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Chapter 2 The soil.

Objectives

Upon completion of this chapter, you should be able to

* state how and why soils differ.

* list the components of soil, major soil separates, and the soil textures they create.

* list the seventeen elements essential to plant growth and their functions and symptoms of their deficiency in plants.

* define good soil structure and list the factors that promote it.

* define soil acidity and alkalinity in terms of pH.

* compare the qualities of fertilizers.

* describe how essential elements in the soil become available for plant use.

WHAT IS SOIL?

If asked to define soil, most people would probably describe where it is rather than what it is. They might also describe what it does, but not how it does it. Like so much of the natural world, soils is taken for granted--praised when the backyard garden is bountiful and disparaged when it is tracked in on the new carpet. Perhaps the greatest evidence of knowing little about the soil is to label it dirt.

Soil is the underground environment of plants, and that part of the definition (where it is) is generally understood by all. What it does and how it originates are less widely understood. Most people are aware that the plant life of the continent changes greatly from region to region. It should not be too surprising then to learn that the soil also changes considerably from place to place. Therefore, any attempt to define and describe soil must be approached in general terms, applicable from the red clay regions of Georgia through the loam fields of Iowa to the deserts of Utah.

Soil is the thin outer layer of the earth's crust, made up of weathered minerals, living and nonliving organisms, water, and air. To understand fully that definition is to understand much of modern soil science.

Weathered Minerals

Imagine a cross-sectional slice made down into the earth's crust (Figure 2-1). This is called a soil profile. The mineral content of soil results from the weathering of solid bedrock or other parent material over long periods of time. The solid rock is acted on by an assortment of natural forces including temperature alternations that crack the rock, water that freezes within the cracks, and plant roots that further pry the cracks open. Then follows abrasive grinding by wind, water, and sometimes ice. Lichens and microbes may produce organic acids that react with the rock to further weaken it. While the weathering of parent material takes place over eons of geological time, it is nonetheless a simple breakdown of large pieces of rock into smaller particles.

[FIGURE 2-1 OMITTED]

Living and Nonliving Organisms

Over time, distinctive layers develop in undisturbed soils. Between the parent layer and the topsoil is the subsoil. It is finely weathered like the topsoil, but it lacks organic matter in the quantity found in the topsoil layer. The roots of green plants flourish in the topsoil, richest in organic matter and shallowest in depth, and rely on it for nutrients, support, water, and air.

The organic matter in soil comes from the decomposition of plant and animal tissue. When green plants are plowed into the soil they are immediately acted on by soil organisms that rapidly break the plant tissue down into a form usable for their own growth. Organic compounds that do not decompose quickly eventually succumb to enzymatic action, forming a complex mixture called humus. Humus as well as green manure (plowed under green plants) are important to the soil's structure. This organic matter increases both the water-holding and mineral-holding capacity of the soil.

Water and Air

Water and air exist around and between the soil particles. As much as 50 percent of the topsoil may be air and water in liquid or vapor form. The ratio of air to water depends on the texture of the soil and how wet it is. A wet soil leaves less space for the air to occupy than a dry soil.

WHY SOILS DIFFER

Soils vary in many ways. They vary in color and weight. Some drain easily whereas others stay wet and bog-like. Some are rocky, breaking tools and backs, while others are easy to dig. Even though the original parent stone may be the same or similar, differences in the subsoil and topsoil may result from variations in:

* weathering elements

* soil movement

* topography

* climate

* amount of organic mater

Soils that weather from bedrock and remain in place are termed sedentary, in contrast to transported soils. Transported soils have been moved by the forces of nature.

1. Colluvial soils have moved in response to gravity, as after a landslide or mudslide.

2. Alluvial soils are carried in water such as rivers. They are eventually deposited on flood plains and at deltas.

3. Aeolian soils are transported and deposited by winds.

4. Glacial till is soil deposited by glaciers.

The best agricultural soils are usually alluvial and glacial till. Colluvial soils are characterized by coarse textures and other undesirable chemical and physical qualities. Aeolian soils are finely textured but vary greatly in their productivity. Sedentary soils may be useful for agriculture if they have not lost their nutrient elements.

