Soil

Green Side Up

As gardeners we often forget that no matter how expensive or highly hybridized our seeds are and no matter how expensive and well maintained our tools are, without adequate soil we are destined to failure. The major job of the gardener is to keep the soil in the best possible condition.

On its most basic level, soil is composed of five elements, which includes an inorganic part that is composed of particles of rocks and minerals, dead and decaying organic matter (also known as humus), water, air and an astonishing community teeming with living creatures ranging from macroscopic organisms such as insects and earthworms to microscopic and submicroscopic fungi, bacteria, protozoa and viruses. It has been said that “dirt” is dead soil. It does not contain any of the living organisms necessary for healthy plants. The exact composition of soil is dependent upon the proportions of each of these components. The proportions of the components are, in turn, dependent upon the location at which they are found as well as the nature of the parent rock layer.

The inorganic, mineral parts of soil are generally classified according to the size of their particles. Sand is the largest particle and can be up to two millimeters (mm) in diameter while typical coarse sand particles are approximately 0.8 mm in diameter. The next smaller particles are called silt. Silt particles are about 0.05 mm in diameter and have been ground down and deposited by running water. For reference, the average adult human hair is up to approximately 0.181 mm in width. The smallest soil particles are clay. The largest clay particles are only 0.002 mm in diameter. These inorganic elements are derived from physical action on the parent materials and make up soil. Wind and water erosion, mechanical breakdown of rocks, and the action of roots all contribute to the decomposition of rocks over a period of time. A soil in good condition contains approximately 50 percent air and water.

Because of the size of the particles, sand feels gritty and its particles do not hold together very well. Sandy soils are easy to work and are called “light” soils. This term is a holdover from the “horse and buggy” days and indicated that the soil could be worked by a light team of horses. Sandy soils drain easily and nutrients are readily leached from the soil. Gardens in sandy soil must be continuously replenished with humus and other nutrients and require frequent watering. Silt, on the other hand, feels smooth and floury between the fingers. It packs together and has fewer air spaces than sand. This slows drainage and can encourage root rot. Silt does not hold together well either and becomes light and powdery when dry. Clay is a “heavy” soil since it takes a heavy team to plow it. Clay packs into clumps that dry into stony clods. Clay retains a shape when it is manipulated (think potter’s clay). It does not drain well. It absorbs water, swelling up in the process and then shrinking again when it dries. This accounts for the deep cracks we often see on the surface after periods of wet followed by extreme heat and dryness.

Loam is a term used to refer to medium textured soil containing sand, silt and clay in well-balanced proportions. Although the term is somewhat ambiguous, it generally means that the soil is good for plants. It is friable which means that a large clump breaks down easily into smaller particles. Tilth refers to the structure of the soil and in general refers to the fitness of the soil for cultivation.

The best thing a gardener can do to revitalize the soil is to add compost. Finished compost can be worked into the soil at any time. Unfinished compost (contains pieces of organic matter that have not rotted) can be worked into the soil in the fall and winter and allowed to rot until spring, or it can be used as a mulch and top dressing. Whether it is done by a professional service such as that offered by Texas A&M through the Extension Service or you purchase a DIY kit, a soil test is highly recommended before doing any major amending of the soil.