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Choosing the Right Growing Medium for Larger Results

By Maximum Yield
Published: December 9, 2020 | Last updated: May 25, 2021
Presented by CANNA Canada
Key Takeaways

When it comes to potted plants, soil is not a one size fits all. When it comes to picking the right growing medium, it's important to consider the porosity and size, along with the nutrients that will be added.

Everyone at one point in their life potted up a plant and for most, it was a straightforward operation: put soil in pot, plant in soil, water and enjoy. Those who enjoy gardening a bit more take the process a step further by taking care to add fertilizers when needed. Sometimes, plants thrive and other times, they don't. When it comes to potted plants, soil is not a one size fits all.

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Passionate growers know that the growing medium plays a vital role that goes beyond holding water and helping the plant to stand up. It needs to offer the plant a hospitable habitat for its roots and its preferred microorganism community.

Over the centuries, plants have developed and adapted their root systems to work with the environments they evolved in. Plants from arid regions have roots that underperform in water-based systems like clay pebbles or gravel. They do much better in an organic medium where the roots see longer cycles between watering. On the flip side, tropical plants prefer frequent watering in shallower well-drained soils and therefore, do very well in these water-based systems. It does not mean that they will not perform, but the troubles that can be associated with these systems are not worth the effort, compared to being grown under ideal circumstances.

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Keeping It Simple

Elaborate systems do not make better growers, they only make things difficult! Sometimes a grower has to use them to overcome obstacles (both physical and economic), but any design should be built with the goal of keeping everything as simple as possible. A good grower works with his or her attributes and limitations.

If time is an issue: automate.

If you're an experienced grower and want to experiment, then try advanced techniques. If you're a new grower, then leave it in a forgiving medium especially if an economic return is imperative.

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Use what works and use it right!

Choosing Your Growing Medium

Roots are somewhat adaptive to a variety of conditions, including pH, fertility, moisture, and aeration. They will grow and generally look good, but they may not provide their maximum potential. Plants have exactly one function: to reproduce. Limited resources will make for limited flower production.

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The wrong pH values will change the ratio of nutrients that become available to the plant, yet as long as they stay within reasonable limits, it will flower well. Low fertility will affect the growth and vitality of a plant, but it will flower. As long as some moisture is available to a plant it will flower. Air is critical to a root system’s survival, not just working. Make sure the medium fits the drainage and water retention characteristics the plant needs. These characteristics equate to one thing: porosity.

Field trial of using different growing mediums in the same set-up at Canna ResearchField trial of using different growing mediums in the same set-up at Canna Research

Porosity

Porosity is the value given for the number of open spaces in the medium. This allows for three types of pores: large pores, small pores, and micropores.

  • Large pores are too big to hold water against gravity and are therefore considered air spaces, although some water will be found there clinging to the walls or immediately after watering.
  • Small pores hold water against gravity under capillary action. These pores will also hold micro life such as fungi and bacteria. Water in the small pores remains accessible by the plant.
  • Micro pores are very small pores that hold water which is not available to a plant and only the smallest of micro life will be found in them.

When we speak of the porosity of a medium, it is in terms of percent (%) porosity. This percent is further broken down into percent large pores vs. percent small pores. A decent medium for container gardening will have a minimum of about 30 – 50% porosity of which about 17 - 25% is air space. This will vary based on the needs of the plant as some prefer less air and others much more. The advantage is that, after draining a newly watered container, the roots will see good air supply and still have a nice water buffer. So porosity relates to drainage as well.

A growing medium designed to grow with organic nutrients, like BioTerra Pro, will have the proper porosity, but also a wide variety of components that will each offer different chemical and physical properties, including porosity. The goal is to offer habitat for a wide array of microorganisms that are needed to convert the organic nutrients into plant available forms, a process called mineralization. This is why uniform growing mediums like rock wool and coco do work consistently better with mineral nutrients like CANNA Coco AB or CANNA Aqua & CANNA Substra.

Container Size

The right grow medium to use is based on all these considerations. Which container to use depends on plant size, the medium used, and the growing environment.

Remember that the root system mass should equal the top mass.

A tree getting to 5 feet tall will have many problems in anything less than a 5 gallon (20 liter) container.

If the crop can produce when smaller, then use a smaller container and flower earlier, but increase the number of overall containers. Keeping in mind that water needs is the same for the same amount of vegetation mass, whether the canopy is from 4 plants or 16.

In short, pick the size of the container based on the crop and the medium. The right system to use will be based on all this information including the ability to provide water when it is needed. If the system is not automated, it can be quite risky to have a situation where the grower needs to provide water once or twice a day...Unless he is chained to the grow room!

Potting-Up

Remember: it is always better to ‘pot up’ plants than to start big.

Potting up refers to taking a smaller container and moving it to a larger container. Un-rooted cuttings are ‘stuck’ into mediums or rooting trays, but rooted cuttings are potted up to beginning containers and then potted up to their final home. This does several things including not overpowering a small plant with a large volume of medium.

Excess medium increases the amount of water that the plant will “see”, creating a situation where the root does not grow out as well and fast as it should. Roots are driven from areas of no water to areas where there is water. This creates a situation where the roots will deplete the nutrients around them, but are not growing to where nutrients are. Eventually, this leads to apparent nutrient deficiencies on the upper part. As a perfect catch 22, when the growers see this he is compelled to add nutrients, which means even more water.

Start small. When a cutting is stuck, leave it just until the new roots reach the outside of the new rooting plug, about an inch long. Then pot up this cutting to a smaller container based on the final container it will go in. For example, use a 2.5-inch square pot for planting to a 6-8 inch container, use a 4-inch container if the final home is 10 – 14 inch. You might need an intermediate potting up stage as well if the target plant is especially susceptible to these issues.

