Chapter 12: Plant nutrition.
After completing this chapter, you should be able to:
* discuss nitrogen nutrition and the nitrogen cycle
* discuss phosphorus nutrition
* discuss potassium nutrition
* answer questions about the secondary nutrients
* answer questions about trace elements
TERMS TO KNOW
Many soil factors, such as texture, structure, and water, affect plant growth. Often these conditions are less than ideal but are not easily or cheaply improved. For instance, growers cannot alter soil texture over large areas. Even irrigation is a costly investment.
The supply of soil nutrient elements, on the other hand, can be more easily controlled. Soil can be tested and fertilized to satisfy crop needs. This chapter takes a detailed look at the essential elements.
Nitrogen, more than any other element, promotes rapid growth and dark green color. Plants need a lot of nitrogen because it is part of many important compounds, including protein and chlorophyll. Plants respond to nitrogen in the following ways:
* Nitrogen speeds growth. Plants receiving adequate nitrogen have vigorous growth, large leaves, and long stem internodes.
* Plants make large amounts of chlorophyll, a dark green pigment. Thus, leaves are dark green on well-fed plants.
* Protein content of plant tissue will be at its best. The higher protein content makes the plant a better source of forage, feed, and human nutrition.
* Plants use water best when they have ample nitrogen.
Plants with too much nitrogen do not grow properly, however. Here are some problems associated with too much nitrogen:
* Soft, weak, easily injured growth is encouraged. For example, plant stems are weaker and more easily topple, or lodge, in the rain. Lodging can turn a good crop into a disaster.
* Soft, high nitrogen growth is more prone to some diseases and insects.
* Overly rapid growth slows maturity and ripening of many crops.
* Too rapid growth also delays the hardening-off process that protects many plants from winter cold. Landscape plants, for instance, may suffer winter damage when nitrogen is applied too liberally.
* Excess nitrogen impairs flavor in several vegetable crops.
* High levels of nitrates may accumulate in some crops, with possible health effects for animals or people consuming them.
About half the nitrogen in a leaf occurs in enzymes involved with photosynthesis, so a well-supplied plant photosynthesizes much more efficiently than a deficient plant. This partially explains why nitrogen stimulates growth. On the other hand, we also know that leaves high in nitrogen also respire--use up the food produced by photosynthesis--more rapidly. In lowlight situations, where photosynthesis is light-limited, high nitrogen merely depletes food more quickly. Plants growing under low light, like shady turf or indoor plants, should be fertilized more lightly than plants grown in the sun.
In general, nitrogen promotes vegetative growth--stems, leaves, and roots--more than the reproductive growth of flowers and fruit. Home gardeners see the effect if they overfertilize their tomato plants, promoting lush growth but few fruit. Also, while nitrogen aids growth in both root and shoot, shoot growth tends to be favored. This can be a problem in turf, where high nitrogen fertilization can yield lush growth with an inadequate root system to support it during times of stress.
In the natural world, low nitrogen tends to be the primary nutrient limiting growth in land ecosystems of cooler climates, on young soils, and many marine ecosystems.
The Nitrogen Cycle. Of the essential elements, nitrogen undergoes the most movement and change. The series of gains, losses, and changes is termed the nitrogen cycle. The central portion of the nitrogen cycle operates by the action of soil microorganisms. To review briefly (see chapter 5), nitrogen comes from nitrogen gas ([N.sub.2]) in the atmosphere, a form unusable to plants. Symbiotic (figure 12-1) or nonsymbiotic bacteria use that nitrogen to form protein for their own bodies or supply it to host plants. When these bacteria, or host plants, die, other microbes mineralize the protein (ammonification) to ammonium ions (N[H.sub.4.sup.+]). These ions can be taken up by plants, but most are converted by bacteria (nitrification) to nitrite ions (N[O.sub.2.sup.-] and then to nitrate ions (N[O.sub.3.sup.-]. Nitrates are taken up by plants or microbes (immobilization) or return to the atmosphere as nitrogen gas through the process of denitrification. The solid lines in the simplified cycle pictured in figure 12-2 summarize this portion of the cycle.
[FIGURE 12-1 OMITTED]
The complete nitrogen cycle includes some nonbiological processes as well, shown in figure 12-2 as broken lines. Two other forms of fixation add usable nitrogen to the soil. First, lightning during storms provides energy to combine gaseous nitrogen and oxygen to form nitrogen dioxide (N[O.sub.2]). The gas dissolves in water vapor to produce nitric acid (HN[O.sub.3]). About five to ten pounds per acre of nitrogen fall to earth yearly in rain and snow from this source. Second, large amounts of nitrogen are fixed from the air in fertilizer factories (see chapter 14) and applied to soil by growers.
