How to mix and prepare a good substrate to grow your marijuana

One of the keys to success is in the quality of the substrate used: having a soil or inorganic substrate with the necessary characteristics and nutrients is essential for plants to grow and develop as well as they can. Whether we use natural soil, rockwool or coconut fibre, in both indoor and outdoor crops, preparing our substrate will be one of the first steps we must take to achieve a top-notch marijuana crop.

Like most plants, marijuana needs a good quality substrate to grow fully. Nature often offers us excellent soil for our outdoor grows. However, in most cases, it will be necessary to add the necessary nutrients in order to optimise results. This must be done early in the plant’s life. Just remember you are preparing the place where it will grow and develop, so the rest of its evolution will largely depend on this part.

First of all, you should bear in mind that whether you choose soil or use other inorganic substrates, these must have the following characteristics:

• An adequate amount of nutrients.
• The right texture for the roots to grow free.
• Enough water retention and drainage capacity.

Many of the organic (and especially inorganic) substrates available on the market incorporate most of these features so that your plants grow in the best conditions. However, we, as growers, can increase their quality by following the steps in this guide.

Keys to preparing a good soil-based substrate

In addition to being the most affordable, using organic soil for the growth of our cannabis plants will provide a more natural aroma and flavour, noticeable upon consumption. This type of soil is easy to find, including those with natural supplements such as the Light·Mix from BioBizz (composed of peat moss, sphagnum peat moss, and perlite).

What should the soil base be like?

However, if you are the type of person who prefers to do everything yourself, you can use natural soil for your mix. In this case, you must make sure that it is suitable for growing cannabis. First, it will need to have the right pH. For marijuana, this is a pH of 6. This pH is usually found more easily in light and loamy soils which drain the water correctly while maintaining the optimum degree of humidity for its growth.

In case of doubt about the most suitable soil for your grow, you can make use of pH measuring strips and distilled water. All you have to do is mix 10 g of earth with 10 ml of distilled water, let it rest for a few minutes, and introduce the strip into the mixture. It will quickly change colour to tell you what the pH of the soil is. Optimal levels in soil would result in a slightly pinkish colour. 

The soil that we have in our garden is usually suitable for growing, so this should not be a problem. However, if the soil comes from the countryside or from some other area without excessive vegetation, you may not get the expected results. Even in such case, you mustn’t worry for there’s always a way through. If the strip has turned blue, it means that the pH is too alkaline and we need to reduce it by adding sulphur, while, if its colour has changed to deep red, this means that it is very acidic and we should use agricultural lime (calcium carbonate).

What will I need to add to it?

Once the pH levels of our soil have been adjusted, it will be time to add some nutrients. Each of these fertilisers has a specific role, from nurturing the soil to activate growth to facilitating the absorption of water.

As nutrients we will use earthworm humus and bat guano. The first, with a high content of hydrogen, will be ideal for the growth stage, while guano will activate plants’ flowering due to its high levels of phosphorus and potassium. 

When using a soil extracted directly from the ground, we can also add dolomite, a mineral-rich in calcium and magnesium that will also help balance the pH.

To improve the sponginess of the soil and its drainage, you can use vermiculite, perlite, or coconut fibre. Vermiculite will boost the absorption of moisture while retaining nutrients and air between its fibres, facilitating root respiration and nourishment. Perlite, volcanic sand, improves the sponginess of the substrate. And coconut fibres are very useful because of their ability to retain liquid.

Also, if you are using pots for growing, you can add expanded clay (Arlita) in the base. Arlita is composed of aggregate clay forming balls that are placed at the base of the pots for drainage. You can even use other additives such as neem powder, which prevents pests, or nettle powder, from the cannabis family, which improves soil conditions and our plants’ assimilation of nutrients.

How should we mix them?

Now that we know how beneficial a good quality substrate can be for your plants, it is time to prepare our own soil mix. After testing many successful combinations, one of the most commonly used ones is as follows:

• 40% soil base
• 20% coconut fibre
• 20% perlite
• 10% vermiculite
• 10% earthworm humus

Make sure you do not exceed the specified quantity of any of the additives since this could be bad for your plants. In most cases, it is easier to bring a plant that shows some kind of deficiency to life than to save one that has been over fertilised.

The size of your crop will also determine how the mixture is prepared. If you have one or two plants, it’ll be easy. Get yourself a large bucket. Start by pouring a quarter of the soil (save the remainder for later). Next, add part of each of the nutrients according to the proportions above. Then cover them with soil and continue alternating the base and the nutrients until you finish them all. Once over, use a shovel to mix it.

However, if your grow is much larger, with a large amount of substrate, you can mix the substrate in bits or use specific machineries such as a substrate mixer or a concrete mixer.

A different mix for each stage of the plant

Some growers recommend changing the substrate when the plant is close to flowering. A good time may be when the plant is to be transplanted into a larger pot.

The main difference lies in the replacement of the earthworm humus, which is perfect for the growth phase, by bat guano, a natural flowering activator. To do so, you must prepare a mix with the same proportions as the previously stated, with the exception of adding 10% guano instead of earthworm humus.

However, if you hadn’t foreseen any transplant for you didn’t want your plants to suffer from any stress, it isn’t necessary to do it just to change the substrate mixture. Adding bat guano to the surface of the soil during the first weeks of flowering should be enough to take advantage of the benefits of this natural fertiliser.

Other enriched substrates without a soil base

The best known of all is coconut fibre, which is often used in soil-based substrates but works perfectly fine on its own. It is recommended for urban gardens thanks to its excellent characteristics that facilitate the growing of cannabis. It is very lightweight, it can hold eight times its weight of water and nutrients and its pH is neutral, which favours cannabis growth. Just a heads-up: it must be mixed with nutrients. The most highly recommended mixture is 60% coconut fibre and 40% earthworm humus in order for the plant to absorb a large amount of nutrients.

Another inorganic substrate that can also be used alone is expanded clay. These balls of clay, in addition to nourishing the soil, can be the living environment for our plants. To prepare them, you will need to carefully wash the clay and let it stand in water with a regulated pH (around 6) for a few days. After this time, it will be ready for use. We can add nutrients to the balls using earthworm humus in the same proportions used with coconut fibre. It is also recommended to use a drip irrigation system in order to maintain high levels of humidity in the substrate and ensure the roots don’t dry out.

Although rockwool is mainly used in seed germination or rooting of cuttings, there are also growers who claim it works great for the plant’s development. In this case, you must immerse the material in a glass of water with a pH of 4.5 for 12 hours beforehand. 

With these tips, you will be able to get the growing of your marijuana off to a good start and thus go a long way to guaranteeing its success. And remember to add compost and fertilisers from time to time. Have a good harvest!