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The Dirt on Potting Soil


Now that the planting season has arrived, many of us will be starting patio pots, filling hanging baskets or starting seeds indoors. The first acquisition will be some potting soil.


All potting mixes are not created equal, however. Some produce strong, well-rooted plants while others seem to encourage plants to languish. What goes into creating a great potting mix, and where can a gardener find a soil blend that meets his or her needs?


Most potting mixes begin with two main ingredients: peat moss for body, and either perlite or vermiculite for drainage.Peat moss is a natural ingredient that is derived from partially decomposed plant material harvested from peat bogs. It retains water well, but is lightweight enough to stay friable, allowing tender roots to grow easily and thrive.The use of peat stirs controversy, as there is some question as to its sustainability.For this reason, coir may be added to the peat or may replace it entirely.Coir is composed of the ground fibers from the outside husk of coconuts, so it is highly sustainable. It is also lightweight, resists compaction and retains water.Some studies have shown that seeds planted in coir germinate more quickly than those planted in peat-based mixes, and coir is also pH neutral whereas peat is acidic. Coir may also reduce the incidences of soil-borne disease such as pythium in greenhouse applications.Both coir-based and peat-based potting mixes retail for between $14.00 for a bag to $45.00 for a 3.2 cubic foot bale. Compared to lower-end mixes, this may seem extravagant but the results of using a good quality potting mix make the purchase worth the additional cost.These mixes expand after hydration, and one 3.2 cubic foot bale of a premium peat-based mix handily fills at least a half-dozen 12Ē patio pots.

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Lower-end potting mixes generally are composed of soil or compost mixed with shredded bark.The bark allows water to drain while the soil provides substance for the plants.However, these mixes tend to compact and will solidify into a rock-hard mass if they are allowed to dry out. Further, as the tree bark decays it robs the soil and plants of nitrogen.This will result in plants that yellow or whose growth is stunted.†† Even with the addition of fertilizers to the potting mix, these blends generally are not the best answer for patio pots and are definitely not appropriate for seed starting.


One more feature of many potting mixes lately has been the addition of mycorrhizae. Mycorrhizae are probiotic fungi, and provide a symbiotic relationship with plantsí roots that resembles that provided to the digestive system by beneficial bacteria that are now added to yogurt and milk drinks. Mycorrhizae attach to roots and act as a virtual root system, allowing for greater nutrient and water uptake by the plants.Plants are able to access and use soil nutrients and more importantly, micronutrients, that impact plant growth, disease resistance, flowering and fruit production.Plants are also better able to withstand water stresses; there will be fewer losses during drought periods or in the case of evergreen plants, during harsh winter weather that desiccates needles or leaves. A small amount of mycorrhizae give plants a lot more bang for the buck, and potting mixes containing these microscopic gems are an excellent choice for seed starting as well as transplanted material from flats or pots.


There are some special mixes on the market that accommodate the needs of plants such as orchids, cactus and citrus or acid-loving plants.These may contain materials such as sharp sand (cactus mix), large pieces of tree bark (orchids) or a soil acidifier such as Ironite.These mixtures are difficult for the home gardener to replicate, so itís a good idea to purchase specialty soils for special planting situations.


Some of the best quality peat-based potting mixes on the market include Premier, Berger and Fafard brands. One coir-based blend is sold under the name Wonder Soil. These mixes are available for sale at many garden stores, nurseries, hardware stores or they can be ordered either through the local agricultural co-op or online.





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