All About Growing Mediums

by James McMillin

What is the best medium of growth for cannabis?

When a prospective gardener is thinking about growing, the first thing they must consider is the medium of growth for their crop.  The medium is debatably the most important part of the horticultural equation, and will directly impact nearly every aspect of harvest.  The medium determines how much food or water the plant receives and has access to, how oxygenated/aerated the roots are, the potential size of the root ball, etc.  While there may be some exceptions, cannabis is generally grown in one of the following mediums, or using one of the following methods:  soil, soil-less mixes, hydroponics, aeroponics, or aquaponics.

Soil

Wikipedia aptly sums up soil, stating "Soil is the mixture of minerals, organic matter, gases, liquids and a myriad of organisms that can support plant life."  However, not all soil is created equally. Nitrogen, phosphorus, and potassium (NPK) value, nutrient density, aeration, diversity and quality of microbial life, ability to drain, etc. is incredibly variable across differing soils.  The main dividing point between soils are whether they are organic or not.

Organic Soil

Soil which is only comprised of organic ingredients and inert mediums can be classified as organic.  The concept of growing in such a soil begins entirely with microbial life in the soil itself.  Since organic matter cannot be directly absorbed or up-taken by plants the same way that salt-based fertilizers can, the plants must have another means of utilizing NPK in the soil.  Plants in organic systems depend on microbial life to break down these "raw" foods into a form that can be used by the plant.  This has to do with the carbon-nitrogen-phosphorus exchange, which I will avoid addressing in this article due to its technicality.

Because organic compounds cannot be fed directly to plants, the gardener should frame her viewpoint in a different way.  The gardener does not feed the plants, but feeds the bacteria and fungi in the soil.  These bacterial and fungal cultures create a symbiotic environment for cannabis plants, encouraging root growth, faster uptake of nutrients, adequate oxygen/aeration, etc.  Thus, organic soil is a living organism, or a collection of organisms (depending on viewpoint).

We have already established that plants grown in organic soil depend entirely on the microbial and biological life in the soil.  So, we should be able to conclude that if the biological life in the soil is disrupted, stressed, or eliminated, the health of the plant(s) will reflect this.  This is why organic soil growers avoid any use of chemicals, salt-based fertilizers, pH adjusters, etc.  If any of these products are added, they can greatly disrupt the microbes in the soil.  This essentially stops the feeding mechanism for the plant(s) as well as retards aeration of the roots.  Such fertilizers and chemicals generally kill off microbes in soil, and make it difficult, if not impossible, to inoculate the soil with beneficial biology.

The incompatibility of organic soil with other fertilizers and pH adjustments pose some difficulties for gardeners and horticulturalists.  When their plant begins to look hungry,  how can they feed it?  What if the pH of the water is off?  These questions, and others like them, can all be ansewred indirectly.  Organic growers can transcend all of these problems by creating a custom soil blend, or using the soil blended from Higher Thought.  Such soils should be the perfect consistency for drainage as well as aeration, should have an adequate amount of immediately-available nutrients as well as slow-releasing fertilizers, and should be pH adjusted.  While everyone has their own markers for what is adequate and what is not, it is generally agreed that the soil should squeeze, but still hold water, should be a dark and rich color when wet, and should have a pH of about 6.5.  Plants should not be fed again until they display signs of "hunger" or nutrient deficiencies.  They should be watered infrequently enough to encourage root growth, but frequently enough so that growth is not retarded.  Adjusting pH of water or tea is a concern for many organic soil growers.  However, it is not relevant.  If an organic soil has been properly blended, pH balanced, and inoculated with the right cultures, it should act as a buffer for high or low pH water/tea, and should be self-regulating.  The pH might move slightly up or down, but in general should stay around 6.5.  This is important as specific nutrient availabilities change according to pH.  Growers who are new to the concept of gardening in organic soil frequently see nutrient imbalances, and try to treat them by adding nutrients to the soil.  This is generally too hasty of an approach, as nutrient imbalances in well-blended organic soil are usually due to pH issues or temperature inconsistencies/extremes.

Non-Organic Soil and Soil-less Mixes

These mediums can be used to great effect in many different horticultural applications, but especially with cannabis.  Examples of soil-less mixes include coco coir or fiber, perlite, seed starting mixes, etc.  These mediums offer exceptional control of root aeration, water retention, drainage, etc.  This is because the mediums themselves do not contain any plant nutrients, but are inert.  These substances provide something for plant roots to grow into and establish themselves, but do not feed the plant the same way organic soil does.

