Soil (Don't call it Dirt)
(Editor's Note: This article was originally published on March 2, 2007)
I'm going to attempt to take the mystery out of that thing called soil. Gardeners seem to have a love, hate relationship with their soil. "Mine is clay," or "mine is sand and dries out too quickly," or "mine is too rocky," seems too be the gist of conversations when gardeners congregate.
I do not believe that any of us think we have perfect soil. When was the last time you heard someone say, "we moved into a new home and we have ideal garden soil." It seems like in new subdivisions all over the country contractors scrape off fertile topsoil and haul it away. The home is completed and what's left is hardpan or clay.
In the paragraphs that follow you will learn exactly what soil is made of. We will explore the management factors that can be employed to change physical properties and aid in plant growth. You will learn what essential plant nutrientsare and tell how they are managed to keep your soil fertile. To a soil scientist, the definition of soil is "A natural body of the earth's surface having characteristics resulting from climate and living organisms acting upon parent material as conditioned by slope over periods of time." To all of us gardeners, soil is a medium for plant growth.
For plants to grow, soils must provide anchorage, to hold the plants into place. Water, provides nutrients and cool the plants as it evaporates from the leaves. Roots need oxygen to breathe and nutrients to build tissue; as well as a growth promoting chemical environment. How soil provides these in relationship with one another determines how a plant grows.Soils are composed of mineral solids, organic solids, pore spaces, water, and air.
Soil texture refers to the percentages of sand, silt, and clay particles that make up the mineral fraction of the soil itself. Common soil textures range from finer to coarser textures as follows:
- clay loam
- silt loam
- sandy loam
- loamy sand
Soil texture greatly affects the type of environment plants roots will be exposed to. It also affects the way the gardener manages the soil to promote plant growth.
Structure of the soil is another physical property that affects plant growth. Decay of organic materials is an important process in building an ideal soil structure.
A key issue to remember is that texture in an unchangeable property of soil but structure may be improved or destroyed by humans.Compaction or excessive tilling is the two primary ways that humans destroy soil structure.Whether you have very sandy soil or clay soil the structure can be improved by the addition of organic materials.Some of the elements found in organic materials and how they interact are:
Bacteria are single celled microorganisms critically important to organic material decomposition. One ounce of soil can contain 30 billion bacteria.
Fungi are multicelled plants that are very important decomposers. Some of theses like bacteria can cause diseases.
Mycorrhizae are plant roots and fungi that infect them. They live off of food produced by the plants and form a large branched network of fungal threads that extend into the soil and act as an extension of the root system.
Protozoa are one-celled animals that are invaluable decomposers.
Invertebrates are animals that have no spine. These include worms, slugs, nematodes, and centipedes plus many more. They play important roles such as tunneling, decomposing and consuming other plants and materials.
As you can see there is a lot of activity going on in our soil much of which is not visible to the naked eye.
Plants need 16 elements from which to grow and develop. These are hydrogen, oxygen, phosphorus, potassium, magnesium, nitrogen, sulfur, calcium, iron, carbon, boron, chlorine, manganese, molybdenum, copper and zinc. Three of these elements do not originate from soil mineral compounds. Hydrogen and oxygen comes from soil and water and carbon comes from the carbon dioxide in the air. The remaining 13 come from soil minerals.
Soil pH is the measure of acidity or alkalinity of the soil. A neutral soil has a pH of 7.0 and the scale increases so does the alkalinity up to a pH of 14.0. As the pH decreases the acidity increases to a pH of 1.0. The soil pH may be altered by addition of certain substances.
Soil pH influences the plant's ability to take up nutrients. It also has a great deal to do with the breaking down of organic materials in the soil.A pH of 6-7 is most ideal for plant growth because most nutrients are available in this range.Many pesticides are designed for specific soil pH's. pH outside of this range may deem the pesticide ineffective or the pesticide may not degrade as expected resulting in high levels of undesirable chemicals.
pH is controlled by several factors. Natural controls are mineralogy, climate and weathering. Soil management often alters the natural pH due to the use of acid forming nitrogen fertilizers. Soil that have sulfur forming minerals can produce very acid soils when exposed to air.
The soil pH should be tested before making management descisions that depend on the soil pH. As you can see, there is a lot of activity going on under the surfaces of our gardens. A key point that I want to stress is don't guess when adding amendments to your soil. Get a soil test to find out exactly what you have or don't have.