The mineral component of soil consists mostly of only 8 elements. Of these, oxygen is the most abundant, and forms a part of most minerals.
|Element||Percent of the Earth’s Crust|
The bedrock on which all soil rests, and from which it is derived, is broken down, ground up, and dissolved by the action of weather, glaciers, water, cycles of freezing and thawing, and the activities of living plants and animals. This resulting material makes up the matrix of soil.
Soil is not composed of inert rock particles, but is a constantly-changing environment in which plants grow. The forces described in the previous paragraph continue to work on the soil as we garden it. In addition, our activities as gardeners change the soil by adding fertilizers and amendments, growing plants, watering, and mulching.
A soil’s texture is created by the proportions of sand, silt, and clay that make it up. Of these, clay has the strongest influence on the soil. Clay particles, being the size of bacteria, have much more surface area with which to react with their environment, so form the most chemically active part of the soil. Many clays form flat platelets that hold molecules between their layers. These clays hold much of the potassium in the soil, as well as many other chemicals. Being held by the clay, potassium leaches out of the soil only very slowly.
The mineral components of the soil also enter into chemical combination with other elements, especially phosphorus, which is chemically highly reactive. Phosphorus—even when in soluble form, such as phosphoric acid—quickly reacts with chemicals in the soil to form almost-insoluble compounds which are very stable and resistant to leaching. In fact, phosphorus is so reactive that phosphorus fertilizers applied to the surface may take years to penetrate beyond the first couple of inches of soil.
Sand is chemically almost inert, and silt is much less chemically active than clay. Beach sand, which is composed of sand particles sorted by the actions of waves, holds so little nutrition that only a few specially-adapted plants grow on it.
It’s usually a mistake to try to change the texture of the soil by adding sand to “lighten” it. The clay in the soil filters between the sand particles, plugging them up and making the whole drain even more slowly than before. In order to get a “lighten” action from sand, the resulting mix must be more than 80% sand. A better way to lighten clay soil is to add organic matter.