SOIL SEPARATES AND SOIL TEXTURE

As the parent rock weathers, it forms particles of differing sizes. Based on their diameter, these particles are classified into groups called soil separates. In decreasing order of size, the separates are:

1. Gravel (coarse and fine)--2.0 mm or more in diameter.

2. Sand (very coarse--[2.00-1.00 mm], medium [.50-.25 mm], fine [.25-.10 mm] and very fine [.10-.05 mm])

3. Silt--(.05-.002 mm in diameter)

4. Clay--less than 0.002 mm in diameter

The relative proportions of these separates of different sizes in any one soil create the soil texture. The proportions can only be determined precisely in a soil laboratory. There, a series of sieves are used to separate out the sand (and gravel, if present), while a suspension and settling technique is used to separate and measure the percentages of silt and clay (Figure 2-2).

[FIGURE 2-2 OMITTED]

Most soils in nature contain sand, silt, and clay in some proportions. The textural names given to soils are ways of describing these proportions. For example, if a soil contains about 40 percent sand, 20 percent of the finer clay, and 40 percent silt separates, it is termed a loam. Loams are generally favored for horticultural field production because they have enough sand to provide good drainage and aeration, yet enough of the finer particles to retain moisture and provide necessary plant nutrients (as will be discussed later). When one or two separates dominate the mixture, the textural name given the soil reflects that domination, as in sandy loam, silt loam, silty clay loam, sandy clay, silty clay, and so on.

In order to appreciate soil textural names, the physical and chemical properties of the separates should be considered.

Sand

Sand particles have assorted shapes and sizes depending on how they were weathered. They range from smooth and round to sharp and angular. The spaces between sand particles are large compared to the spaces between silt and clay particles. Water passes through sand quickly because of this large pore space, and air is present in greatest quantity in sand. Sand is low in mineral nutrients and is generally inactive chemically.

Silt

Silt particles are irregularly shaped and much smaller than most sand particles. Given identical volumes of sand and silt, there would be many more silt than sand particles; hence silt has a greater surface area than sand. Since water clings to particle surfaces, the result is that silt holds water in the soil far better than sand. It does not provide as much space for air, however. Like sand, silt has a low nutrient level and is not very actively chemically.

Clay

Clay has very small, plate-like particles. It possesses the greatest surface area of all the separates. Water is held tightly to the clay particles and passes very slowly through the soil. Predictably, air is often in short supply in a heavy clay soil, especially when it is wet. Clay has an adhesive quality when moistened and squeezed. This is what gives cohesiveness to soil, sometimes too much, creating sticky, hard-to-plow fields. Clay is active chemically. Many of the particles have surface charges, which attract water and ions. The term for such a chemical state is colloidal. It is the colloidal quality of clay that makes it important for chemical activity and nutrient exchange in the soil.

The names of soil textures should now assume descriptive meaning. For example, a sandy loam would possess the following characteristics:

* sand, silt, and clay separates present, with sand dominant

* drainage good and perhaps slightly excessive

* nutrient content good since clay is present

* aeration good due to the sand, assuming that it is not too fine

* water-holding capability fair to good, depending on the amount of organic material and clay

The United States Department of Agriculture (USDA) soil texture triangle illustrates how soils are named, based on a laboratory determination of their composition (Figure 2-3).

SOIL NUTRIENTS

As described in Chapter 1, green plants require nutrients which they obtain from the soil. A rich loam soil will provide a balanced supply of the elements essential to the growth of most plants. A soil that is predominantly sand or silt will be nutritionally poor.

The need by green plants for at least 17 separate chemical elements has been proven repeatedly through tests that demonstrate growth abnormalities when any one of these essential elements is lacking. As previously mentioned, the amount of the chemical element required by a plant is not a measure of the element's essentiality. Whether required in large amounts (macronutrients) or very small amounts (micronutrients), the element is essential if the plant cannot grow and develop normally without it.

The elements presently known to be vital to the survival of green plants are shown, along with their chemical symbols, in Table 2-1. One method of remembering the essential elements is to associate them with a catchy phrase. For example:

See MG men mob Cousin Hopkins nice clean cafe C Mg Mn MoB CuZn HOPKNS Ni Cl CaFe

[FIGURE 2-3 OMITTED]

Of the essential elements, the plant obtains only carbon, hydrogen, and oxygen from sources other than the soil. The remainder are absorbed as minerals from the soil around the plant's roots.

Knowing that the elements are necessary still does not address the question of what each element does for the plant that makes it so essential. The roles of several have not yet been clearly defined; they are believed to allow certain enzyme systems to function normally in the plant. At least some of the functions of other essential elements are known, as well as the symptoms shown by the plant when the element is lacking or in short supply. A brief summary of the functions of 14 essential mineral elements and the symptoms of their deficiency is given in Table 2-2. Both the functions and symptoms are discussed further in later chapters.

