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SOILS |
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KEY CONCEPTS
II. Soil Horizons
III. Soil Formation
IV. Soil Characteristics
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SOIL CHARACTERISTICS
There are several ways to describe soil. Understanding these characteristics of soil helps the gardener amend soil. Urban soil is often quite different from the native soil. The native soil may have been compacted by heavy equipment and traffic. It may be removed before construction and sold as TOPSOIL. Subsoil excavated from basements is often dumped on the surface.
Soil texture is the relative volume of SAND, SILT and CLAY particles in a soil. Soil texture affects the water-holding capacity of soil, movement of water through the soil and ease of cultivation.
The following diagram illustrates the dramatic difference in the size of the three types of soil particles: sand, silt and clay. Sand is visible, silt particles are the size of talc and clay particles are microscopic.
Sand is obviously the largest particle. Soils that are coarsely textured are sandy. Medium-textured soils have equal parts sand, silt and clay. Finely textured soils are mostly clay or clay and silt. The same weight of clay can hold 50 times as much water as very fine sand particles. Soils containing a high percentage of clay are undesirable because the small particles pack tightly together, leaving little pore space for air and available water. This type of texture makes digging difficult. Although, clay is usually nutrient rich, nutrients are too tightly bound to be easily released. Ideal garden soil, called LOAM, is a mixture of all three particle sizes. To experience soil texture, wet a small handful of soil and rub it between the thumb and forefinger. A gritty feel indicates the presence of sand. A silky or a smooth feel indicates silt. Moist fine clay particles that stick together cause a sticky sensation. Moisten a small amount of soil to the consistency of putty. Roll it into a 1/2-inch diameter ball. Press the ball. If the ball breaks apart, the soil is sandy. Clay can be worked to form a ribbon of soil. Loam, which contains approximately equal volumes of sand, silt and clay particles, will stick together when pressed, but will not form a ribbon of more than a half inch in length.
Soil texture is important because it affects how easily soil can be worked, how well it holds water, and how quickly it warms. For example, coarsely textured, or sandy soils, allow water to enter and pass through more quickly. Sandy soils warm up more quickly than finer soils and can be tilled more easily; however, they dry out more quickly and are not as rich in nutrients.
Soil structure results from the binding together of soil particles into aggregates or clumps of varying sizes and shapes. Aggregates can be as small as a grain of sand or as large as a pea. A well-structured soil is made up of aggregates of varying sizes that allow maximum space for air and water (ideally about 20 to 25% for each by volume). These aggregates form as a result of physical forces: freezing and thawing cycles, wetting and drying cycles. Organic matter promotes stabilization of the aggregate particles. Decaying organic matter acts like a glue to hold soil particles together, much like flour moistened and used as paste. Unnecessary digging or rototilling may break down aggregates into a fine powder, reducing pore space.
Soil structure is critical to plant health because it affects root development. Soil structure is more important than texture, color or parent material. It is also more important than any fertilizers that can be applied. Good soil structure allows water and air movement, and it provides channels through which roots grow. Overworked, poor soils show a very indistinct soil structure. Aggregates do not exist, so there are few air spaces between particles. Roots can suffocate and die where large amounts of water have forced air out of the space. TILTH is a term gardeners use to describe how easily soil can be tilled. Soil with good tilth allows seedlings to emerge easily. It allows roots to penetrate soil. Soil with good tilth has good structure.
Living organisms impact soil structure. Organic matter attracts earthworms that drill channels deep into the subsoil. These channels provide needed space for plant roots, air and water. Worms leave behind "castings" as they tunnel, which improve nutrient levels in the soil. In addition to earthworms, microbes live in soil. Microbes break some nutrients and organic matter into simpler units that plants can use. The organic remains of microbes improve soil structure. Some plant roots and microbes have a SYMBIOTIC relationship. These microbes depend on plant roots to provide them with food. In exchange, microbes make nutrients available to the roots. Some microbes attach themselves to roots and form extensions called NODULES. These extensions increase the root's surface. Microbes chemically change nutrients, transforming them into usable forms for the plant. An example of these microbes is nitrogen-fixing bacteria on the roots of beans and peas. Even growing plants change soil structure as they send their roots down into the soil. Roots enlarge openings in the soil and provide organic matter when they die and decay.
