Things to Consider When Selecting Soilless Substrates

Wednesday, February 2, 2022

Soilless substrates are used across the world, offering a solution to those with difficult soils to grow on, or those who are looking to increase efficiencies when it comes to nutrient and water inputs. These substrates are comprised of non-soil components, but still provide the structural support needed by the plant in order to grow.

When it comes to the base material used for soilless grow media, there are two groups: synthetic and non-synthetic products. 

How do I chose what substrate to use?

While both synthetic and non-synthetic substrates are good options for providing structural support, nutrients and water to the plant root system through the cropping cycle, there are a few key questions to consider when purchasing a growing media:

1) What is the cost? 

Substrates are not created equally when it comes to cost. It is not recommended, however to chose the cheapest/most expensive one without understanding what you are getting with it. It is definitely an important factor to consider, but make sure that you understand the rest of the list below when making your choice. 

2) Where is the material coming from?

In COVID times, shipping delays are all too real for producers of all shapes and sizes. This should definitely be a consideration if you have yet to order your growing media for the upcoming season. There are many local substrate providers who might be able to fill your needs without having to ship overseas. 

3) What is the baseline nutritional status of the media?

While soilless substrates do not have much to offer in terms of baseline nutrients, it is important to realize that there are differences between the base materials. Some substrates are going to be more prone to having high initial salt levels compared to others, and if the media is not treated properly, these salts can be very damaging to your new crop. Be sure to ask your provider about your substrate and the treatments it has received. 

4) What is the particle density, porosity and particle size range of my substrate? 

These are key parameters that are going to determine how well or how poorly your substrate holds on to water. A substrate with tiny particles, means that the air and water spaces within your media are also going to be very small. This makes it harder for the plant to pull water out of those air spaces. It is also difficult for this water to drain out of the media, meaning the roots are more prone to sit in a wet matrix, regardless of the time of day. This has a tendency to lead to root disease and sub-optimal plant productivity. On the other end of that spectrum, large particles have very little capacity to hold onto water, meaning you are significantly increasing the number of irrigation events throughout the day to sustain plant growth. 

5) How do I want to handle my substrate?

Substrate is available for purchase in a few different forms. It can be purchased in large bulk orders, where the producer must hydrate, divide up into the individual growing containers, and then move those containers into the growing space. The other option is to purchase pre-filled slabs (also referred to as grow bags). Some crops have the option for specialized containers that help mitigate some production issues that typically develop down the line, while others don't. It is up to the producer, the target crop, and the labour force available, to decide which method is going to work best for that particular operation. 

6) What am I going to do with my substrate now that I'm finished with it?

This is where things get a little hairy. Synthetic substrates cannot be composted, and usually require transport off site for disposal. Non-synthetic substrates do have the option for composting, but must be done in a strategic way to prevent the accidental creation of an oasis for lingering insects and disease. This plays in to point number 5 as well - removing the inner contents of a slab wrapped in plastic is much more time consuming to compost compared to a crop grown in a container.   

For those who are new to growing in substrate, these questions can be overwhelming...but never fear! Your Protected Crop Specialist is here. If you have any questions on soilless substrates check out some of Perennia's resources: