The soil that sustains plant life is very complex and quite variable. One classic definition for soil states that it (soil) is the "chemically and physically weathered, biologically molded upper layer of the earth's regolith." The definition, in part, describes the intricate nature of soil both from the standpoint of its origin and composition.
The source of all soil is rock. In particular, it results from the weathering of the mantle rock which is the layer of disintegrated rock fragments that exist in varying depths above the soil rock of the earth's crust. This component of the earth's cover often is referred to as the regolith.
In the upper portions of the regolith, rapid weathering takes place. It is this weathering that leads to the development of soil as we know it. Soil covers the earth in a very thin layer when compared to the regolith and bedrock below it. Remarkably, it is this thin layer combined with the proper amounts of air and water that supports life. Only when soil exists can land plants and all the animals they sustain grow and thrive.
For those who have gardens and landscapes, the details of soil complexity may not seem that important. However, without any special training we know that plants grow very well in good topsoil, and very poorly in subsoil. Erosion, construction, and a variety of other processes often leave gardeners and homeowners only with subsoil, or something closely akin to it in structure and appearance, after the topsoil has eroded away or removed purposefully by man.
The question arises then, "Is it possible to grow plants in subsoil?" The answer is "yes," but with the understanding that the conditions needed for good topsoil—proper proportions of air, water, mineral materials, and organic matter—must be developed in what currently is subsoil. In essence, gardeners and homeowners need to create topsoil as much as possible from subsoil in a few years—a process that takes nature thousands of years to accomplish.
The major portion of any soil consists of its mineral components. The mineral component of soil is made up of particles raging in size from large sand particles, to smaller silt particles, and (finally) to very fine particles of clay. The ratio of sand, silt, and clay is known as soil texture. A mixture of these three particle sizes (in proper proportions) creates a soil texture known as loam. The latter is considered to be soil good for gardening because of its ability to retain water and nutrients, while at the same time allowing excess water to drain through freely.
Another important component of a garden loam is organic matter which is essential for good plant growth. Organic matter improves soil structure by acting as a bonding agent that holds soil particles together in aggregates. Without organic matter, aggregates are less stable and can be easily broken apart. Soil aeration, water drainage, root growth, and biological activity all are affected by the organic matter content of soil.
The average upland mineral topsoil contains from only two to four percent organic matter (by weight). This rather small amount is caused by the continual decay of organic matter by soil microorganisms. Since organic matter is constantly being broken down, it needs to be replaced either by nature of by the gardener on a regular basis. Ideally, soils destined for growing ornamental flowers and vegetables should contain a minimum of five to ten percent organic matter.
Subsoil differs from topsoil in several ways. Perhaps the most important of these is that in subsoil organic matter content is greatly reduced to less than one percent or, in certain cases, none. As mentioned above, organic matter is important for creating soil structure. The latter, in turn, greatly influences the pore space of soil which represents the area in which air and water move, providing essential nutrients and oxygen to plant roots.
In a good soil, such as a silt loam, the pore space may comprise as much as 50 percent of the soil (by volume). Of the 50 percent, ideally half (25 percent) is air space and half (25 percent) is made up of water-holding space. Subsoils, and particularly clay subsoils, contain much less pore space which reduces root activity and (consequently) shoot growth as well.
Since, in nature, organic matter is important in converting subsoil into topsoil, the frequent application of organic matter to poor garden soils will start the process of topsoil development. Compost is ideal but, in many cases, in short supply. Therefore, materials such as tree leaves, grass clippings, straw, organic mulches such as bark and wood chips can be added to decompose and contribute to soil organic matter.
Where large areas must be improved, the use of green manure crops is an inexpensive way to help build organic matter in soil. Grasses with fine, extensive root systems such as annual ryegrass (Lolium multiflorum) are ideal. Because of its vigor and rapid growth rate, a warm-season grass such as a Sorghum-Sudangrass hybrid (a.k.a., Sudex) can add significant amounts of organic matter to poor soil at a time when cool season grasses go dormant.
With the addition of nitrogen and other nutrients to assist organic matter decomposition by soil microorganisms, the process of soil improvement can be hastened. Tilling soil to work the organic matter more deeply into the soil in the early stages of development also speeds the development of a good growing medium from poor soil.
In closing, it is important to be patient since it takes time to accomplish the transition of creating topsoil out of subsoil or, "turning lemons into lemonade."