Cannabis root systems seem to love soil-less mixes due to how well-aerated and porous such mediums are.  This encourages root growth, but does not provide the plant with any nutrients.  Thus, in such mediums, additional nutrients must be used to feed the plants.  Generally, salt-based fertilizers are used in these mediums as they can be dissolved in water and fed to the plants this way.  Soil-less mixes and non-organic soil will absorb water the same way that soil does, and will hold the fertilizers in the medium until the plant uses them.  Unlike hydroponics, this means that trace minerals can be added to the medium, and will not have to be added to the nutrient solution.  This poses one advantage to hydroponic or aeroponic horticulture.  Additional precautions must be taken when feeding as the plants can be overfed or nutrient "burned."  The medium can accumulate more nutrients than the plant can use, and can cause all kind of nutrient deficiencies/toxicities.  Therefore, the medium is usually "flushed" in the same manner that hydroponic and aeroponic growers flush their final product.  Flushing is the process of removing all fertilizers from the medium.  This forces the plant to pull nutrients from old foliage to put into its new growth.  Smokers report that this process reduces chlorophyll and other undesirable contaminants in the finished flower. 

Hydroponics

Hydroponics refers to the method of horticulture which makes water into the medium of growth.  Roots grow into water and nutrient solution, and absorb food and water directly this way.  The roots must be exposed to air as well, or the plant will end up drowning.  Many clever growers have devised all kinds of hydroponic techniques to achieve this delicate balance between air, nutrients, and water for the roots.  These systems pose several advantages, namely the amount of control over the grow.  In hydroponic systems, the grower can quite literally have quantified data about her garden, and know exactly how much food they are eating, how much water they use, etc.  In an organic soil system however, there is no meter which can measure amount of microbial life.  Monitoring the pH of water and the parts per million of nutrient is extremely important in hydroponics, as these two values determine nearly every aspect of growth (provided there is adequate air exchange, temperature, etc.).  Changing pH is very important as the pH determines which nutrients are available in which quantities.

Hydroponic methods can use both organic as well as inorganic fertilizers, but growers generally report much more difficulty growing with organics in such systems.

Deep Water Culture (DWC)

Deep water culture is a very straight-forward concept of hydroponic growing.  In these systems, the root system of a plant is completely submerged in water.  Normally, the plant will die soon under such circumstances.  However, air pumps and air stones are used to pump air into the root chamber.  This creates a constant flow of both water/nutrient solution as well as air over the roots.  This gives the plant as much water as it needs, access to nutrients and food, and adequate oxygen to its roots.

Ebb and Flow/Flood Tables

Many growers swear by the simplicity of flood tables.  In such systems, plants grow roots into inert mediums such as clay pellets.  This allows for plenty of air flow, but usually dries out the root systems somewhat quickly.  To combat this problem, growers flood the tables containing the plants until the root systems are completely submerged in nutrient solution.  They remain submerged for several minutes, while the nutrient solution drains from the tables back into the reservoir.  This process repeats in a cycle, alternating between giving the plants food/water, and exposing the roots to air.  This gives the plants everything they need to grow aggressively.

Drip Systems

Drip systems are the polar opposites of DWC systems.  In drip systems, the roots are suspended in air, hydroton, or some other inert media.  These roots would quickly dry out, but the root systems are continuously dripped with water and nutrient solution.  Again, the default in this system is that the roots are getting plenty of air, and the grower is dumping water/nutrient solution on top of them when it is necessary.  This gives the plant adequate food, water, and air. 

Nutrient Film Technique (NFT) and Gutters

These systems of growing combine many of the favorable aspects from other hydroponic systems, making it a favorite method for commercial farmers and the initiated.  In such systems, gutters are usually slightly angled downward, to power the flow of nutrient solution with gravity.  Root systems are placed in a row along the gutters.  The plant roots are suspended in air in the gutters, but have continuous access to nutrient solution which is flowing at the very bottom of the gutter.  The key to this system is that the nutrient solution is in such small quantities that the root systems have continuous access to air, but can reach down to the flowing solution to uptake food or water at any time.  This creates a film of nutrients along the bottom of the gutters if the amount of water used is small enough.

Aeroponics

To my knowledge, aeroponics is the most advanced and newest horticultural method in use.  Debates about just exactly what "aeroponics" is typically ensue around horticulturalists with some background knowledge.  Most simply, the term refers to a method of growing where the growth medium for the plants is air itself.  Root systems are totally suspended in air, and come into contact with nutrient solution periodically.  Enthusiasts generally divide aeroponics into two categories:  high pressure aeroponics, also referred to as "true" aeroponics, and low pressure aeroponics, also referred to as regular aeroponics.

