Chapter 5: soil.
A horizon adsorption aggregate anion B horizon C horizon cation cation exchange capacity composite sample compost E horizon eluviation hardpan horizon O horizon organic matter parent material ped R horizon soil profile soil structure soil texture subsoil tilth topsoil
Soil can be the gardener's best friend or worst enemy. The luckiest homeowner has rich, dark, crumbly soil that drains well and holds adequate moisture for good plant growth. Unfortunately, the soil in many urban environments falls far short of this ideal. Poor soil is the norm for many homeowners. Besides being difficult to dig and cultivate, it also encourages weeds and is not conducive to good growth of our desired cultivated plants. In this chapter we will examine what makes soil good or bad for plant growth and what to do to amend soil for improved plant growth.
Of all of earth's natural resources, soil is perhaps the most important one. Soil is the matrix in which plant roots are anchored and from which they obtain nutrients, water, and air (Fig. 5-1). The ability of soil to provide adequate plant nutrients, store water, and provide air for gas exchange is a direct result of its origin and development. Human activity on soils greatly affects its ability to provide a healthy environment in which plants can grow. By studying the physical and chemical properties of soil we can better understand the ability of different soils to provide the necessary environment for healthy plants. We can also begin to understand how to manage this valuable resource.
Where does soil come from? Geologic events, such as volcanic eruptions, glaciation, flooding, and landslides, contribute to the formation of new soil. Soil microorganisms, such as protozoa, bacteria, and fungi, and other organisms, such as earthworms, nematodes, and insects, all contribute to the development of healthy soil.
There are two general categories of soil: organic and inorganic. Most of the soils that cover the earth are mineral soils derived from inorganic parent material. Some examples of parent material are limestone, granite, basalt, shale, and sandstone. Parent materials are broken down by weathering processes, most notably the freezing and thawing action of water, and by movement of water. Plant activity also contributes to soil formation. The physical action of roots growing through the soil results in a kind of microtillage. Another way root growth contributes to the development of soil is by the carbon dioxide released by plant roots that then is dissolved in soil water, forming carbonic acid. This acid contributes to chemical weathering of soil material.
The formation of soil may take a hundred to thousands of years. The length of time depends on the hardness of the parent material, its chemical composition, the slope and landform, the presence of living organisms and organic residues in the soil, and the climate and environmental conditions of the area.
Organic soils are derived from decayed or decaying plant material. A soil that contains 20% or greater organic matter is classified as an organic soil. Organic soils retain water well and may become waterlogged, hold nutrients well, and exhibit some nutrient deficiencies. An example of an organic soil is the peat bog. Peat bogs are wet, poorly draining areas in which plant decay is very slow. This leads to the accumulation of a great deal of organic matter. Peat bogs are important to horticulture, as most of our potting soil (soilless media) contains a high level of peat harvested from bogs. Peat moss is also used extensively in the landscape industry as a soil amendment. Other organic soils are tundras and muck soils. Together, organic soils comprise less than 1% of the global land surface.
The physical properties of soil determine how well it holds water, how easily plant roots can obtain air, and how well-draining the soil is. Tilth is a term that is used to describe the physical characteristics of soil and its moisture condition. Soil texture and soil structure are important features of soil that determine its physical properties.
Soil texture is a basic physical property of soil that is determined by the percentages of different soil particle size groups. The size groups are called soil separates and fall into three general classes: sand, silt, and clay, designated by the U.S. Department of Agriculture (Table 5-1). The largest soil particles are classified as sand, medium-sized particles are silt, and the smallest particles are clay. To understand the sizes of soil separates relative to one another, imagine that if sand were a basketball, silt would be a baseball, and clay a marble.
Sand, silt, and clay each have characteristics that are particular to them. For example, sand, being the largest sized particle, tends to have large pore sizes, or spaces between particles. The larger pores allow better drainage but poorer water-holding ability. The relative surface area of a grain of sand is low, so sand cannot hold as many nutrients as the same volume of silt or clay particles. Sand grains are generally visible to the unaided eye and feel gritty to the touch. If you fill your palm with sand and form it into a ball, when you open your palm, the ball will not stick together very well.
