The Dirt on Dirt
By Randy Johnson
Of all the factors involved in growing plants outdoors, soil is the most complex. It has its own ecology, which can be modified, enriched, or destroyed; the treatment it receives can ensure crop success or failure.
There is no such thing as universal soil for outdoor gardening. Each variety of plant can grow within a wide range of soil conditions. Your goal is garden soil within the range for healthy growth: well-drained, high in available nutrients, and with a near neutral (7.0) pH. Plants grow poorly, if at all, in soils which are extremely compacted, have poor drainage, are low in fertility, or have an extreme pH.
There are several soil factors that are important to a grower; these include soil type, texture, pH, and nutrient content. We will begin by discussing each of these topics in succession.
Types of Soil
Each soil has its own unique properties. These properties determine how the soil and plants will interact. For our purposes, all soils can be classified as sands, silts, clays, mucks, and loams. Actually, soils are usually a combination of these ingredients. If you look carefully at a handful of soil, you may notice sand granules, pieces of organic matter, bits of clay, and fine silty material.
Sands are formed from ground or weathered rocks such as limestone, quartz, granite, and shale. Sandy soils may drain too well. Consequently, they may have trouble holding moisture and nutrients, which leach away with heavy rain or watering. Some sandy soils are fertile because they contain significant amounts (up to two percent) of organic matter, which also aids their water-holding capacity. Sandy soils are rich in potassium (K), magnesium (Mg), and trace elements, but are often too low in phosphorous (P) and especially nitrogen (N). Nitrogen, which is the most soluble of the elements, is quickly leached from sandy soil. Vegetation on sands which are pale, yellowed, stunted, or scrawny indicates low nutrients, and usually low Nitrogen.
Sandy soils can be prepared for cultivation without much trouble. They must be cleared of ground cover and treated with humus, manure, or other N-containing fertilizers. In dry areas, or areas with a low water table, organic matter may be worked into the soil to increase water-holding capacity, as well as fertility. Sandy soil does not usually have to be turned or tilled. Roots can penetrate it easily, and only the planting row need be hoed immediately before planting. Growers can fertilize with water-soluble mixes and treat sandy soil almost like a hydroponic medium.
Sandy soils are also good candidates for a system of sheet composting (spreading layers of uncomposted vegetative matter over the garden), which allows nutrients to gradually leach into the soil layers. Sheet composting also prevents evaporation of soil water, since it functions as mulch.
Silts are soils composed of minerals (usually quartz) and fine organic particles. To the casual eye, they look like a mucky clay when wet, and resemble dark sand or brittle clods when dry. They are the result of alluvial flooding, that is, deposits from flooding rivers and lakes. Alluvial soils are usually found in the Midwest, in valleys, and along river plains. The Mississippi Delta is a fertile alluvial plain.
Silts hold moisture but drain well, are easy to work when moist, and are considered among the most fertile soils. They are frequently irrigated to extend the length of the growing season. Unless they have been depleted by faulty farming techniques, silts are rich in most nutrients. They often support healthy, vigorous vegetation. This indicates a good supply of Nitrogen.
Mucks are formed in areas with ample rainfall which supports dense vegetation. They are often very fertile, but may be quite acidic. They usually contain little potassium.
Mucks range from very dense to light sandy soils. The denser ones may need heavy tilling to ensure healthy root development, but the lighter ones may be cleared and planted in mounds. Mucks can support dense vegetation, and are often turned over so that the weeds thus destroyed form a green manure.
Clays are composed of fine crystalline particles which have been formed by chemical reactions between minerals. Clays are sticky when wet, and can be molded or shaped. When dry, they form hard clods or a pattern of square cracks along the surface of the ground. Clays are usually hard to work and drain poorly. Roots have a hard time penetrating clay soils unless these soils are well-tilled to loosen them up. Additions of perlite, sand, compost, gypsum, manure, and fresh clippings help to keep the soil loose. Clay soils in low-lying areas, such as stream banks, may retain too much water, which will make the plants susceptible to root and stem rots. To prevent this, some growers construct mounds about six inches to one foot high, so that the stems and tap roots remain relatively dry.