In addition to the 14 essential mineral elements and three essential nonmineral elements, there is another group known as the beneficial elements.

They have been found to promote plant growth in many species, but have not been proven to be absolutely necessary for completion of the plants' life cycle. Future research may yet prove them to be essential. Currently regarded as beneficial elements are silica, sodium, cobalt, and selenium. Other elements being considered for inclusion as beneficial are chromium, vanadium, and titanium.

Plants may exhibit symptoms of nutrient deficiency for several reasons:

* The element may be lacking totally or not be present in sufficient quantity.

* The element may be bound in a chemical form unavailable or too slowly available to the plant.

* There may be an overall imbalance of nutrients in the soil.

While micronutrients can be and often are deficient in soils, macronutrients are most often deficient. Nitrogen is foremost among the elements regularly lacking in sufficient quantities to produce strong, healthy plants. When nitrogen, in the nitrate form, is not absorbed by the colloidal particles of the soil, it passes quickly through the root region of the soil in an action called leaching.

SOIL STRUCTURE AND ORGANIC MATERIAL

In good loam soils, small soil particles adhere together to form larger particles or aggregates. This arrangement of soil particles into aggregates is termed the soil structure. Structure resulting from small porous aggregates is highly desirable since it blends the desirable qualities of looseness, drainage, and aeration with water and mineral retention. In the bare hand, good loam soil feels like short pastry dough. This may explain why such a structure is often referred to as a crumb structure or a granular structure.

Since the granular structure of soil is a desirable attribute, resulting in high-quality horticultural and field crops, it is important to understand how such soil structure develops and can be encouraged.

The most significant factor in the development of a granular structure is organic matter, including green manure, soil organisms, decomposing plant roots, and especially humus. The organic material and colloidal clay bind the small mineral particles together as crumb-like aggregates. Humus is highly significant as a binding agent.

The characteristics of organic matter have already been described briefly. When a field of grasses, weeds, and other herbaceous plants is turned under, the green manure represented by their plant parts is rapidly acted on by organisms of the soil. Macrobial life, such as insects and earthworms, feed on the plant parts to obtain the chemical energy bound within them. Microbial life--principally algae, fungi, bacteria, and actinomycetes--may directly attack the decomposing plant parts or contribute indirectly through digesting the excretions of the macrobes and the dead macrobes themselves.

The breakdown of dead plant tissue and other organic material by the microorganisms of the soil is accomplished with digestive enzymes in a manner similar to the way an animal's stomach digests food. Some chemical compounds within the plant break down more quickly than others. The simple proteins, sugars, and starches decompose quickly; the more complex organic compounds, like lignin, a component of the cell wall, decompose more slowly. Eventually, though, all organic matter decomposes into either humus, energy, or a number of other end products including carbon dioxide, water, nitrates, phosphates, sulfates, and calcium compounds. The energy release explains why the temperature rises inside a compost pile.

SOIL ACIDITY AND ALKALINITY

The soil's water, held between the particles and granules of the soil, contains dissolved mineral salts. This liquid is known as the soil solution. The way the soil solution reacts determines the acidity, alkalinity, or neutrality of the soil. Many farmers and home gardeners still refer to the "sweetness" or "sourness" of the soil, harking back to a time when differences in the soil's reaction could be observed and dealt with even if the causes were not fully understood. Today it is understood that some soils contain more hydrogen ions ([H.sup.+]) than hydroxyl ions (O[H.sub.-]). This makes them acidic. Other soils contain more hydroxyl ions than hydrogen ions. They are termed alkaline. When a soil contains equal concentrations of hydrogen and hydroxyl ions, it is termed neutral. The exact relationship between the hydrogen and hydroxyl ions is expressed as a pH number (Figure 2-4).

* A pH of 7.0 is neutral.

* A pH of less than 7.0 is acidic.

* A pH of more than 7.0 is alkaline.

The pH of a soil can only be measured precisely using an instrument known as a pH meter (Figure 2-5). Commercial growers and homeowners can either send soil samples to state or private testing laboratories for a pH test at a nominal charge or purchase their own portable pH meter for immediate results. There are also pH test kits on the market, but their results are imprecise compared to those obtained with a pH meter. In situations where the crop is very sensitive to soil pH, the pH meter test should be used.

