Soil and Plant Nutrition

1
Chapter 37
Soil and Plant Nutrition
2
Overview A Horrifying Discovery

• Carnivory by pitcher plants is well-documented
• An extreme example is Nepenthes rajah, a pitcher
  plant large enough to catch a rat
• N. Rajah lives in very unproductive soil and uses
  carnivory to obtain nutrients such as calcium,
  potassium, and phosphorus

3
Figure 37.1
4
Concept 37.1 Soil contains a living, complex
ecosystem

• Plants obtain most of their water and minerals
  from the upper layers of soil
• Living organisms play an important role in these
  soil layers
• This complex ecosystem is fragile
• The basic physical properties of soil are
• Texture
• Composition

5
Soil Texture

• Soil particles are classified by size from
  largest to smallest they are called sand, silt,
  and clay
• Soil is stratified into layers called soil
  horizons
• Topsoil consists of mineral particles, living
  organisms, and humus, the decaying organic
  material

6
Figure 37.2
A horizon
B horizon
C horizon
7

• Soil solution consists of water and dissolved
  minerals in the pores between soil particles
• After a heavy rainfall, water drains from the
  larger spaces in the soil, but smaller spaces
  retain water because of its attraction to clay
  and other particles
• The film of loosely bound water is usually
  available to plants
• Loams are the most fertile topsoils and contain
  equal amounts of sand, silt, and clay

8
Topsoil Composition

• A soils composition refers to its inorganic
  (mineral) and organic chemical components

9
Inorganic Components

• Cations (for example K, Ca2, Mg2) adhere to
  negatively charged soil particles this prevents
  them from leaching out of the soil through
  percolating groundwater

10

• During cation exchange, cations are displaced
  from soil particles by other cations
• Displaced cations enter the soil solution and can
  be taken up by plant roots
• Negatively charged ions do not bind with soil
  particles and can be lost from the soil by
  leaching

Animation How Plants Obtain Minerals from Soil
11
Figure 37.3
Soil particle
?
?
K?
K?
?
?
?
?
?
?
?
Ca2?
Mg2?
Ca2?
K?
H?
H2O ? CO2
H?
HCO3? ?
H2CO3
Root hair
Cell wall
12
Organic Components

• Humus builds a crumbly soil that retains water
  but is still porous
• It also increases the soils capacity to exchange
  cations and serves as a reservoir of mineral
  nutrients
• Topsoil contains bacteria, fungi, algae, other
  protists, insects, earthworms, nematodes, and
  plant roots
• These organisms help to decompose organic
  material and mix the soil

13
Soil Conservation and Sustainable Agriculture

• Soil management, by fertilization and other
  practices, allowed for agriculture and cities
• In contrast with natural ecosystems, agriculture
  depletes the mineral content of soil, taxes water
  reserves, and encourages erosion
• The American Dust Bowl of the 1930s resulted from
  soil mismanagement

14
Figure 37.4
15

• At present, 30 of the worlds farmland has
  reduced productivity because of soil
  mismanagement
• The goal of sustainable agriculture is to use
  farming methods that are conservation-minded,
  environmentally safe, and profitable

16
Irrigation

• Irrigation is a huge drain on water resources
  when used for farming in arid regions
• For example, 75 of global freshwater use is
  devoted to agriculture
• The primary source of irrigation water is
  underground water reserves called aquifers
• The depleting of aquifers can result in land
  subsidence, the settling or sinking of land

17
Figure 37.5
18

• Irrigation can lead to salinization, the
  concentration of salts in soil as water
  evaporates
• Drip irrigation requires less water and reduces
  salinization

19
Fertilization

• Soils can become depleted of nutrients as plants
  and the nutrients they contain are harvested
• Fertilization replaces mineral nutrients that
  have been lost from the soil
• Commercial fertilizers are enriched in nitrogen
  (N), phosphorus (P), and potassium (K)
• Excess minerals are often leached from the soil
  and can cause algal blooms in lakes

20

• Organic fertilizers are composed of manure,
  fishmeal, or compost
• They release N, P, and K as they decompose

21
Adjusting Soil pH

• Soil pH affects cation exchange and the chemical
  form of minerals
• Cations are more available in slightly acidic
  soil, as H ions displace mineral cations from
  clay particles
• The availability of different minerals varies
  with pH
• For example, at pH 8 plants can absorb calcium
  but not iron

22
Controlling Erosion

• Topsoil from thousands of acres of farmland is
  lost to water and wind erosion each year in the
  United States
• Erosion of soil causes loss of nutrients

23

• Erosion can be reduced by
• Planting trees as windbreaks
• Terracing hillside crops
• Cultivating in a contour pattern
• Practicing no-till agriculture

24
Figure 37.6
25
Phytoremediation

• Some areas are unfit for agriculture because of
  contamination of soil or groundwater with toxic
  pollutants
• Phytoremediation is a biological, nondestructive
  technology that reclaims contaminated areas
• Plants capable of extracting soil pollutants are
  grown and are then disposed of safely

26
Concept 37.2 Plants require essential elements
to complete their life cycle

• Soil, water, and air all contribute to plant
  growth
• 8090 of a plants fresh mass is water
• 4 of a plants dry mass is inorganic substances
  from soil
• 96 of plants dry mass is from CO2 assimilated
  during photosynthesis

27
Macronutrients and Micronutrients

• More than 50 chemical elements have been
  identified among the inorganic substances in
  plants, but not all of these are essential to
  plants
• There are 17 essential elements, chemical
  elements required for a plant to complete its
  life cycle
• Researchers use hydroponic culture to determine
  which chemical elements are essential

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Figure 37.7
TECHNIQUE
Control Solutioncontaining all minerals
Experimental Solutionwithout potassium
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Table 37.1
30

• Nine of the essential elements are called
  macronutrients because plants require them in
  relatively large amounts
• The macronutrients are carbon, oxygen, hydrogen,
  nitrogen, phosphorus, sulfur, potassium, calcium,
  and magnesium

31

• The remaining eight are called micronutrients
  because plants need them in very small amounts
• The micronutrients are chlorine, iron, manganese,
  boron, zinc, copper, nickel, and molybdenum
• Plants with C4 and CAM photosynthetic pathways
  also need sodium
• Micronutrients function as cofactors, nonprotein
  helpers in enzymatic reactions

32
Symptoms of Mineral Deficiency

• Symptoms of mineral deficiency depend on the
  nutrients function and mobility within the plant
• Deficiency of a mobile nutrient usually affects
  older organs more than young ones
• Deficiency of a less mobile nutrient usually
  affects younger organs more than older ones
• The most common deficiencies are those of
  nitrogen, potassium, and phosphorus

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Figure 37.8
Healthy
Phosphate-deficient
Potassium-deficient
Nitrogen-deficient
34
Improving Plant Nutrition by Genetic
Modification Some Examples

• Plants can be genetically engineered to better
  fit the soil

35
Resistance to Aluminum Toxicity

• Aluminum in acidic soils damages roots and
  greatly reduces crop yields
• The introduction of bacterial genes into plant
  genomes can cause plants to secrete acids that
  bind to and tie up aluminum

36
Flood Tolerance

• Waterlogged soils deprive roots of oxygen and
  cause buildup of ethanol and toxins
• The gene Submergence 1A-1 is responsible for
  submergence tolerance in flood-resistant rice

37
Smart Plants

• Smart plants inform the grower of a nutrient
  deficiency before damage has occurred
• A blue tinge indicates when these plants need
  phosphate-containing fertilizer

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Figure 37.9
No phosphorusdeficiency
Beginningphosphorusdeficiency
Well-developedphosphorusdeficiency
39
Figure 37.9a
No phosphorusdeficiency
40
Figure 37.9b
Beginningphosphorusdeficiency
41
Figure 37.9c
Well-developedphosphorusdeficiency
42
Concept 37.3 Plant nutrition often involves
relationships with other organisms

• Plants and soil microbes have a mutualistic
  relationship
• Dead plants provide energy needed by
  soil-dwelling microorganisms
• Secretions from living roots support a wide
  variety of microbes in the near-root environment

43
Soil Bacteria and Plant Nutrition

• The layer of soil bound to the plants roots is
  the rhizosphere
• The rhizosphere contains bacteria that act as
  decomposers and nitrogen-fixers

44
Rhizobacteria

• Free-living rhizobacteria thrive in the
  rhizosphere, and some can enter roots
• The rhizosphere has high microbial activity
  because of sugars, amino acids, and organic acids
  secreted by roots

45

• Rhizobacteria can play several roles
• Produce hormones that stimulate plant growth
• Produce antibiotics that protect roots from
  disease
• Absorb toxic metals or make nutrients more
  available to roots

46
Bacteria in the Nitrogen Cycle

• Nitrogen can be an important limiting nutrient
  for plant growth
• The nitrogen cycle transforms nitrogen and
  nitrogen-containing compounds
• Plants can absorb nitrogen as either NO3 or NH4?
• Most soil nitrogen comes from actions of soil
  bacteria

