If you’re an indoor grower, you’re probably using a soilless potting mix in some way shape or form. This might be a loose mix or plugs bound with something like a polymer (FlexiPlugs are an example of a polymer bound plug, while some peat plugs are simple contained in a thin baggie).
Soilless potting mix might look like soil, but it’s actually just a mix of organic matter like peat, wood chips, or coco coir, perlite and/or vermiculite, and often a slow release fertilizer. There’s no inorganic matter like sand, silt, or clay involved, which means that the mix technically isn’t soil.
Each component of the soilless mix serves a purpose.
Coco coir & peat moss
Coco coir and peat moss are the fine fibers in the mix, and provide the main structure of the mix. This gives the mix volume, provides biological surface area, and to a point even determines porosity and water holding capacity.
Coco coir is being used more and more these days as it is more immediately sustainable than peat moss, which is harvested in great volumes. Coco coir can be salty at times and can change the EC of a hydroponic system. Be sure that you’re getting coco coir from a name brand source to avoid this.
Perlite is essentially superheated and expanded obsidian. The small white media gives the soilless mix artificial air pores, keeping the mix oxygenated. Vermiculite (superheated and expanded mica) does the same thing, as well as holding water.
Slow release fertilizer
Many soilless potting mixes also have grains of slow release fertilizer included to provide nutrients to growing seedlings. If growers are using a dilute nutrient mix, this isn’t necessary, but seedlings will need nutrition in one form or another.
Benefits of using soilless mixtures
Many farmers – indoor farmers especially – use a soilless mix for seedlings and beds because it’s easy to source, make, and use and it’s compatible with almost every type of growing system because unlike many media, it’s fairly inert. (It won’t affect pH or EC.)
For systems where seedlings are transplanted from a separate seedling system (like in most ZipFarms), soilless potting mixes trump soil mixes because they don’t break down and create system debris (and therefore clogging) like soil will. Seedling roots bind the organic matter in the mix together well, and very little volume of the plug is washed away or shaken loose into the hydroponic system.
Limits of soilless mixes
While soilless mixes won’t break down and create debris like soil mixes often do, the root binding does have its limits. In open irrigation systems like NFT (nutrient film technique) systems, a polymer bound plug is better suited for seed starting.
On the downside, soilless mixes are biodegradable in the long term, and although they don’t initially compact like soil, they will eventually compact as the components break down. This compaction causes problems in the root zone, anaerobism, and decomposition.
Sterilized media is an important part of most IPM strategies to avoid harmful pathogens and inoculum that could be carried into the system in the seedling media. Usually, getting sterilized media entails ordering from a reliable source. For growers mixing their own media, they may need to sterilize the media themselves. This can be done with a steam sterilizer, which can usually rented from a nearby landscape and agricultural equipment company.
Where to get a soilless mix
Pre-mixed soilless potting mixes are common and available everywhere, from Ace to Walmart.
Growers can also mix their own using the components with their own ratios of coco coir, perlite, vermiculite, and fertilizer. This gives growers more control over the structure and characteristics of their mix.
Most growers who mix their own use a recipe similar to this:
- 1 Part (buckets are an easy way to measure) coir or peat
- 1 Part perlite or vermiculite
- 2 Parts compost
Individual components of the mix are available in many home improvement stores like the Home Depot or Lowe’s, or online with companies like HortAmericas.
Other add-ins include:
- Worm castings
- Slow-release fertilizers
Growers creating a mix for long-term growth in pots or beds often add other fertilizers and additives, but for a hydroponic grower only using the mix to raise seedlings, these additions will only get in the way.
Ready to grow seedlings?
One of our favorite seed companies is Baker Creek Seeds. They sell high quality, high germination rate seeds with a wide variety and many interesting heirloom varieties. You can see our favorite vertical crops here and order some seeds of your own.