Two nonbiological losses of nitrogen from soil may be important as well. The nitrate ion is negatively charged and so is not adsorbed by soil colloids. Nor is it held in soil by other means, so nitrates easily leach from the soil. Although ammonia does not leach readily (being adsorbed by soil colloids), it too can be lost by a process called ammonia volatilization. Ammonium ions react with hydroxyl ions in the following reaction:
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The smell of an open bottle of household ammonia (ammonia gas dissolved in water) is a result of this reaction. Normally, this is a balanced reaction in the soil, with nitrogen changing back and forth between the two forms. However, the balance can be shifted by soil conditions to cause a loss of ammonia. If soil dries, for instance, water is lost from the right side of the equation. As a result, the reaction shifts to the right and releases ammonia gas (see appendix 1 for an explanation). If the soil is made more alkaline (by liming, for instance), the reaction again shifts to the right because of an excess of hydroxyl ions. Thus, ammonia losses may occur in dry, alkaline, or recently limed soil.
In native habitats, including virgin forests or prairies, gains and losses in the cycle balance over time. However, farming changes the balance greatly in ways that increase nitrogen losses:
* Nitrogen is removed by crop harvest (figure 12-3).
* Cropped soil is more likely to erode, so nitrogen and other nutrients are carried off in running water.
* Irrigation increases percolation of water through the soil profile. Thus, losses of nitrate nitrogen by leaching increase on irrigated land.
* Liming may increase the loss of ammonia by volatilization.
To compensate for increased nitrogen losses and to meet the needs of modern high productivity, growers supply more nitrogen by manuring, growing legumes, or by fertilization. Figure 12-4 shows the nitrogen cycle as it operates on modern farms that raise both crops and animals. There is a strong trend away from a mixed farming operation toward one that specializes in either cash crops or raising animals. This trend improves economic efficiency, but exacts an obvious penalty in view of the nitrogen cycle. For the cash crop grower, more money must be spent on fertilizers. For the animal raiser, manure becomes a waste disposal problem (see chapter 15).
[FIGURE 12-2 OMITTED]
Forms of Nitrogen in the Soil. About 97 percent of soil nitrogen resides in organic matter, the soil's storehouse of nitrogen. At any time, only a small percentage of nitrogen is mineralized to usable forms. On the average, decay makes available about 90 pounds of mineral nitrogen per acre per year (see figure 12-5). However, it also follows that cropping systems that preserve soil organic matter, like no-till and some organic farming, also retain more soil nitrogen. The difference can be substantial. In one 1996 study, after twenty-three years of continuous corn, there were nine milligrams of mineralizable nitrogen per kilogram of soil (in the top 7.5 centimeters) under no-till, compared to 1.4 mg N under conventional tillage. (1)
Both mineral forms of nitrogen, ammonium and nitrates, are taken in by plants. In forest and woodland, ammonium is the most common form. Farm crops usually make more use of nitrate, either from nitrate fertilizer or from nitrified ammonium. The two ions behave very differently in the soil (figure 12-6).
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Ammonium nitrogen bears a positive charge. Negatively charged soil colloids attract the cation, protecting it from leaching. The nitrate ion, by contrast, moves freely in the soil because of its negative charge. Free movement allows nitrate to diffuse easily through soil to plant roots. However, nitrate losses from soil can be high. Nitrate ions leach out of the soil readily, and some may disappear as nitrogen gas in wet soil.
The amount of ammonium and nitrate nitrogen in the soil depends on the amount and type of nitrogen applied to the soil and the rates of nitrification and denitrification. Some nitrogen fertilizers contain nitrates. Most modern fertilizers, however, mainly provide ammonium nitrogen. Nitrifying soil bacteria change this to nitrates, the preferred form for crops. Nitrifying bacteria grow best in moist, loose, well-drained soil at a pH of 6.0 to 7.5. Nitrifying bacteria function poorly below 41[degrees]F and reach maximum activity between 85[degrees]F and 95[degrees]F. Thus, cold, wet, or acid soils slow the conversion of ammonium nitrogen to nitrate nitrogen.
Waterlogged soil prevents nitrifying bacteria from thriving. However, anaerobic denitrifying bacteria thrive in the same conditions. Denitrification causes the greatest loss of nitrogen when soil users apply nitrate fertilizers to wet soils. Similarly, overirrigation of turf wastes fertilizer by stimulating both denitrification and leaching.
Because of potential losses of nitrates, it is useful to control the ammonium nitrification rate. Several chemicals have been developed to inhibit (but not stop) nitrification. In practice, results have been variable, often of benefit but sometimes not.