Clay, on the other hand, holds a larger portion of nutrients and water but does not drain easily because of its smaller pores. Clay has the additional disadvantage of being easily compacted, especially when wet. It is very important not to cultivate the soil when it is wet. Compaction destroys soil structure, another important physical characteristic of soil. Clay is silky smooth to the touch. It sticks together well, as is evidenced in the making of pottery and other clay items. If you form a ball of clay in the palm of your hand, when you open your hand, the ball will retain its shape.
Water adheres to soil particles in the same way it does to the side of a glass. The surface tension on the soil particles holds the water very tightly. When soil is watered thoroughly and then allowed to drain, the remaining water that is held to soil particles against the force of gravity is known as the soil's water-holding capacity. Soils with greater surface area in a given volume have a higher water-holding capacity than those with less surface area in the same volume (Fig. 5-2). Therefore, clay soil holds more water than sand.
If a soil is very uniform in particle size, it may be classified as clay or sand. Both of these soil types are undesirable because clay tends not to drain well and sand drains too quickly. However, soils are typically composed of a wide variety of particle sizes (Fig. 5-3). The combination of soil separates determines the type of soil. Figure 5-4 shows a soil texture triangle that can be used to identify soil types. See the subsection on the jar test under soil testing to learn how to use the soil texture triangle. In that section also, the subsection on the ribbon test demonstrates another method that is often used to determine soil texture.
The ideal soil is able to provide air space for gas exchange at the root surface as well as pore spaces to hold water for some period of time for plant use. Therefore, the ideal soil matrix for plant use is approximately 50% soil plus organic matter, 25% air, and 25% water. For this, a combination of particle sizes is required. The ideal soil texture for a majority of horticultural plants is loam to sandy loam. Loam contains an ideal mix of sand, silt, and clay. However, because of the pronounced effects of clay, which are due to its relatively great amount of surface area for a given volume, the ideal soil does not contain equal amounts of sand, silt, and clay. To achieve a balance of water holding and aeration, a higher proportion of larger particles is required.
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Soil structure, like soil texture, is built over a period of time. Soil structure refers to the tendency of soil particles to form clusters called aggregates. Aggregation is brought about by freezing and thawing of water, wetting and drying, earthworm activity, and root growth. These aggregates are held together by iron oxides, clays, carbonates, silica, and humic acid, a kind of glue that is a by-product of the decomposition of organic matter. The organic matter may be present from decaying leaves and roots, or it may be added as a soil amendment, as in the case of adding peat moss or other organic matter to a flower bed or vegetable garden. Aggregates naturally form into different ped shapes that may be platy, blocky, subangular blocky, prismatic, columnar, granular, and crumb. Granular and crumb shapes are both spheroidal aggregates. Granular and crumb soil structures are both ideal for healthy plant growth because they provide ample aeration and drainage but also hold water well.
Tilling and plowing benefit finer soils by increasing aeration and drainage. But after a few years, plowing can destroy desirable soil structure and reduces aeration because of the formation of an impermeable layer caused by compaction, called a hardpan. Plowing and tilling also accelerate decomposition of organic matter. Although planting of cover crops can aid in replacing organic matter, the use of newly developed low-till and no-till practices may prove to be more efficient. Such practices result in improved soil structure and subsequently improved crop yields by allowing larger sized crop residues to remain after harvest. Decomposition is slower, and more nutrients remain in the root zone for the following growing season. Low-till and no-till practices also aid soil retention in rainy and windy conditions.
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If you dig down through the soil until you came to bedrock, you could go through as many as four distinct layers, or horizons, as soil scientists refer to them. A soil profile is a vertical section of soil that reveals the layers present in the soil in a particular location (Fig. 5-5). Each horizon has different chemical and physical properties, and each is designated with a letter. The horizons, or layers, are O, A, B, C, E, or R. The O horizon is the thin (less than 2 inches) layer of organic matter lying on the very top of the soil profile. The top mineral layer is the A horizon. This is the highly treasured topsoil. Under the topsoil lies the B horizon, subsoil, followed by the C horizon, partially decomposed parent material. The E horizon is a subsurface horizon that has been deposited by the movement of water (eluviation). Parent material is called the R horizon. As it decomposes it contributes to the C horizon.
The layers in the soil profile may be present in varying amounts, or they may be absent altogether. Recall the times you have seen rock outcroppings with a thin layer of soil and perhaps a tree or shrubs clinging to the side (Fig. 5-6). In these instances, very little topsoil is present and perhaps no subsoil. Mountains generally have little topsoil on which to cultivate crops. On the other hand, good agricultural soils may be as deep as 25 feet or more. Deep soil is any soil that is 36 inches or more and free of root-restricting materials (Table 5-2).
