What is soil?

Soil is composed mostly of mineral particles from weathered rock. They are classified by size; they are sand, silt and clay. Sand is the largest sized particle, silt intermediate and clay particles are so small one needs an electron microscope to see them. An ideal soil is about 50% minerals and 50% air and water. The air and water are located in the voids or pores of the mineral structure. The amounts of air and water is very dependent on the looseness and amount of open space in the structure.

Organic material composes only about 1% of the soil in Oklahoma. It may be higher, up to 4-5% in the cooler upper mid West. Organics are extremely important in terms of improving water handling, nutrient retention and as a home of the enormous number of beneficial soil organisms. Organics are also a key source of nutrients as they decompose.


Soil Texture: Not everyone has ideal loamy soil; often it is too sandy or has too much clay. The amount of sand, silt and clay in soils contributes to what is known as soil texture. Ideal loamy soil has about 40% sand, 40% silt and 20% clay. To determine the texture of your soil, there is a home test you may perform. It is described in the soil classification section. Determining which type of soil texture you have has practical applications.

Soil Function: Soils with excessive amounts of sand retains water and nutrients poorly. These soils require more irrigation and nitrogen fertilizer than high clay soils.

Clay soil has a difficult time absorbing water and fertilizer, but once absorbed, it holds them very well. Clay also compacts easily making it difficult for plants roots to penetrate. Clay serves an important function in soils in terms of its chemistry. Clay has the ability to bind and release many chemicals, an activity which contributes to fertility and acidity.

Another important aspect of soil is the soil pH. This is a measure of soil acidity reported on a scale of 1 to 14. One is very acid, 14 is alkaline. Water is neutral with a pH of 7. All plants have a range of pH preferences at which they absorb nutrients best. Many plants, including many vegetables, prefer a pH between 6 and 7.

There are 17 plant nutrients, including the big three, nitrogen, phosphorus and potassium which are found in soils. Most of these nutrients are attached to and retained by, organic material and clay particles. Soil tests for homeowners measures soil pH, nitrogen, phosphorus and potassium.

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