Become a Grow Media Master! How to Grow Like a Pro with the Right Substrate
Armando Suarez offers some perspectives from the commercial side of plant production as he describes the virtues of some of the most popular plant growth mediums out there.
As an avid home gardener who also works in the commercial hydroponic industry, I am often reminded of how different commercial gardens are.
At a time when many hobby growers are taking steps towards becoming professional growers, it is important to understand that successful commercial growing operations do not operate like regular gardens, as labor and resource management becomes more important as the business grows.
In a commercial garden, tasks as simple as putting dirt in a pot and sticking a plant into it become more complex when you need to perform said task several thousand times or more. Saving just a few seconds per task ends up being measured in hundreds of dollars.
It is also important to consider how the end-product will be sold. Will it be sold as a complete plant in a container? If so, it will require different handling methods and perhaps a different substrate than a plant being harvested bit by bit. Let’s take a closer look at the different growing media options available for commercial growers.
Peat moss is the partially decayed remains of sphagnum, a type of moss that accumulates in cold-weather swamps in the northern latitudes of North America, where it is fairly cheap and plentiful. Peat is acidic and soggy in its natural state, and once it is dried, it tends to repel water. To make peat moss work as a growing media, wetting agents are often added, and lime is often added to decrease acidity.
Additives such as perlite are also necessary to increase aeration. Commercially, peat moss is ideal for potted ornamental annuals or vegetable field transplants. It is light, even in humid conditions, which makes it easily transportable. As a growing media, peat moss is primarily used for short-term crops because it compacts readily, and once the wetting agents are leached away, it becomes tricky to irrigate and fertilize.
Wood products are common and inexpensive in places where commercial logging is a big industry. Their suitability as a growing media depends on a vast number of factors, including the species of trees used, the parts of the tree included in the mix, the size of the particles and how long the material was left to compost.
All these factors result in a wide range of growing medium qualities that can either be incredible to grow all kinds of plants in, or produce a toxic substrate. Commercially, non-composted wood products are mostly used as landscaping mulch, and sawdust is generally avoided altogether.
Partially composted bark is used in a variety of sizes for potted perennial ornamentals like shrubs or trees, as it can remain stable without decaying further or compacting for a couple of years. It is also lightweight.
Coco coir is the external, fleshy husk of the coconut. There are two different products that come from the husk: the pith and the fibers. Generally, the more fiber the product has, the more aerated the product will be. Pith (a.k.a. coco peat) has almost no fiber in it and tends to retain a lot of water. The big challenge with coco is the presence of high salt concentrations. Coconut trees are mostly found near the ocean, which tends to concentrate sodium in the pith.
Unless the manufacturer of the substrate has repeatedly washed the material, it will contain high salt levels that are unsuitable for many crops. Most of the coco that ends up in grow media tends to be mostly pith, which requires many of the same additives peat moss does to work well as a medium.
One advantage of pith is that it is highly compressible, which makes it extremely economical to ship. It also re-wets readily, which makes it helpful in mixes that tend to dry out quickly. Chunky coco, or coco chips, are the chopped pieces of husks that contain all the fibers and pith. Coco chips are a well-aerated media that is popular in hydroponic vegetable production and high-end ornamental production.
Gravel and Sand
Most natural soils are composed of more than 80% minerals rather than organic materials. The earliest hydroponic trials were performed in sand and gravel, which both work fine as growing media. Most mineral substrates are actually variations of these. To make gravel and sand work as intended, choose rocks or sands that do not interact with nutrient solutions.
Since many rocks are alkaline, they raise the pH excessively, which may result in nutrient deficiencies. Another problem with using gravel and sand is that they are especially heavy, which complicates the shipping and handling process, especially considering the vast quantities required in commercial operations. Commercial growers favor lighter substitutes—cooked rocks—over natural rock substances.
Perlite, Expanded Glass, Vermiculite & Clay Pellets
These grow mediums often get grouped together because of how they are manufactured (cooked), despite having vastly different roles in commercial gardens. Although popular among hydro gardeners, these are predominantly used as amendments or additives to peat moss in potted plants, or in the case of clay pellets, as heavy landscaping mulch (mostly in Europe).
Perlite and expanded glass are good at increasing the drainage of peat mixes without increasing nutrient retention, while vermiculite tends to have the opposite effect. In commercial hydroponic vegetable production, only perlite is still used in isolated facilities.
The limiting factor of these options, as with all the media discussed so far, is that they are bulk products that must be contained in a pot. Over time, they settle and compact, which changes their behavior over the course of the season.
Stonewool is made with volcanic basalt that is melted and spun in a process similar to the way cotton candy is made. It is popular with commercial hydroponic growers because it is clean, does not interact with the nutrient solution, does not compact and retains its ability to dry and re-wet throughout the season.
Stonewool is consistent from bag to bag, and is ideal in recirculating hydro systems because it does not leach anything that was not already in the nutrient solution. Stonewool comes self-contained in pre-formed shapes that facilitate easy handling and eliminate the need for machines to handle bulk materials. Also, because of its smaller volume, spent stonewool is easier to dispose of. Stonewool also comes in bulk form, but the bulk form is rarely used commercially because it requires potting.
Another way to grow plants is by using no medium at all. Commercially, such options have proven impractical for anything beyond the size of a transplant or a head of lettuce, but this method of growing is especially successful in those applications.
Many growers might argue that most of these techniques, such as nutrient film technique, deep water culture and aeroponics, still use substrates, if only in trace amounts, to hold the plant into the system or to germinate or root the plant during the first few days of its life.
Handling horticultural substrates at the commercial level either requires immense machinery or a lot of people. Factors like dry weight, wet weight, ease of unpacking, dust, wettability and cost of transport are all factors that need to be considered when choosing one growing media over another. Performance of the substrate still matters, though, as the goal should always be growing the most crops with the least amount of resources.