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Whereas topsoil is generally the best layer in which to cultivate plants, subsoil is composed of mainly clay. This presents a problem in areas of recent construction where topsoil is often removed during the grading process. Heavy equipment used in construction of new homes compacts the remaining soil or subsoil so that it is virtually unusable by subsequent residents. To landscape or cultivate flower beds and vegetable gardens where only subsoil is available, homeowners must bring in topsoil from another location. It is not feasible to try to amend such poor soil with peat moss, compost, or other organic material. It would be better to just build beds on top of the poor-quality subsoil, using imported topsoil and organic material. In some cases developers will save the topsoil and replace it after construction is complete.
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The chemical properties of soil affect the nutrient status and availability of essential nutrients to plants. The most important of these for plants are pH and cation exchange capacity.
The acid or alkaline nature of soil is measured in units of pH, a chemical term that specifically measures the concentration of hydrogen ions. It is represented in a logarithmic fashion such that a higher concentration of hydrogen ions results in a lower pH and a lower concentration results in a high pH. The pH is measured on a scale from 0 to 14 (Fig. 5-7). Zero is most acid and 14 is most alkaline. Table 5-3 lists pH values of common household substances.
The reason soil pH is important to plants is because plant nutrients (these are discussed further in chapter 6) are more or less available, depending on soil pH (Fig. 5-8). At optimal pH levels, plant nutrients are most readily available to plants. When the soil pH is too high or too low for a given plant nutrient, it gets bound up into chemical compounds that make it unavailable for plant uptake. Even though the nutrient may be present in adequate amounts in the soil, the plant is unable to use it. Addition of fertilizer would be wasteful in this case. It would be more advantageous to adjust the pH to acceptable levels to correct for nutrient problems. Some plant diseases are more or less prevalent, depending on soil pH. For example, potato scab is less of a problem in acid soil, and club root of crucifer is less of a problem in alkaline soils.
Adjusting the pH is a common practice in horticulture. If the pH is too low, or acidic, then a calcium-containing compound is recommended. Limestone products are commonly used. If the pH is too high, or alkaline, then sulfur-containing compounds are used to lower it.
Rainfall has an acidifying effect on soil pH because it leaches calcium and magnesium out of the root zone of plants and replaces it with hydrogen and aluminum ions. Rainwater normally has a pH of about 5.5. Acid rain, which has a pH as low as 4.3, is a phenomenon caused by air pollution, particularly from by-products of the burning of fossil fuels in electric generation plants. The primary by-products that contribute to acid rain are sulfur dioxide and nitrogen oxide compounds.
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Cation Exchange Capacity
The ability of a soil to hold plant nutrients to its surfaces makes it ideal for supporting plant growth. Just as with water-holding ability, the surface area of a soil particle affects its nutrient-holding capacity. The name designated for this ability of soil is cation exchange capacity (CEC). Plants take up many plant nutrients in the form of positively charged ions (Table 5-4). Ions are chemicals having either a negative or positive charge. Negatively charged ions are anions and positively charged ions are cations. Soil particles are negatively charged (Fig. 5-9), and so positively charged particles are attracted to them and are held by a force known as adsorption. The cations adsorbed to soil particles can be exchanged for other cations in solution, such as when a fertilizer is applied. Thus, a cation exchange occurs. As might be expected, clay and organic particles have numerous exchange sites, whereas sand has fewer than either, because it has less surface area in a given volume. Organic particles hold as much as 30 times as many cations as clay particles do.
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Before beginning any gardening or landscape project, you must first know the characteristics of your soil. Will it be heavy and poorly drained? Will it retain water long enough for plants to use it? Does it have an acceptable level of important plant nutrients? Does it require a pH adjustment?
Before collecting samples, you can determine some characteristics of your soil by simply digging a hole into your soil and observing its color and looseness. In general, darker color indicates more organic matter and more fertile soil. If the soil is hard to dig and feels heavy in the shovel, it is likely to be more clayey than sandy or silty. If it is lightly colored, it probably has low fertility. If it is loose and tends to crumble, you have good soil structure. You can feel the soil between your hands and determine whether it is grainy, as a sandy soil would be (see ribbon test below). Clay, at the other end of the spectrum, feels very smooth and lacks graininess.
