Food is an essential commodity that separates prosperous nations fromstruggling ones. For instance, North Korean agriculture met that en-tire country’s food needs until about a decade ago. The country’s farm-ers were highly efficient and productive. Its food crisis began with the

collapse of the Soviet Union, which had provided North Korea with chemicals andpetroleum. This loss of support was followed by three years of drought, hailstorms,and floods. Today, North Korea is a starving country with a failed farming system.

Why should a desperate shortage of chemicals and petroleum affect a nation’sagriculture? Crop production depends on several factors, but the one that is mostcommonly limiting is a supply of nitrogen in a form usable by plants. All plants re-quire the element nitrogen, which is an abundant component of proteins and nucleicacids as well as chlorophyll and many other important biochemical compounds. If aplant cannot get enough nitrogen, it cannot synthesize these compounds at a rate ad-equate to keep itself healthy. To meet their crops’ need for nitrogen and other miner-als, farmers in all parts of the world apply fertilizers of one kind or another. The in-dustrial production of fertilizers is an energy-intensive process, and the energyneeded is most commonly obtained from petroleum. Without petroleum, North Korea cannot begin to provide the fertilizer needed to restore its crop production.

In addition to nitrogen, plants need othermaterials from their environment. In thischapter, we will explore the differences be-tween the basic strategies of plants and of an-imals for obtaining nutrition. Then we willlook at what nutrients plants require and howthey acquire them. Because most nutrientscome from the soil, we will discuss the for-mation of soils and the effects of plants onsoils. As any farmer can tell you, nitrogen isthe nutrient that most often limits plantgrowth, so we will devote a section specifi-cally to nitrogen metabolism in plants. Thechapter concludes with a look at carnivorousand parasitic plants, which supplement theirnutrition in special ways.

The Acquisition of Nutrients

Every living thing must obtain raw materialsfrom its environment. These nutrients include

Plant Nutrition37

Nitrogen is Essential for Plant GrowthIn this experimental wheat field inBangladesh, nitrogen was withheld fromthe plot on the left. The resulting plantswere stunted and unhealthy.

PLANT NUTRITION 717

the major ingredients of macromolecules: carbon, hydrogen,oxygen, and nitrogen. Carbon and oxygen enter the livingworld in the form of atmospheric carbon dioxide through thecarbon-fixing reactions of photosynthesis. Hydrogen entersliving systems through the light reactions of photosynthesis,which split water. For carbon, oxygen, and hydrogen, pho-tosynthesis is the gateway to the living world, and these elements are in plentiful supply.

In the remainder of this chapter, we shall focus our atten-tion on nitrogen, which is in relatively short supply forplants. The movement of nitrogen into organisms beginswith processing by some highly specialized bacteria living inthe soil. Some of these bacteria act on nitrogen gas, convert-ing it into a form usable by plants. The plants, in turn, pro-vide organic nitrogen (and carbon) to animals, fungi, andmany microorganisms.

In addition to nitrogen, other mineral nutrients are es-sential to living organisms. The proteins of organisms con-tain sulfur (S), and their nucleic acids contain phosphorus (P).There is magnesium (Mg) in chlorophyll, and iron (Fe) inmany important compounds, such as the cytochromes.Within the soil, these and other minerals dissolve in water,forming a solution—called the soil solution—that contactsthe roots of plants. Plants take up most of these mineral nu-trients from the soil solution in ionic form.

Autotrophs make their own organic compoundsPlants, some protists, and some bacteria are autotrophs; thatis, they make their own organic (carbon-containing) com-pounds from simple inorganic nutrients—carbon dioxide,water, nitrogen-containing ions, and a few other soluble min-eral nutrients. The plants provide carbon, oxygen, hydrogen,nitrogen, and sulfur to most of the rest of the living world.Heterotrophs are organisms that require preformed organiccompounds as food. All heterotrophs depend directly or in-directly on autotrophs as their source of nutrition.

Most autotrophs are photosynthesizers—that is, they uselight as their source of energy for synthesizing organic com-pounds from inorganic raw materials. Some autotrophs,however, are chemosynthesizers, deriving their energy notfrom light, but from reduced inorganic substances, such ashydrogen sulfide (H2S), in their environment. All chemosyn-thesizers are bacteria. As we’ll see below, some chemosyn-thetic bacteria in the soil contribute to the nutrition of plantsby increasing the availability of nitrogen and sulfur.

How does a stationary organism find nutrients?Many heterotrophs can move from place to place to find thenutrients they need. An organism that cannot move, termeda sessile organism, must obtain nutrients and energy from

sources that are somehow brought to it. Most sessile animalsdepend primarily on the movement of water to bring themraw materials and energy in the form of food, but a plant’ssupply of energy arrives at the speed of light from the sun.However, with the exception of carbon and oxygen in CO2,a plant’s supply of nutrients is strictly local, and the plantmay use up the water and mineral nutrients in its local envi-ronment as it develops. How does a plant cope with theproblem of scarce nutrient supplies?

One way is to extend itself by growing in search of newresources. Growth is a plant’s version of movement. Amongplant organs, the roots obtain most of the mineral nutrientsneeded for growth. By growing through the soil, they minethe soil for new sources of mineral nutrients and water. Thegrowth of leaves helps a plant secure light and carbon diox-ide. A plant may compete with other plants for light by out-growing and shading them.

As it grows, a plant—or even a single root—must deal witha variable environment. Animal droppings create high localconcentrations of nitrogen. A particle of calcium carbonate inthe soil may make a tiny area alkaline, while dead organic mat-ter may make a nearby area acidic. Such microenvironmentsencourage or discourage the proliferation of a root system.

Mineral Nutrients Essential to Plants

As roots grow through the soil, what important mineral nu-trients do plants take up from their environment, and whatare the roles of those nutrients? Table 37.1 lists the mineralnutrients that have been determined to be essential for plants.Except for nitrogen, they all derive from rock. All of them areusually taken up from the soil solution.

There are three criteria for calling something an essentialelement:

� The element must be necessary for normal growth andreproduction.

