Issue 1, April 24, 2017

Is It Really Soil?

We all know "soil" is the medium for growing trees, vegetables and lawns, not to mention all the corn and soybeans around the state.  But the definition of "soil" gets stretched when it comes to growing plants in pots.

In Soils 101 we learn soil is composed of three particle sizes:  clay, silt and sand.  An ideal soil will have all three particle sizes, which helps with water draining (the larger sand particles) as well as retention (the small clay particles), and allows root growth to take place.  On the other hand, an ideal soil doesn't have them in equal parts.  What you really are looking for is 60-70% silt, and equal parts of sand and clay for the rest.  This gives you a loamy soil.

Organic matter, decomposed plant and animal matter, is the fourth component.  Typically, a good black soil is 3-5% organic matter.  Remember, organic matter continually breaks down, and unlike the soil particles, changes yearly.  Prairie grasses produce more organic matter, through their deeper root system and continual dieback each year, not to mention all those prairie fires centuries ago.  Contrary to popular opinion, forest soils typically have a shallow organic layer as the roots don't go down as deep nor do the leaves produce as much biomass as prairie grasses.

It should be noted that certain pesticides have a tendency to bind with organic matter and clay particles.  In those cases, the label usually indicate a higher application rate for control.

The broader definition of "soil" now includes any growing medium for a plant.  In some cases, there may be no actual soil (sand, silt, clay) in the mix.  These are technically called "soilless mixes" but many still refer to them as soil.  High concentration of organic products such as bark, coconut fibers (sometimes called coconut coir), milled sphagnum peat moss, and/or plain peat may actually substitute for soil particles.  Finely milled compost also can be sold as a soilless mix.  To these mixes, vermiculite and perlite is often added, as well as coarse sand.

These specialized loose growing media facilitate root growth, which can lead to faster top growth.  The downside is they can dry out quicker and don't retain as high a nutrient level, meaning you'll have to water and fertilize more.  Peat mosses can hold water, but often are difficult to wet.  In containers, particularly on hot summer days, you may have to water daily to keep plants from wilting.  You also may need to fertilize weekly with a one-quarter-strength water-soluble fertilizer or monthly at normal strength as the frequent watering can leach nutrients from the mix.  On the other hand, unless there is poor drainage in the container itself, it's hard to overwater, though not impossible.

Adding these soilless organic mixes to a garden soil, or just plain compost and other decomposed organic products, to improve drainage will help, although not permanently.  Still, it's the preferred method.  Contrary to popular belief, sand will not facilitate drainage and compaction unless it's added at the rate of 8:1, which means for every inch of clay, you need to work in 8 inches of sand, or for every foot of clay, you need to add 8 feet of sand.  Otherwise, you'll end up with a concrete-like substance, which will be worse than the clay itself. (David Robson)

David Robson

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