Nitrogen Deficiency. In all plants, slow growth and stunting are the most obvious signs of nitrogen shortage. Because nitrogen is part of chlorophyll, nitrogen-deficient plants lack the dark green color of well-fed plants. This symptom is called chlorosis. Leaves turn light green, then yellow, starting with the lower leaves. In grasses, yellowing starts at the blade tips, progresses down the midvein, and finally the entire leaf yellows. In extreme cases, the leaf dries up, a symptom called firing. In broadleaf plants, leaves are small with overall yellowing.
Phosphorus also spurs growth but to a lesser extent than nitrogen (figure 12-7). Phosphorus affects plant growth in a number of ways:
* Phosphorus is part of genetic material (chromosomes and genes) and so is involved in plant reproduction and cell division.
* Phosphorus is part of the chemical that stores and transfers energy in all living things. Without it, all biological reactions come to a halt.
* Phosphorus spurs early and rapid root growth and helps a young plant develop its roots.
* Phosphorus helps plants use water more efficiently by improving water uptake by roots.
* Phosphorus helps plants resist cold and disease, speeds crop maturity, aids blooming and fruiting, and improves the quality of grains and fruits.
* Phosphorus improves the efficiency of nitrogen uptake by plants, making better use of fertilizer nitrogen and reducing the risk of groundwater pollution due to nitrate leaching.
* Adequate soil phosphorus ensures that animal feeds will supply sufficient phosphorus.
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In many ways, phosphorus acts to balance nitrogen. While nitrogen delays maturity, phosphorus hastens it. Nitrogen aids vegetative growth; phosphorus aids blooming and fruiting. As a rule of thumb, phosphorus is most important for crops from which we use the floral parts--that is, flowers, fruits, or seeds. One should not overapply this simple rule, however: nitrogen and phosphorus must both be sufficient for both vegetative and flower growth, and supplying more phosphorus than necessary does not stimulate more bloom.
Because phosphorus is needed for root growth, it is often a major element in starter fertilizers, those applied at planting. However, there is no evidence that amounts of phosphorus greater than adequate encourage heavier rooting. In fact, at low phosphorus levels, plants tend to favor roots over shoots to improve uptake, and in greenhouse production of bedding plants, the best root systems are achieved under low phosphorus rates.
In the natural world, phosphorus tends to be the primary nutrient limiting growth in tropical land ecosystems, on old soils, and in freshwater and some marine ecosystems.
Forms of Phosphorus in the Soil. Soil phosphorus is provided by the weathering of minerals like the apatites, which are calcium phosphate minerals. As apatite weathers, it releases anions that can be used by plants. These anions are primary orthophosphate ([H.sub.2]P[O.sub.4.sup.-] and a secondary orthophosphate (HP[O.sub.4.sup.-2]. For simplicity, the text refers to them both as phosphates.
Many soils contain large amounts of phosphate, but much is unavailable to plants. Phosphate in insoluble forms that are not free for plant growth is said to be "fixed." The reactions that fix phosphate depend on soil pH. In strongly acid soil (pH 3.5-4.5), insoluble iron phosphate forms. Between pH 4.0 and 6.5, phosphorus reacts with aluminum. Calcium phosphates are important between pH 7.0 and 9.0. Maximum availability lies at pH 6.5 in mineral soils, but 6.0 to 7.0 is satisfactory for most crops.
Between 25 percent and 90 percent of all soil phosphorus resides in organic matter, an important store-house of phosphorus. Figure 12-8 summarizes the forms of phosphate in the soil.
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A typical acre of soil holds between 800 and 1,600 pounds of phosphorus in the plow layer. Of that total, only about four pounds is in solution at any time. As plants remove phosphate from solution, mineral and organic phosphate become soluble by mineralization and the activities of P-dissolving bacteria. At the height of the growing season, soluble phosphorus may be replaced from soil stores several times daily.
Because phosphorus availability is low in so many of the world's soils, plants have developed adaptations to improve access to it. These include mycorrhizal associations, specialized root systems, high root length densities, longer root hairs, and exudates given off by roots and mycorrhizae that free phosphorus. While these can be effective responses in natural ecosystems, crops often cannot tap soil stores fast enough to produce a full crop. For this reason, growers fertilize soil with phosphate to compensate for fixation.
Movement and Uptake in the Soil. Phosphorus moves very little in mineral soil, diffusing over a distance as small as one-quarter inch. This limited movement has important implications for soil management. It cannot leach downward in soil as do nitrates. Instead of leaching, phosphorus is more commonly lost by runoff, erosion, or blowing soil. It also increases the difficulty of plants in obtaining adequate phosphorus. Because of low mobility, it is critical that phosphate fertilizer be placed near seed when planted or mixed into soil near plant roots.
