Soil, coco coir or rockwool: What’s the best way to grow marijuana?

Choosing the right substrate is one of the most important tasks for a grower. They need to choose the type that best suits their needs in order to develop healthy, strong, and vigorous cannabis plants. The most popular and well-known growing medium is soil, which has been used in cannabis cultivation for centuries, but there are other soilless alternatives that offer excellent results. Here we explain the benefits of the most frequently used options.

One of the most frequent mistakes marijuana growers make is to underestimate the importance of the type and quality of the substrate. It is commonly believed that just using quality cannabis seeds and good fertilisers is enough to obtain a successful harvest. And even though cannabis can easily adapt to different environmental conditions and soil types, the choice of substrate can affect the plant’s yielding capacity both in terms of quantity and quality.

A good substrate for growing marijuana needs to offer a balanced and stable supply of water, oxygen, and nutrients. The composition of natural substrates usually includes some mineral and organic compounds, with a typical presence of biological organisms and a certain amount of water and air: depending on the balance between these components, the substrate will be more or less suitable for growing marijuana plants.

A substrate for cannabis cultivation should ideally have the following characteristics:

• Maintain a firm and stable structure, without breaking, splitting, or expanding.
• Have good absorption capacity, retention, and homogeneous distribution of water, thereby maintaining adequate levels of humidity.
• Maintain good porosity to allow the entry of air and oxygenation of the root system.
• Have the right pH and EC (electroconductivity) levels.
• Be clean and free of pests or pathogens, whilst promoting the development of beneficial microorganisms.

Under these premises, there are several substrate types which are suitable for marijuana growing. Choosing one or another will have an influence on the crop, as well as on the type of nutritional plan required. Let’s have a look at the different types and their characteristics.

Growing in soil: a safe method

Growing marijuana in soil is the most natural and simple method that there is, which is why this is one of the most popular substrates. Nonetheless, there are many types of soil, and depending on their composition, pH levels, and nutrient content, they may be more or less suitable for the growth and flowering phases of cannabis plants.

Most of the commercialised soil for growing marijuana contains a mixture of black and blonde peat, coco coir, pieces of bark, worm castings, and perlite or vermiculite. Depending on the amount of nutrients present in the soil and the soil’s ability to release them, fertilisers will have to be added through irrigation at a certain point in the cycle.

It is essential that the soil is not too compact so that the roots can develop easily and are provided with good oxygenation at the same time. This also prevents moisture excess, which can lead to rotting, since the soil usually absorbs a large amount of water and retains it for several days.

Once we are aware of everything that a high-quality substrate can contribute to our plants, it is time to prepare it with the precise amounts. There are numerous combinations that have been tested with excellent results, but one of the most frequently used is as follows:

• 50% of the capacity of the pot of simple soil, without added nutrients or fertilisers.
• 25% perlite or coco coir to promote aeration.
• 25% worm castings: do not mix with the other 75% and try to place most of them at the bottom of the pot, so that the roots can only access them when the plant is more mature.
• Optional: mycorrhizae, i.e. beneficial fungi which act in symbiosis with the plants and promote the quick development of a healthier and stronger root system. The dosage will vary depending on the type of commercial mycorrhizae purchased.

Benefits

• Growing in soil is the simplest method. As nutrient uptake is slower and pH levels are generally stable, this growing medium allows for a greater margin of error.

If the right type of soil is used, it will be possible to simply irrigate with water every few days and only add fertilisers at certain specific times, such as for instance at the beginning of the flowering. Thanks to all these features, growing in soil is the most suitable method for novice growers.

Coco coir and perlite: maximum accuracy

Coco coir is used to create a very spongy and aerated substrate, which allows for frequent watering and facilitates the maximum root development of marijuana plants, preventing them from rotting. It is in fact so highly aerated that some growers add some perlite to increase water retention.

Coco coir is a neutral and inert substrate, which means that it doesn’t contain any nutrients. Therefore, it needs to be fertilised by adding the necessary amount of fertilisers at each irrigation event, whose pH and EC levels must be accurately monitored at all times.

Benefits

• If irrigation times and fertiliser quantities are followed rigorously, growing in coco coir and perlite will help increase the speed of growth of the roots. This will in turn reduce the vegetative and structural growth period.

Growing in coco coir requires lots of perseverance and precision. This method enables you to keep the pH levels and the amount of fertiliser needed for each cannabis plant under control, according to every stage of their life. Having said that, this might not be the most suitable growing medium for rookie growers: as the plant depends solely on the food that it is provided through irrigation, a failure in this regard would be a dear price to pay. Therefore, there’s not too much room for error, so you can end up ruining your grow in just a matter of days.

Rockwool: maximum oxygenation

Rockwool is a substrate of very spongy mineral origin, characterised by a high permeability and a remarkably high oxygenation capacity. For these reasons, it requires the use of an automatic irrigation system, something that makes this medium one of the most frequently used in hydroponics. Rockwool is an inert medium (i.e. it doesn’t contain any nutrients), so, as with coco coir, fertilisers need to be added to the water for each irrigation event, and pH and EC values also need to be constantly monitored.

Benefits

• The porosity of rockwool facilitates rapid and dense root development, prevents the appearance of pathogens, promotes vegetative growth, and helps to keep the fertilisation of the grow under control.

As with the previous medium, this may not be the most suitable substrate for beginners. However, when monitored appropriately, it can help accelerate the plant’s metabolic processes, leading to spectacular results.

Conclusions

There is no perfect growing method for everyone, but there certainly is a system suited to every situation. If you’re starting in the world of cannabis growing, the best advice we can give you is to try the different techniques, starting with soil, which is the recommended growing method for both rookie and expert growers, or for those who need a substrate for outdoor cultivation.

If you’re an experienced grower who wants to grow indoors, it’s best to try coco coir and perlite first, before venturing into the world of rockwool, which is only recommended for real cannabis virtuosos.

And remember: a cannabis plant can grow and flower regardless of the growing medium, as long as its roots have enough space to expand and access fresh oxygen, water, and the right nutrients. Nonetheless, every grower has a strong personal preference for a specific substrate depending on factors such as yielding potential, ease of use, and, above all, the space available for cultivation