There isn’t one “good” soil that’s right for all plants. Desert plants need excellent drainage, while bog plants prefer standing water. However, there is a “good garden soil” that suits many plants. You might need to create special situations for your rock garden or your bog, but “good garden soil” will grow most plants.
Just as you can say a person in “good health” is not only free from disease, but is also fit, you can say that “good garden soil” is free from problems, and is also “fit”. Fitness in soil is composed of its tilth and its chemical balance.
It’s easiest to think of a good soil as one that serves the needs of plants. The most important needs are:
The soil acts as a reservoir for the air and water a plant needs to grow. Air and water circulate through channels and pores in the soil. Water is held in soil pores until the plant needs it. The ability of soil to supply air and water is related to its tilth, or workability.
The soil also supplies minerals vital to plant growth. Some of these minerals dissolve out of the mineral parts of the soil and others are held by the soil until the plant needs them.
The “tilth” of a soil results from its texture and structure. The texture—the sizes of the soil mineral particles—and the structure, or the way those particles clump together, together make the “feel” of the soil in your hand or under your shovel. They cause the soil to resist or assist root growth and are responsible for the amount of air and water it holds.
The texture of a soil determines whether it is sticky like clay, silky like silt, or gritty like sand. Most soils aren’t pure sand, silt, or clay, but a mix of particle sizes. This mix determines its texture. A mix that is not dominated by one of the three textures is called “loam”.
The clay portion of the soil determines its structure, or the way the particles aggregate. Soil with poor structure forms into masses like potter’s clay. Well-structured soil clumps into aggregates that break apart easily. The spaces between these aggregates allow the free flow of water and air.
A well-structured soil is porous and crumbly. When moist, it looks and feels like chocolate cake. When dry, the clods break when attacked with a rake or hoe.
Fertility is determined by the plant nutrients in the soil, and also by how available those nutrients are to the plants. If the soil chemistry is unbalanced, nutrients can be present in large amounts, but not be available. From the plants’ point of view, they are missing.
Nutrient availability is a result of the acidity or alkalinity of the soil, a measure called pH. Soil that is just slightly acid holds nutrients in forms available to the plants. Soil that is too acid or too alkaline holds tightly to some nutrients. For example, soil that is too acid holds phosphorus, a vital plant nutrient, so tightly that plants can’t use it. Soil that is too alkaline holds iron—another essential nutrient—in insoluble forms.
So how can you tell if your soil is fertile? Fertility is best measured by observing the growth of plants. If the soil is covered with vigorous plants—even if they’re weeds—it is fertile. A soil test will also determine soil fertility. However, because fertilizers are so inexpensive, many gardeners choose to fertilize regularly rather than have a test done. Fertility is easily improved with fertilizer.
Soil pH can be tested with a simple kit purchased at a garden center, or a sample can be sent to a soil test laboratory. The acidity can be balanced by adding lime, sulfur, or other chemicals to the soil.
Fertile soils in good tilth may still have problems that keep them from being “good” soils. The soil may be too shallow, too rocky, too steep, have too many weeds, or have one of a myriad of other problems. Problems of this sort must be overcome with determined effort, just as diseases are fought in the body. For more see Problem Soils
If your soil is not perfect—or even if it is not “good”—you can make it into good soil. Any soil, from the heaviest clay to the droughtiest sand, can be made into good garden soil. This is part of the art of the gardener.