� The element cannot be replaceable by another element.� The requirement must be direct—that is, not the result of

an indirect effect, such as the need to relieve toxicitycaused by another substance.

In this section, we’ll consider the symptoms of particularmineral deficiencies, the roles of some of the mineral nutri-ents, and the technique by which the essential elements forplants were identified.

There are two categories of essential elements: macronu-trients and micronutrients (see Table 37.1).

� Plants need macronutrients in concentrations of at least1 gram per kilogram of their dry matter.*

*Dry matter, or dry weight, is what remains after all the water has beenremoved from a plant tissue sample.

� Plants need micronutrients in concentrations of lessthan 100 milligrams per kilogram of their dry matter.

These two categories differ only with regard to the amountsrequired by plants. Both the macronutrients and the mi-cronutrients are essential for the plant to complete its life cy-cle from seed to seed. How do we know if a plant is gettingenough of a particular nutrient?

Deficiency symptoms reveal inadequate nutritionBefore a plant that is deficient in an essential element dies, itusually displays characteristic deficiency symptoms, such asdiscoloration or deformation of its leaves. Table 37.2 describesthe symptoms of some common mineral deficiencies. Suchsymptoms help horticulturists diagnose mineral nutrient de-ficiencies in plants. With proper diagnosis, appropriate treat-ment can be applied in the form of a fertilizer (an addedsource of mineral nutrients).

Nitrogen deficiency is the most common mineral defi-ciency in both natural and agricultural environments. Plantsin natural environments are almost always deficient in nitro-gen, but they seldom display deficiency symptoms. Instead,their growth slows to match the available supply of nitrogen.Crop plants, on the other hand, show deficiency symptoms ifa formerly abundant supply of nitrogen runs out. The visiblesymptoms of nitrogen deficiency include uniform yellowing,or chlorosis, of older leaves. Chlorophyll, which is responsiblefor the green color of leaves, contains nitrogen. Without ni-trogen there is no chlorophyll, and without chlorophyll, theyellow carotenoid pigments in the leaves become visible.

Nitrogen deficiency is not the only cause of chlorosis. In-adequate iron in the soil can also cause chlorosis becauseiron, although it is not contained in the chlorophyll molecule,is required for chlorophyll synthesis. However, iron defi-ciency commonly causes chlorosis of the youngest leaves,with their veins sometimes remaining green. The reason forthis difference is that nitrogen is readily translocated in theplant and can be redistributed from older tissues to youngertissues to favor their growth. Iron, on the other hand, cannot

718 CHAPTER THIRT Y-SEVEN

Mineral Elements Required by Plants

ELEMENT ABSORBED FORM MAJOR FUNCTIONS

MacronutrientsNitrogen (N) NO3

– and NH4+ In proteins, nucleic acids, etc.

Phosphorus (P) H2PO4– and HPO4

2– In nucleic acids, ATP, phospholipids, etc.Potassium (K) K+ Enzyme activation; water balance; ion balance; stomatal openingSulfur (S) SO4

2– In proteins and coenzymesCalcium (Ca) Ca2+ Affects the cytoskeleton, membranes, and many enzymes; second messengerMagnesium (Mg) Mg2+ In chlorophyll; required by many enzymes; stabilizes ribosomes

MicronutrientsIron (Fe) Fe2+ In active site of many redox enzymes and electron carriers; chlorophyll synthesisChlorine (Cl) Cl– Photosynthesis; ion balanceManganese (Mn) Mn2+ Activation of many enzymesBoron (B) B(OH)3 Possibly carbohydrate transport (poorly understood)Zinc (Zn) Zn2+ Enzyme activation; auxin synthesisCopper (Cu) Cu2+ In active site of many redox enzymes and electron carriersNickel (Ni) Ni2+ Activation of one enzymeMolybdenum (Mo) MoO4

2– Nitrate reduction

37.1

Some Mineral Deficiencies in Plants

DEFICIENCY SYMPTOMS

Calcium Growing points die back; young leaves are yellow and crinkly

Iron Young leaves are white or yellow with greenveins

Magnesium Older leaves have yellow in stripes betweenveins

Manganese Younger leaves are pale with stripes of deadpatches

Nitrogen Oldest leaves turn yellow and die pre-maturely; plant is stunted

Phosphorus Plant is dark green with purple veins and is stunted

Potassium Older leaves have dead edges

Sulfur Young leaves are yellow to white with yellow veins

Zinc Young leaves are abnormally small; older leaves have many dead spots

37.2

be readily redistributed. Younger tissues that are activelygrowing and synthesizing compounds needed for theirgrowth show iron deficiency before older leaves, which havealready completed their growth.

Several essential elements fulfill multiple rolesEssential elements may play several different roles in plantcells—some structural, others catalytic. Magnesium, as wehave mentioned, is a constituent of the chlorophyll moleculeand hence is essential to photosynthesis. It is also required asa cofactor by numerous enzymes involved in cellular respi-ration and other metabolic pathways.

Phosphorus, usually in phosphate groups, is found inmany organic compounds, particularly in nucleic acids andin the intermediates of the energy-harvesting pathways ofphotosynthesis and glycolysis. As we saw in Chapter 7, thetransfer of phosphate groups occurs in many energy-storingand energy-releasing reactions, notably those that use or pro-duce ATP. The addition or removal of phosphate groups isalso used to activate or inactivate enzymes.

Calcium plays many roles in plants. Its function in the pro-cessing of hormonal and environmental cues is a subject ofgreat biological interest, as we’ll see in the next chapter. Cal-cium also affects membranes and cytoskeletal activity, partic-ipates in spindle formation for mitosis and meiosis, and is aconstituent of the middle lamella of cell walls. Other elements,such as iron and potassium, also play multiple roles in plants.

All of these elements are essential to the life of all plants.How did biologists discover which elements are essential?

