Dirt vs. Soil

What is the difference between dirt and soil? If you said "none," you're not wrong. The dictionary definitions are very close synonyms that can be used interchangably. However, there is another sense in which the two are very different.

Organic growers often find it useful to distinguish between the finely divided weathered rock that makes up the mineral content of "dirt" and the more complex growing medium that also includes organic matter and a complex web of micro-organisms that comprise living "soil." The distinction is widely and fairly consistently applied.

This is not to say that dirt isn't important -- it's the basis and foundation of good soil. The point is that dirt is not the whole story. Let's start by looking at dirt. There is no shortage of books on soil science, from those focussing almost entirely on compost to engineering texts interested only in structural capabilities. It's a very important topic. I'm only going to touch upon a couple of very basic points here.

First, dirt is broadly classsified according to particle size. We think of grains of sand as very small, but they are the coarsest particles in the category. Somewhat smaller particles are called silt, which when dry, has a consistency and feel similar to flour. The smallest, microscopic particles are found in clay.

There is no single "ideal" soil: plants have been adapting to an variety of soils for millions of years, and no matter what kind of soil you have something will grow in it. Many culinary and medicinal herbs, for example, thrive in what are generally considered "poor" soils. For the most part, though, plants grow best in a mixture of the three particle sizes called loam.

Take some dry soil and grind it into a loose powder. If you have a graduated cylinder, that would be great, but any tall skinny jar such as the ones olives are sometimes packed in will work. Mix a bit of dirt with several times as much water and shake it up. Then set it down and watch.

Much if not all of the soil organic matter will float to the top. The sand will settle almost immediately. In a half hour or so the silt will drop out of suspension. It may take a couple of days for the clay to sink to the bottom of the jar, leaving relatively clear water on top. Now you have a sort of dirt parfait that allows you to estimate how much of each particle size your soil contains -- very useful information.

Plants like soils that are friable -- that is, soils that will stick together in clumps, but can easily be broken apart. All soils are friable for some range of moisture content. Clay soils easily become gooey plastic masses when wet or hard as rocks when dry. Sand won't stick together unless its very wet. But if the moisture content is just right both of these problem soils exhibit friability.

Organic matter can absorb many times its own weight in water. Thus, it can both keep soils from becoming too wet and from drying out too quickly. As little as 5% organic content can greatly improve the growing properties of any dirt. Organic loam is friable over a wide moisture range, and this promotes good soil structure.

We all know that roots neeed water, but when we think about it, we also know that they need air. We put rocks in the bottom of potted plants to permit excess water to drain out the bottom, because we know that too much water is as much a problem as under-watering. Friable loams maintain good soil structure or "crumb" which means there are spaces in the soil that absorb water, and just as readily draw in air as the rain seeps down into the groundwater.