In these conditions, decomposition is very rapid – but charcoal decays much more slowly than fresh material, as it contains less moisture and is less chemically reactive.
Soil Composition
To fully understand soil, it’s also important to consider the specific elements and compounds that make up soil particles.
In particular, clay particles are crucial to determining soil composition, as they have larger surface areas and greater reactivity compared to larger soil particles.
Soil composition is incredibly complex, and certainly can’t be covered in one short article – but here’s one example of why it matters!
- High presence of calcium or hydrogen ions in clay soils causes more flocculation, as their double positive charge strongly attracts negatively-charged clay particles and causes them to clump.
- Conversely, high presence of sodium or potassium ions, with a weaker single positive charge, causes less flocculation and more dispersal of particles.
Dispersed clay soils are a real nightmare for cultivators. As soon as it rains, they can turn into a homogenous, sticky goo with practically no structure, and can rapidly become waterlogged and anaerobic.
If you are working with dispersed, heavy clay soils, you can add lime or gypsum to introduce calcium and encourage flocculation. However, this will also increase pH, so be sure that you test your soil’s pH to make sure it is optimum (see below for more on soil pH).
Ultimately, soil composition comes down to a dozen or so essential elements, with nitrogen, phosphorus and potassium being the most important of all, and copper, manganese, calcium, magnesium, sulfur, molybdenum, and many other micronutrients also playing a part.
But how they break down and distribute themselves determines how the soil will behave, and how well plants will grow in it.
Soil pH
Soil pH is determined by the composition of the particles, as well as the proportion and type of organic material within the soil.
Again, soil pH is a complex subject worthy of reading up on, although here are a few basic points to consider if you need to raise or lower your pH:
- Cannabis prefers a soil pH of around 6.0 to 7.0.
- Most clay soils tend to be alkaline
- Many sandy soils are acidic
When using soil amendments to alter pH, it’s important to pay attention to potential salt buildup. Some types of manure and compost can be very high in salt, and that can harm plant growth if allowed to build up. To avoid this, use sphagnum moss and plant-based compost, which contain fewer salts.
Soil Aeration & Drainage
As we’ve mentioned, soil structure, texture and composition influence other fundamental properties of soil, including aeration and drainage.
We’re not going to get too deep into the specifics of drainage and aeration here, but it’s important to keep in mind its place in the network!
Of course, plants need water and air at the roots in order to survive.
But it’s much more complex than that – the levels of moisture and air in the soil (as well as the temperature, pH, and other environmental variables) determine the types of microbe that can survive and thrive in that environment, and the rate that soil microbes can react with organic matter in the soil and break down into its constituent parts.
Ultimately, all these factors working together determine how easy it is for a seed to germinate, for roots to form, and for seedlings to emerge and grow.
Soil Microbiome
In a healthy, well-aerated and sufficiently moist soil, there should be a thriving community of aerobic bacteria, fungi, and viruses (collectively known as microflora).
They work together with a bunch of amoebae, protozoa and nematodes (collectively known as microfauna) to break down organic matter into forms that plants can easily use.
There may be as many as 50,000 to one million microbes living in every single gram of soil!
In very general terms, aerobic bacteria are more useful for plant growth, and excessive levels of anaerobic bacteria are generally not conducive to healthy plant life.
- Well-aerated soils allow aerobic bacteria to thrive, whereas poor aeration encourages anaerobic microbes.
- But too much aerobic bacterial activity can lead to rapid oxidization of organic matter in the soil. This leads to soils becoming rapidly depleted of nutrients.
So, as with all things when it comes to growing cannabis, balance is everything.
Soil Macrobiome
As well as microscopic organisms, various larger creatures also help to break down soil. These creatures are known as soil macrofauna, and include everything from earthworms, ants and millipedes to rabbits, moles, badgers and even deer!
To be precise, anything larger than 2mm is considered macrofauna, while anything in between 2mm and microscopic is considered mesofauna – including mites, insect larvae, some spiders, tardigrades, and so on.
Soil macrofauna and mesofauna are incredibly useful because:
- They break down organic matter into smaller particles more easily digested by plants.
- Their burrows alter the structure of the soil, often improving aeration and drainage.
Of course, this discussion would not be complete without a mention of macroflora too – the plants large enough to be seen with the naked eye, which includes trees, shrubs, herbs and so on, as well as fungi, mushrooms, toadstools, mosses, lichens and much, much more.
These plants contribute to the soil by:
- Physically breaking up soil via penetration of roots.
- Deposition of organic material from roots, including sugars and mucigel.
- Contributing more organic matter when they die and break down!
Rather than things that live in or on soil, macroflora and macrofauna can really be considered a fundamental part of soil, just as much as any microorganism!
How Can We Optimize Our Soil For Cannabis?
Cannabis is very adaptable and has shown remarkable ability to survive in a very wide range of different soil types and conditions.
However, ability to survive does not necessarily equate to an ability to thrive!
Cannabis has certain preferences: if you stray outside the rules a little, your plants may not suffer too much, but if you disregard them entirely, your crop will suffer or simply die.
As we’ve mentioned, cannabis prefers a soil pH of around 6.0 – 7.0. You can amend your soil’s pH by following the tips in the Soil pH section.
Soils with pH higher than 7.5 will be unproductive for cannabis (and most other plants!) because they lock out the vital micronutrients iron, copper, zinc, manganese, and boron.
Soils with pH lower than 5.5 will lock out magnesium, calcium and phosphoric acid.
Cannabis grows best in light, loamy soils that drain well but also retain a degree of moisture.
Whatever your soil type, the best approach is to add organic matter to improve the structure, drainage and aeration of your soil.
Before you start amending your soil, keep pH in mind.
If you amend an acidic soil with acidic additives, you’ll compound the problem and threaten your entire crop.
Don’t just check the soil pH, also check the pH of your water supply – and check the pH of the additives you plan to use before you add them!
That’s it for our guide to soil types for cannabis!
Using this guide as a starting point, you can now go and read up thoroughly on all these complex aspects of soil, in order to maximize your understanding and ability to manipulate your soil safely to provide exactly what you need to your plants.
You may also want to check out Creating A Healthy Root Zone for Your Cannabis Plants and Cannabis Growing Insights: Understanding Terroir for more insight into cannabis and your soil.