Experiments were designed to identify essential elementsAn element is considered essential to plants if a plant fails tocomplete its life cycle, or grows abnormally, when that elementis not available, or is not available in sufficient quantities. Theessential elements for plants were identified by growing plantshydroponically—that is, with their roots suspended in nutrientsolutions without soil (Figure 37.1). In the first successful ex-periments of this type, performed a century and a half ago,plants grew seemingly normally in solutions containing onlycalcium nitrate, magnesium sulfate, and potassium phosphate.Omission of any of these compounds made the solution inca-pable of supporting normal growth. Tests with other com-pounds including these elements soon established the sixmacronutrients— calcium, nitrogen, magnesium, sulfur, potas-sium, and phosphorus—as essential elements.

Identifying essential elements by this experimental ap-proach proved to be a more difficult task in the case of themicronutrients. In the nineteenth-century experiments onplant nutrition, some of the chemicals used were so impure

that they provided micronutrients that the investigatorsthought they had excluded. Furthermore, because some mi-cronutrients are required in such tiny amounts, there may beenough in a seed to supply the embryo and the resultant sec-ond-generation plant throughout its lifetime and leaveenough in the next seed to get the third generation wellstarted. Indeed, simply touching a plant may give it a signif-icant supply of chlorine in the form of chloride ions fromsweat. Such difficulties make it necessary to perform nutri-tion experiments in tightly controlled laboratories with spe-cial air filters (to exclude microscopic salt particles in the air)and to use only chemicals that had been purified to the high-est degree attainable by modern chemistry. Only rarely arenew essential elements reported now. Either the list is nearlycomplete, or perhaps, we will need more sophisticated tech-niques to add to it.

Where does the plant find its essential mineral nutrients?How does it absorb them?

Soils and Plants

Most terrestrial plants live their lives anchored to the soil. Ofcourse, soils offer mechanical support for growing plants, butthere are many other plant-soil interactions, some of which

RESULTS

METHOD Grow seedlings in a medium that lacks the element in question (in this case, nitrogen)

EXPERIMENT

Conclusion: Nitrogen is an essential plant nutrient.

Question: Is a particular ingredient of a growth medium anessential plant nutrient?

Seedling grownin a completegrowth medium.

Seedling grown in a medium lacking nitrogen.

Growth is normal. Growth is abnormal.

37.1 Identifying Essential Elements for Plants This dia-gram shows the procedure for identifying nutrients essentialto plants, using nitrogen as an example.

are much more complex. Plants obtain their mineral nutri-ents from the soil solution. Water for terrestrial plants alsocomes from the soil, as does the supply of oxygen for theroots. Soil harbors bacteria, some of which are beneficial toplant life. Soils may also contain organisms harmful to plants.

In this section, we will examine the composition, structure,and formation of soils. We will consider their role in plant nu-trition, their care and supplementation in agriculture, andtheir modification by the plants that grow in them.

Soils are complex in structureSoils are complex systems made up of living and nonlivingcomponents. The living components include plant roots aswell as populations of bacteria, fungi, protists, and animalssuch as earthworms and insects (Figure 37.2). The nonlivingportion of the soil includes rock fragments ranging in sizefrom large rocks through sand and silt and finally to tiny par-ticles called clay that are 2 µm or less in diameter. Soil alsocontains water and dissolved mineral nutrients, air spaces,and dead organic matter. The air spaces are crucial sourcesof oxygen (in the form of O2) for plant roots. The character-istics of soils are not static. Soils change constantly throughnatural phenomena such as rain, temperature extremes, andthe activities of plants and animals, as well as human activi-ties—agriculture in particular.

The structure of many soils changes with depth, revealinga soil profile. Although soils differ greatly, almost all soils con-sist of two or more recognizable horizontal layers, called

horizons, lying on top of one another. Mineral nutrients tendto be leached from the upper horizons—dissolved in rain orirrigation water and carried to deeper horizons, where theyare unavailable to plant roots.

Soil scientists recognize three major horizons (A, B, and C)in the profile of a typical soil (Figure 37.3). Topsoil is the Ahorizon, from which mineral nutrients may be depleted byleaching. Most of the dead and decaying organic matter inthe soil is in the A horizon, as are most plant roots, earth-worms, insects, nematodes, and microorganisms. Successfulagriculture depends on the presence of a suitable A horizon.

Topsoils are composed of different proportions of sand,silt, and clay. In pure sand there are abundant air spaces be-tween the relatively large particles, but sand is low in waterand mineral nutrients. Clay contains many mineral nutrientsand more water than sand does, but the tiny clay particlespack tightly together, leaving little space to trap air. A littlebit of clay goes a long way in affecting soil properties. A loamis a soil that has significant amounts of sand, silt, and clay,and thus has sufficient levels of air, water, and nutrients forplants. Loams also contain organic matter. Most of the besttopsoils for agriculture are loams.

Below the A horizon is the B horizon, or subsoil, which isthe zone of infiltration and accumulation of materials leachedfrom above. Farther down, the C horizon is the parent rockthat is breaking down to form soil. Some deep-growing rootsextend into the B horizon to obtain water and nutrients, butroots rarely enter the C horizon.

720 CHAPTER THIRT Y-SEVEN

H2OAirAir AirAirQuartz

Air

Aggregates ofclay particles 25 µm

Air and water

Organic matter (from plants, animals, and fungi)

Bacteria

37.2 The Complexity of Soil Even a tiny crumb of soil has bothorganic and inorganic components.

A horizonTopsoil

B horizonSubsoil

C horizonWeatheringparent rock

(bedrock)

37.3 A Soil Profile The A, B, and C horizons can sometimes be seenin road cuts such as this one in Australia. The dark upper layer (the Ahorizon) is home to most of the living organisms in the soil.

PLANT NUTRITION 721

Soils form through the weathering of rockThe type of soil in a given area depends on the type of par-ent rock from which it formed, the climate, the landscapefeatures, the organisms living there, and the length of timethat soil-forming processes have been acting (sometimes mil-lions of years). Rocks are broken down into soil in part bymechanical weathering, which is the physical breakdown—without any accompanying chemical changes—of materialsby wetting, drying, and freezing. The most important partsof soil formation, however, include chemical weathering, thechemical alteration of at least some of the materials in therocks.