The uptake of phosphorus depends on a number of soil conditions:
* Soil pH largely sets the degree of fixation. Phosphorus is most free at a pH of 6.5 to 6.8.
* Dry soil stalls the diffusion of phosphorus to roots. Therefore, plants take up phosphate best in moist soils.
* Oxygen is needed for the breakdown of organic phosphates. Roots also need oxygen to take up nutrients. Thus, a loose, well-drained soil improves phosphorus uptake. Compacted or poorly drained soil reduces access.
* Cold soil slows the activity of microorganisms that place phosphorus in solution, slows diffusion to roots, and retards root growth. Root respiration also declines, depriving roots of the energy needed to absorb phosphorus. Phosphate shortages are commonly seen on cold, wet soils.
* The total nutrient balance is also important. Nitrogen, for instance, improves phosphorus uptake. Too much zinc seems to lower it.
* Mycorrhizae infection of plant roots helps the plant absorb phosphorus, especially in phosphorus-deficient soils.
A crop uses only 10 percent to 30 percent of the phosphate fertilizer applied to it. The rest goes into reserve and may be used by later crops. Many growers, in fact, have built up large reserves of soil phosphorus. With annual fertilizer applications, many turf and landscape areas are even better supplied. Only soil testing can tell soil users how much phosphorus crops need.
Deficiency. A shortage of phosphorus can cause stunting and fewer, smaller leaves. Plants remain dark green or may even become darker green than normal. Phosphorus-deficient plants often have a purple tint to leaves and stems, starting on lower, older leaves. A phosphorus shortage may delay maturity of several crops, including corn, cotton, soybeans, and others. Some crops, like carrots, develop poor root systems. On the other hand, excess soil phosphorus ties up several plant nutrients, such as iron.
Potassium, often called potash, is a key plant nutrient. Plants consume more potassium than any other nutrient except nitrogen, and some plants, like Kentucky bluegrass, may use more. No organic compounds in a plant contain potassium, but many life processes need it. Potassium is dissolved in plant fluids, filling several regulatory functions. Potassium activates enzymes needed in formation of protein, starch, cellulose, and lignin. Thus, it is necessary for the development of thick cell walls and strong, rigid plant stems. Potassium regulates the opening and closing of leaf stoma (pores in the leaf that pass oxygen, carbon dioxide, and water vapor into and out of the leaf ). Therefore, potassium is involved in the gas exchange needed for photosynthesis and in transpiration.
Potassium is instrumental in moving sugars produced by photosynthesis within the plant, so it is important in the development and ripening of fruits like apples or tomatoes. Similarly, potassium is needed for proper growth of root and tuber crops.
Potassium acts to balance the effects of nitrogen, and a particular nitrogen:potassium ratio is suggested for many crops. Nitrogen leads to soft growth, but potassium promotes a tougher growth. The toughness results from thicker cell walls. This increased toughness improves crops in a number of ways:
* Plants well stocked with potassium have strong stems that are less prone to lodging (figure 12-9). In corn, reduced lodging also results from the greater number of brace roots (figure 12-10).
* Well-fed plants fight disease. Potassium reduces diseases such as mildew in soybeans, wildfire in tobacco, and leaf and dollar spot in turfgrass.
* Potassium makes plants more winter-hardy and less likely to be injured by spring or fall frosts.
* Potassium, by regulating the stoma, influences the transpiration rate. A plant well supplied with potassium transpires less and so makes better use of water supplies.
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As an example, with adequate potassium, turf is less disease-prone, more winter hardy, and better resists wear and tear.
The more potassium in soil, the more plants take up. However, there is no evidence that supplying potassium beyond plant needs will additionally increase hardiness or toughness. In addition, excess potassium uptake may inhibit uptake of calcium or magnesium.
Forms of Potassium in the Soil. Weathering releases potassium ions into the soil solution from a number of common minerals such as feldspars and micas. This ion can be easily taken up by plant roots. Little potassium becomes part of soil organic matter, so most is stored in soil by adsorption and fixation.
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Potassium ions bear a positive charge and so are adsorbed on soil colloids. In most mineral soils, a few pounds of potassium are dissolved in the solution of an acre of soil at any one time. In contrast, several hundred pounds per acre of exchangeable potassium occupy cation exchange sites.
Potassium can also be fixed by certain 2:1 clays, trapped between the 2:1 layers, as shown in figure 12-11. This potassium can be released slowly if the potassium level in the soil solution declines. Montmorillinite clay layers are so loose that potassium ions can enter and leave easily, allowing potassium to remain available. Figure 12-12 shows the forms of potassium.
