1. Soil and Plant Nutrition
Chapter 37
Soil and Plant Nutrition

2. Overview: “The Nation that Destroys Its Soil Destroys Itself”
Farmland productivity often suffers from chemical contamination, mineral deficiencies, acidity, salinity, and poor drainage
Healthy soils improve plant growth by enhancing plant nutrition
Typically, plants obtain carbon dioxide from the air, and water and minerals from the soil

3. Fig. 37-1
Figure 37.1 Could this happen again?

4. Soil Bacteria and Plant Nutrition
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
Soil Bacteria and Plant Nutrition
The layer of soil bound to the plant’s roots is the rhizosphere
The rhizosphere has high microbial activity because of sugars, amino acids, and organic acids secreted by roots

5. Rhizobacteria
Free-living rhizobacteria thrive in the rhizosphere, and some can enter roots
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
Inoculation of seeds with rhizobacteria can increase crop yields

6. Bacteria in the Nitrogen Cycle
Nitrogen can be an important limiting nutrient for plant growth
The nitrogen cycle transforms nitrogen and nitrogen-containing compounds
Most soil nitrogen comes from actions of soil bacteria
Plants absorb nitrogen as either NO3– or NH4+
Bacteria break down organic compounds or use N2 to produce NH3, which is converted to NH4+
Nitrification is carried out by bacteria that convert NH3 into NO3–

7. Bacteria in the Nitrogen Cycle
Atmosphere N2 N2
Atmosphere
Soil
Nitrate and
nitrogenous
organic
compounds
exported in
xylem to
shoot system N2
Nitrogen-fixing bacteria
Denitrifying
bacteria H+
(from soil)
NH4+
Soil
NH3
(ammonia)
NH4+
(ammonium)
NO3–
(nitrate)
Ammonifying
bacteria
Nitrifying
bacteria
Figure 37.9 The role of soil bacteria in the nitrogen nutrition of plants
Organic material (humus)
Root
Nitrogen can be an important limiting nutrient for plant growth
The nitrogen cycle transforms nitrogen and nitrogen-containing compounds
Most soil nitrogen comes from actions of soil bacteria

8. N2 Nitrogen-fixing bacteria convert gaseous nitrogen into NH3
Fig N2
Nitrogen-fixing bacteria convert gaseous nitrogen into NH3
NH3 (ammonia) picks up another H+ in the soil solution to form NH4+, which plants
can absorb
Figure 37.9 The role of soil bacteria in the nitrogen nutrition of plants
Ammonifying
bacteria
Organic material (humus)

9. N2 N2 Atmosphere Soil Nitrate and nitrogenous organic compounds
Fig N2 N2
Atmosphere
Soil
Nitrate and
nitrogenous
organic
compounds
exported in
xylem to
shoot system
Nitrogen-fixing bacteria
Denitrifying
bacteria H+
(from soil)
NH4+
NH3
(ammonia)
NH4+
(ammonium)
NO3–
(nitrate)
Figure 37.9 The role of soil bacteria in the nitrogen nutrition of plants
Ammonifying
bacteria
Nitrifying bacteria
oxidize ammonium
to nitrite and then to
nitrate
Organic material (humus)
Root

10. Nitrogen-Fixing Bacteria: A Closer Look
N2 is abundant in the atmosphere, but unavailable to plants
Nitrogen fixation is the conversion of nitrogen from N2 to NH3
Symbiotic relationships with nitrogen-fixing bacteria provide some plant species with a built-in source of fixed nitrogen
Key symbioses occur between nitrogen-fixing bacteria and plants, including those in the legume family (peas, beans, and other similar plants)
Inside the root nodule, Rhizobium bacteria assume a form called bacteroids, which are contained within vesicles formed by the root cell
The bacteria of a root nodule obtain sugar from the plant and supply the plant with fixed nitrogen

11. Fig a
The bumps on this pea plant root are nodules containing bacteria. The bacteria fix nitrogen and obtain photosynthetic products supplied by the plant.
Nodules
Figure Root nodules on legumes
Roots
(a) Pea plant root

12. Fig b
Inside the root nodule, Rhizobium bacteria assume a form called bacteroids, which are contained within vesicles formed by the root cell
The bacteria of a root nodule obtain sugar from the plant and supply the plant with fixed nitrogen
Bacteroids
within
vesicle
Figure Root nodules on legumes
5 µm
(b) Bacteroids in a soybean root
nodule

13. 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
The following slide is interesting however, do not memorize.