Both the physical and chemical properties of soils dependon the amounts and kinds of clay particles they contain.These tiny particles, which bind mineral nutrients and ag-gregate into larger particles, are extremely important to plantgrowth. Clay is not produced merely by the mechanicalgrinding up of rocks. In addition to mechanical weathering,several types of chemical weathering are required:

� Oxidation by atmospheric oxygen makes some essentialelements more available to plants.

� Reaction with water (hydrolysis) releases some mineralnutrients from the rock.

� Acids, carbonic acid in particular, free some essential ele-ments from their parent salts.

These reactions leave the surface of clay particles with anabundance of negatively charged chemical groups, to whichcertain mineral nutrients bind. Let’s see how roots take upthese mineral nutrients from clay particles.

Soils are the source of plant nutritionThe availability of mineral nutrients to plant roots dependson the presence of clay particles in the soil. The negativelycharged clay particles bind the cations of many mineralsthat are important for plant nutrition, such as potassium(K+), magnesium (Mg2+), and calcium (Ca2+). To becomeavailable to plants, these cations must be detached from theclay particles.

This task is accomplished by reactions with protons (hy-drogen ions, H+). These protons are released into the soil byroots, which also release CO2 through cellular respiration.The CO2 dissolves in the soil water and reacts with it to formcarbonic acid, which then ionizes to form bicarbonate andfree protons (CO2 + H2O ~ H2CO3 ~ H+ + HCO3

–). Theseprotons bind more strongly to the clay particles than do themineral cations, so they trade places with the cations in aprocess called ion exchange (Figure 37.4). Ion exchange putsimportant cations back into the soil solution, from which theyare taken up by the roots. The capacity of a soil to support

plant growth, called soil fertility, is determined in part by itsability to provide nutrients in this manner.

Clay particles effectively hold and exchange cations, andcations tend to be retained in the A horizon. However, thereis no comparable mechanism for exchanging anions, thenegatively charged ions. As a result, important anions suchas nitrate (NO3

–) and sulfate (SO42–)—the primary and direct

sources of nitrogen and sulfur, respectively—leach rapidlyfrom the A horizon. As a consequence of this leaching, theprimary soil reservoir of nitrogen is not in the form of nitrateions. Most of the nitrogen in the A horizon is found in theorganic matter in the soil, which slowly decomposes to re-lease nitrogen in a form that can be absorbed and used byplants.

Fertilizers and lime are used in agricultureAgricultural soils often require fertilizers because irrigationand rainwater leach mineral nutrients from the soil and be-cause the harvesting of crops removes the nutrients that theplants took up from the soil during their growth. Crop yieldsdecrease if any essential element is depleted. Mineral nutri-ents may be replaced by organic fertilizers, such as rotted manure, or by inorganic fertilizers of various types.

ORGANIC AND INORGANIC FERTILIZERS. The three elementsmost commonly added to agricultural soils are nitrogen(N), phosphorus (P), and potassium (K). Commercial fertil-izers are characterized by their “N-P-K” percentages. A

Root hair

The cations are exchanged for hydrogen ions obtainedfrom carbonic acid (H2CO3) or from the plant itself.

K+

––––––– – – – –

ClayH+

H+

H+

H+HCO3– +H2CO3CO2 + H2O

A clay particle, which is negatively charged, binds cations.

37.4 Ion Exchange Plants obtain mineral nutrients from the soilprimarily in the form of positive ions; potassium is the exampleshown here.

5-10-10 fertilizer, for example, contains 5 percent nitrogen,10 percent phosphate (P2O5), and 10 percent potash (K2O)by weight.* Sulfur, in the form of a sulfate, is also occasion-ally added to soils.

Either organic or inorganic fertilizers can provide the nec-essary mineral nutrients for plants. Organic fertilizers releasenutrients slowly, which results in less leaching than occurswith a one-time application of an inorganic fertilizer. How-ever, the nutrients from organic fertilizers are not immedi-ately available to plants. Organic fertilizers also containresidues of plant or animal materials that improve the struc-ture of the soil, providing spaces for air movement, rootgrowth, and drainage. Inorganic fertilizers, on the otherhand, provide a supply of soil nutrients that is almost im-mediately available for absorption. Furthermore, inorganicfertilizers can be formulated to meet the requirements of aparticular soil and a particular crop.

pH EFFECTS ON NUTRIENTS. The availability of nutrient ions,whether they are naturally present in the soil or added asfertilizer, is altered by changes in soil pH. The optimal soilpH for most crops is about 6.5, but so-called acid-lovingcrops such as blueberries prefer a pH closer to 4. Rainfalland the decomposition of organic substances lower the pHof the soil. Such acidification can be reversed by liming—the application of compounds commonly known as lime,such as calcium carbonate, calcium hydroxide, or magne-sium carbonate. The addition of these compounds leads tothe removal of H+ ions from the soil. Liming also increasesthe availability of calcium to plants.

Sometimes, on the other hand, a soil is not acidic enough.In this case, sulfur can be added in the form of elemental sul-fur, which soil bacteria convert to sulfuric acid. Iron and someother elements are more available to plants at a slightly acidicpH. Soil pH testing is useful for home gardens and lawns aswell as for agriculture. The test results indicate what amend-ments should be made to the soil.

SPRAY APPLICATION OF NUTRIENTS. Spraying leaves with anutrient solution is another effective way to deliver someessential elements to growing plants. Plants take up morecopper, iron, and manganese when these elements areapplied as foliar (leaf) sprays than when they are added tothe soil. Such foliar application of mineral nutrients isincreasingly used in wheat production, but fertilizers arestill delivered most commonly by way of the soil.

The relationship between plants and soils is not a one-wayaffair—soils affect plants, but plants also affect soils.