Also use similar materials throughout, rock wool if the final medium is inert, coco or peat or sand if organic. This keeps the roots working on the same level and avoids big delays in production. It's especially important to use nutrients developed for specific grow mediums, such as the CANNA Terra line of nutrients designed specifically for peat-based potting mixes.

Potting up means starting small and transplant to a bigger container step by stepPotting up means starting small and transplant to a bigger container step by step

One of the most commonly seen paradigms with inexperienced growers is to use much too large containers for much too small plants, the idea being saving time and work. Soon enough these growers realize the plants are not growing as fast as they should. Their next step is to try a growing medium with higher porosity, sometimes even adding things like perlite to further increase air content and drainage as the observation leads them to believe that the plants were seemingly “drowning”. More often than not, this still does not work so well.

Then they'll try other approaches such as using air pots or fabric containers. This does help them get within their hopes in the early days, but shows later to be difficult to maintain moisture levels and controlling salt levels becomes a hefty task. The answer all along was a better technique in the production process through the use of proper container size for the root system, not blaming the growing medium.

High-quality growing mediums are carefully designed, with specific components to ensure the proper physical and chemical properties. With this in mind, nutrient companies that also produce growing mediums can provide specific nutrients that will guarantee optimal availability as well as maintaining other factors like pH within the ideal range.

This can be to improve ease of use, like having a one-part nutrient that covers all of the plant's needs like CANNA Terra nutrients and the Terra Pro Plus growing medium.

Or, to assure that the particular chemistry of certain mediums like coco fibers is properly handled. CANNA has strict control over-production of their coco mediums which leads to having precise knowledge of the chemistry within. Their Coco A&B nutrients handle both feeding the plants and maintenance of the proper chemistry for nutrients to be fully available. This eliminates the need for cal-mag additives as well as extending the durability of the medium, making it suitable for long term crops and re-use.

Canna Terra, a pre-fertilized soil, compared to Canna CocoCanna Terra, a pre-fertilized soil, compared to Canna Coco

Table 1 gives a very brief rundown of some of the types of medium currently available. There are many additional characteristics that can and should be taken into account when choosing. How much labor is required to use a medium, what effects will the plant produce, how much time can be spent monitoring the crop, and how is it disposed of after use, are but a few of the questions a grower must answer with the choice of medium. How to crop and grow a plant are just a couple of questions to answer in addition to the ones just mentioned when picking a container to use.

Table 1

Medium
Drainage
Support
Aeration
Suitable Systems
Notes
Mineral Soil
low to high
excellent
low to high
Container, bed, Run to Waste
Many types and compositions
Soilless Mix, Peat based
low to high
medium to excellent
low to high
Container, bed, Run to Waste
Composition of aggregates determine porosity, low pH
Soilless Mix, Coco
low to high
medium to excellent
low to high
Container, bed, Run to Waste
Age of Coco an issue, chemical composition issues, pH stable, aggregate and mulch size determine porosity
Inert Mediums, sand, perlite, vermiculite etc.
high
low to medium
medium to high
NFT, container, bed, Run to Waste
pH stable, higher the aeration less water buffer
Inert Mediums, rock wool
medium
medium (light and depends on size)
medium
Ebb and Flood, containers, Run to Waste
High initial pH, pH can rise again after buffering, solid blocks restrict additional aeration
Gravel, pebbles
high
little
high
Hydroponic recirculation in containers
Neutral pH, wash before use, reusable, requires constant monitoring, no nutrient or water buffer
Water
none
none
none
Hydroponic recirculation in a tank
pH varies widely, need aeration pumps, continuous nutrient monitoring, fast negative effects
Aeroponic
high
none
high
Hydroponic recirculation in special systems to confine water
pH varies widely, continuous nutrient and water monitoring, no buffers, chemical composition varies with air quality, no root temp buffering


Table 1 also lists the mediums in order of ease-of-use. A clay/sand soil will have the greatest buffers for nutrients and water; aeroponic systems will have the least. Rock wool takes more time to prepare and will have recurring pH issues. Inert mediums can have decent water retention with no nutrient buffering.

Organic materials add back to drain water and should not be recirculated. Inert mediums require monitoring and precise measuring of nutrients and pH. It also means needing to invest in specialized equipment to monitor these components. This increases labor costs as well.

A beginning grower should start with an organic medium which will provide the greatest margin for error and only progress to the next one once all the elements of good growth are understood and achieved such as light, water intervals, temperature control and humidity levels.

Choices Made by the Grower

In short, keep the system simple, grow in an easy medium and the right size container, and enjoy the garden. Some growers think that the best system to use is the one they will try next. Keep it simple; achieve your results first before moving on. The right set of nutrients, composed correctly with correct ratios of fertilizer will produce similar results in any medium as long as all the other variables are in line; it is the mistakes that limit production. Remember that it is not the system that makes the plants, it is the grower and the choices made by the grower that determine success.



Canna LogoSince 2002, CANNA is an established manufacturer of plant fertilizers and growing mediums. CANNA is serving high-quality products to the Hydroponics and greenhouse industry. Visit CANNA's website now to learn more.


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Written by Maximum Yield | Publisher

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Maximum Yield Inc. is the print and online publisher behind Maximum Yield and Maximum Yield Cannabis magazines. With topics such as cannabis cultivation and consumption, hydroponics and controlled environment cultivation, as well as greenhouse, container, urban, and vertical growing, Maximum Yield is focused on teaching you how to reach your maximum yield by providing informative articles on the latest technologies and plenty of tips and tricks from grow experts.

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