Movement in the Soil. Potassium moves more readily in soil than does phosphorus, but less readily than nitrogen. Because potassium is held on clay or other colloids, it is least mobile in fine-textured soil and most readily leached from sandy soils.
Most plant uptake of potassium occurs by diffusion. Because the element moves more readily than phosphorus, fertilizer placement is less crucial.
Deficiencies. Growers see potassium deficiencies less often than those of other primary nutrients. Shortages occur primarily in sandy, heavily leached soils, especially if irrigated, or in organic soils. Overfertilization with nitrogen can cause plant tissues to lack potassium. Dry, cold, or poorly aerated soil may also slow uptake. Potassium uptake is most rapid near neutral pH.
Plants show a lack of potassium by a "marginal scorch," or burnt look on the edges of the lower, older leaves. This symptom can be easily mistaken for moisture shortage during hot dry weather or for salt damage. In some cases, the margins merely turn yellow.
Calcium. Calcium, the nutrient used in third greatest amounts by most plants, is a critical component of both cell walls and membranes. In cell walls, much is found as calcium pectates, located especially in a layer in the outer part of the cell wall where it lends strength (figure 12-13). The crispness of apples, for instance, derives from a high calcium pectate content. Pectins are the same materials used to jell preserves, and one can make jellies using apple pectins. Calcium also stabilizes plant cell membranes to prevent leakiness.
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Calcium also plays a role in protein formation and carbohydrate movement in plants, and plays a signaling, or regulatory, role in several plant functions--like directing roots to grow down rather than up. It also largely controls soil pH and helps soil aggregation.
Because of its role in cell walls and membranes, calcium shortages present the greatest problems where cells are actively dividing and enlarging, such as root and shoot tips (figure 12-14) and developing fruit. Calcium, being relatively immobile in the plant, may not reach developing fruit or other fleshy plant parts rapidly enough to supply their needs. Water core of apples is a collapse of cells in rapidly growing fruit, leading to soft areas in the fruit. Similarly, blossom end rot in tomatoes is a weakening of tomato fruit at the end furthest from the sap stream, leading to cell collapse and attack by rot organisms. Similar conditions include bract burn in poinsettia and black-heart of celery. Growers often spray plants with calcium to reduce these problems. Adequate calcium, on the other hand, reduces insect and disease infestation and post-harvest decay of many fruits. While several horticultural crops often suffer these calcium shortages, calcium problems are more rare in other crops, especially in grasses, which have low calcium needs. Calcium shortages are most likely on acid, irrigated sands or where excessive potassium levels inhibit calcium uptake.
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Calcium relations in plants also impact human diets. Much of plant calcium is tied up in forms unusable by humans. Those who cannot or do not consume dairy products must rely largely on vegetable sources, and care should be taken to find foods with more available calcium content, like green beans.
Calcium comes from weathering of common minerals including feldspars, limestone, or gypsum. These materials are so common that most soils contain enough calcium to supply most plant needs. Calcium is neither fixed in the soil nor held in organic matter. It is the main occupant of the cation exchange complex, and calcium storage depends on cation exchange capacity (figure 12-15).
Magnesium. Magnesium resembles calcium chemically and in its action in soil. Its role in the plant differs, however. Magnesium is the essential ingredient in chlorophyll--each molecule has one magnesium atom at its center (see figure 12-17). Magnesium also aids the uptake of other elements, especially phosphorus. Like potassium, magnesium activates a number of important enzyme systems. Magnesium is involved in protein, carbohydrate, and fat synthesis, as well as a wide range of other compounds. Deficient plants offer less resistance to drought, cold, and disease.
Magnesium weathers from minerals as a cation (figure 12-15). However, clay holds magnesium less strongly than calcium, so it is more easily leached. As a result, low-magnesium soils are more common than low-calcium soils. Highly leached coarse soils are most likely to need fertilization with magnesium, especially if treated with low-magnesium lime. High levels of soil potassium may also induce a magnesium shortage in plants.
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Hunger signs resulting from low levels of chlorophyll include chlorosis, a yellowing of the leaf, beginning with the older leaves. Forage low in magnesium is known to cause grass-tetany disease in cattle.
Sulfur. Crops need less sulfur than the other macronutrients, but it is still a crucial nutrient. Several proteins include sulfur, and it is needed for making chlorophyll. It aids nodulation of legumes and seed production of all plants. Overall, sulfur improves protein and chlorophyll content, stress tolerance, animal nutrition, and the appearance of plant products. Alfalfa, members of the mustard family (including cabbage), and members of the onion family need much sulfur. The pungent flavors of those plants derive from sulfur compounds.