Improving soil structure is an ongoing goal of gardeners. Soil structure cannot be changed easily. However, the addition of organic matter and working the soil at the proper moisture level improve soil structure. Correct soil moisture level exists when soil crumbles easily when turned over with a fork or spade. Soil should not stick to the fork or spade. Working wet soil compacts structure and reduces air and water spaces.
Drainage is the RATE and AMOUNT of water movement through and across soil. Water is a solvent for vital plant nutrients. The presence and movement of water through the soil affect the availability of these nutrients to the plant. Water will drain quickly across a sloped surface. This is called surface drainage. Additionally, there is internal drainage. Gravity moves water vertically through the soil. Horizontal drainage results from CAPILLARY ACTION. Even in highly sloped terrain, water drains internally through the soil as well as over the surface. Texture, structure and physical condition of surface and subsoil layers affect vertical drainage and capacity of soil to store water. The ideal moisture level is reached when water occupies one half of the pore space in soil structure. Soil is saturated when all the pore space is filled with water. Saturated soil has no oxygen in its pore spaces. This dramatically reduces root growth because roots require oxygen for growth. If saturation continues for extended periods, roots will die.
Gravity will pull a certain amount of water out of saturated soils if there is somewhere for it to go. The water remaining after gravity removes what it can is called the field capacity. Water also evaporates or is taken up by plants. A certain amount of water is unavailable to plants because it adheres to the surface of soil particles. Clay soils have a higher moisture-holding capacity than sand; many fine particles of clay have more surface area to which water can adhere. Wilting point is the point at which plants cannot absorb any more water from soil to remain turgid. Any water remaining in soil is unavailable because it is strongly adhering to the soil particles. Soils with high available water normally have a high organic matter content. Any organic matter increases the water-holding capacity of soil. Peat moss, for instance, retains more than 100 percent of its weight in water. Peat moss must be worked into the soil to be effective.
Gardeners can alter soil to improve drainage. Adding organic matter may improve drainage by improving soil structure. In low areas, drainage ditches, drain tile, swales or dry wells help drain overly wet areas. Soil depth has a direct effect on drainage. A deep, well-structured soil provides plenty of space for vertical and horizontal drainage. Deep soils with a large capacity for water and nutrients have a positive effect on crop yields.
The pH scale is a measure of the degree of acidity or alkalinity of the soil. The pH scale has 14 divisions ranging from 1 to 14. At the midpoint of this scale, 7, soil is neutral (neither acid or alkaline). Acid soils, those between 1 and 6.9, are sometimes called "sour." Soils that measure 7.1 to 14 are increasingly alkaline or "sweet." Increments between each number represent a tenfold increase. For example, at pH 5 it is 10 times more acidic than at pH 6. Soil pH of 6.0 to 7.0 is suitable for most plant growth. A soil test is necessary to determine pH of soil. Gardeners should know the pH of their soil because it affects the availability of plant nutrients. Nutrients become less available at pH extremes. Some plants, mostly broadleaf evergreens (azalea, rhododendron, holly, blueberry, etc.), prosper in acidic soils.
The pH of soil is usually correct for most plants. However, most soils in Ohio must be modified for acid-loving evergreens. Use of commercial fertilizer can cause soil to become more acidic, resulting in the need to add lime. Lime is applied to make the soil more alkaline. Ground agricultural limestone is an inexpensive soil amendment. Lime is safe to use, nontoxic to humans and does not cause pollution problems. Dolomitic lime offers the added benefit of magnesium. Wood ashes can raise pH, but are not recommended. There is no way to determine accurately the volume to add for the desired change. They are very messy and leach out of the soil so rapidly that they are ineffective.
If pH is too alkaline, add sulfur to lower the pH. Chelated iron sulfate or aluminum sulfate has an acid reaction and is commonly added to correct chlorosis or yellowing of leaves. Overly alkaline soil interferes with plant uptake of iron. Iron sulfate reduces alkalinity and adds iron. Specific plants affected by chlorosis include evergreen shrubs, raspberries, roses, mountain ash, pin oak and currants.
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