Plants affect soil fertility and pHThe soil that forms in a particular place depends on the typesof plants growing there. Plant litter, such as dead fallenleaves, is the major source of the carbon-rich materials thatbreak down to form humus—dark-colored organic material,each particle of which is too small to be recognizable with thenaked eye. Soil bacteria and fungi produce humus by break-ing down plant litter, animal feces, dead organisms, andother organic material. Humus is rich in mineral nutrients,especially nitrogen that was excreted by animals. In combi-nation with clay, humus favors plant growth by trappingsupplies of water and oxygen for absorption by roots. Look-ing at the big picture, we see that successful plant growth cancreate conditions that support further plant growth.

Plants also affect the pH of the soil in which they grow.Roots maintain a balance of electric charges. If they absorbmore cations than anions, they excrete H+ ions, thus lower-ing the soil pH. If they absorb more anions than cations, theyexcrete OH– ions or HCO3

– ions, raising the soil pH.The mineral nutrient most commonly in short supply, in

both natural and agricultural situations, is nitrogen, despitethe fact that elemental nitrogen makes up almost four-fifthsof Earth’s atmosphere. What is the reason for this scarcity?Let’s consider how nitrogen is made available to plants.

Nitrogen Fixation

The Earth’s atmosphere is a vast reservoir of nitrogen in theform of nitrogen gas (N2). However, plants cannot use N2 di-rectly as a nutrient. It is a highly unreactive substance—thetriple bond linking the two nitrogen atoms is extremely sta-ble, and a great deal of energy is required to break it. How,then, is nitrogen made available for the synthesis of proteinsand nucleic acids?

722 CHAPTER THIRT Y-SEVEN

*The analysis is by weight of the nutrient-containing compound and notas weights of the elements N, P, and K. A 5-10-10 fertilizer actually doescontain 5 percent nitrogen, but only 4.3 percent phosphorus and 8.3 per-cent potassium on an elemental basis.

37.5 Root Nodules Large, round nodules are visible in the rootsystem of a pea plant. These nodules house nitrogen-fixing bacteria.

PLANT NUTRITION 723

A few species of bacteria have an enzyme that enablesthem to convert N2 into a more reactive and biologically use-ful form by a process called nitrogen fixation. These prokary-otic organisms—nitrogen fixers—convert N2 to ammonia(NH3). There are relatively few species of nitrogen fixers, andtheir biomass is small relative to the mass of other organismsthat depend on them for survival on Earth. This talentedgroup of prokaryotes is just as essential to the biosphere asare the photosynthetic autotrophs.

Nitrogen fixers make all other life possibleBy far the greatest share of total world nitrogen fixation isperformed biologically by nitrogen-fixing prokaryotes, whichfix approximately 170 million Mg (megagrams or metrictons) of nitrogen per year. About 80 million Mg is fixed in-dustrially by humans. A smaller amount of nitrogen is fixedin the atmosphere by nonbiological means such as lightning,volcanic eruptions, and forest fires. Rain brings these atmos-pherically formed products to the ground.

Several groups of bacteria fix nitrogen. In the oceans, vari-ous photosynthetic bacteria, including cyanobacteria, fix nitro-gen. In fresh water, cyanobacteria are the principal nitrogen fix-ers. On land, free-living soil bacteria make some contributionto nitrogen fixation, but they fix only what they need for theirown use and release the fixed nitrogen only when they die.

Other nitrogen-fixing bacteria live in close association withplant roots (Figure 37.5). They release up to 90 percent of thenitrogen they fix to the plant and excrete some amino acidsinto the soil, making nitrogen immediately available to otherorganisms. The plant obtains fixed nitrogen from the bac-terium, and the bacterium obtains the products of photosyn-thesis from the plant. Such associations are excellent exam-ples of mutualism, an interaction between two species inwhich both species benefit. They are also examples of sym-biosis, in which two different species live in physical contactfor a significant portion of their life cycles.

Bacteria of the genus Rhizobium fix nitrogen only in close,mutualistic association with the roots of plants in the legumefamily. The legumes include peas, soybeans, clover, alfalfa,and many tropical shrubs and trees. The bacteria infect theplant’s roots, and the roots develop nodules in response totheir presence. The various species of Rhizobium show a highspecificity for the species of legume they infect. Farmers andgardeners coat legume seeds with Rhizobium to make sure thebacteria are present. Some farmers alternate their crops,planting clover or alfalfa occasionally to increase the avail-able nitrogen content of the soil.

The legume–Rhizobium association is not the only bacterialassociation that fixes nitrogen. Some cyanobacteria fix nitro-gen in association with fungi in lichens or with ferns, cycads,or nontracheophytes. Rice farmers can increase crop yieldsby growing the water fern Azolla, with its symbiotic nitrogen-fixing cyanobacterium, in the flooded fields where rice isgrown. Another group of bacteria, the filamentous actino-mycetes, fix nitrogen in association with root nodules onwoody species such as alder and mountain lilacs.

How does biological nitrogen fixation work? In the foursections that follow, we’ll consider the role of the enzyme ni-trogenase, the mutualistic collaboration of plant and bacter-ial cells in root nodules, the need to supplement biologicalnitrogen fixation in agriculture, and the contributions ofplants and bacteria to the global nitrogen cycle.

Nitrogenase catalyzes nitrogen fixationNitrogen fixation is the reduction of nitrogen gas. It proceedsby the stepwise addition of three pairs of hydrogen atoms toN2 (Figure 37.6). In addition to N2, these reactions requirethree things:

Nitrogenase

N N

Substrate:Nitrogen gas (N2)

Binding ofsubstrate

Reduction

+ 2H

Reduction

+ 2H

ReductionNitrogenase

+ 2H

Product:Ammonia (NH3)

N N

N N N N N N

N N

The enzyme nitrogenase binds a molecule of nitrogen gas.

1

A reducing agent transfers three successive pairs of hydrogen atoms to N2.

2 The final products—two molecules of ammonia—are released, freeing the nitrogenase to bind another N2 molecule.

3

H HH

H

H H

H

H

H

H H

H HH

H

H

H

H

37.6 Nitrogenase Fixes Nitrogen Throughout the chemical reac-tions of nitrogen fixation, the reactants are bound to the enzymenitrogenase. A reducing agent transfers hydrogen atoms to nitrogen,and eventually the final product—ammonia—is released.