Low Pressure Aeroponics

Low pressure aeroponics (LPA), or aeroponics, use air as the medium of growth for plant roots.  Roots are generally sprayed with water or some sort of nutrient solution, giving them adequate moisture.  However, the default in these systems is that the roots are suspended in air, allowing them virtually unlimited access to oxygen and air.  This encourages incredibly fast root growth, but many growers argue that LPA is virtually indistinguishable from hydroponics.  After all, water is still the medium carrying plant nutrients, and the concept does not sound radically different from the hydroponic methods previously-discussed.

High Pressure Aeroponics (HPA)

HPA is also referred to as true aeroponics by its more enthusiastic proponents.  Fundamentally, it is the same as LPA, but there is a technical difference between the two systems which is a significant divider.  In HPA systems, the roots are suspended into air, just the same as in LPA.  Again, this allows roots to uptake as much oxygen as required, and encourages trichoblasts and rapid root growth.  HPA growers often report that their plants produce a type of root that is more "fuzzy" than hydroponic or LPA plants, and that this root can more efficiently uptake nutrients. 

The fundamental difference between HPA and LPA obviously involves pressure.  This refers to the pressure at which water or nutrient solution is coming out of nozzles or emitters.  While this sounds like a small difference, Richard Stoner and NASA would argue it is the biggest difference one could make.  Stoner and NASA discovered that root systems can most-efficiently and rapidly uptake and utilize nutrient solution when the water droplets are sized at 30-80 microns in diameter (this ideal size varies according to plant species, maturity, etc.).  Hydroponics usually submerges root systems in water, or constantly flows over the roots.  This does not create any of these small droplets.  In HPA systems, nozzles with extremely small orifices are used in conjunction with high pressure pumps.  This forces water out of the nozzles, and when they emerge, they are sized in the sweet spot illustrated by Stoner and NASA.  This is when HPA gets very technical.  If the emitters are turned on constantly, these water droplets will bounce around the root chamber, condense, and become larger droplets.  This essentially turns the system into a hydroponic or LPA system.  To avoid this, the amount of time that nutrient solution is sprayed is cycled.  Usually, growers report that anywhere from 1-5s on and 1-10m off is a good cycle.  Again, plant size, species, root size, root chamber size, etc. will all play factors in determining the ideal cycling for your system.  This amounts to a very small period of time that roots are exposed to nutrient solution and water.  According to the NASA studies, this cycling with precisely-sized nutrient solution droplets is the fastest, most efficicent way to grow plants.  Not only is it the fastest, but it also reduces water and nutrient use by about 90% when compared to hydroponic systems. 

Here is an except from an article I wrote for Multiponics.com/multiponics.biz.  Much of the information is redundant, but the article supplies some additional information as well as clarifies some points.

What is Aeroponics?
Aeroponics is a method of growing plants which uses air as a growth medium. When growing in a soil medium, roots grow through and around soil. In a hydroponic system, water takes the place of soil, and roots grow in and through water. In an aeroponic system, air is the growth medium. Roots are
suspended in air and are periodically misted with a nutrient solution. Roots are constantly in contact with the air, allowing for an increased uptake of nutrients, water, and oxygen. Proponents of this method argue that aeroponic growing is superior to both hydroponic and soil grow systems. Increased efficiency means less nutrients, water, and oxygen are required. This makes aeroponic gardening a viable means for urban farming, greenhouse growing, rooftop gardens, etc.

Aeroponics vs. High Pressure Aeroponics (HPA)
HPA is a specific subset of aeroponic growing, developed by NASA and Richard Stoner.  NASA’s goal with HPA was to create the largest amount of bio-mass with the smallest amount of input. All HPA systems are aeroponic, but not all aeroponic systems are HPA. HPA is a kind of aeroponic method in which the nutrient solution is misted or sprayed on the roots at a high pressure, usually around 120 PSI or so. This solution is sprayed via nozzles, which are generally quite small. This high pressure forces the nutrient solution out of the nozzles in the form of water droplets. The higher the pressure, the smaller the resulting water droplet. NASA studies have indicated that there is a specifically-sized water droplet that is taken up by plant root systems more efficiently than other sizes. This sweet spot is within the 30-80 micron diameter range. Water, oxygen, and nutrient solution are more efficiently and rapidly processed when roots are exposed to nutrient solution droplets of this size. High pressures also atomize the nutrient solution, furthering positive benefits. Consequently, less water, oxygen, and nutrients are
required than in a conventional growing system. HPA systems can cut nutrient use by 90%, meaning lower costs, a “greener” product, and farming that is more environmentally friendly. Clearly, HPA is a superior method of growing.