Additions to the soil that increase the number of [H.sup.+] ions will lower the pH of the soil; conversely, soil additions that increase the number of O[H.sup.-] ions will raise the soil pH. Many of the materials used to improve the structure and texture of the soil will also modify its pH. For example, peat moss is highly acidic, and its addition to the soil as a source of organic material will have a direct impact on the acidity of the soil solution. Limestone has the opposite effect, contributing alkalinity to the solution. These changes in the pH may or may not be desirable. Thus, additives should be used with caution and with a knowledge of their total impact. Obviously, it is easier to adjust the pH of a greenhouse bench or pot crop where the soil mixture is totally within the control of the grower than it is to change the pH of a 100-acre nursery field, especially since many soils have a strong buffer resistance to pH change. Buffering occurs when hydrogen ions that are held in adsorbed form dissociate from the clay particles and enter into the soil solution to replace those hydrogen ions neutralized by the addition of lime. No pH change will result until enough lime is added to deplete the supply of hydrogen ions that constitute a reserve of acidity in the soil. If the reserve acidity is strong and the field is large, a significant change in pH may be impossible. Where strong buffer resistance is not a factor, the pH of nursery fields can be altered to improve crop production.

[FIGURE 2-4 OMITTED]

[FIGURE 2-5 OMITTED]

Within the pH range of 4.0 to 9.0, the availability of many mineral nutrients is determined by the acidity or alkalinity of the soil. For example, many plants will exhibit a distinctively patterned yellowing, or chlorosis, when grown in soil having a high pH. The cause of the chlorosis is lack of iron in the plant tissue, and it results because iron compounds, needed by the plant, are precipitated out of the soil solution and rendered unavailable to the plant. At a lower pH, the iron will remain in the soil solution and be available for plant uptake.

CATION EXCHANGE

To understand how colloidal clay particles and humus contribute to the chemical reactions of the soil, how the soil's pH can be modified, and how the application of chemical fertilizer can increase the nutrients in soil requires an understanding of cation exchange. The term refers to the capacity of colloidal particles to attract positively charged ions (cations) and to exchange one ion for another. Without cation exchange, nutrients would be readily leached from the soil. With cation exchange, the hydrogen cations held by the colloidal particles can be replaced by cations furnished through the decomposition of organic material, the weathering of rocks, or the application of fertilizers. It follows that soils having a higher percentage of colloidal particles, such as clay soils and organic soils, have a higher capacity for cation exchange than sandy soils that are lower in colloidal particles. Therefore, they have a higher capacity to hold available nutrients and resist loss due to leaching.

Role in pH

It is the replacement of hydroxyl cations on the colloidal particles of the soil by hydrogen ions that makes a soil acidic. To make a soil more acidic (lower the pH), elemental sulfur is usually added. In the soil, bacteria convert the sulfur to sulfuric acid. To make the soil more alkaline (increase the pH), calcium or calcium-magnesium compounds are commonly used.

The influence of pH on cation exchange is centered around the availability of nutrients as described earlier and illustrated by iron chlorosis at high soil pH. There are other elements that become bound tightly within the soil and are unavailable to plants when the pH is high, just as there are elements made unavailable when the pH is too low. At extremely low or high pH ranges the mere excess of [H.sup.+] or O[H.sup.-] ions can be toxic to the plants.

Figure 2-6 illustrates how various nutrients become more or less available to the plants as the pH of the soil increases or decreases.

Role in Mineral Absorption

It seems logical to assume that minerals are absorbed into plant roots as water is absorbed. It is a logical assumption but an incorrect one. The uptake of water and the uptake of minerals are independent processes.

[FIGURE 2-6 OMITTED]

Minerals enter root cells through a permeable membrane when the concentration of the mineral salts in the soil solution is greater than in the root cell. Such a condition creates a concentration gradient. Since plants are continually using the mineral salts within the roots, the concentration gradient serves to explain how certain elements are absorbed. With others, absorption occurs even against a concentration gradient. The explanation is thought to reside with ion exchange or with contact exchange.

In ion exchange, a positively charged ion may be absorbed by a root cell if another positively charged ion is released from the cell. Another form of ion exchange can occur when both a positively charged ion (cation) and a negatively charged ion (anion) are absorbed by the root cell together, thus maintaining the electrostatic equilibrium in the cell.