Taking a Soil Sample
When taking a soil sample, you should obtain a composite sample over the entire area to be planted (Fig. 5-10). You may want separate samples for vegetable or flower beds than for areas of turf. But, otherwise, you should combine samples from different areas of the yard to get an average reading. Soil can vary quite a bit from one area of your yard to another, and you are not going to treat each little area differently. By obtaining a composite sample, you can treat a large area with the average fertilizer, lime, or other recommended amendment and achieve a happy medium for your whole lawn or garden.
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You may hire a turf or landscape company to do the testing for you, but some people prefer to test the texture, pH, and nutrients themselves. You may use a home-test kit or send samples to a soil testing laboratory for analysis. Either way, you should collect the samples in a similar manner. Soil test laboratories may provide special bags for collecting. Otherwise, place your sample in a clean, pint- to quart-sized plastic bag. If it is moist, allow it to air-dry on a paper towel. DO NOT bake it in the oven or microwave, as this will change the chemical composition of the soil!
Collect the soil sample from the depth at which your planned plants will grow. For turf, this is about 4 to 6 inches deep. For vegetables and annual flowers, collect from the top 6 to 10 inches. Although herbaceous and woody perennials will develop deeper roots, many of the roots will be concentrated in the top 12 inches, so do not worry about going deeper for your sample.
Remove any sticks, leaves, and other large organic matter from the surface of the soil. You should collect samples from random spots in the area to be planted, making sure they are fairly evenly spaced throughout the entire area. Place all the samples together and allow them to mix.
The jar test is used to determine soil texture (Fig. 5-11). Place some soil in a canning jar that has measurements marked on the side. Fill it about one-fourth full. Add enough water to fill it to the three-fourths line. Add a drop of dish detergent to suspend small particles. Shake gently for a full minute to bring all soil particles into solution. Set the jar on a level surface and watch as the heaviest particles sink to the bottom. This is your sand; it takes about 30 to 40 seconds to settle out. After 30 minutes the silt will have settled. The remaining particles are clay, and they will require about 24 hours to settle. Now you can see the sand, silt, and clay fractions and estimate their percentages. Using the soil texture triangle (see Fig. 5-4), determine the soil texture.
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Follow the steps below to determine the soil texture of your sample.
Place a fistful of moistened soil into the palm of your hand. Moisten it further and form it into a ball shape.
If it does not form a ball, it is sandy. If it forms a ball shape, then proceed to the next step.
Massage the soil and roll it between your thumb and forefinger to make a ribbon.
If it will not form a ribbon it is loamy sand or silty sand.
If a ribbon forms of about 1/2 to 1 inch long before breaking off, it is sandy loam.
If a 1-inch ribbon forms, it is loam.
If it makes flakes instead of a ribbon, it is silt or silt loam.
Sandy clay loam makes a ribbon of 1 to 2 inches.
Sandy clay makes a ribbon of 2 to 3 inches.
If it continues to form a ribbon of more than 3 inches long, it is clay.
Soil amendments may be added to garden soil to improve the physical qualities of the existing soil. Soil amendments can be classified into inorganic or organic types. Some inorganic amendments are sand or pea gravel, perlite, and vermiculite. Organic amendments include sphagnum peat, wood chips, grass clippings, straw, compost, and manure. Table 5-5 lists valuable organic amendments that are commonly available.
Most garden soils can benefit from the addition of organic soil amendments. Organic matter protects soil against erosion, whether as a cover crop, a living mulch, or green manure. Organic matter produces a cementing agent known as humus during decomposition that is important in building good soil structure. Because of its great importance to a healthy soil, especially one used for crop production, organic matter and crop residues should be allowed to remain on or in the soil. The larger the residues are, the longer the organic matter will contribute to the health of the soil. Its decomposition can act as a slow-release fertilizer, whereas disking and turning under the organic matter causes it to decompose more quickly, accelerating the release and subsequent loss of plant nutrients from the plant root zone. Regardless of the speed with which organic matter decomposes, it is in constant flux and must be replaced regularly to maintain soil productivity.
