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What is Soil Made of?

By Harold Willis

My, it’s dark down here in the soil. No wonder most people know so little about it. But that’s why we’re here, so let’s learn. Soil is the absolute basis of agriculture, and thus of all human existence, for as we have seen, we either eat plants grown in soil, or animals which eat plants grown in soil. Our soil has been called our most important national resource. Wise use and management of the relatively thin upper layer, the topsoil, is vital for maintaining good health and a high standard of living.

But through misuse, about 7-10 tons of topsoil per acre are being lost to erosion each year in the Midwest (the figure can be much higher in the worst areas). It may take several hundred years for 1 inch of soil to form. Obviously, we can’t keep on sending our topsoil down the river much longer.

Besides that, most once-fertile soil is now polluted by toxic substances, along with the groundwater and our wells. We are literally fouling our own nest, destroying the hand (or land) that feeds us! We had better understand more about this important, but neglected and abused part of the food chain. Let’s take a tour through this dark and mysterious land that lies beneath our feet.

healthy soil
Closeup of a plowed field, fertile, black soil.

What makes up the soil?

What is soil? It is a very complex substance, not “just dirt.” Soil is a mixture of several components, sometimes defined as that part of the earth’s surface capable of supporting plant life. It originally formed by the weathering of the rocks of the earth’s crust. Typical soil contains approximately the following proportions of four constituents:

Components found in soil:

  1. Minerals (about 45%), some of which are insoluble and not used by plants (sand, clay, iron oxides) and others which are soluble and provide valuable plant nutrients (calcium, potassium, magnesium). Mineral particles range in size from coarser gravel and sand to finer silt and the smallest clay particles, which are in the size range called colloidal.
  2. Water (about 25%), which is needed as a part of plant cells and to dissolve and carry nutrients. Too much water in soil can exclude needed air.
  3. Air (about 25%), which provides oxygen to roots and soil microorganisms, and nitrogen to nitrogen-fixing bacteria. Good aeration is vital to fertile soil.
  4. Organic matter (about 1–5%), which includes the living soil organisms and the dead organic matter which decomposes to form humus. Humus has been broken down to very small particles in the colloidal size range. A “good” soil should have 2–5% organic matter, and up to 10% can be beneficial.


In the above four components of soil, two contain colloidal particles, clay and humus. Colloidal particles, which are particles less than 0.002 mm (1/5000 inch) in size, are important in soil because they have a great ability to hold certain plant nutrients. Humus colloids can hold three times the nutrients that clay can. This is one of the reasons humus is so valuable in soil.


A “good” soil should have a loose, almost spongy texture because the tiny soil particles (sand, silt, clay) are clustered into small clumps or “crumbs,” also called aggregates. This condition is called good soil structure, or good tilth. Soil structure affects the ease of water penetration and aeration, root growth, the activity of soil organisms, and the availability of nutrients. Good tilth is generally only found in the upper layers of soil, with the lower, harder layer often being called a “hardpan,” “plowpan,” or “claypan.” Soil of good tilth is easy to plow, soaks up water like a sponge, and resists erosion.

Factors that contribute to good soil structure include freezing and thawing, wetting and drying, penetration by plant roots, animal burrows, soil colloids, and most important, a “glue” secreted by roots and soil microorganisms. This is one of the many reasons soil organisms are important.

In poor soils, too much tillage can destroy tilth, as can leaving the soil without a vegetative cover to cushion raindrops and to encourage microorganism growth. Also, too acid or too alkaline conditions can destroy good tilth.

Soil pH

One soil condition that agricultural “experts” concern themselves about excessively is pH. This is a measure of acidity or alkalinity of any substance using a scale of numbers, from 0 (most acid) to 14 (most alkaline), with 7 being neutral. The pH of some common materials includes: lemon juice = 2, vinegar = 2.5, black coffee = 5, pure water = 7, sodium bicarbonate solution = 8.2, ammonia water =11. Soil pH generally varies from 4 to 10, but most crops do best under slightly acid conditions (6.0–6.8). Soil pH affects the availability of nutrients, which may be connected with nutrient deficiencies and toxicities (for example, manganese can be toxic to plants at pH 4.5 or below). Soil pH also affects the types of soil organisms and their ability to flourish, including those that fix nitrogen. Most bacteria cannot live in very acid conditions, while many fungi can.

