Greenhouse plants are grown in a wide variety of root media, including those that contain actual soil as well as those that are entirely soil-less. The media can be formulated by the grower, adapting and modifying one of the traditional mixes such as those described in Chapter 14, or the media can be purchased from commercial sources.
Whether soil-containing or soil-less, root media must all do the same things: provide nutrients to the plant, retain water for use by the plant, allow aeration for the roots, and anchor the plant firmly in the container or bench.
The research comparisons between greenhouse crops grown in soil versus those grown in artificial media are extensive. However, the grower’s decision to use one medium rather than another is likely to be based on economic factors more than research findings. Is there a close source of good field soil that can fill ground beds or benches? Is the potted crop going to be shipped over a long distance, making weight a cost and handling factor? Does the grower have the facilities and desire to mix and pasteurize large quantities of soil, or is a ready-mixed product more practical?
When field soils are used in greenhouse crop production, they must be conditioned and pasteurized. Drainage is essential, so sand, peat, vermiculite, fired clay, and even gravel may be added. Even a loam soil, ideal for nursery and landscape use, is not satisfactory for greenhouse use without additives. Pasteurization with steam or chemicals is needed because of the presence of weed seeds, pathogens, and insects in the soil. The specifics of pasteurization are described later in this chapter. Once in the greenhouse bench, the soil may serve for many years if properly reconditioned after each crop is harvested.
While good field soil in a bench offers the cultural predictability that is essential to high-quality crop production, the unpredictability of field soil taken from different sources each time is what led to the development and widespread use of soil-less mixes for potted crops.
The commercially prepared soil-less mixes differ in their formulations but not in their functions. Regardless of brand names, the artificial mixes are either bark-based or peat moss-based mixes. Brand name examples of each formulation include:
Bark-based growing media
Ball Growing Mixes I and II Choice Container Mix Fafard Mix, Number 3 and 4 Metro Mixes 300, 350, and 500 Pro-Mix Peat-Bark Mix Strong-Lite Bark Mix VJ #1 Mix and #3 Mix
Peat-based growing media
Ball Germinating Mix Fafard Peat-Lite Mix Jiffy Mix Jiffy Mix Plus
Ogilvie Professional Mixes 2 and 5 Premier Germinating Mix Pro-Mix A
Redi-earth Peat Lite Mix Sunshine Mixes
The principal advantage of artificial root media is their uniformity of composition and the predictable response of plants to fertilization, watering, and other production techniques when grown in them. By using soil-less media, a grower eliminates one of the variables that can make production difficult. An added advantage is that these media are lightweight, making their handling less burdensome. Some also contain small amounts of starter fertilizers and/or a gel to retain moisture.