Don't Treat Your Soil Like Dirt

You've probably heard that the ideal garden soil is a rich loam. But what exactly is this?

A rich, loamy soil contains a range of mineral particle sizes -- from microscopic clays to relatively large grains of sand. These particles are bound together into groupings of various sizes called aggregates. Soil composed of these aggregates has lots of pore spaces of various sizes, and these spaces contain water and air. If all the spaces contain water, the soil is waterlogged; few plants can withstand such saturated soils for long, because plant roots also need air.

2. Soil Shake. Fill a large glass jar about two thirds full of water, then add enough soil to almost fill the jar. Shake the jar vigorously, then let it settle for a few days. The larger particles -- gravel and sand -- will settle first, followed by silt-sized particles, and, finally, microscopic clay particles. In fact, the clay may stay suspended in the water for quite some time. Organic matter will float at or just below the surface of the water. By looking at the layers, you can find the approximate ratio of sand to silt to clay in your soil. "Ideal" loamy garden soil contains 40% sand, 40% silt, and 20% clay.

Very sandy soils have excellent drainage, but can drain so quickly that plants suffer from drought. Clay soils, on the other hand, hold water so well they can easily become waterlogged. Loamy soil provides a happy medium, retaining adequate water but draining well.

If you have a loamy soil, congratulations! But if yours is on the sandy or clayey side, don’t despair. (See the sidebars for some tips on improving these soils.)

Soil acidity is measured using the pH scale, which runs from 0 to 14, with a pH of 7 being neutral. The lower the number, the more acidic; the higher the number, the more alkaline. Blueberries and rhododendrons thrive in soil with a pH in the range of 4.0 to 5.2; garden vegetables generally prefer a pH of 6.5 to 6.8.

In general, soils in regions with high rainfall tend to be acidic; this includes much of the eastern part of the U.S. Soils in arid regions tend to be alkaline. This isn't a hard-and-fast rule, however, and soil pH can vary even within a small yard.

You can raise the pH of acidic soils by adding limestone or wood ashes; you can lower pH by adding soil sulfur, peat, or pine needles. Don’t try to change soil pH quickly by adding lots and lots of these materials; this can disrupt soil life. It can take several years of modest additions to alter pH significantly.

Organic Matter
People talk a lot about organic matter, but what exactly does the term mean? Technically, the term organic describes substances that contain carbon. For gardeners, it’s best to think of organic matter as any material that was once living, has died, and has decomposed to one degree or another. Organic matter familiar and useful to gardeners includes horse, cow, and other animal manures, straw and hay, grass clippings, vegetable wastes, bark mulch, wood chips, sawdust, and garden debris.

Amazingly, adding organic matter improves both heavy clay and light, sandy soils. In heavy soils organic matter improves drainage; in dry, sandy soils it increases the water-holding capacity.

So should you add these materials whenever you want?  Surprisingly, the answer is no. Always compost materials before adding them to the garden. Otherwise, if you add fresh organic matter to the soil, the decomposition process can temporarily tie up soil nutrients, making them unavailable to plants. We'll go into some details about composting later in the course.

That’s all for this class. In our next class we’ll "put pencil to paper" and plan a sample garden. See you then!