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Growing with Phipps: Mixture must be in tune with plant
Saturday, May 19, 2001 By Curt Pesanka, Phipps Conservatory and Botanical Gardens
Choosing correct potting mixtures is very important in gardening.
Notice I use the term "potting mixture" and not "potting soil." Potting soil is a mixture of weathered stone -- ranging in size from a small rock to the tiniest specks of clay -- combined with decaying organic material from plants and animals. Potting mixtures, on the other hand, are composed of little or no soil; instead, it is a mixture of lightweight organic and inorganic materials.
Previous articles in a series
All-America Selections For 2001
This is one of a series of periodic columns by staff members of Phipps Conservatory and Botanical Gardens. Curt Pesanka of Munhall is the foreman of the horticulture department.
Peat moss, the decomposed remains of spagnum moss, is the most widely used component in potting mixtures. Peat moss resists the leaching of fertilizers, has good water-holding capabilities and provides good aeration. It cannot be stated more strongly that good drainage or aeration is the most important feature of potting mixes.
Perlite, expanded volcanic rock, is a lightweight amendment that will not decompose, provides excellent aeration and also absorbs some moisture. Perlite contains soluble chemicals that may injure sensitive plants, so leach these plants heavily with water a few days before using perlite.
Vermiculite, expanded mica or clay material, is another amendment that provides excellent aeration and absorbs moisture. But unlike perlite, it decomposes after a period of time.
Sand adds excellent aeration to a potting mix if the particle size is correct. However, be careful what kind of sand you use; sand made of particles that are too fine will add excessive weight and actually plug pore spaces. Examples of this sand are builders' sand and play sand.
Compost, tree bark or wood shavings all have benefits for potting mixes, but they do have notable differences. For example, the three materials decompose at different rates. After decomposing, these materials must be re-added to the soil mixture; otherwise, the mixture will begin to compact. In addition, any vegetative material must be aged properly before using in a soil mixture. Inadequately aged vegetative material will compete with the plant for available nutrition.
Why are there so many different kinds of amendments? Specific plants have specific needs! Orchids use a higher percentage of tree bark. Desert plants use a higher percentage of sand and perlite. African violets use a special mixture of coarse peat moss and perlite. To use soil mixtures in your garden, research your plant needs, match it up with the correct potting mix and begin what will surely be a "sunsational" growing season.
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