Good Soil Means An Excellent Garden! January 3, 2016

excellent-gardenThomas Jefferson wrote to portraitist Charles Willson Peale in 1811, “though an old man, I am but a young gardener.” Rarely has a truer line been writ, for though I’ve been gardening since the age of five–now a respectable three decades–I am always learning new things. This is particularly true when it comes to soil. For years, I admit, I never paid much attention to the stuff, even when I dealt with my favorite tree service in San Diego, CA.¬† It was, well, just dirt. If you planted something, tossed in a little fertilizer, and kept it watered, it generally grew, right? What I had failed to notice was how well the plant in question actually grew.

It wasn’t until I started my garden design practice in 1986 and had a chance to see how the same type of plant performed in different locations that I became aware of just how essential good soil is. I began to notice that, given similar light and watering conditions, in just a few years a tree or shrub planted in one spot was sometimes twice the height as the same type set in a comparable location–a size discrepancy largely due to differences in soil quality. I began to pay closer attention to dirt.

One of the most important things to understand about your soil is its structure. Before you start building up and amending its quality, you need to know what type of soil you have–sandy, clayey, or in between. One of the easiest ways to determine this is to take a handful and let it run through your fingers. What does it feel like? Does it crumble easily into small, gritty grains? Or is it smooth, like talcum powder, and fairly sticky? This simple test is important, because it gives you a good idea of how your soil is composed. Soil consists principally of two things: a small quantity of organic matter (about three to five percent) and a mixture of eroded rocks and other minerals. The nonorganic component can be further divided, according to size, into distinct elements the relative quantities of which can vary considerably from place to place and have a direct impact on how things grow in your garden. Not only do these mineral components determine the ability of water to drain through the soil but also, equally important, they affect the ability of soil to retain nutrients.

The largest components of soil are the rocks and stones we often dig up in the garden; although a nuisance, they actually constitute the smallest percentage of overall soil. Next in size is sand, whose individual granules range from .01 to .02 inch in diameter. The number of sand particles in your soil is important because the gaps between sand particles allow for water and nutrients to pass easily. The next-smallest particle is silt, even harder to measure, which retains more water and nutrients than does sand (although it is more difficult to wet initially). Finally, there is clay–the smallest of the components–whose particles can’t be seen without the aid of an electron microscope. Clay particles are so tiny and tightly bound that it takes a long time for water, even on the molecular level, to pass between them; once they’ve worked their way inside, it is equally difficult for the water molecules to detach themselves.

Look again at your handful of soil. Squeeze it into a ball and then open your fist. If the ball immediately falls apart, your soil is sandy. If it stays together until tapped, and then falls apart, you are the lucky possessor of “loam.” If the ball of soil stays together when tapped, then your soil has a large clay component. In general, sandy soils are not good for gardening because they dry our quickly and lose nutrients almost immediately. Soils with a heavy clay component are not good for just the opposite reason: They are hard to wet, equally hard to dry, and water and nutrients are so often rightly bonded to the clay at the molecular level that plants are unable to rap them through their roots. It’s generally not practical, or a good idea, to add sand or clay to compensate for any deficiencies (see “Tips for Improving Soil Consistency,” page 74). The best thing you can do to improve your soil is to enrich it with organic matter.

Organic matter is a catchall term used to describe plant and animal residues in various stages of decomposition. It may constitute only a small portion of soil, but its importance cannot be overstated. For the gardener, there’s a better term: “black gold.” Organic matter benefits the garden in two principal ways. First, it contains a considerable quantity of essential nutrients that are gradually released into the soil as the material decays, thereby increasing the soil’s overall fertility. More important in the long term is organic matter’s second effect: amelioration of the soil structure. Adding organic matter to clay soils, for example, essentially isolates the day into smaller lumps, allowing water and nutrients to penetrate the soil more freely. In sandy soil, organic matter helps to capture water and keep nutrients from draining immediately through the soil.

Organic matter comes in many forms: humus, manure and other animal wastes, sawdust, and compost. Each  material has a slightly different use and benefit, and a few even have their downsides. Still, their benefits far outweigh the occasional problems they might engender. Your choice should be determined by which one is least expensive and most readily available in your area.

Gardeners often ask me how much manure or compost they should add to their soil. The answer depends on the type of soil. For sandy and loamy soils, add at least two inches of organic matter once a year in either the spring or fall. For clay soils, one inch is better, added in the fall. Using too much organic material at once can add to clay soil’s inability to shed excess moisture. For new beds, till the organic matter in as deeply as you can, preferably to a depth of 10 inches or more. For existing beds, you can top-dress with manure. The action of worms and gravity will eventually transport the organic matter down to the root level. Organic matter also makes an effective mulch and hasthe added benefit of improving the soil while helping to suppress weeds. One final tip about improving your soil’s structure: Try to keep heavy machinery and unnecessary foot traffic our of the garden, as soil compaction is a major problem. Even the best of soils, when heavily compressed, will lose the essential spaces between particles that allow water and nutrients to penetrate. As a result, rain and irrigation water will run off the surface and root growth of all but the most vigorous plants can be stunted, resulting in poor plant performance. This is all in addition to the purely practical fact that it’s almost impossible to dig in compacted soil. If you are lucky enough to have good soil, or are in the process of creating it, you should treat it like treasure. There is nothing more valuable in your garden.

Leave a Reply