Most soil sulfur comes from the weathering of sulfate minerals such as gypsum. The sulfate anion is the form used by plants. Organic matter contains 70 percent to 90 percent of the soil sulfur; it is neither adsorbed nor fixed to any degree. Because it is readily leached, surface layers of soil are often low in sulfur. Figure 12-16 reviews the sulfur cycle.
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Interestingly, acid rain supplies sulfur in many areas. In many parts of the country, sulfur from acid precipitation has been reduced by burning low-sulfur coal and by better clean-air controls on exhaust stacks. Older fertilizer types contained sulfur as a by-product of their manufacture. The fertilizers that are most popular now are much purer. Since pollution- and fertilizer-supplied sulfur have both been reduced, shortages are increasingly common. Use of sulfur fertilizer has increased rapidly, especially in the southeastern states. Leached and low-organic-matter soils are likely candidates for sulfur shortage. Soils high in organic matter or soils located near industrial centers are least likely to be short of sulfur.
Plants that are short of sulfur may be stunted and older leaves will be pale green, like those of nitrogen-deficient plants.
Trace elements play many roles in plants, many difficult to understand without knowing plant chemistry. With the exception of boron and chlorine, trace elements are metals. These metals interact with special molecules, called enzymes, that control important biological reactions. Enzymes are "keys" that activate biological reactions in living systems. They are not consumed in the process. For instance, an iron enzyme controls one step in the formation of chlorophyll, but is not itself part of chlorophyll.
Very little of each enzyme is needed, because each is reused repeatedly. Therefore, very little of the trace elements that are part of enzymes is needed. Without this tiny amount, however, important processes suffer. On the other hand, an excess of a trace element can be toxic to plants or the animals feeding on them. The difference between enough and too much can be quite narrow, sometimes only a few pounds per acre. Growers should apply trace elements with caution, after proper soil and tissue testing.
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Trace elements are stored in the soil in a somewhat different manner than macronutrients. Some trace elements are stored in slightly soluble compounds, or are involved to a small extent in cation exchange. Many trace elements combine with organic molecules in the soil to form very complex molecules called chelates. A chelate is a metal atom surrounded by a large organic molecule (figure 12-17). Chelates are an important form of storage for many trace elements.
Iron. Iron is part of many enzymes necessary in the formation of a number of chemicals, especially chlorophyll. Iron minerals are widespread in soil. Most soils have sufficient iron, but much is in the form of insoluble compounds, such as ferric hydroxide, Fe [(OH).sub.3]. Organic matter chelates some iron in the soil. Interestingly, some soil microbes living in the rhizosphere emit compounds that chelate iron, probably improving iron uptake by plants.
The solubility of iron compounds relates directly to pH, declining about 100 times for each rise of one pH point. Acid-loving plants suffer iron shortages when the pH rises above 5.0 or 6.0, while many plants become deficient at higher pH. Sorghum, soybeans, and flax are field crops sensitive to an iron shortage. Iron hunger is most likely in alkaline or calcareous soils, or with excesses of phosphate, zinc, copper, or manganese. Anything that inhibits nutrient uptake, like cold, wet soils, or drought, may induce iron deficiencies.
Iron chlorosis is the usual symptom of iron hunger. It is easy to see as an interveinal chlorosis on new, growing leaves (see figure 10-2). While leaf veins remain green, tissue between the veins becomes light green or yellow. In trees, branches begin to die back. Fruit and ornamental crops commonly show these symptoms. Examples include azaleas, pin oaks, and blueberries.
Various treatments are available to overcome a lack of iron: (1) soil pH can be lowered to free the iron; (2) soluble iron compounds such as iron sulfate may be mixed into the soil, sprayed on leaves, or even injected into the trunks of trees; (3) artificially prepared chelates may be used in the same way; and (4) animal manures can be mixed into the soil.
Manganese. Manganese resembles iron in that weathering releases a cation that is tied up in nonacid soil. Manganese acts with iron in the formation of chlorophyll. Manganese speeds seed germination and crop maturity and helps plants take up several other nutrients.
Manganese deficiencies are usually seen on calcareous soils or on soils that have been overlimed. The solubility of manganese decreases a hundredfold for each rise of one pH point. Soybeans grown on some slightly acid to alkaline soils of the Atlantic Coastal Plain are known to suffer manganese shortages. Dwarfing is a common symptom of manganese deficiency and is often seen in combination with chlorosis. Flecks of dead tissue, along with chlorosis, often appear on new leaves, as shown by some species of maple trees growing on alkaline soil (figure 12-18).When soil pH is below 5.0, so much manganese may be free that it reaches toxic levels.
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Deficient soil can be treated by mixing manganese sulfate into the soil. Lowering pH by applying sulfur may also be helpful. Leaves may be sprayed with a solution of manganous sulfate or chelate. Oats, soybeans, sugar beets, and several vegetables are most likely to respond to manganese treatments. Liming cures manganese toxicity in acid soil.
