8+ Important Things to Consider Before Choosing a Growing Media

Last Updated on December 17, 2018

The growing medium is any material in which the plant grows; when growing your plants in your garden the growing medium is the soil. Choosing the optimal growing media (substrate) requires special consideration because it is much more than just an anchor for the plant: it can be crucial to a successful crop.

Growing medium has three main functions.

  1. Supply roots with nutrients, air, and water
  2. Allow for maximum root growth
  3. Physically support the plant
  4. Also serves to moderate the temperature around the plant roots.

It basically has to replace the soil and do all function which soil do for plants.

In aquaponics, The grow bed media also provides a surface area for the nitrifying bacteria to colonize, facilitates the mechanical filtration for the solid waste and provides a home for beneficial worms in aquaponics systems. 

The kind of expand media you use will count on the atmosphere of your method. The effects of which media you choose will be both useful and economical. Some elements to consider are pH of the material, ability to hold water, weight, and affordability. you have to understand the general constraints, tradeoffs, and benefits surrounding media types.

The best Grow media is

  • Nutrient free
  • Able to retain air
  • Neutral pH
  • Able to retain water
  • Able to drain quickly so that roots don’t get waterlogged.

Different hydroponics/aquaponic media come with different benefits and challenges. Commercial growers especially need to be aware of these so that they can plan costs accordingly.

Factors to Consider when Choosing Media

1. Biological Surface Area

The biological surface area is determined by surface area and particle size of your grow media. The direct benefit of surface area is to cation exchange capacity (CEC). CEC is the ability of media particles to hold exchangeable cations. (Cations are positively charged atoms in a solution.) Since many of the nutrients that crops need are salts which dissociate in water, cation holding equates to nutrient holding for many nutrients.

Surface area is so integral to the engines of the aquaponic system — the microbes — that it deeply affects other functions. There are two things in particular that are directly affected by the amount of surface area.

Nutrient cycling is the conversion of ammonia to nitrates and is key to plant health. Cycling is powered by nitrifying bacteria. Surface area acts as a habitat for those bacteria and can be directly linked to the speed and amount of nutrient cycling that occurs in the system.

When hosting microbes, the surface area is called biological surface area, or BSA. BSA is one of the direct benefits of high surface area. Systems with high BSA recover from disturbance better, hold pH better, and respond to feeding faster.

2. Particle size

Your growing media should have a moderate size of particles and suitable pore spaces between them.

The problem is that both large particles and small particles have benefits and challenges.

Large particles with large pore spaces between the particles. This will make it light and fluffy (well-aerated) encouraging fast seed germination, strong root growth, and good water drainage. Large particles like hydroton are easier to move and plant with, easier to keep clean, and have great water movement. The pore space between particles is so large that solids rarely gather and cause blockage or anaerobic zones.

Small particles offer much higher Biological Surface Area and nutrient holding capacity but can be more difficult to work with. Smaller pore space means that solids can accumulate, which can lead to anaerobic decomposition. Sometimes smaller particles can become compacted as well. This introduces a whole new set of issues with planting and maintenance.

 Small ParticlesLarge Particles
  • More Biological Surface Area
  • Better Nutrient holding capacity
  • Better cycling
  • Easy to Plant and move
  • Higher void space (Large pores)Easy to clean
  • Better percolation
  • Compaction
  • Solid accumulation
  • Lower Biological Surface Area

Because the benefits of each particle size are playing on different sides, growers can’t get all of the benefits in one aquaponic media type. They have to pick and choose the benefits they get.

Fibers are the exception.! They ignore the normal surface area trade-off.

Fibers turn the competing benefits of particle size and surface area on their head: They

  • Have extremely high BSA, which hosts microbes good for cycling and increases cation holding capacity.
  • Come in one piece, which is simple to pick up and move
  • Can be quite tough, easy to clean and difficult to compact
  • Have high void space, meaning good water movement
  • Catch solids without accumulating them, meaning they can act as a filter without causing anaerobic zones
  • Can be natural or synthetic. Synthetic like Rockwool is created from recycled plastic and reusable for many years. while organic is something that is repurposed which would otherwise be wasted like rice hulls, coco coir, and pine bark.
Fiber acts as better grow media
Fiber as grow media has many benefits

As you can see, fibers are the best fit for many systems. It’s likely that fibers are even the best choice for you, although there are a few situations where fiber is not appropriate.

Tubers and root crops don’t always grow well in fibers, and DWC (deep water culture) doesn’t require any media. Even in these systems, however, growers should be deliberately adding biological surface area.

3. Air-holding Capacity

Plant roots require air for oxygen supply and gas exchange, and therefore, aeration is critical for optimum plant development. Lack of adequate aeration results in poor plant growth, susceptibility to diseases and nutrient deficiencies.  you want your growing medium to retain oxygen; plants need oxygen to grow, and it will greatly benefit your plant’s growth rates if the growing medium holds onto some air, even when saturated.

4. Water-retention/absorbing Ability

Finally, you want to select your growing medium on its ability to hold water. This will depend completely on the type of plant that you are growing, but some mediums will absorb water — holding it for longer will allow your plants to access it for longer — whilst other mediums will see it run straight through.

Ideal growing media provides the plant with an adequate water supply and at the same time contain enough air to allow gas exchange in the root system.

Good growing media are also characterized by high hydraulic conductivity, i.e. ability to transmit water.

A balance between air content and available water is one of the most important requirements of a good growing medium.

5. pH

Local crushed rock or gravel media can have a high pH level, a classic example of this is limestone, which can cause your pH to go very high. High pH can be a bit of a problem in an aquaponic system. You want to be a little careful if you’re using any type of rock media.  Some can have high levels of limestone and other high pH minerals within them which can lead to nutrient lockout.

How to do a pH test on Grow Media?

If you are unsure, a quick way to check any rock you are thinking of using is to do the vinegar test. Get a handful of the media you want to use, drop it into a jug or cup of normal household vinegar, if the rocks appear to be visibly bubbling, releasing bubbles from the rock, then chances are it has a high pH and best if you can look at an alternative.

6. Availability

Availability of different media differs based on where you live. Many types of rock media are probably available locally for you, things like river-stone, coco coir, shale, scoria and many others. You can always buy some grow media from amazon.

7. Weight

Another important property is the growing media’s weight: it should be lightweight for easy and less expensive transport and handling. But it should also be heavy enough to provide physical support to the plant. Rock media or gravel is also very heavy so you need to plan to have enough support for it when building your grow-beds and their stands and supports.

8. Renewability & Decomposable

The organic and natural growing media like rice hulls and coco coir re-purposes something that would otherwise be thrown away. These have good fiber ratio providing benefits irrespective of particle sizes. However, The trade-off of having them is that it decomposes over the time. In the case of aquaponics, Your grow media must never decompose otherwise the levels of the pH and nutrient will fluctuate out of control. It can also turn your water dark which will make it hard to see your fish because of tannin leaking into the water as a result of decomposition.

You should not use these type of growing media standalone but mixed with other types of media

Decomposable grow media includes Coco Coir, Fiber & Chips, pine bark, and rice hulls.

Reusable media include Rockwool, hydroton, grow stones and polymers. Rockwool is synthetic but also fibrous providing benefits of both large and small particle size.

Before choosing a media, choose a design.

Media choice depends on your growing technique, design, and needs.  Check the types of hydroponics and guidance about it here. You may be using Bato Buckets, ZipGrow, Deep Water Culture (DWC), Aeroponics, and Decoupled Systems.

Check the list of all growing media here


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