In contact exchange, the intimate association between the soil particles and the root hairs is the key. A direct exchange occurs between the ions adsorbed to the particles of soil and those of the root cells.

FERTILIZERS

Fertilizers are nutrient additives applied to the soil periodically to maintain optimum crop productivity. The need for fertilization may result from a deficiency of one or more mineral elements in the soil, their presence in a form unavailable to the plant, or the leaching of elements into the soil to a depth below the root zone.

Chemical Fertilizers

Since nitrogen, phosphorus, and potassium are the soil elements used in greatest quantity by the green plant, a fertilizer that provides all three elements is termed a complete fertilizer. The actual percentage by weight of each of the three primary elements in a fertilizer determines its analysis; for example, 100 pounds of 10-6-4 analysis fertilizer contains 10 pounds of nitrogen (N), 6 pounds of phosphoric acid ([P.sub.2][O.sub.5]), and 4 pounds of potash ([K.sub.2]O). The analysis figures are always expressed in the same order and represent the same nutrients. Note that phosphorus and potassium are not present in elemental form, but as chemical compounds. Thus the amount of actual element provided for the plant is less than the analysis implies (Figure 2-7).

[FIGURE 2-7 OMITTED]

With simple arithmetic, fertilizers can be compared on the basis of their nutrient ratio. The ratio is a reduction of the analysis to the lowest common denominator. For example, a 5-10-10 analysis has a ratio of 1-2-2. (Each of the numbers has been reduced through dividing by a common factor of 5.) A fertilizer analysis of 10-20-20 also has a ratio of 1-2-2. Therefore, a 5-10-10 fertilizer supplies the three major nutrients in the same proportion as a 10-20-20 fertilizer, but twice as much of the actual product must be applied to obtain the same amount of nutrients (Table 2-3).

When less than 30 percent of a complete fertilizer's weight represents available nutrients, it is termed a low-analysis fertilizer. When the amount of available nutrients is 30 percent or more, the product is a high-analysis fertilizer. The remaining material in a fertilizer is filler, either organic or snythetic. The filler may provide some additional essential elements, and may even be important as a source of micronutrients, but essentially the filler is a carrier for the available macronutrients. It allows them to be applied evenly to the soil and crop. It also adds weight and bulk, both undesirable features. Table 2-4 compares high-analysis and low-analysis fertilizers.

The more common chemical fertilizers include: anhydrous ammonia, ammonium nitrate, urea, sodium nitrate, and ammonium sulfate as carriers of nitrogen; superphosphates, ammoniated phosphates, and round rock phosphate as carriers of phosphorus; and potassium nitrate, potassium chloride, potassium sulfate, and potassium-magnesium sulfate as carriers of potassium.

Organic Fertilizers

Organic fertilizers tend to be low in nutrient content, especially nitrogen. Although they enjoy some popularity with organic gardeners and hobbyists, they have limited application to commercial ornamental horticulture. Not only are the nutrients limited in the organics, but they are often more slowly available to the plant than those in chemical fertilizers. Organic fertilizers include materials such as dried blood, cocoa meal, animal manures, dried sewage sludge, and bone meal. The latter two materials have some commercial usage.

Sewage sludge is used as a top-dressing on golf greens and bone meal as a high-phosphorus fertilizer for flower bulbs.

Ammonification and Nitrification

The presence of an element in the soil does not guarantee that the plant can make use of it. It may be an essential element but in a form that must undergo chemical change before being available to the plant. Nitrogen is made available in green plants as nitrate salts (N[O.sup.-.sub.3]) or ammonium salts (N[H.sup.+.sub.4]). In turn, members of the animal kingdom depend almost exclusively on green plants for their nitrogen which they take from plants' proteins and amino acids.

Nitrogen salts are not found as minerals in the soil. They are formed by the decomposition of nonliving plants, plant parts, animals, or excretory products. Integral to this degradation are several separate groups of bacteria that are key factors in the twin conversions known as ammonification and nitrification.

Ammonification is the conversion of nitrogen in organic compounds to ammonia. Nitrification is the conversion of ammonia to nitrite, then to nitrate as shown.

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

The energy produced by these reactions is used by the bacteria for their growth and development. Collectively, the two processes can be diagrammed as follows:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

Three additional points deserve mention. First, the nitrifying bacteria are sensitive to low temperatures, acidity, and poor drainage. In such conditions, the bacteria do not drive the biochemical changes as rapidly as they do under more favorable conditions. Second, the microorganisms of the soil need nitrogen for growth as much as green plants or animals do. As a result, the addition of large quantities of organic matter to the soil can result in a temporary decline in the amount of nitrogen available to crop plants as the soil microbial population increases. Finally, three genera of bacteria (Azotobacter, Clostridium, and Rhizobium) are able to convert atmospheric nitrogen gas ([N.sub.2]) into organic compounds. After the bacteria die, the processes of ammonification and nitrification release the nitrogen for use by green plants. Especially important is the relationship between the Rhizobium bacteria and members of the bean family (the legumes or Leguminosae). The bacteria live in the legume roots where they obtain organic food materials. In return, the bacteria capture or fix nitrogen gas, making it available to the green plant.

Phosphorus

Phosphorus is present in very small amounts in mineral soils. An even smaller amount is present in a form usable by plants at any one time, since even simple phosphorus compounds are usually insoluble in the soil solution. Along with nitrogen, which is also present in small amounts, phosphorus is often a limiting factor in soil nutrition.

Much of the soil's phosphorus is bound within the organic matter of the soil and becomes available to plants as organic material decomposes. The phosphorus held within the mineral matter of the soil is much more slowly available to the plant. Only through a slow interaction between the insoluble phosphate, water, carbon dioxide, and various root exudates does the phosphate become water-soluble, and, hence, available. Even then, the soluble phosphates can quickly revert to complex, insoluble forms, especially as the pH increases or decreases.

Potassium

Potassium is present in soil in much larger quantities than either nitrogen or phosphorus. Nearly all the soil's potassium is in inorganic forms, however, and inorganic potassium is not readily available to plants. It becomes available slowly through the reaction of water and carbonic acid in the soil with the feldspars, micas, and other sources of insoluble potassium.

The other essential elements require similar chemical reaction in the soil to become available for absorption by the plant. Soil chemistry is a complex field of study and the brief treatment given the subject here has only touched on a few of the points necessary to an understanding of crop production.

SUMMARY

Soil is the thin outer layer of the earth's crust, made up of weathered minerals, living and nonliving organisms, water, and air. It provides the underground environment of plants. In profile, three distinctive layers can be seen: bedrock or parent material, subsoil, and topsoil. Both subsoil and topsoil are finely weathered from the parent material, but topsoil contains more organic matter than subsoil. As a result, it is more supportive of plant growth.

Organic matter is composed of plant and animal compounds that soil microbes can rapidly break down, and of humus. Humus is a complex colloidal mixture that originates with organic compounds that do not decompose easily. Humus and other organic matter are important in the development of good soil structure, causing small soil particles to bind and form larger particles or aggregates.

Soils differ in many ways such as color, weight, drainage, rockiness, and texture. The differences result from weathering elements, soil movement, topography, climate, and variations in the amount of organic matter.

When parent material weathers, particles of differing sizes are formed. The four groups of weathered particles or separates are gravel, sand, silt, and clay. The relative proportion of these separates in any one soil creates the soil texture. The textural name of a soil roughly describes the particular mixture of separates that it contains.

Because of their colloidal properties, clay and humus are instrumental in making nutrients available to plants. Seventeen nutrients have been found essential to the growth of green plants (C, H, O, P, K, N, S, Ca, Mg, Fe, Mo, B, Cu, Mn, Zn, CI, Ni). Of these essential elements, only carbon, hydrogen, and oxygen are obtained by the plant for sources other than the soil. The remainder are absorbed as minerals from the soil around plant roots. When one or more element is lacking totally or in part, is bound in a form unavailable to the plant, or is part of an overall nutrient imbalance in the soil, the plant will exhibit symptoms of nutrient deficiency.

Several other elements have been found to be beneficial to plant growth, even though they are not essential to completion of the plant life cycle. The soil solution is a liquid composed of water held within the soil and mineral salts dissolved in the water. How the soil solution reacts chemically determines its acidity or alkalinity and is expressed as a pH number. Soils that contain more hydrogen ions ([H.sup.+]) than hydroxyl ions (O[H.sup.-]) are acidic and have a pH of less than 7.0. Soils containing more hydroxyl ions than hydrogen ions are alkaline and have a pH greater than 7.0. A soil with equal concentrations of hydrogen and hydroxyl ions is neutral and has a pH of 7.0. Soil additives can raise or lower the pH of the soil depending on whether they increase the number of H+ ions or O[H.sup.-] ions.

Modification of pH and nutrient uptake by plants depend on cation exchange: the capacity of colloidal particles to attract positively charged ions (cations) and exchange one ion for another. Those soils with a higher percentage of colloidal particles, such as clays and organic soils, have the highest cation exchange capacity. If soil pH becomes too high or too low, certain elements become bound tightly within the soil and unavailable to the plants.

Fertilizers are nutrient additives applied to the soil periodically to maintain optimum crop productivity. They may be complete or incomplete, high-analysis or low-analysis, inorganic (chemical) or organic. Usually, the elements within the fertilizer must undergo chemical changes before becoming available to the plant. Ammonification and nitrification exemplify such changes.

Achievement Review

A. SHORT ANSWER

Answer each of the following questions as briefly as possible.

1. Define the following terms:

a. soil

b. topsoil

c. subsoil

d. humus

e. soil texture

f. essential element

g. macronutrients

h. micronutrients

i. leaching

j. soil structure

k. cation

l. cation exchange

m. complete fertilizer

n. fertilizer analysis

o. nutrient ratio

p. high-analysis fertilizer

q. low-analysis fertilizer

2. List five ways in which the same parent material could become quite different subsoil or topsoil.

3. Match the type of transported soil with the correct agent of transport.

a. aeolian soil

b. coluvial soil

c. glacial till

d. alluvial soil

1. water

2. sedentary

3. gravity

4. ice

5. wind

4. List the four soil separates in order of increasing particle size.

5. From the soil texture triangle, identify soils having the following analysis:

a. 60 percent sand 30 percent silt 10 percent clay

b. 25 percent sand 55 percent silt 20 percent clay

c. 25 percent sand 30 percent silt 45 percent clay

d. 80 percent sand 10 percent silt 10 percent clay

6. Based on the textural name, arrange the following soils in decreasing order of water retention: sandy loam, silty clay, clay, clay loam, sand.

7. Indicate whether the following statements apply most appropriately to clay, silt, or sand.

a. It is the most chemically active particle in the soil.

b. The size of the air spaces between the particles is greatest.

c. Water passes most quickly through this separate.

d. This separate has small particle size and low nutrient value.

e. The particles are small and plate-like.

f. It is the separate most important to soil nutrition.

8. List the 17 essential elements and their chemical symbols.

9. Compare high and low analysis fertilizers by placing an X in the appropriate column after each of the characteristics listed below.

[ILLUSTRATION OMITTED]

B. ESSAY

1. Describe the natural decomposition of organic matter in the soil and its relationship to soil structure.

2. Explain how nutrients move from the soil into the plant.

C. TRUE/FALSE

Indicate if the following statements are true or false.

1. Excess hydrogen ions in the soil make it acidic.

2. Excess hydroxyl ions in the soil make it alkaline.

3. An equal concentration of hydrogen and hydroxyl ions results in a pH of 6.0.

4. A pH of 7.0 represents a state of chemical neutrality in the soil.

5. A large nursery field with strong buffer capacity could have its pH changed easily.

6. The soil pH of potted greenhouse crops can usually be changed easily.

7. A field test kit is the most precise way to measure soil pH.

8. Organic matter can lower soil pH.

9. Sulfur can lower soil pH.

10. Limestone can raise soil pH.
TABLE 2-1. The Essential Elements

Macronutrients    Micronutrients

Calcium (Ca)      Boron (B)
Carbon (C)        Chlorine (Cl)
Hydrogen (H)      Copper (Cu)
Magnesium (Mg)    Iron (Fe)
Nitrogen (N)      Manganese (Mn)
Oxygen (O)        Molybdenum (Mo)
Phosphorus (P)    Nickel (Ni)
Potassium (K)     Zinc (Zn)
Sulfur (S)