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Organic materials, such as bark mulch, peat moss, or dead leaves, have a high CEC and also provide nutrients as they decompose. The CEC of organic matter is greater than that of clay. Organic matter also produces humic acid, or humus, as it decomposes, often imparting a rich, dark color to topsoil. Good soil amendments include various animal manures, peat, wood ash, rice hulls, and activated sewage sludge, such as the commercial product Milorganite. Some other benefits of organic matter are the following:
* Provides 90% to 95% of the nitrogen in unfertilized soils
* Provides phosphorus and sulfur
* Supplies a source of carbon for beneficial soil microbes
* Increases the water-holding ability, which is particularly important in sandy soils
* Improves aeration
* Improves water flow rate through the soil
* Serves as a chelate for micronutrients such as iron, zinc, manganese, and copper
* Reduces erosion
* Shades the soil in summer and insulates it in winter
Compost is formed by the decomposition of organic matter. Composting is an excellent way to recycle waste products that can enrich the fertility of soil for crop production. Composting systems can be low or high management, depending on the desired outcome. Either way, the final result will be a rich, dark, fertile substance that is good for plant growth.
Ingredients for Compost
Many different plants and plant by-products, including kitchen scraps, can be used in compost (Table 5-6). Only plant materials should be used in a compost pile, as meat attracts raccoons and other undesirable pests. Many gardeners keep a container in the kitchen for easy collection of scraps. All types of scraps from coffee grounds and filters and tea bags to apple cores and banana and zucchini peels can be added. Some nonplant materials that are acceptable in compost are included in Table 5-6. Under ideal conditions and depending on the material used, compost may require only 3 to 4 weeks to develop, or it may take as long as 6 months.
How the Composting Process Works
During the decomposition process, heat builds up in the compost pile, which speeds the decay of plant matter. To be successful, compost piles have several basic requirements:
* The correct combination of carbon- and nitrogen-containing plant materials
* The correct microorganisms for decomposition of organic matter
* Adequate moisture
* The ability to build up heat
The microorganisms that are responsible for the decomposition process require adequate nutrition for reproduction. These microbes are present in native soils, but inoculum can also be purchased. Plant materials containing differing levels of carbon and nitrogen should be included in such a way as to achieve a balance of 30 parts of carbon to every 1 part of nitrogen. This 30:1 ratio can be achieved if plenty of green vegetative plant parts are included in higher numbers than woody plant parts (Table 5-7) because fresher, greener materials are relatively higher in nitrogen than dry materials or well-rotted manure (Fig. 5-12). Wood is difficult to break down because of the lignins present in its cellular structure. Sawdust is not recommended for compost piles either because it is relatively low in nitrogen. If it must be used, then plenty of blood meal or some other high nitrogen-containing material should be added so that decomposition is not unduly impeded. A higher nitrogen content than the recommended amount will result in a speedier breakdown of organic material.
MANAGEMENT OF THE COMPOST PILE
Managing the compost pile requires checking for the correct temperature, maintaining adequate moisture, and turning the pile when needed. Aeration is a very important component, and the compost bin or pile should be designed to allow adequate aeration.
Soil or compost thermometers can be used to monitor temperatures in the compost pile. The optimal temperature range for decomposition is between 104 and 130[degrees]F. The higher end of this range is adequate for killing many weed seeds and plant pathogens. If the temperature drops below this range or exceeds it, it is a good time to turn the pile. If weed seeds appear in the pile or if they have been a problem in the past, allow the pile to remain at 140 to 150[degrees]F for 1/2 hour to kill them.
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Watering is not necessary if rainfall is adequate, but microbial activity will be reduced in a dry compost pile. On the other hand, an excessively wet pile is anaerobic and will smell bad. If the pile is too wet, add dry materials or turn it over to help aerate it. Some people insert aeration tubing consisting of plastic tubes or pipes with holes drilled in them. The holes may be covered with screen to prevent the pipes from clogging.
Turn the compost pile regularly if you want to speed up the decomposition process. Turning the pile will distribute the heat evenly and allow different materials to be in the center of the pile, where heat builds up most. Turning increases aeration in the pile. New materials should be turned in whenever they are added to an existing pile.
A rapid way to obtain compost is to shred the material before it is added to the pile and to add a nitrogen supplement such as cottonseed meal, dried manure, or blood meal at 1 part for every 2 parts of shredded plant material.
