Stu Campbell - Improving Your Soil

Stu Campbell

The key to understanding your garden soil is to understand where it came from, what it is, and what it is becoming.

We can understand a lot about how soil is formed if we remember just a little school science. Large rocks are broken down by weathering. Over centuries wind, water, and ice gradually reduce large boulders to small particles. As Richard Langer puts it in Grow It!, A cupful of water shattered a house-size boulder here, a thimbleful of water a six-foot rock there, not a million pieces at once, but a million pieces in a million years. Glaciers ground the boulders into earth, pulverizing pebbles between them as the millstone does wheat. From this stone flour came soil.

While weathering is making physical changes in rock masses, a series of organisms go to work. Minuscule single-cell bacteria can feed on the minerals in rock particles. They extract nutrients from the rock particles and nearly barren dust, which in turn allow other bacteria to survive. Simple mosses and lichens can also eke out a living on bare rock. When they die, they leave behind organic material to mix with the inorganic minerals, and to feed microorganisms. Soil is being formed, very slowly, and in teaspoon-size quantities at first.

Once this whole biomechanical process is set in motion, things begin to happen faster, and soil as well as all the life that can now spring from it becomes more abundant. Next, plant roots go to work on the rock pile. A hungry root in search of moisture and nutrients can split a rock if it has to. This, plus weathering, plus the action of chemicals formed as vegetable matter decays, helps to further increase the organic matter content of whats gradually becoming soil.

It can take 1,000 years to build a single inch of topsoil through the natural biomechanical process. But on some sites, an inch of cultivated topsoil can be removed by a single heavy thunderstorm. Humans have a long history of causing soil erosion. Plato and Aristotle give accounts of serious soil erosion in the fourth century B.C. Human activity stripped the once fertile soils (along with the trees) from the slopes of ancient Greece to create the stark landscape seen today. Chuck Pendergast, an organic gardener and writer, claims that in the past 200 years more than 60 percent of the topsoil in the United States has been totally lost or destroyed because of ignorance and poor farming methods. Though progress has been made in the past 20 years, loss of topsoil continues to be a serious problem in the United States, on both agricultural and nonagricultural lands. It seems absurdly simple to say that we should preserve this all-important, life-giving resource better than we do, and that what soil we have left should be treated with reverence.

Soil particles. About 95 percent of soil consists of minerals. Most of this mineral material has been weathered into particles smaller than pebbles and gravel; these are classified by decreasing size as sand, silt, or clay. You can visualize the relative size of soil particles by thinking of a grain of coarse sand as a large beach ball, a particle of silt as a marble, and a particle of clay as a pinpoint. Sand includes particles with a diameter smaller than 2 mm but larger than 0.05 mm; it feels gritty between your fingers. Silt ranges from 0.05 mm down to 0.002 mm, too small to see with the naked eye. Pure silt feels smooth and fine like talcum powder or white flour. Clay includes anything smaller than 0.002 mm. Thats about one as large as the average grain of coarse sand, so fine it can be seen only with the aid of a powerful microscope. Pure clay feels very smooth, and if wet it feels sticky and clings to your fingers. Soils contain a mix of particle sizes; the relative proportions of each determine texture (whether a soil is silt loam, sandy clay loam, silty clay, etc.).

Organic matter and humus. Only 3 to 5 percent of soils total weight is organic matter, formerly living creatures (primarily plants) in various stages of decomposition. But you should never underestimate the importance of this once living material. Its something that must be renewed constantly if the soil is to stay in good condition. Its an important source of phosphorus, nitrogen, and sulfur (major nutrients that plants require), plus the main source of food (energy) for microorganisms.

Humus is a term that gardeners use all the time, but that few really understand. Its more than just rotting organic matter. True humus is well aged, fine and dark, and decomposed to the point where the original material cant be recognized any longer. Yet its much more resistant to breaking down than is organic material in earlier stages of decay; some humus lasts many, many years in the soil. Its a bit like Jell-O in texture; the humus that surrounds soil particles helps hold them together into nice, porous crumbs.

The Soils Profile

Soil exists in more or less horizontal layers; you can see this layercake effect if you dig down about 3 feet. The thickness and nature of the layers will vary in different soils because of differing interactions among the sites original rocks, climate, slope or shape of the land, animals, plants, and microorganisms, which together shape a particular soil. A typical soil profile, or cross section showing all the layers, is illustrated below.

Air and water. When were standing on the ground it feels pretty solid, but only half of soil is solid particles. The other half is a combination of space (air) and water. When everything is just right, there is plenty of room for air to occupy the spaces between soil particles. These spaces are known as soil pores. They allow gases such as oxygen and carbon dioxide to move in and out of soil, so that plant roots wont asphyxiate.

When too much water saturates soil, it fills all available air spaces and plant roots cant get any oxygen. Ideally, about half of the empty space in soil is filled with water. If present in the right amount, water should encase each soil particle in a thin film of moisture. An alphabet soup of chemicals is dissolved in this soil water, including plant nutrients; as a result, its often called the soil solution. The only way these vital minerals can move through soil and be absorbed by plants is when theyre dissolved in water.

Most plants need an astonishing amount of moisture. Water is what keeps many plants upright, by filling cells until theyre turgid, or stiff. Thats why some plants wilt and flop over when deprived of water. Plant roots will grow remarkably deep in search of water and nutrients. Alfalfa roots have been known to bore as deep as 30 feet, and if the root system of a single wild oat plant were put end to end and measured, the whole thing might total more than 54 miles not even counting the tiny root hairs.

Water and Air in the Soil

Everybody has had some experience with soil, even if it was just wiping sticky clay off boots or pouring sand out of sneakers after a walk on the beach. Chances are you know more about different soils than you think.

There are six things a plant needs to grow successfully: