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Soil Management Specialty Gardens
Healthy soil and how to make it

What Is Soil?

Excerpted from: Teaming With Microbes

Technically, soil is all the loose, unconsolidated, mineral and organic matter in the upper layer of the earth's crust. The standard comparison uses an apple to represent the earth. Carve off approximately 75% of the skin which represents all the water, another 15% which represents the deserts and mountains — land too hot, too cold, too wet or too steep to be usable for growing plants. The remaining 10% represents the earth's soils — soils with the necessary physical, chemical and biological properties to support plant life.

When we take into account the footprint of cities, roads and other man-made infrastructure, the surface are of usable soil is further reduced.

  How Is Soil Created?

Soil is in large part a product of weathering. Weathering is the impact of all the natural forces that decay rocks. The forces can be physical, chemical or biological.

  Physical Forces

The action of wind, rain, snow, sun and cold along with glacial grinding, bumps along river beds, scrapes against other rocks and rolls in ocean waves physically breaks down rocks into tiny mineral particals. This starts the process of soil formation.

  Chemical Forces

Chemical weathering dissolves rocks by breaking down the molecular bonds that holds them together through exposure to water oxygen and carbon dioxide. Some materials in rock go into solution, causing the rock to lose structural stability and making them more susceptible to physical weathering.

Soil fungi and bacterial also contribute to chemical weathering by producing chemicals as they decay their food — fungi produce acids and bacterial produce alkaline substances. Besides carbon dioxide, microbes produce ammonia and nitric acids which act as solvents.

  Biological Activity

Biological activity also causes weathering. Mosses and lichens, or more precisely the fungi in them, attach themselves to rocks and produce acids and chelating agents that dissolve little bits of rock to use as nutrients. This action result in small fissures that fill up with water. Freezing and thawing further breaks apart the parent mater and the roots of larger plants penetrate the crevices and widens them.

  The Role Of Organic Matter In Soil Creation

Chart showing composition of soil

For soil to support plant life, this requires more than just minerals. On average, good garden soil is 45% mineral, 25% of both air and water, and 5% organic material, built up as organisms above and in the soil go about their daily activities. As plants and animals on the surface die and are decayed by bacteria and fungi, they are converted into humus, a carbon-rich coffee-colored, organic material.

Humus consists of very long, hard-to-break chains of carbon molecules with a larger surface area. These surface areas carry electrical charges, which attract and hold mineral particles. The molecular structure of the long chains resembles a sponge — lots of nooks and crannies that service as condominiums for soil microbes.

  Air And Water

Humus and minerals make up the solid phase of soil, but plants require oxygen and water, the gaseous and liquid phases, as well. The voids between individual mineral and organic particles filled by air or water. Just about half the soil pore spaces are filled with water and the other half are filled with air.

Water moves between smail pore space in two ways: (1) by the pull of gravity or (2) by the pull of individual water molecules on each other (capillary action).

Gravitational water moves freely through the soil. Large pores in the soil structure promote the flow of gravitational water. As water fills the pores, it displaces and pushes out the air in front of it. When the water flows through, it allows a supply of fresh, oxygenated air to move in.

Smaller soil pore spaces contain a film of capillary water that is NOT influences by gravity and is left behind after gravitation water passes through.

  Soil Color

Color can be an easy indicator of what is in your soil as color is partially dependent on the soils mineral and organic components.

Organic components in soil are very strong coloring agents and produce dark soils. The organic components can accumulate or can dissolve and coat other particles of soil with black color.

Iron-rich soils: Iron rusts and soil particles are coated with red and yellowish tints.
Manganese oxide-rich soils: Soil particles take on a purple-black hue.
Depleted organic material soils: Gray soils indicate a lack of organic material.

  Soil Texture

Soil scientists describe the size of soil particles in terms of texture. There are 3 categories of soil texture: (1) sand, (2) silt and (3) clay. All soil has a specific texture that enables its ability to support a healthy food web and thus healthy plants. The key point is that texture has to do with the size of particle, NOT the composition of particles.

What difference does texture make? The size of the soil particles had everything to do with their surface are and the size of the pore spaces between individual particles.

Graphic showing comparison of soil particle size
If a clay particles were the size of a marigold seed, a silt particle would be the size of a radish and a sand grain would be the size of a large wheelbarrow.

When you look at garden soils, you don't see individual particles but rather aggregates of these particles. The biology of the soil produces the glues that bind individual soil particles into aggregates.


Sand particles can be seen with the naked eye and range in size from 0.025 to 2 millimeters in diameter. Anything larger has far too much space between particles to support plant life.

Sand particles are just small enough to hold some water when aggregated, but most of it is gravitation water and readily drains. This action leads to lots of air and only a little capillary action.


You will need a microscope to see individual silt particles. Like sand, these consist of weathered rock, only much smaller in size. Silt particles are between 0.004 and 0.0625 millimeters in diameter.

The pore spaces between silt particles are much smaller and hold a lot more capillary water than sand. Particles of silt are also influenced by gravity and will settle out when put in water. The texture of silt when rubbed between your fingers is like flour.


Clays are formed during intense hydrothermal activity or by chemical action. Clay particles are readily distinguished from silt, but this time with an electron microscope. Clay particles are the smallest that make up soil and are 0.004 millimeters in diameter or less.

Clay particles are 'plastic' and somewhat slippery when rubbed between your fingers. This is because clay particles absorb and hold lots of water.

Further Information

 Improving Soil Health
 Using Soil Inoculants
 The Wood Wide Web
 Soil In Winter
 Benefits of Mycorrhizae Fungi

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