How to Make a Compost Bin
The ideal size of a compost pile is 1 cubic yard (3 feet x 3 feet x 3 feet). If one prefers to contain the pile or disguise it from view, then a bin should be purchased or built. A compost bin of 1 cubic yard allows optimal temperatures to be reached during the decomposition process. A bin can be built from various materials or by constructing an enclosed area from chicken wire with or without wooden posts or from wooden pallets, snow fencing, or concrete blocks (Fig. 5-13). In cold winter areas, compost piles may be dug up to 3 feet into the ground to enhance heat retention for the decomposition process. Another method for dealing with compost is to use a large garbage can with holes cut into the sides for aeration and drainage of excess water. If you have a lot of material available, you may want to have more than one bin. Place bins together for ease of adding material and storing excess material close by. Locate the compost bin or pile in a convenient place where it does not create an eyesore. Be mindful of neighbors and visitors, but do not place it where it will be too inconvenient for regular use.
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Compost can be used as top-dressing for flower and vegetable beds and as an ingredient in homemade potting soil. It can even be used as a mulch to reduce moisture loss and prevent weed seed germination.
In the horticulture industry, many crops are grown in containers in greenhouses and nurseries. The use of containers compounds the effects of poorly drained soil to the extent that clay or even silty clay soil hardens and may become almost brick-like in a container. The smaller the container, the more pronounced is this effect. Therefore, greenhouse and nursery growers cannot grow their crops in field soil. In some cases with the use of larger containers, field soil may be mixed in with other components, but in others no field soil is used at all. Instead, nonsoil components are used to create soilless media. Several components have been determined to support plant growth in the absence of field soil.
The primary concerns with use of soilless media are that they provide optimal drainage and good water- and nutrient-holding ability and that they are able to provide anchoring while allowing healthy plant growth. The components commonly used are partially decomposed peat moss, compost from various sources, perlite, and vermiculite. Sometimes sand is added, and a few other components have been used on a limited basis, for example, coconut coir (from coconut husks) and Styrofoam beads.
Peat moss is the primary ingredient in commercially available soilless media mixes. It is also the primary ingredient in recipes developed to meet the concerns stated above. For example, Peat-Lite mixes, developed at Cornell University, contain 1 part peat and 1 part either vermiculite or perlite. Soilless mixes developed at the University of California were some of the earliest used in the United States and their contents range from 100% peat moss to 100% sand, with several gradients of the two in a variety of mixes. A popular recipe is 6 parts peat moss, 3 parts perlite, and 1 part vermiculite, with calcium or lime added to raise the pH to 5.5 to 6.0. Partially composted pine bark is added to some commercially available mixes, and sand may be added or substituted for all or some of the perlite to provide drainage.
The best quality peat moss is harvested from peat bogs in Canada and the northern United States (Fig. 5-14). Peat is the partially decomposed plant material that is found in wetland areas. The anaerobic conditions prevent plants that grow there from decaying very quickly. The partially decomposed plant material accumulates and increases over a number of years. Bogs from which peat is harvested are centuries old. Harvesting of peat from these bogs requires draining the bog and allowing the plants to dehydrate for several days before harvesting. Large vacuums are used to remove the material from the field. The harvest process disrupts the natural ecosystem of the bog. Bogs can be restored, but hundreds of years are required for them to be restored to the depth that was removed during harvest. Some alternatives to peat include coconut coir, composted pine bark, and commercially produced compost. To be a good alternative, the material should be able to retain moisture and nutrients, while allowing adequate aeration. A combination of materials may be required to adequately meet these conditions.
Plants can be grown in the absence of soil, as long as their primary requirements are met: support or anchoring, aeration (oxygen), water, and essential nutrients. History provides many examples of plants growing in water, including the famous Hanging Gardens of Babylon. Yet, modern hydroponics has been made possible in part by the scientific understanding of plant nutrition and the nutrients essential for plant growth. Two scientists, Hoagland and Arnon, published an important article that had lasting effects for the field of growing plants without soil (see box).
Hydroponics (hydro- water and -ponics labor), working water, is a method of growing plants in an aerated nutrient solution, with the plant often being anchored by an inorganic media such as rockwool, Styrofoam, or plastic. Hydroponics also includes a method of suspending plants and providing continuous misting of an aerated nutrient solution.
Hydroponics is popular as a hobby and also has limited commercial potential. It could have use in growing food crops on space stations, while providing the additional benefit of generating oxygen. There are many components of growing plants hydroponically that are crucial to a successful outcome. One of the more important factors in hydroponics is the nutrient solution. It must be balanced (with correct ratios of all the essential nutrients) and at the correct concentration for the size of container used. Another important consideration is the temperature of the solution. It should be maintained between 68 and 86[degrees]F. In addition, adequate aeration is imperative, and the pH of the solution is important for availability of nutrients to plant roots.