� a strong reducing agent to transfer hydrogen atoms toN2 and to the intermediate products of the reaction

� a great deal of energy, which is supplied by ATP� the enzyme nitrogenase, which catalyzes the reaction

(Depending on the species of nitrogen fixer, either respirationor photosynthesis may provide both the necessary reducingagent and ATP.)

Nitrogenase is so strongly inhibited by oxygen that itspresence in biochemical extracts was obscured and its dis-covery delayed because investigators had not thought to seekit under anaerobic conditions. It is therefore not surprisingthat many nitrogen fixers are anaerobes and live in environ-ments with little or no O2. Because this crucial enzyme is soinhibited by O2, it was at first surprising that legumes respireaerobically, as do Rhizobium. Investigation of the root noduleswhere nitrogenase is found revealed how the enzyme couldoperate there.

Within a root nodule, O2 is maintained at a low level suf-ficient to support respiration, but not so high as to inactivatenitrogenase. The plant makes this possible by producing the

protein leghemoglobin in the cytoplasm of the nodule cells.Leghemoglobin is a close relative of hemoglobin, the oxy-gen-carrying pigment of animals. Some plant nodules con-tain enough of it to be bright pink when viewed in cross sec-tion. Leghemoglobin, with its iron-containing heme groups,transports enough oxygen to the bacteroids to support theirrespiration.

Some plants and bacteria work together to fix nitrogenNeither free-living Rhizobium species nor uninfected legumescan fix nitrogen. Only when the two are closely associated inroot nodules does the reaction take place. The establishmentof this symbiosis between Rhizobium and a legume requiresa complex series of steps, with active contributions by boththe bacteria and the plant root (Figure 37.7). First the root re-leases flavonoids and other chemical signals that attract soil-living Rhizobium to the vicinity of the root. Flavonoids trig-ger the transcription of bacterial nod genes, which encodeNod (nodulation) factors. These factors, secreted by the bac-

Bacteroids in infected cell

Uninfected cell

Root hairs Root hair

Nodule

Rhizobia

Nodule

Bacteroids

Infectionthread

Root hairs release chemical signals that attract Rhizobium.

1

Rhizobium proliferates and causes an infection threadto form.

2

The infection thread grows into the cortex of the root.

3

The infection thread releases bacterial cells, which become bacteroids in the root cells. Nod factors from bacteria cause cortical cells to divide.

4

The nodule forms from rapidly dividing, infected cortical cells.

5

Root tip

Cortical cells

37.7 A Nodule Forms Rhizobium develops the ability to fixnitrogen only after entering a legume root. The diagrams show thesequence of events in nodule formation. The photograph showsbacteroids of Rhizobium japonicum in vesicles within a soybeanroot cell. A portion of an uninfected root cell is seen on the right.

PLANT NUTRITION 725

teria, cause cells in the root cortex to divide, leading to theformation of a primary nodule meristem. The meristem givesrise to the plant tissue that constitutes the nodule.

Among the products of the meristem is a layer of cells thatexcludes O2 from the interior of the nodule. The function ofleghemoglobin is to carry O2 across this barrier. Within a nod-ule, the bacteria take the form of bacteroids within membra-nous vesicles. Bacteroids are swollen, deformed bacteria thatcan fix nitrogen—in effect, nitrogen-fixing organelles.

The partnership between bacterium and plant in nitrogen-fixing nodules is not the only case in which plants depend onother organisms for assistance with their nutrition. Anotherexample is that of mycorrhizae, root–fungus associations inwhich the fungus greatly increases the absorption of waterand minerals (especially phosphorus) by the plant (see Fig-ure 31.16). A growing body of evidence suggests that noduleformation depends on some of the same genes and mecha-nisms that allow mycorrhizae to develop.

Biological nitrogen fixation does not always meetagricultural needsBacterial nitrogen fixation is not sufficient to support theneeds of agriculture. Traditional farmers used to plant deadfish along with corn so that the decaying fish would releasefixed nitrogen that the developing corn could use. Industrialnitrogen fixation is becoming ever moreimportant to world agriculture becauseof the degradation of soils and the needto feed a rapidly expanding population.

Most industrial nitrogen fixation isdone by a chemical process called theHaber process, which requires a greatdeal of energy. An alternative is urgentlyneeded because of the rising cost of en-ergy. At present, in the United States, themanufacture of nitrogen-containing fer-tilizer takes more energy than does anyother aspect of crop production. In bio-logical systems, nitrogen fixation re-quires a great deal of ATP.

Research on biological nitrogen fixa-tion is being vigorously pursued, withcommercial applications very much inmind. One line of investigation centerson recombinant DNA technology as ameans of engineering new plant–bac-terium associations that produce theirown nitrogenase. Currently there are at-tempts to transfer genes from Rhizobiuminto bacteria that already live in theroots of cereal plants.

Plants and bacteria participate in the global nitrogen cycleThe nitrogen released into the soil by nitrogen fixers is pri-marily in the form of ammonia (NH3) and ammonium ions(NH4

+). Although ammonia is toxic to plants, ammoniumions can be taken up safely at low concentrations. Soil bacte-ria called nitrifiers, which we described in Chapter 27, oxidizeammonia to nitrate ions (NO3

–)—another form that plantscan take up—by the process of nitrification (Figure 37.8). SoilpH affects the uptake of nitrogen: Nitrate ions are taken uppreferentially under more acidic conditions, ammonium ionsunder more basic ones.

The steps that we have followed so far are carried out bybacteria: N2 is reduced to ammonia in nitrogen fixation andammonia is oxidized to nitrate in nitrification. The next stepsare carried out by plants, which reduce the nitrate they havetaken up all the way back to ammonia. All the reactions ofnitrate reduction are carried out by the plant’s own en-zymes. The later steps, from nitrite (NO2

–) to ammonia, takeplace in the chloroplasts, but this conversion is not part ofphotosynthesis. The plant uses the ammonia thus formed to

NITROGENFIXATION

NITRIFICATION

Denitrifyingbacteria

Nitrosomonas,Nitrosococcus

Oxidation

Recycling to soil

Nitrogen-fixingbacteria

Nitrobacter

N2

NH3NH4

+

NH4+

DENITRIFICATION

NO2–

NO3–

NITRATEREDUCTION

Bacteria fix N2 from the atmosphere producing ammonia and ammonium ions.

Plants reduce nitrate ions back to ammonia, the form in which nitrogen is incorporated into proteins.

Some denitrifying bacteria can oxidize ammonia back to nitrogen gas, which returns to the atmosphere.

Nitrifying bacteria oxidize ammonia to nitrate ions.

Recycling to soil

37.8 The Nitrogen Cycle Nitrogen fixation, nitrification,nitrate reduction, and denitrification are the components of anessential chemical cycle that converts atmospheric nitrogen gasinto ammonium ions and nitrate ions—forms of nitrogen that

can be taken up by plants—and returns N2 to the atmosphere.

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manufacture amino acids, from which the plant’s proteinsand all its other nitrogen-containing compounds are formed.Animals cannot reduce nitrogen, and they depend on plantsto supply them with reduced nitrogenous compounds.

Bacteria called denitrifiers return nitrogen from animalwastes and dead organisms to the atmosphere as N2. Thisprocess is called denitrification (see Chapter 27). In combi-nation with leaching and the removal of crops, denitrifica-tion keeps the level of available nitrogen in soils low.

This global nitrogen cycle is complex. It is also essential forlife on Earth: Nitrogen-containing compounds constitute 5 to30 percent of a plant’s total dry weight. The nitrogen contentof animals is even higher, and all the nitrogen in the animalworld arrives there by way of the plant kingdom.

Carnivorous and Heterotrophic Plants

Some plants that are found primarily in nitrogen-deficientsoils augment their nitrogen and phosphorus supply by cap-turing and digesting flies and other insects. There are about450 of these carnivorous species, the best-known of which areVenus flytraps (genus Dionaea; Figure 37.9a), sundews (genusDrosera; Figure 37.9b), and pitcher plants (genus Sarracenia).

Carnivorous plants are normally found in boggy regionswhere the soil is acidic. Most decomposing organisms requirea less acidic pH to break down the bodies of dead organisms,so relatively little nitrogen is recycled into these acidic soils.Accordingly, the carnivorous plants have adaptations that al-low them to augment their supply of nitrogen by capturinganimals and digesting their proteins.

The Venus flytraps have specialized leaves with twohalves that fold together. When an insect trips trigger hairson a leaf, its two halves come together, their spiny marginsinterlocking and imprisoning the insect. The leaf then se-cretes enzymes that digest its prey. The leaf absorbs theproducts of digestion, especially amino acids, and uses themas a nutritional supplement.

Pitcher plants produce pitcher-shaped leaves that collectsmall amounts of rainwater. Insects are attracted into thepitchers either by bright colors or by scent and are preventedfrom getting out again by stiff, downward-pointing hairs.The insects eventually die and are digested by a combinationof enzymes and bacteria in the water. Even rats have beenfound in large pitcher plants.

Sundews have leaves covered with hairs that secrete aclear, sticky, sugary liquid. An insect touching one of thesehairs becomes stuck, and more hairs curve over the insectand stick to it as well. The plant secretes enzymes to digestthe insect and eventually absorbs the carbon- and nitrogen-containing products of digestion.

None of the carnivorous plants must feed on insects tosurvive. They can grow adequately without insects, but intheir natural habitats they grow faster and are a darker greenwhen they succeed in capturing insects. They use the addi-tional nitrogen from the insects to make more proteins,chlorophyll, and other nitrogen-containing compounds.

Thus far in this chapter we have considered the mineralnutrition of plants. As you already know, another crucial as-pect of plant nutrition is photosynthesis—the principal

source of energy and carbon for plants them-selves and for the biosphere as a whole. Notall plants, however, are photosynthetic au-totrophs. A few, in the course of their evolu-tion, have lost the ability to sustain them-selves by photosynthesis. How do theseplants get their energy and carbon?

A few plants are heterotrophic parasitesthat obtain their nutrients directly from theliving bodies of other plants. Perhaps themost familiar parasitic plants are the mistle-toes and dodders (Figure 37.10). Mistletoesare green and carry on some photosynthesis,but they parasitize other plants for water andmineral nutrients and may derive photosyn-thetic products from them as well. Mistletoesand dodders extract nutrients from the vas-cular tissues of their hosts by forming ab-sorptive organs called haustoria, which invadethe host plant’s tissues. Another parasiticplant, the Indian pipe, once was thought toobtain its nutrients from dead organic matter.It is now known to get its nutrients, with the

(a)

Dionaea muscipula

(b)

Drosera rotundifolia

37.9 Carnivorous Plants Some plants have adapted to nitrogen-poor environmentsby becoming carnivorous. (a) The Venus flytrap obtains nitrogen and phosphorus fromthe bodies of insects trapped inside the plant when its hinges snap shut. (b) Sundewstrap insects on sticky hairs. Secreted enzymes will digest the carcass externally.

PLANT NUTRITION 727

help of fungi, from nearby actively photosynthesizing plants.Hence it, too, is a parasite.

Dwarf mistletoe is a serious parasite in forests of the west-ern United States, destroying more than 3 billion board feetof lumber per year. However, parasitic plants are a muchmore urgent problem in developing countries. Striga (witch-weed) imperils more than 300 million sub-Saharan Africansby attacking their cereal and legume crops. In the MiddleEast and North Africa, Orobanche (broomrape) ravages manycrops, especially vegetables and sunflowers.

Chapter Summary

The Acquisition of Nutrients� Plants are photosynthetic autotrophs that can produce all theorganic compounds they need from carbon dioxide, water, andminerals, including a nitrogen source. They obtain energy fromsunlight, carbon dioxide from the atmosphere, and nitrogen-containing ions and mineral nutrients from the soil.� Plants explore their surroundings by growing rather than bymovement.