Zinc. The zinc cation is weathered out of soil minerals, where it can be adsorbed, form a chelate, or form slightly soluble zinc compounds. Several biological reactions use zinc, including chlorophyll and protein production. Low zinc levels are widespread in many crops, including beans, corn, and rice. Some nutritionists have voiced fears that these shortages could be passed on to human consumers. The symptoms of zinc deficiency in plants are quite varied but include tight growth of small, closely spaced leaves, interveinal chlorosis, and dead spots on leaves.
Zinc is most available in acid soil, least available on alkaline or recently limed soils. Soils that have lost topsoil by leveling, terracing, or erosion may also be zinc poor. Low levels may also appear on very coarse soils, because the parent materials lacked zinc and the soils tend to be low in organic matter. Cold soils or excess levels of phosphate inhibit uptake. Like iron, a lack of zinc can be treated by fertilizing soil or spraying foliage with zinc compounds or chelates. Sewage sludge is an excellent source of zinc (see chapter 15). Corn, rice, and onions are most likely to respond to zinc treatment.
Copper. Copper is held by cation exchange and combines chemically with organic matter. Some organic-copper complexes are so stable that the copper is unavailable to plants. Copper is part of a number of important enzymes, especially for the formation of chlorophyll and lignin. Copper affects how well a plant resists disease and how well it controls moisture. Copper shortages also inhibit pollen formation, reducing fruit yields. Shortages are not common, but symptoms include poor fruiting, distorted new growth, stunting, and leaf bleaching. Shortages are most likely to be seen in either leached sands or peats and mucks. A few pounds per acre of copper sulfate mixed into the soil usually supplies all the copper that is needed. Carrots grown on organic soils may need extra copper, but small grains and other vegetables sometimes suffer as well.
Boron. Boron exists in the soil largely as boric acid, [H.sub.3]B[O.sub.3], which is taken up by plants and gathers in organic matter near the soil surface. Fixation at high pH and leaching limit the amount of boron plants can use. Shortages sometimes appear if a soil is overlimed. Conditions that limit organic matter decay also limit the amount of free boron.
The functions of boron are not well understood. Unlike other micronutrients, it does not appear to be involved with any enzymes. It appears to be important for making cell membranes and walls, and for cell enlargement, an important growth process.
Boron deficiencies are fairly widespread in alkaline soils that limit its availability, in high rainfall areas where it leaches readily, and under drought conditions. A shortage of boron often appears as death of terminal buds, followed by tight, bushy growth, known as a "rosette." Thick, fleshy tissues like celery stems or sugar beets may get heart rot, and seed may fail to form in many plants. Boron toxicities are also common, mostly in arid regions.
A number of boron fertilizers may be applied to the soil or sprayed on plant leaves. The oldest form, the common laundry product borax, may be applied at the rate of a few pounds per acre. However, even slightly high boron levels hurt plants, so it should not be used without first testing the soil. Especially sensitive to excess boron are indoor foliage plants.
Molybdenum. Molybdenum, the nutrient with the smallest plant requirement except nickel, is necessary for proper nitrogen metabolism by plants and for nitrogen fixation by both symbiotic and free-living bacteria. Molybdate, [Mo[O.sub.4.sup.-2], gathers in soil organic matter.
Unlike other micronutrients, it is most available at a high soil pH. Shortages are most common on acid, leached, and low-organic-matter coarse soils, as well as acid peats.
Several crops, in addition to legumes, respond to treatment. Crops in the mustard family are especially sensitive. Whiptail of cauliflower, for instance, results from a lack of molybdenum. An ounce of a soluble molybdenum material, often mixed with phosphate fertilizer, will usually treat an acre of deficient soil. Frequently, liming releases enough of this trace element to cure shortages.
Chlorine, Nickel, and Others. The function of chlorine, a recently identified essential element, is not well understood. It is known to play a role in photosynthesis and may help regulate opening and closure of stomata. Chlorine is needed in very small amounts and is commonly found in the soil. Chlorine is thought to be never lacking in farm soils. However, it has been shown to increase grain yields in some soils of the Great Plains, most commonly where plant diseases have been a problem. Toxicity is observed in such sensitive species as beans and cotton, particularly in drier regions of the world.
Nickel is needed by plants and microorganisms for the proper metabolism of the simple nitrogen compound urea and possibly for other uses. It may also help plants resist disease. Nickel is the element most recently shown to be an essential element, making it the seventeenth essential element.