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Soil can provide some of the most important necessities for plant growth and health: air, water, and nutrients. Soils have derived from decomposition of parent material, displacement through flooding or other water movement, glaciers, and volcanic activity. Most soils in the world are inorganic, deriving from mineral rocks, but there are also organic soils that derive from the decomposition of organic plant material.
Soil texture is a measure of the size of particles in the soil. These are generally classified into sand, silt, and clay. The various soil textures impart different properties to the soil, such as good or poor drainage, aeration, and nutrient-holding ability. Soil structure is the tendency of the soil to form aggregates. Soil can be improved by the addition of organic matter, which provides nutrients, aeration in poorly draining soil, and moisture-holding ability to fast-draining soils. In the soil profile, topsoil, subsoil, and partially decomposed parent material may be present.
Cation exchange capacity and soil pH are chemical components of soil. Cation exchange capacity is an expression of the nutrient-holding ability of a soil. pH determines the availability of necessary plant nutrients.
* Do a jar test and/or a ribbon test. If you do both, compare the results to see whether you come to the same conclusion about soil texture in both tests.
* Using a store-bought kit or one provided by your instructor, test your soil for pH, nitrogen, phosphorus, and potassium.
* Try vermicomposting: buy a kit or construct a module that can be used in the classroom and create your own compost using worms.
1. Discuss the properties of organic soil.
2. What determines soil texture? What are the three major soil separates that determine soil texture?
3. What is the ideal soil texture? What are the properties that make it ideal?
4. What is soil structure? What is the optimal soil structure for growing plants?
5. Identify the layers in soil horizons and briefly discuss the properties of each.
6. In what way is pH important to plant growth?
7. Discuss cation exchange capacity (CEC): what is it and how does it work?
8. Name three benefits of organic soil amendments.
9. Name four basic requirements for successful composting.
10. What is vermicomposting?
Jones, J. B., Jr. (1997). Hydroponics: a practical guide for the soilless grower. Boca Raton, FL: CRC Press.
Plaster, E. J. (2003). Soil science and management, 4th ed. New York: Thomson Delmar Learning.
Stewart, A. (2004). The earth moved: On the remarkable achievements of earthworms. Chapel Hill, NC: Algonquin Books of Chapel Hill.
Who Was Hoagland and What Is His Solution?
In 1950, D. R. Hoagland and D. I. Arnon from the University of California published the article "The Water Culture Method of Growing Plants without Soil." In this experiment station circular, they described growing a tomato plant in 4 gallons of nutrient solution, which was replaced once a week. The nutrient solution was made up following a recipe that included the known nutrients required for healthy plant growth at the time. The formula has stood the test of time, and it or a modified version of it is widely used in soilless plant culture today. An interesting development in the Hoagland's solution story is that the formula works well enough to have been adapted for use in tissue culture media, which includes an agar base for growing plants in vitro. Whereas Hoagland's solution is a familiar name to hydroponic growers and tissue culture laboratories, many other nutrient solutions have been developed over the years.
Dr. Marietta Loehrlein currently teaches horticulture classes at Western Illinois University in Macomb, Illinois. She earned both her bachelor's degree in Agronomy and her master's degree in Plant Genetics at The University of Arizona. Her master's research project was concerned with germination problems associated with triploid seeds, from which seedless watermelons grow. Following that she worked for 5 years in a breeding and research program for Sunworld, International near Bakersfield, California. She worked with peaches, nectarines, plums, apricots, and cherries. Then she returned to school to earn her Ph.D. in Horticultural Genetics at The Pennsylvania State University. Her Ph.D. research focused on flowering processes in regal pelargonium (also called Martha Washington geraniums). While at The Pennsylvania State University, she bred a new cultivar of regal pelargonium, "Camelot." At Western Illinois University, Dr. Loehrlein teaches nine courses: Greenhouse and Nursery Management, Introductory Horticulture, Landscape Design, Landscape Management, Home Horticulture, Plant Propagation, Turf Management, and two courses in Plant Identification.
