A plant’s health is rooted in the right growing media.
A good potting media has to multitask. It has to absorb and exchange nutrients, retain water (but not too much), allow for the exchange of gases by plant roots, be heavy enough to anchor the plant, but not so heavy that shipping costs are prohibitive. On top of that, it needs to be affordable.
It’s a tall order, and there are many different components that can be used in a mix. Your choice will vary based on your location, the crop and the cost. Some materials are more regional in nature, such as peanut shells and types of bark. The choice of media is probably the most important single choice a grower makes, because it so directly affects so many aspects of production.
What Makes a Good Growing Media?
Large operations (more than 100,000 square feet) will often mix their own media from bulk ingredients. Smaller operations will usually find it’s more economical to buy a commercially prepared bagged mix. Either way, it’s important to understand what properties are required in a soilless mix and how the various components provide them, so you can make the right choice.
The spaces between the media particles are even more important than the media itself. The physical components of the mix — the bark, peat, perlite, etc. — should only take up 30 to 50 percent of the total volume, leaving the other 50 to 70 percent as pore space, which is occupied by air and water. Handling of the media, the height of the container (or more specifically, the height of the media within the container), and the amount and frequency of irrigation impact the amount of air and water within it.
The size of particles make the biggest difference in porosity; the larger the particles, the more space between them. Peat moss usually makes up 30 to 60 percent of commercial greenhouse mixes, and if the particles are too fine it will have a significant impact on drainage and aeration. Materials such as pine bark, perlite, rice hulls, or coir are also included in the mix, because they have larger particles and increase porosity. They are also lightweight.
The height of the container matters, too. It may seem odd to think gravity affects a four-inch pot differently than a six- or eight-inch pot, because the height difference is so small. But this effect can be easily visualized. Saturate a sponge and hold it horizontally until the
dripping stops. Then turn it so the longer side is vertical, and gravity will pull more water out.
When a container is watered, some of the water is absorbed by the media, some drains out quickly, and some stays between the particles. The key is to have just enough of this water to be available to the plant, but not too much. If water occupies too much of this pore space for too long, roots can begin to die or become vulnerable to root rot pathogens such as Pythium.
The percentage of pores filled with water after the initial drainage is called the volumetric moisture content (VMC), or water-holding capacity. A good percentage to aim for is between 45 and 65 percent by volume. A good-quality sphagnum peat moss will have a VMC around 60 percent. Materials such as perlite or coarse sand that do not absorb water at all have a VMC of about 25 percent. The VMC and the porosity of your media can be evaluated using a variety of relatively simple testing techniques or by an outside laboratory.
Handle Media With Care
Growing media starts out with its maximum porosity, and it decreases over time as it settles, materials decompose, and as it is handled. Peat-based media should be moistened before filling trays or pots. The moisture actually makes it expand and increases aeration. Peat moss shrinks as it dries, so planting should take place soon afterwards so watering will keep the media moist. Once containers are filled, they should be stacked in such a way that the weight does not compress the media. Compaction from stacking trays can cause a significant loss of aeration in the media.
Use the filled trays or containers quickly, especially if the media contains fertilizer — because the nutrients break down. Once bags are opened or the media is mixed and in piles, cover any unused media to avoid contamination from weeds, pests and pathogens, and to keep it from drying out.
Air, Water and Media…Putting Them All Together. (n.d.). Retrieved April 04, 2017, from http://aggie-horticulture.tamu.edu/ornamental/greenhouse-management/air-water-and-media-putting-them-all-together/
Fisher, P., Huang, J., & Argo, B. (n.d.). Selecting Propagation Media for Rooted Liners. Retrieved April 04, 2017, from http://hort.ifas.ufl.edu/yprc/resources/media/pdfs/Selecting%20propagation%20media.pdf
“PB1618-Growing Media for Greenhouse Production,” The University of Tennessee Agricultural Extension Service, , http://trace.tennessee.edu/utk_agexcomhort/28. Accessed April 2, 2017.