47
Figure 37.10
N2
N2
ATMOSPHERE
ATMOSPHERE
Nitrate andnitrogenousorganiccompoundsexported
inxylem toshoot system
SOIL
Nitrogen-fixingbacteria
N2
Denitrifyingbacteria
H?(from soil)
NH4?
SOIL
NH3(ammonia)
NH4?(ammonium)
NO3?(nitrate)
Nitrifyingbacteria
Ammonifyingbacteria
Organicmaterial (humus)
Root
48
Figure 37.10a-1
N2
Nitrogen-fixingbacteria
NH3(ammonia)
Ammonifyingbacteria
Organicmaterial (humus)
49
Figure 37.10a-2
N2
N2
ATMOSPHERE
Nitrate andnitrogenousorganiccompoundsexported
inxylem toshoot system
SOIL
Nitrogen-fixingbacteria
Denitrifyingbacteria
H?(from soil)
NH4?
NO3?(nitrate)
NH3(ammonia)
NH4?(ammonium)
Ammonifyingbacteria
Nitrifyingbacteria
Organicmaterial (humus)
Root
50

• Conversion to NH4?
• Ammonifying bacteria break down organic compounds
  and release ammonia (NH3)
• Nitrogen-fixing bacteria convert N2 into NH3
• NH3 is converted to NH4?
• Conversion to NO3
• Nitrifying bacteria oxidize NH3 to nitrite (NO2)
  then nitrite to nitrate (NO3)

51

• Nitrogen is lost to the atmosphere when
  denitrifying bacteria convert NO3 to N2

52
Nitrogen-Fixing Bacteria A Closer Look

• Nitrogen is abundant in the atmosphere, but
  unavailable to plants because of the triple bond
  between atoms in N2
• Nitrogen fixation is the conversion of nitrogen
  from N2 to NH3
• N2 ? 8e? ? 8 H? ? 16 ATP ? 2 NH3 ? H2 ? 16
  ADP ? 16 Pi
• Symbiotic relationships with nitrogen-fixing
  Rhizobium bacteria provide some plant species
  (e.g., legumes) with a source of fixed nitrogen

53

• Along a legumes roots are swellings called
  nodules, composed of plant cells infected by
  nitrogen-fixing Rhizobium bacteria

54
Figure 37.11
Bacteroidswithinvesicle
Nodules
Roots
5 ?m
(a) Soybean root
55
Figure 37.11a
Nodules
Roots
(a) Soybean root
56

• Inside the root nodule, Rhizobium bacteria assume
  a form called bacteroids, which are contained
  within vesicles formed by the root cell

57
Figure 37.11b
Bacteroidswithinvesicle
5 ?m
58

• The plant obtains fixed nitrogen from Rhizobium,
  and Rhizobium obtains sugar and an anaerobic
  environment
• Each legume species is associated with a
  particular strain of Rhizobium
• The development of a nitrogen-fixing root nodule
  depends on chemical dialogue between Rhizobium
  bacteria and root cells of their specific plant
  hosts

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Figure 37.12
Rhizobiumbacteria
Infectionthread
Dividing cellsin root cortex
Chemical signalsattract bacteria andan
infection threadforms.
Bacteroids form.
Infectedroot hair
Bacteroid
Nodulevasculartissue
Dividing cells in pericycle
Bacteroid
Bacteroids
Root hairsloughed off
Developing rootnodule
Growth continuesand a root nodule forms.
Sclerenchymacells
The mature nodulegrows to be many times the
diameterof the root.
The nodule developsvascular tissue.
Nodulevascular tissue
Bacteroid
60
Nitrogen Fixation and Agriculture

• Crop rotation takes advantage of the agricultural
  benefits of symbiotic nitrogen fixation
• A nonlegume such as maize is planted one year,
  and the next year a legume is planted to restore
  the concentration of fixed nitrogen in the soil

61

• Instead of being harvested, the legume crop is
  often plowed under to decompose as green manure
• Nonlegumes such as alder trees and certain
  tropical grasses benefit from nitrogen-fixing
  bacteria
• Rice paddies often contain an aquatic fern that
  has mutualistic cyanobacteria that fix nitrogen

62
Fungi and Plant Nutrition

• Mycorrhizae are mutualistic associations of fungi
  and roots
• The fungus benefits from a steady supply of sugar
  from the host plant
• The host plant benefits because the fungus
  increases the surface area for water uptake and
  mineral absorption
• Mycorrhizal fungi also secrete growth factors
  that stimulate root growth and branching

63
Mycorrhizae and Plant Evolution

• Mycorrhizal fungi date to 460 million years ago
  and might have helped plants colonize land

64
The Two Main Types of Mycorrhizae

• Mycorrhizal associations consist of two major
  types
• Ectomycorrhizae
• Arbuscular mycorrhizae

65
Figure 37.13
Epidermis
Cortex
Mantle (fungal sheath)
Epidermalcell
(Colorized SEM)
Endodermis
Fungalhyphaebetweencorticalcells
1.5 mm
Mantle(fungal sheath)
(LM)
50 ?m
(a) Ectomycorrhizae
Cortical cell
Epidermis
Cortex
Endodermis
Fungalvesicle
Fungalhyphae
Casparianstrip
10 ?m
Root hair
Arbuscules
(LM)
Plasmamembrane
66

• In ectomycorrhizae, the mycelium of the fungus
  forms a dense sheath over the surface of the root
• These hyphae form a network in the apoplast, but
  do not penetrate the root cells
• Ectomycorrhizae occur in about 10 of plant
  families including pine, spruce, oak, walnut,
  birch, willow, and eucalyptus

67
Figure 37.13aa
Epidermis
Cortex
Mantle (fungal sheath)
Epidermalcell
(Colorized SEM)
Endodermis
Fungalhyphaebetweencorticalcells
1.5 mm
Mantle(fungal sheath)
(LM)
50 ?m
(a) Ectomycorrhizae
68
Figure 37.13ab
(Colorized SEM)
1.5 mm
Mantle(fungal sheath)
69
Figure 37.13ac
Epidermalcell
Fungalhyphaebetweencorticalcells
(LM)
50 ?m
70

• In arbuscular mycorrhizae, microscopic fungal
  hyphae extend into the root
• These mycorrhizae penetrate the cell wall but not
  the plasma membrane to form branched arbuscules
  within root cells
• Hyphae can form arbuscules within cells these
  are important sites of nutrient transfer
• Arbuscular mycorrhizae occur in about 85 of
  plant species, including grains and legumes

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Figure 37.13ba
Cortical cell
Cortex
Epidermis
Endodermis
Fungalvesicle
Fungalhyphae
Casparianstrip
10 ?m
Root hair
Arbuscules
(LM)
Plasmamembrane
72
Figure 37.13bb
Cortical cell
10 ?m
Arbuscules
(LM)
73
Agricultural and Ecological Importance of
Mycorrhizae

• Farmers and foresters often inoculate seeds with
  fungal spores to promote formation of mycorrhizae
• Some invasive exotic plants disrupt interactions
  between native plants and their mycorrhizal fungi
• For example, garlic mustard slows growth of other
  plants by preventing the growth of mycorrhizal
  fungi

74
Figure 37.14a
EXPERIMENT
75
Figure 37.14b
RESULTS
300
200
Increase inplant biomass ()
100
0
Sterilizedinvaded
Sterilizeduninvaded
Invaded
Uninvaded
Soil type
40
30
Mycorrhizalcolonization ()
20
Seedlings
10
Sugar maple
0
Red maple
Invaded
Uninvaded
White ash
Soil type
76
Epiphytes, Parasitic Plants, and Carnivorous
Plants

• Some plants have nutritional adaptations that use
  other organisms in nonmutualistic ways
• Three unusual adaptations are
• Epiphytes
• Parasitic plants
• Carnivorous plants
• An epiphyte grows on another plant and obtains
  water and minerals from rain

77
Figure 37.15a
Staghorn fern, an epiphyte
78

• Parasitic plants absorb sugars and minerals from
  their living host plant

79
Figure 37.15b
Mistletoe, a photo-synthetic parasite
Dodder, anonphotosyntheticparasite (orange)
Indian pipe, a nonphoto-synthetic parasite
ofmycorrhizae
80
Figure 37.15ba
Mistletoe, a photo-synthetic parasite
81
Figure 37.15bb
Dodder, a nonphotosyntheticparasite (orange)
82
Figure 37.15bc
Indian pipe, a nonphoto-synthetic parasite
ofmycorrhizae
83
Figure 37.15c
Sundews
Pitcher plants
Venus flytrap
84
Figure 37.15ca
Pitcher plants
85
Figure 37.15cb
Pitcher plants
86
Figure 37.15cc
Venus flytrap
87
Figure 37.15cd
Venus flytrap
88
Figure 37.15ce
Sundews
89

• Carnivorous plants are photosynthetic but obtain
  nitrogen by killing and digesting mostly insects

Video Sun Dew Trapping Prey
90
Figure 37.UN01
N2
(to atmosphere)
N2
(from atmosphere)
Nitrogen-fixing bacteria
H?(from soil)
Denitrifyingbacteria
NH4?
NH4?(ammonium)
NH3(ammonia)
NO3?(nitrate)
Nitrifyingbacteria
Ammonifyingbacteria
Organicmaterial (humus)
Root
91
Figure 37.UN02