Every gardener is curious about the soil in which a garden grows. As you sift it through your fingers, remember that the parent material of every particle of soil is rock. The soil that we so carefully alter, amend, save, and endlessly fuss over begins its existence as solid rock.
If you look around, you can probably come upon remnants of the rock that your soil came from. The granite coast of Maine attests to long-ago volcanic activity. Manhattan schist still rears artistically above verdant Central Park. Ice Age rocks still rise to the surface in garden soils across northern reaches of the continent with ongoing soil-making activity while the chocolate-cake soil of Iowa is a young soil, having been laid down only 10,000 years ago. Violent upheavals resulted in the now-ancient Appalachians as well as the relatively youthful Rocky Mountains. They continue to expose their crags to the weathering forces of the wind, water, and the freeze-and-thaw cycle.
Volcanic activity produced the different kinds of lava that long ago flowed out upon the land. Seismic activity broke up the cooled lava in time for two Ice Ages to grind it slowly to a mixture of cobble, boulders, and sand called glacial till. Roaring torrents still carve the terrain to create gullies that become canyons, and as the speed of the water diminishes, these sediments harden as sandstone or limestone. Not with blaring trumpets, but with steady, ongoing weathering forces, are born sand, silt, and clay soils.
Not with blaring trumpets, but with steady, ongoing weathering forces, are born sand, silt, and clay soils.
Soil contains both parent material and organic matter. When you dig through your soil, you may see strands of a root, a shred of rotten leaf, or a miniscule clump of something unidentifiable. These particles mean that organic matter is there (despite the typical gardener’s continual lament that there’s never enough).
Organic material is important in soil, but it is the continued chemical and physical activity on the soil’s parent material that releases the nutrients necessary for plant growth. The organic matter doesn’t feed the plant, but it combines with minerals in the soil to feed the plant. By itself, organic matter, including compost, contains almost no plant nutrients. Its main function is to free up the minerals to make them available to the plant. Your plants don’t care whether the organic matter in your soil was laid down in layers millions of years ago or whether it came from your backyard compost pile. They simply enjoy the three most important assets of organic matter—water-holding capacity, aeration, and insulation from the vagaries of weather. Everyone wishes for more of it, unless, of course, you are the owner of Florida Everglades muck.
If you add soil to your compost to augment the decomposition process, you are also adding soil minerals. Many gardeners assume that because green stuff goes into the compost pile and comes out brown stuff, it must be rich in nitrogen. Regretfully, no. Nitrogen is a gas in the air, but it is ultimately the source of soil nitrogen. It must change by means of chemical and biological reactions to a more stable form in order to become available to a plant.
Also needed for plant health are phosphorous and potassium. They are soil minerals that occur in varying amounts depending on geography. For example, in some areas of the East, potassium is often lacking in the soil because it is washed away by heavy rains. In the West, potassium is abundant. Phosphorous is also short in the soils of the East; in the West, the abundant phosphorous is in a form that is not available to the plant. Only the addition of organic matter and tilling it in at root depth will free it up for plant use.
Examining your soil
When we look at particles of soil, the first thing we notice is the size of the particles. Sand is a big thing, silt is a medium thing, and clay is a wee thing but mighty in its power to carry plant nutrients. The much-maligned clay particle carries the weight of the world’s crop capacity and the high quality of our nation’s nutrition. Silt is also a highly desired particle, laid down as loess (pronounced LO-is) by wind eons ago. Along the midlands of America, it is said that the depth of loess along the Mississippi River has been measured at 40 feet.
To get a rough idea of your soil’s classification, moisten a handful and squeeze it. If the handful crumbles easily, it is a sandy soil. If the handful sticks together, try forming it into a ribbon with thumb and forefinger. If it holds together but has sharp tooth-like jags along the edges, it is a silty soil. If it makes a solid, strong ribbon, it is formidable clay.
Sprucing up your soil
If you are one of the legion of gardeners who never knows despair and vows to fix whatever ails your soil, call your local Cooperative Extension Office to learn how to have your soil tested in the laboratory. (Additional resources of soil information and testing are listed on page 42.) A soil test will give you the percentage of each of the macronutrients (nitrogen, phosphorous, and potassium) as well as some of the micronutrients necessary for good plant growth, such as iron, magnesium, calcium, copper, sulfur, manganese, and zinc. The report may also tell you how much of each deficient element to add to your soil. See a sample soil test above.