TABLE 2-2. Essential Elements with Their Functions and Symptoms of
Deficiency

Element            Function in the Plant

Boron (Bo)         * Role not clearly defined except in translocation
                     of sugar. It has a role in flower and fruit
                     development as well.
Calcium (Ca)       * A component of the cell wall
                   * Needed for cell division and growth
Chlorine (Cl)      * Role not clearly defined, but important to
                     shoot and root development
Copper (Cu)        * A catalyst for respiration
                   * Needed for photosynthesis
Iron (Fe)          * Needed for chlorophyll synthesis
Magnesium (Mg)     * Essential for photosynthesis as a component
                     of chlorophyll molecule
                   * An important enzyme activator
                   * Needed for sugar and fat formation
Manganese (Mn)     * An enzyme activator in respiration
                   * Makes nitrogen available for plant use
Molybdenum (Mo)    * Makes nitrogen available for plant use
                   * Needed for protein synthesis
Nickel (Ni)        * Role not clearly defined, but believed to be
                     instrumental in enzymatic functions
Nitrogen (N)       * Important to synthesis and structure of
                     protein molecules
                   * Encourages vegetative growth
                   * Promotes rich green color
Phosphorus (P)     * Essential to energy transfer
                   * Stimulates cell division
                   * Needed for flowering
                   * Promotes maturation
                   * Promotes disease resistance
                   * Promotes root development
Potassium (K)      * Role not clearly defined
                   * Believed to activate important plant systems
                     and enzymes
Sulfur (S)         * Important to structure of protein molecules
                   * Needed for enzyme activity to occur
Zinc (Zn)          * Important to the synthesis of plant auxins
                   * An important enzyme activator
                   * Needed for protein synthesis

Element            Symptoms of Deficiency

Boron (Bo)         * Dead shoot tips
                   * Leaves thicken, curl, and become brittle
                   * Flowers fail to form
                   * Stunted roots
Calcium (Ca)       * Stems, leaves, and roots die at tips, where
                     growth is normally most active
                   * Chlorosis of young leaves, then necrosis
                     along margins
Chlorine (Cl)      * Stunting
                   * Chlorosis
Copper (Cu)        * Necrosis is young leaf tips and margins
                   * Stunting
Iron (Fe)          * Chlorosis of younger leaves only, usually in
                     an interveinal pattern
Magnesium (Mg)     * Interveinal chlorosis, appearing first in older
                     leaves, followed by red or purple color and
                     necrotic spots
Manganese (Mn)     * Interveinal chlorosis and necrosis
Molybdenum (Mo)    * Interveinal chlorosis in lower leaves
                   * Marginal necrosis
                   * Flowers fail to form
Nickel (Ni)        * None have been observed
Nitrogen (N)       * Chlorosis, first noticeable in older leaves
                   * Stunting of growth
Phosphorus (P)     * Red or purple discoloration of older leaves
                   * Premature leaf drop
                   * Stunting of growth
Potassium (K)      * Mottled chlorosis first noticeable on lower
                     leaves
                   * Necrosis at tips and margains of leaves
Sulfur (S)         * Leaf chlorosis first noticeable in younger
                     leaves
                   * Weak stems
Zinc (Zn)          * Interveinal chlorosis on younger leaves
                   * White necrotic spots
                   * Leaf drawing
                   * Distortion of leaves

TABLE 2-3. Comparison of Fertilizers with Same
Nutrient Ratio

50 Pounds of                      50 Pounds of
5-10-10                           10-20-20
Fertilizer Contain:               Fertilizer Contain:

2 1/2 pounds of                   5 pounds of N
  N (nitrogen)
5 pounds of [P.sub.2][O.sub.5]    10 pounds of [P.sub.2][O.sub.5]
  (phosphoric acid)
5 pounds of [K.sub.2]             10 Pounds of [K.sub.2]O
  O (potash)

TABLE 2-4. Comparison of High- and Low-
Analysis Fertilizers

High-Analysis                   Low-Analysis
Fertilizers                     Fertilizers

* Contain more                  * Contain fewer nutrients
  nutrients and less filler       and more filler
* Cost less per pound of        * Cost more per pound of
  actual nutrient                 actual nutrient
* Weigh less; less labor        * Weigh more and are
  required in handling            bulkier; more labor
                                  required in handling
* Require less storage          * Require more storage
  space                           space
* Require less material to      * Require more material
  provide a given amount          to provide a given
  of nutrients per                amount of nutrients per
  square foot                     square foot
* Require less time to          * Require more time to
  apply a given amount            apply a given amount
  of nutrients                    of nutrients
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Title Annotation:SECTION 1 The Science of Ornamental Horticulture
Author:Ingels, Jack E.
Publication:Ornamental Horticulture, Science, Operations & Management, 3rd ed.
Geographic Code:1USA
Date:Jan 1, 2001
Words:6456
Previous Article:Chapter 1 The green plant.
Next Article:Chapter 3 Describing and identifying plants.
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