Mineral Nutrients Essential to Plants� Plants require 14 essential mineral elements, all of whichcome from the soil solution. Several of these essential elementsfulfill multiple roles. Review Table 37.1� The six mineral nutrients required in substantial amounts arecalled macronutrients; the eight required in much smalleramounts are called micronutrients. Review Table 37.1

� Deficiency symptoms suggest what essential element a plantlacks. Review Table 37.2� Biologists discovered the requirement for each essential ele-ment by growing plants on hydroponic solutions lacking thatelement. Review Figure 37.1. See Web/CD Tutorial 37.1

Soils and Plants� Soils are complex systems with living and nonliving compo-nents. They contain water, air, and inorganic and organic sub-stances. They typically consist of two or three horizontal zonescalled horizons. Review Figures 37.2, 37.3� Soils form by mechanical and chemical weathering of rock.� Plants obtain some mineral nutrients through ion exchangebetween the soil solution and the surface of clay particles.Review Figure 37.4� Farmers use fertilizers to make up for deficiencies in soil min-eral nutrient content, and they apply lime to raise low soil pH.� Plants affect soils in various ways, such as by adding organicmaterial, removing nutrients (especially in agriculture), andchanging pH.

Nitrogen Fixation� A few species of soil bacteria are responsible for almost allnitrogen fixation. Some nitrogen-fixing bacteria live free in thesoil; others live symbiotically as bacteroids within the roots ofplants.� In nitrogen fixation, nitrogen gas (N2) is reduced to ammonia(NH3) or ammonium ions (NH4

+) in a reaction catalyzed bynitrogenase. Review Figure 37.6� Nitrogenase requires anaerobic conditions, but the bacteroidsin root nodules require oxygen for their respiration. Leghemo-globin helps maintain the oxygen supply to the bacteroids at theproper level.� The formation of a nodule requires an interaction betweenthe root system of a legume and Rhizobium bacteria. ReviewFigure 37.7� Nitrogen-fixing bacteria reduce atmospheric N2 to ammonia,but most plants take up both ammonium ions and nitrate ions.Nitrifying bacteria oxidize ammonia to nitrate. Plants take upnitrate and reduce it back to ammonia, a feat of which animalsare incapable. Review Figure 37.8. See Web/CD Activity 37.1� Denitrifying bacteria return N2 to the atmosphere, completingthe global nitrogen cycle. Review Figure 37.8

Carnivorous and Heterotrophic Plants� Carnivorous plant species are autotrophs that supplementtheir nitrogen supply by feeding on insects.� A few heterotrophic plants are parasitic on other plants. Someparasitic plants have major effects on crops, especially in devel-oping countries.

Self-Quiz1. Macronutrients

a. are so called because they are more essential thanmicronutrients.

b. include manganese, boron, and zinc, among others.c. function as catalysts.d. are required in concentrations of at least 1 gram per

kilogram of plant dry matter.e. are obtained by the process of photosynthesis.

Dodder flowers

Tendrils ofdodder

Host stem

37.10 A Parasitic Plant Tendrils of dodder wrap around otherplants. This parasitic plant (genus Cuscuta) obtains water, sugars, andother nutrients from its host through tiny, rootlike protuberances thatpenetrate the surface of the host.

2. Which of the following is not an essential mineral elementfor plants?a. Potassiumb. Magnesiumc. Calciumd. Leade. Phosphorus

3. Fertilizersa. are often characterized by their N-P-O percentages.b. are not required if crops are removed frequently enough.c. restore needed mineral nutrients to the soil.d. are needed to provide carbon, hydrogen, and oxygen to

plants.e. are needed to destroy soil pests.

4. In a typical soil,a. the topsoil tends to lose mineral nutrients by leaching.b. there are four or more horizons.c. the C horizon consists primarily of loam.d. the dead and decaying organic matter gathers in the B

horizon.e. more clay means more air space and thus more oxygen

for roots.

5. Which of the following is not an important step in soil formation?a. Removal of bacteriab. Mechanical weatheringc. Chemical weatheringd. Clay formatione. Hydrolysis of soil minerals

6. Nitrogen fixation isa. performed only by plants.b. the oxidation of nitrogen gas.c. catalyzed by the enzyme nitrogenase.d. a single-step chemical reaction.e. possible because N2 is a highly reactive substance.

7. Nitrification isa. performed only by plants.b. the reduction of ammonium ions to nitrate ions.c. the reduction of nitrate ions to nitrogen gas.d. catalyzed by the enzyme nitrogenase.e. performed by certain bacteria in the soil.

8. Nitrate reductiona. is performed by plants.b. takes place in mitochondria.c. is catalyzed by the enzyme nitrogenase.d. includes the reduction of nitrite ions to nitrate ions.e. is known as the Haber process.

9. Which of the following is a parasite?a. Venus flytrapb. Pitcher plantc. Sundewd. Doddere. Tobacco

10. All carnivorous plantsa. are parasites.b. depend on animals as a source of carbon.c. are incapable of photosynthesis.d. depend on animals as their sole source of phosphorus.e. obtain supplemental nitrogen from animals.

For Discussion1. Methods for determining whether a particular element is

essential have been known for more than a century. Sincethese methods are so well established, why was the essen-tiality of some elements discovered only recently?

2. If a Venus flytrap were deprived of soil sulfates and hencemade unable to synthesize the amino acids cysteine andmethionine, would it die from lack of protein? Explain.

3. Soils are dynamic systems. What changes might result whenland is subjected to heavy irrigation for agriculture afterbeing relatively dry for many years? What changes in thesoil might result when a virgin deciduous forest is cut downand replaced by crops that are harvested each year?

4. We mentioned that important positively charged ions areheld in the soil by clay particles, but other, equally impor-tant, negatively charged ions are leached deeper into thesoil’s B horizon. Why doesn’t leaching cause an electricalimbalance in the soil? (Hint: Think of the ionization ofwater.)

5. The biosphere of Earth as we know it depends on the exis-tence of a few species of nitrogen-fixing prokaryotes. Whatdo you think might happen if one of these species were tobecome extinct? If all of them were to disappear?

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