A variety of commercially prepared potting mixes are available for today’s commercial growers. Selecting the formulation that performs best for your particular situation can be a difficult task, unless you know the specific needs of the crop, your cultural practices and the characteristics of the growing medium. To get the best results for your crop, the key is to work closely with your supplier.
The “stuff” we use to fill containers is called a medium. When containing proper amounts of water and air, it provides mechanical support and sustenance for cultivation of plants. It functions as a water reservoir, nutrient holding and exchange system, a framework for gaseous exchange and anchorage for root systems and plant support. Medium is singular for one type of mixture, whereas several blends are referred to as media. The majority of what is used in greenhouses and nurseries today is made from organic-base materials, such as sphagnum peat moss, bark and/or coir that are blended with aggregates, liming materials and fertilizers. Whether you buy it pre-formulated or blend your own, peat- and peat/bark-based media is the mainstay for the industry.
There is a wide variety of components and various combinations that can be used to formulate growing media. Popular aggregates, such as perlite, vermiculite and calcined clay, are sometimes incorporated separately or in combination to achieve desired water-holding, aeration and drainage properties. Calcitic and dolomitic limestone are used for buffering, adjusting pH and as a minor calcium source. Calcitic is used for quick pH adjustment compared to dolomitic limestone, which has a longer staying power for pH management. Dolomitic limestone also provides a small amount of magnesium as well as calcium. Gypsum is sometimes used as a calcium and sulfur source, but has no effect on pH. Chemical ingredients, such as fertilizers, wetting agents and/or polymers, are used for nutrition, ease of wetting and water retention, respectfully. Combining different ingredients results in various physical and chemical properties.
Media selection is best determined by the plant species grown, stage of plant development, container size and cultural requirements. In comparison, peat-based media is lightweight, has higher water retention characteristics and is composed of smaller particle fractions. These characteristics are ideal for small containers, propagation of cuttings, seed starting and general greenhouse potting applications. Bark-based media tends to have higher bulk density, lower water retention and larger particle fractions. These characteristics are desirable for improved container stability, large containers and growing situations that require low water retention. Outdoor nurseries typically use bark-based media, since rainfall is variable, and overhead irrigation systems are often used to deliver large volumes of water over large nursery areas.
All media manufacturers blend a variety of formulations for different applications. How do manufacturers know what you need? They should know the crop requirements and match them to the characteristics of the growing medium. If the growing medium is a commercial blend, ask the manufacturer for technical information and speak with the sales representatives. Many companies also employ technical representatives who can assist with specific crop questions. The manufacturer should be able to give you this information readily. If not, consider a different supplier. If you currently blend your own and are thinking of switching to a pre-formulated medium, use a reputable laboratory that is familiar with horticulture testing. Send several samples of your growing medium for analysis of its chemical and physical characteristics. Once you have the lab results, share these with the media manufacturer and discuss with your sales representative and define your requirements. Together you can determine the right product to evaluate.
Considering the number of manufacturers and the quantity of standard formulations on the market, there is most likely a pre-formulated medium that will meet your growing needs. However, occasionally there is a situation when a specially formulated media may be needed for a unique growing situation. Usually this is due to a water-quality issue (low alkalinity, high alkalinity, high salts, etc.) or a specific crop requirement (nutrient modification, long-term fertilizer, etc.). In addition to lab analysis, give your potential supplier several cubic feet of your growing medium to examine. Actual samples help the manufacturer better understand your requirements, such as size of aggregates, quality of peat/bark used and other visual aspects. The manufacturer should be able to supply you with a sizeable sample for you to evaluate.
Would you buy a car without taking it for a test drive? The same applies to growing media. Allocate a space in your greenhouse/nursery to sample and evaluate any growing medium before you use it. In some cases, the potential supplier can help assist in a growing evaluation by giving you tools, such as ideas for layout, identification labels and tracking forms.
Use the medium as you normally would by filling containers, planting, watering and growing plants in the greenhouse. The evaluation space in the growing area should represent the “where, how and what” of the crops to be grown. Use your current growing medium alongside as a reference. Remember that watering and fertilizer requirements for the evaluated medium may vary slightly from your current medium. Use separate irrigation zones for each medium so water requirements can be closely monitored. Be careful not to favor one medium over the other during the growing evaluation. At the crop term, be objective in your evaluation. When you do the evaluation, don’t forget to inform your sales representative. After all, they are interested in the results also.
Once you have established which medium meets your parameters, you need some assurance that you will receive what you requested. Before you commit to purchase, you should have these items on your checklist:
When you receive a delivery, check the product as it arrives. Look at the package, and open it to make sure you are getting the product you ordered. As with most deliveries, if something doesn’t look right, contact the manufacturer while the truck is at your location. After delivery, it is a good idea to record the manufacturing number on the package. You can also go one step further by testing some of the product. If you don’t have one already, an EC and pH meter is a tool well worth the investment. You can use it to test media prior to use, during the crop cycle and to monitor water/fertilizer solution. If you have a question contact your media manufacturer for assistance.
Most growing media contains short-term fertilizers that are incorporated to help plants acclimate after initial planting. During the crop cycle media pH, EC and nutrient content will change with fertilizer application, irrigation water and nutrient uptake by plants. Horticulture laboratories can test growing media as well as plant tissue, irrigation water and nutrient solutions. Periodic testing is an invaluable tool to monitor crop requirements and establish growing trends. With lab analyses, growers can make small modifications, if necessary, during the crop cycle to fine-tune cultural practices and achieve the best performance from growing media.
Working with your media supplier to make sure you’re getting what you asked for.