Soil vs. Soilless- How to Choose Your Growing Medium

Before chiming in with our two cents on this highly debated topic, it’s safe to say there is a WIDE spectrum of opinions and methods when it comes to choosing a growing medium. Whether you grow in traditional soil, strict hydroponic solutions, or with soilless media, all growers are entitled to their own opinions. Our goal is provide a basic overview of two prominent choices for growing and create a space where other growers can share their experience and views. Everything we do is for the benefit of education so if this answers even one question you might have, we consider it a good day in the soil world.

Let’s being by establishing what constitutes a good growing medium for marijuana cultivation:

• Anchorage: a place for roots to extend and become stable.
• Nutrient retention: must be able to retain and absorb enough nutrients to sustain the plant system.
• Gas exchange/aeration: your medium must be sufficiently porous to allow air to move through the root system and gasses to escape.
• Water retention and drainage: your medium must be able to retain enough liquid to sustain the plant between waterings, BUT still be able to drain quickly enough to be fed aggressively depending on what stage of growth your plants are in.

With that in mind, let’s do a quick overview of what makes up soil or soilless mediums. Traditional soil has four basic components:

• Air (gaseous phase of soil)
• Water (liquid phase of soil)
• Mineral Particles
• Organic Matter

Air and water can account for up to 25% of traditional soil. Mineral particles are the most recognized component of traditional soil because they make up the solid, tangible matter. Typically, soil particles will consist of varying amounts of sand, silt, and clay. The physical characteristics of these particles effects soil behavior and management needs (i.e. how and with what you will need to amend your soil). Lastly, organic matter includes plant and animal residues at various stages of decomposition. OM can also include the cells and tissues of soil organisms and soil microorganisms. Even though this aspect makes up a relatively small percentage of soil composition (roughly 5%), it plays a huge role in creating the living competent of soil and allows biology to work its magic.

While traditional soil obviously has its benefits for raised bed, traditional garden beds, and general outdoor applications, attempting to cultivate medical marijuana in traditional soil comes with a number of setbacks. Mainly, you don’t always know what you’re getting in native soil. Disease spores, unregulated bacteria, planting eating insects, weed seeds, and other various unwanted company can easily infect your soil. When used outdoors Mother Nature has all kinds of tricks up her sleeve to keep these issues in check, but in order to bring native soil indoors it must go through a sterilization treatment first.

Another issue that keeps traditional soil from being the prime choice for growing cannabis is its lack of drainage. Native soil tends to be somewhat heavy so if you don’t go through the trouble of tilling constantly, it quickly becomes very compact and will essentially suffocate your roots because they won’t be able to soak up water and nutrients. Precise feeding also becomes more difficult because native soil comes with its own nutrients and characteristics that interfere with the distribution and absorption of individual nutrient elements required for bud growth. When growers try and remedy traditional soil problems using ferts that boost phosphorus, calcium, or potassium it often causes more problems, such as salt build up or burned roots.

The other method of choice we’ll explore today is typically referred to as soilless cultivation. This term generally means the media does not contain any traditional or native soil (i.e. the stuff you find in the ground in your backyard or at a local park) and is typically pH neutral.

Methods of growing plants without soil can be divided into two general categories:

•  Liquid culture (“true hydroponics”): where the nutrient solution is recirculated and adjusted for desired pH and nutrient levels.
• Aggregate culture (“soilless culture”): where the nutrient solution is supplied to plants via a soilless media and excess solution is allowed to run to waste or be recirculated.

Check out this awesome table which breaks down the differences of soilless cultivation systems, courtesy of the Agricultural University of Athens. Check out the full study here.