One of those little "quirks" I have is that I do not like people calling soil, dirt. I was taught that soil is a combination of minerals, air, water and organic matter that has a structure capable of supporting plant life. Dirt is what you sweep up off the floor. It has no structure and contains all sorts of unidentified "stuff" that would not help a plant to grow.

I have come to the belief that those people we say have "a green thumb" are really people who understand soil. Most of us concentrate on which plants we want to grow. This is the fun part of gardening which includes searching through beautifully illustrated catalogs and nurseries with fantastic display gardens. It is exciting.

The "green thumbers", however, have learned that they need to devote at least some of their precious time to the less glamorous task of building their soil. They do not mind getting their hands and clothes dirty (yes, once it leaves the garden enmeshed in the knees of your pants, soil becomes dirt). One way or another, they have learned that in order to grow good plants, you have to maintain balance in the soil.

Soil Composition

An ideal soil for landscape plants is generally defined as one that has 25% air spaces, 25% water, 45% minerals and 5% organic matter. Before planting anything in the landscape (except trees), we should try to re-establish this ratio in our soils. After things are planted, we should strive to maintain this balance of factors by not compacting the soil by walking on it or driving equipment across the surface.

Air - Plant roots need oxygen to grow. Without an adequate supply of free oxygen in the soil, roots will die back and the ones that survive will not be able to absorb water and nutrients. Turning the soil during tillage and the tunneling activity of critters such as earthworms helps to keep the soil properly aerated. Soil compaction is an enemy of plants because it causes the loss of air spaces in the soil.

Water - Of course, is one of the basic factors required for plant growth. Too much of a good thing, however, can be as bad as not enough. If the water level goes above 25%, it squeezes out the oxygen. If adequate drainage is allowed by the soil structure, enough water will be left in the root zone so that plants can absorb it when needed.

Minerals - The basic building blocks of standard soils are the inorganic particles that came originally from minerals. Most come from the original rocks that have been broken down into smaller particles over the eons through the action of rain, wind and freezing water. Some minerals such as calcium act as both a nutrient element for plants and also impact the pH of the soil. Other minerals such as quartz are more or less neutral in their impact on plant life.

The minerals in the soil are defined by three types of particles:

  1. Sand - These particles are the largest of the three and range from 0.5 to 2.0 mm in size. Sand particles fit together loosely leaving large pores resulting in rapid drainage of water. Generally, sand particles have a very weak electronic charge and, therefore, do not hold onto nutrients very well.

  2. Silt - Between sand and clay in size at 0.002 to 0.05 mm, silt is an inorganic particle that is gritty in nature. It also has other characteristics between the two other particles. Silt holds onto nutrients better than sand but not as tightly as clay. It drains better than clay but not as fast as sand. Soils with a higher percentage of silt tend to be good soils for plant growth.

  3. Clay - These extremely tiny particles are less than 0.002 mm in size. They are often flat and plate-like so that particles fit very tightly together which accounts for the fact that soils heavy in clay do not have space for water and air. So the water does not move through very quickly and the soils are poorly drained.

A simple method for getting a handle on the amounts of each of these three particles in your native soils is to take a cup of topsoil (usually the top 8 to 12 inches) and place it in a quart jar. Fill it with water and then shake it up until everything in the jar is mixed thoroughly. Let the solution sit quietly for a day or two and then measure the size of each layer as shown in the illustration. Using a little arithmetic, figure out the percentage represented by the depth of each soil particle.

By applying those percentages to a soil triangle, we can come up with a name that will describe the soil type. In the example shown in the illustration, we have a soil with 20% clay, 40% silt and 40% sand. When you follow the connecting lines from each percentage across the triangle, they intersect in the "Loam" portion of the graphic.

Without going into a lot of detail, just be aware that any soil type with the term “loam” in its name is probably pretty good for plant growth. If clay dominates, the soil will have its type of characteristics i.e. poor drainage, high nutrient holding capacity. If sand dominates, the soil will probably be hard to keep moist and will not hold nutrients well.

Organic Matter - Although the smallest fraction of a good soil, organic matter is one of the more important ingredients. Micro-organisms such as fungi and bacteria feed on it and reduce it to the basic components such as nitrogen which can then be used by plants. Earthworms eat organic matter and provide air spaces through their tunneling. The structure of organic matter in the soil also provides air spaces and it has a high cation exchange capacity so it helps hold onto nutrients.

So, avoid walking in those flower beds, aerate those lawns and add organic matter whenever possible to keep your soils in shape. Your plants will thank you.

More on Organic Matter

Note: We have provided some general information and observations on this topic aimed at the home gardener. Before you take any serious action in your landscape, check with your state's land grant university's Cooperative Extension Service for the most current, appropriate, localized recommendations.

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