A number of other elements contribute to nutrition of certain plants, though they are not currently considered to be universal essential elements. Legumes need cobalt for nitrogen-fixing. Some grasses and horsetail need silicon; it also is needed for best yields in rice and sugarcane. Research also shows that high silicon content strengthens cell walls and reduces disease infections and insect attack. Sodium appears to be required for many plants native to sodium-rich soils. Plants that need sodium also include many species that have special types of photosynthesis adapted to hot, sunny climates. These include cacti, succulents, and many warm-season grasses. In some areas of the country, selenium and cobalt may be needed in animal forage.
The fourteen mineral nutrients perform many important tasks in the plant. Of the major elements, nitrogen promotes rapid succulent growth. Phosphorus gives early root growth, blooming, and resistance to pest and weather damage. Potassium lends toughness, strength, and pest resistance. Plants need a balance of these three nutrients for strong, vigorous, and healthy growth.
Anyone who grows crops should know how nutrients behave in the soil. An important consideration, for instance, is how a nutrient is stored in the soil. Some nutrients, such as nitrogen and boron, are stored predominantly in organic matter. Some nutrients, such as calcium and magnesium, are adsorbed primarily on soil colloids. Many nutrients are part of slightly soluble compounds, including phosphorus and iron. Many trace elements, like copper, react with organic matter in the soil to form chelates. Most nutrients are found in several of these forms.
Other important traits of nutrients include their solubility and mobility. The solubility of most nutrients depends on pH. For example, phosphorus compounds are most soluble between pH 6.5 to 6.8. Highly mobile nutrients, like nitrate nitrogen, can leach easily from the soil. Elements that move only a short distance, like phosphates, must be placed where roots or seeds will use them.
Plants grow best when each nutrient is present in the right amount. A lack of any one nutrient causes poor or abnormal growth. In addition, plants need a balance of nutrients. To achieve this balance, soil testing should be completed before fertilization is started.
1. Prepare a table with four columns for the four major pools of nutrients in the soil. Place each of the macronutrients in the correct columns. In the soil solution column, write the chemical form the nutrient is in.
2. Soil professionals whose background is agronomic often consider calcium deficiencies an uncommon problem, but horticulturists deal with them often. Explain the difference.
3. Compare and contrast the roles of the primary macronutrients in top growth, root growth, flowering, hardiness, toughness, and pest resistance.
4. How is nitrogen lost from the soil? How can these losses be reduced?
5. Soils in natural ecosystems tend to contain very low amounts of dissolved nitrogen at any given moment compared to agricultural soils. Why might this be true? What could be ecological consequences of the higher amounts in agricultural soils?
6. You want to fertilize a large tree with N, P, and K. Discuss the importance of fertilizer placement for each of these elements for successful fertilization.
7. Some elements move readily in the plant, and when deficiencies occur, the plant moves those elements out of the older leaves into new growth. Deficiency symptoms thus tend to occur on old leaves first. Other elements do not move so readily, and symptoms occur on new leaves first. Go through this chapter and categorize the nutrients on this basis.
8. Excess amounts of many nutrients can also have negative consequences on plant growth. Describe several examples.
9. Compare the kinds of natural ecosystems where nitrogen tends to be growth-limiting to those where phosphorus tends to be. These are broad-scale generalizations.
10. Both Chapter 10 and this one describe reasons why plants growing under low light should be fertilized less than those in full sun (especially with nitrogen).What are those reasons?
1. For color images of nutrient deficiencies, try this Web site and answer these questions: <http://www.back-to-basics.net/nds>
a. Describe deficiency symptoms of iron on apples.
b. Describe deficiency symptoms of phosphorus on corn.
c. Describe deficiency symptoms of calcium on tomatoes.
Note that symptoms are often different on different plants. If you click back to the site's home page, you will find other interesting articles about soil fertility.
2. The nitrogen, phosphorus, and potassium content of the tissues in many crops can be found at: <http://www.nrcs.gov/technical/land/pubs/nlapp1a.html>.
Compare the NPK content of alfalfa hay to small grain hay. Using information on the site, calculate how much NPK is removed from a field in each ton of corn silage.
3. For more information on the fate of nutrient ions in the soil, try this Web site: <http://www.uog.edu/cals/site/users/soil/soil/fertft2a.html>.
4. For more information on the nitrogen cycle, especially N fixation, see: <http://helios.bto.ed.ac.uk/bto/microbes/nitrogen.htm>.
FIGURE 12-6 Characteristics of three types of soil nitrogen. Organic N Ammonium N Nitrate N Storage In humus Adsorbed Little storage Losses Mineralization, Volatilization, Leaching, erosion erosion denitrification Plant Use Not used Usable Usable Changes Mineralization Immobilization, Immobilization, nitrification denitrification