The report will also give you the pH of your soil. A pH above 7.0 is alkaline; below is acid. The Lime Line of the United States follows the course of the Mississippi River at the 100th meridian. Soils east of this line are usually acid. Those to the west are alkaline, except for the Northwest coast of Washington and Oregon. The reason for the acid goes back to the weathering forces of rain, snow, and wind. Areas with higher amounts of precipitation result in acid soil.
Forty inches of precipitation annually is average for the eastern portion of the nation; in the Midwest and West, the rainfall amount ranges from moderate to almost nothing in the deserts. This disparity shows the importance of organic matter in soil building. The deciduous hardwood forests dominant in the East lay down layer upon layer of leaves and other debris every year. Soaking rains and melting winter snow quickly decompose the debris to rich leaf mold that further drives the pH into the acid range.
In the West, however, forested land exists at widely spaced intervals and is largely evergreen. The soils of the West, only lately broken down from rock, are abundant in minerals and lack only organic matter and water to be productive. The soils of the East may experience drought intermittently, but they have been leached by heavy precipitation of many of the minerals necessary for good plant growth, and the organic matter, once abundant, is depleted gradually. Both Eastern and Western gardeners indulge in wishful daydreams of the perfect soil.
The truth about topsoil
Topsoil is a much-loved word among gardeners. Actually, all it means is that any soil on top is better than what’s underneath, which is true. If you are an average gardener, you leave some of the fallen organic matter during your fall or spring cleanup. If you are a model gardener, you also add compost to your soil each year. The fungi, bacteria, worms, and other minute organisms in the compost and debris continue to work in your soil to add more organic matter each season, gradually building up the topsoil.
But what of the prospective new home builder? Before you sign a contract with a builder or developer to have a home built, think about preservation of the topsoil. Often the builder will maintain that there is no topsoil—that it’s all the same ten feet down. Wrong. Again, what’s on top is better than what’s underneath. A developer in a subdivision has upfront costs that can be defrayed by selling the topsoil as each home is built. In most states there is no law against this practice. But stand your ground! You will eventually win. You are buying the lot and that means you will own the soil.
Make sure that the developer or builder places the topsoil in an out-of-the-way corner, covers the pile, and surrounds it with orange plastic construction-zone fencing. Be on hand when the topsoil is spread around the property to make sure the grade is to your specifications. Sometimes the topsoil is stripped off and sold before work on a subdivision is even begun. If the topsoil was sold before you bought the lot and you are left with only subsoil, all is not lost. There’s one small consolation to dealing with subsoil—it seldom contains weed seeds!
Topsoil can be purchased, but be careful that what you buy is actually topsoil and not subsoil amended with coal dust and undigested sewage sludge to make it look rich. The soil test will not say “sewage sludge” or “coal dust,” but the test’s results will indicate poor fertility. If the cost of topsoil is out of the question, you can amend your subsoil with sphagnum peat moss, coir, weathered manure, or sawdust fortified with nitrogen, in addition to the mineral nutrients specified by a soil test.
Barbara Hyde holds a bachelor’s degree and a master’s degree in horticulture from Colorado State University and has served as the Horticulture Extension Agent in Boulder County, Colorado, for seventeen years. She has written Gardening in the Mountain West (1999) and Progress of a Gardener (1993). You can order one of her books by contacting her at 2431 W. Peakview Ct., Littleton, CO 80120 or by calling (303) 794-7088.
902 13th St. North
Benson, MN 56215
A&L Western Agricultural Labs
10220 S.W. Nimbus Ave.
Portland, OR 97223
PO Box 50084
St. Louis, MO 63105
Mowers Soil Testing Plus, Inc.
117 E. Main St.
Toulon, IL 61483-0518
Soil and Plant Laboratory Inc.
1594 N. Main St.
Orange, CA 92667
Texas Plant and Soil Lab, Inc.
5115 W. Monte Cristo
Edinburg, TX 78539