Clay soils are often very fertile. How well plants do in clay soils usually depends on how well these soils drain. In certain areas “clay” soils regularly support corn cotton. This type of soil will support a good crop. Red colour in clay soil (red dirt) indicates good aeration and a “loose” soil that drains well. Blue or gray clays have poor aeration and must be loosened in order to support healthy growth.
This is a typical schedule for preparing a heavy clay soil In the late fall: Before frost, turn the soil. Add fresh soil conditioners, such as leaves, grass clippings, fresh manure, or tankage. Gypsum may also be added to loosen the soil. Spread a ground cover, such as clover, vetch, or rye. In early spring, make sure to break up the large clods, and add composts and sand if needed. At planting time, make sure to till the soil with a hoe wherever the seeds are to be planted.
As the composts and green manure raise the organic level in the soil, it becomes less dense. Each year, the soil becomes easier to work and easier for the roots to penetrate it. After a few years, you may find that you only need to turn under the cover crop. No other tilling will be necessary.
Loams are a combination of about 40 percent each of sand and silt, and about 20 percent clay. Organic loams have at least 20 percent organic matter. In actuality, a soil is almost always a combination of these components, and is described in terms of that combination, e.g., sandy silt, silky clay, sandy clay, or organic silty clay. Loams range from easily worked fertile soils to densely packed sod. Loams with large amounts of organic matter can support a good crop with little modification.
Humus and Composts
Humus and composts are composed of decayed organic matter, such as plants, animal droppings, and microbes. Their nutrient contents vary according to their original ingredients, but they most certainly contain fungi and other microorganisms, insects, worms, and other life forms essential for the full conversion of nutrients. As part of their life processes, these organisms take insoluble chemicals and convert them to soluble forms, which plant roots can then absorb.
Humus and composts hold water well and are often added to condition the soil. This conditioning results from the aerating properties and water-holding capacity of humus and composts, as well as balanced fertility.
Humus and composts have a rich, earthy smell, they look dark brown to black, and may contain partially decayed matter such as twigs or leaves. They are produced naturally as part of the soil’s life process or can be “manufactured” at the site by gathering native vegetation into piles. Composts cure in one to three months, depending on both ingredients and conditions.
Decomposition can be sped up by turning and adding substances high in Nitrogen. Composts are frequently acidic and are sweetened with lime when they are piled. This also shortens curing time, since the desirable microbes prefer a neutral medium.
Soil texture refers to density, particle size, and stickiness, all of which affect the soil’s drainage and water-holding characteristics. The most important quality of the soil is that it drains well – that is, water does not stand in pools after a rain, and the soil is not constantly wet. In a well-drained soil, the roots are in contact with air as well as water.
Plants do best in medium-textured soils: soils that drain well, but can hold adequate water. Loams, silts, and sands usually drain well and are loose enough to permit good root development. Some clays and most mucks are too compact to permit the lateral roots to penetrate and grow. In addition, they often drain poorly, and when dry they may form hard crusts or clods, a condition most plants cannot tolerate.
Several simple tests will indicate the consistency and drainage qualities of your soil. Test when the soil is moist but not wet. First, dig a hole three feet deep to check the soil profile. In a typical non-desert soil, you will find a layer of decaying matter on the surface, which evolves into a layer of topsoil. Most of the nutrients available to the plant are found at this level or are leached down from it. The topsoil layer is usually the darkest. It may only be an inch thick or may extend several feet. When in good condition, the topsoil is filled with life. Healthy topsoil contains abundant worms, bugs, and other little animals, and is interlaced with roots. If you can easily penetrate the underlying topsoil with your hands, its texture is light enough for healthy root growth.
The next layer, or subsoil, may be composed of a combination of sand, clay, and small rocks, or you may hit bedrock. Sandy, rocky, and loamy subsoil presents no problems as long as the topsoil is at least six inches thick. Clay or bedrock often indicates drainage problems, especially if the spot has a high water table and stays wet.
Next, scrape up a handful of soil from each layer. Press each handful in your fist, release it, and poke the clump with a finger. If it breaks apart easily, it is sandy or loamy. Clods that stick together, dent, or feel sticky indicate clay or muck.
Now that you know all about the dirt under your feet, try not to track it across the carpet. You don’t want to have to pick up an article entitled “The DIRT on the DOGHOUSE!”