The Homeowners Column

The Homeowners Column

Mixing Soils for Containers

Photo of Sandra Mason

Sandra Mason
Extension Educator, Horticulture
slmason@illinois.edu

One of the first things I learned in college was Soils 101 is not Dirt 101. Dirt is the stuff in your vacuum cleaner bag. But not only is dirt not soil, but soil is not always the same soil. Through my own inexperience, I also learned that soil from the garden placed into pots produces concrete; great if you were worried the plant might walk away.

Wonderful well-drained garden soil takes on a whole new personality once it is placed in a pot. The shorter the pot, the less well drained the soil will be. The moral is: potting soils for containerized plants need soil amendments such as perlite, or compost.

Even some commercial potting soil mixes do not provide the proper amount of drainage and water retention. If the mix contains predominantly soil, than add soil amendments. Many recipes are available. The taller the container the more natural drainage it provides.

In general if using:

Heavy clay soil - (By volume) use one part soil with three parts sphagnum peat moss or comparable product with two parts coarse aggregate such as perlite or vermiculite.

Medium textured soil (silt loam or sandy clay loam) - Use two parts soil with three parts sphagnum peat with two parts coarse aggregate.

Garden soil should be pasteurized. Spread moist soil on a cookie tray and bake at 180 degrees F for 30 minutes. Or use the charcoal grill outside if you don't like that "earthy" smell.

Peat Moss or coco fiber - Peat moss is partially decomposed sphagnum moss. When purchasing peat moss, look for sphagnum peat moss. Michigan peat, peat humus and native peat are usually too decomposed. Coco fiber is a byproduct of the coconut industry and much more environmentally sound than peat moss. Coco fiber is sold in bricks, which are easy to transport. Once wet the coco fiber swells to 3 cubic feet. Both are free of weed seeds and diseases. Both hold nutrients and water in the soil. Apply warm water and wait at least on hour to wet products throughly. Pasteurized compost can also be used.

Perlite - Perlite is a sterile white material produced by super heating volcanic rock until it pops. The result is a very lightweight, porous material that improves aeration and drainage. Perlite can cause fluoride burn, which shows as brown tips on plants such as spider plants. Moisten perlite before using to reduce dust.

Vermiculite - Vermiculite is a sterile, lightweight mineral that is super heated to expand its platelike structure. Vermiculite will hold large quantities of air, water, and nutrients needed for plant growth. Vermiculite is available in four particle sizes. Use the large size for better soil aeration. The smaller size may be used for germinating seeds. Do not use the vermiculite sold for house insulation since it may be treated to repel water.

Sand - Sand is the least expensive additive and can add soil aeration when used at the proper volume. If sand is used, it should be the coarse construction grade sand. Silica sand and beach sand has too fine of particle size. Sand adds weight to pots.

Soilless mixes - Or forget all the mixing and use soilless mixes that contain no soil but use organic materials such as peat moss, ground bark, vermiculite or perlite. Be sure to use a fertilizer with trace elements with soilless mixes.

Idea Garden workshop

Saturday April 29 at 10 am, New Plants for 2000.

During the months of May and June the University of Illinois Master Gardener Champaign County office will be open until 8 pm on Mondays and Thursdays. Bring us your plant problems or give us a call at 333-7672.

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