The traditional belief is that soil acidity is bad and should be counteracted by applying lime. But some acidity is necessary for plants to absorb certain nutrients from soil colloids. The experts usually base their liming recommendations on a single soil pH test, but testing several times during a year will reveal that pH can change markedly through a growing season, even daily. Also, various fertilizers and soil conditioners can have short-term and long-term effects on pH. So, as long as the pH does not become extremely acid or alkaline, pH “correction” and testing for many soils are not as important as some seem to think. The use of lime to “sweeten” soil is a case of doing the right thing for the wrong reason. Crops benefit more from the calcium supplied than from pH control.

Water in Soil

The movement of water in soils is little known or often misunderstood by most people. The average person assumes that water simply moves downward in soil, but if water is applied to soil at a single point, it defies the law of the gravity and moves just as fast horizontally as vertically; thus it soaks into the soil in a spherical pattern. Of course, rain normally falls all over the surface, not at a single point, so generally a horizontal “front” of water will soak in.

One of the most amazing facts of water penetration is the barrier effect of a soil of different texture, such as a subsoil “hardpan” or simply a change in texture, such as from fine to coarse. When water encounters such a change in texture, its rate of penetration will be considerably slowed, and in reverse, a later rise of water to supply roots will also be slowed by such a barrier.

The role of porous organic matter, such as plant residue, in absorbing water can be important under the right conditions. Organic matter which is thoroughly incorporated into the upper layers of soil or which is partly exposed to the surface acts as a wick to increase water penetration. A compact plowed-under mass of organic matter acts as a barrier.

Organic Matter (Humus)

The importance of organic matter in soil cannot be overemphasized. William A. Albrecht, former head of the Soils Department at the University of Missouri, once called organic matter the “constitution” of the soil. As mentioned previously, soil organic matter consists of the dead decomposed humus and the living soil organisms. First let’s look at humus, which has been called “the most important source of human wealth on this planet” (S. Waksman, Humus, 1938, p. 414).

Humus is a structureless colloidal material resulting from the decomposition (humification) of any type of dead organic matter (mostly plant residues and manures). It is a complex chemical mixture including proteins, lignin (originally part of plant cell walls), fats, carbohydrates, and organic acids. Its great ability to hold nutrients because of its colloidal nature has already been mentioned. The beneficial aspects of humus include:

  1. It provides a storehouse of essential plant nutrients; for example, it stores over 95% of the nitrogen, 60% of the phosphorus, and 98% of the sulfur available to plants.
  2. It helps make some nutrients more soluble and available to plants. Because of increased microorganismic activity during the warmer months, nutrients are released at the time of the plants’ greatest need. Acids in humus also slowly dissolve soil minerals and release nutrients.
  3. It contains substances that stimulate plant growth and improve crop quality and resistance to pests and diseases.
  4. It provides a high water absorption and holding capacity because of its spongy nature.
  5. It contributes to good soil structure (good tilth) by helping make soil crumbly and porous. It also reduces wind and water erosion and makes soil easier to work.
  6. It buffers the soil and protects plants from high salt levels, toxic chemicals, and drastic changes in pH.
  7. It provides food for the beneficial soil organisms, especially in the “pre-humus” form of fresh organic matter.

So we see that humus is a marvelous substance, with many functions, and “worth its weight in gold.” Now let’s look at its equally important partner, the living part of the soil.

Fertile soil is literally teeming with an amazing variety of plant and animal life, most of which are microscopic and thus little known nor appreciated by the average person. They form a valuable “work force,” performing a multitude of chemical transformations, as well as many other services.

Editor’s Note: This is an excerpt from the book Foundations of Natural Farming. You can buy it at the Acres U.S.A. bookstore here.

About the Author

Dr. Harold Willis was born and raised in Kansas, eventually going on to earn B.A. and Ph.D degrees in Biology and Entomology. Dr. Willis taught in Wisconsin at the university level for 15 years before entering the field of agricultural consulting and then writing. He is now retired.