TABLE 5-1 U.S. System of Classification of Soil Particles PARTICLE DIAMETER (INCHES) Clay <0.00008 Silt 0.00008-0.002 Sand 0.002-0.08 TABLE 5-2 Soil Depth Categories DEPTH (INCHES) Deep [greater than or equal to]36 Medium 20-36 Shallow 10-20 Very shallow <10 TABLE 5-3 pH Values of Some Common Substances SUBSTANCE pH VALUE Battery acid 0 Hydrochloric acid 1 Stomach acid 1.2-1.4 Lemon juice 2.2-2.4 Orange juice 3.0 Vinegar 3.0-4.0 Beer 4.0-5.0 Hydrogen peroxide 4.5 Coffee 5.0 Aspirin 5.0 Tea (black) 6.0 Milk (cow's) 6.5 Human saliva 6.0-7.5 Water 6.5-8.0 Baking soda 8.0 Sea water 8.3 Borax 9.0 Lava soap 10.0 Ammonia 10.0 Milk of Magnesia 10.6 Oven cleaner 13 Liquid drain cleaner 14 TABLE 5-4 Ions Taken up by Plants NUTRIENT CATIONS ANIONS Nitrogen NH4+(ammonium) N[O.sub.3-] (nitrate) Phosphorus HP[O.sub.4.sup.2-], [H.sub.2]P[O.sub.4-] (phosphate) Potassium [K.sup.+] Calcium [Ca.sup.2+] Magnesium [Mg.sup.2+] Sulfur S[O.sub.4.sup.2-] (sulfate) Iron [Fe.sup.2+], [Fe.sup.3+] Manganese [Mn.sup.2+], [Mn.sup.4+] Boron B[O.sub.3.sup.3-] (borate) Molybdenum M[O.sub.4.sup.2-] Zinc [Zn.sup.2+] Chlorine [Cl.sup.-] TABLE 5-5 Some Organic Soil Amendments and Important Properties Compost Can easily be made at home from kitchen scraps and yard waste. See section on compost for details. Grass Decompose rapidly. Provide high level of nitrogen and clippings some phosphorus and potassium. Leaves Use leaves that dry well and do not become moldy. Oak leaves are preferred; maple leaves can become moldy. Shredded leaves will decompose more quickly, allowing nutrients to become available. Milorganite Activated sewage sludge (dried microbes) from the Milwaukee Municipal Sewerage District. Is granular and porous. Some claim it to have deer repellent properties. Peat moss Has a low pH, may require adjusting with lime or calcium. Readily available. More costly than other choices. Rice hulls Maintains uniformity and stability in the container. Low water-holding and nutrient-storing capacities. Should be mixed with other, more fertile components. Well rotted Cow and horse manure cure the quickest. May contain manure weed seeds if not properly cured. Sheep and hog manure may require 5 years before it is no longer too hot. TABLE 5-6 Compost Ingredients TYPE OF MATERIAL C/N DETAILS Algae, seaweed, and N Good nutrient source. lake moss Cardboard C Shred into small pieces for quicker decomposition. Coffee grounds and N Good for earthworms. filters Cornstalks, corn cobs C Shred and balance with nitrogen- rich materials. Hair N Mix in well so it does not clump. Manure (horse, cow, pig, N High in nitrogen. sheep, goat, chicken, rabbit) Newspaper C Shred for quicker decomposition. Avoid slick colored pages. Oak leaves C Shredded leaves break down faster. Pine needles and cones C Acidic and decomposes slowly. Use sparingly. Sawdust and wood C Slow to decompose; add nitrogen- shavings (untreated rich material to speed up. wood) Avoid treated woods. TABLE 5-7 Average Carbon/Nitrogen Ratio of Some Compost Ingredients * MATERIAL C:N RATIO Blood meal 3:1 Cardboard 400:1 Coffee grounds 25:1 Dry leaves 60:1 Fruit waste 16:1 Grass clippings 15:1 Manure, fresh 8:1 Manure, well-rotted 20:1 Sawdust 225:1 Small sticks and twigs 375:1 Weeds (vegetative parts) 10:1 * Actual ratios vary, depending on the source of the material. A 30:1 ratio can be achieved by mixing ingredients in the appropriate proportions. FIGURE 5-1 Relative volume of soil, water, air, and organic matter in a typical mineral soil. Air 25% Pore Space 50% Water 25% Organic Matter 5% Solids 50% Minerals 45% Note: Table made from pie chart.
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|Author:||Loehrlein, Marietta M.|
|Publication:||Home Horticulture: Principles and Practices|
|Date:||Jan 1, 2008|
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