14. Rhizobium bacteria Infection thread Chemical signals Bacteroids form
Fig
Rhizobium
bacteria
Infection thread 1
Chemical signals
attract bacteria 2
Bacteroids form
Bacteroid 1
Infected root hair
Dividing cells
in root cortex 2
Dividing cells in
pericycle
Developing
root nodule
Bacteroid 3 4
Figure Development of a soybean root nodule 3
Nodule
forms 4
Nodule develops
vascular tissue
Nodule
vascular
tissue
Bacteroid

15. Nitrogen Fixation and Agriculture
Crop rotation takes advantage of the agricultural benefits of symbiotic nitrogen fixation
A non-legume 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
Instead of being harvested, the legume crop is often plowed under to decompose as “green manure” and reduce the need for manufactured fertilizer
Non-legumes such as alder trees, certain tropical grasses, and rice benefit either directly or indirectly from nitrogen-fixing bacteria

16. 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
Mycorrizal relationships are common and might have helped plants to first colonize land

17. 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
Epidermis
Cortex
Mantle
(fungal
sheath)
100 µm
Endodermis
Fungal
hyphae
between
cortical
cells
Mantle
(fungal sheath)
Figure 37.12a Mycorrhizae
(colorized SEM)
(a) Ectomycorrhizae

18. 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
Epidermis
Cortex
10 µm
Cortical cells
Endodermis
Fungal
hyphae
Fungal
vesicle
Casparian
strip
Root
hair
Arbuscules
Figure 37.12b Mycorrhizae
Plasma
membrane
(LM, stained specimen)
(b) Arbuscular mycorrhizae (endomycorrhizae)

19. 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

20. plant biomass (%) Increase in colonization (%) Mycorrhizal
Fig
EXPERIMENT
RESULTS
300
200
plant biomass (%)
Increase in
100
Invaded
Uninvaded
Sterilized
invaded
Sterilized
uninvaded
Soil type 40 30
colonization (%)
Mycorrhizal
Figure Does the invasive weed garlic mustard disrupt mutualistic associations between native tree seedlings and arbuscular mycorrhizal fungi? 20
Seedlings 10
Sugar maple
Red maple
Invaded
Uninvaded
White ash
Soil type

21. Epiphytes, Parasitic Plants, and Carnivorous Plants
Some plants have nutritional adaptations that use other organisms in nonmutualistic ways

22. Staghorn fern, an epiphyte
Fig a
An epiphyte grows on another plant and obtains water and minerals from rain
Figure Unusual nutritional adaptations in plants
Staghorn fern, an epiphyte

23. Mistletoe, a photosynthetic parasite
Parasitic plants absorb sugars and minerals from their living host plant
Figure Unusual nutritional adaptations in plants
Mistletoe, a photosynthetic parasite

24. Dodder, a nonphotosynthetic parasite draws its nutrients from the host
Fig c-1
Host’s phloem
Dodder
Haustoria
Figure Unusual nutritional adaptations in plants
Dodder, a nonphotosynthetic parasite draws its nutrients from
the host

25. Indian pipe, a nonphotosynthetic parasite absorbs nutrients from
Fig d
Indian pipe, a nonphotosynthetic
parasite absorbs nutrients from
the fungal hyphae of
mycorrhizae of green plants.
Figure Unusual nutritional adaptations in plants

26. Fig e-1
Carnivorous plants are photosynthetic but obtain nitrogen by killing and digesting mostly insects
Figure Unusual nutritional adaptations in plants
Venus flytrap

27. Pitcher plants have water-filled funnels that insects
Fig f-1
Figure Unusual nutritional adaptations in plants
Pitcher plants have water-filled funnels that insects
drown in and then are digested

28. Sundews exude a sticky fluid that glitters like
Fig g
Figure Unusual nutritional adaptations in plants
Sundews exude a sticky fluid that glitters like
Dew. Insects get stuck to the leaf hairs, which
enfold the prey.

29. You should now be able to:
Summarize the ecological role of each of the following groups of bacteria: ammonifying, denitrifying, nitrogen-fixing, nitrifying
Describe the basis for crop rotation
Distinguish between ectomycorrhizae and arbuscular mycorrhizae
Describe the adaptations for nutrition of parasitic, epiphytic, and carnivorous plants