chapter 5

Question 1

What is soil formed from?Or how is soil formed?

Answer 1

the weathering of rocks
—-> it is the highly weathered outer layer of the Earth’s crust

Question 2

What is soil composed of?

Answer 2

-Inorganic material: rocks, sand, silt and clay
-humus= partly decayed organic matter
-pore spaces (filled with water or air)
-living microorganisms (ex. Bacteria that decompose organic debris)

Question 3

How many natural occurring elements does the earth include? And how are most found?in the form of what?

Answer 3

92
Most are found in the form of inorganic compounds called minerals.

Question 4

What do rocks consist of? And what does this result in?

Answer 4

Most rocks consist of several different minerals (ex. Iron, calcium, potassium, sulphur, magnesium…)
This results in having different types of rocks

Question 5

What are the 3 main types of inorganic soil particles?

Answer 5

sand, silt and clay. Different types of soil are mixtures of these particles in different proportions

Question 6

What does the breaking down of rocks yield?

Answer 6

–Gravel(>2mm and mainly considered inert)

–Sand is large, coarse and gritty; it drains water quickly, but does not hold much water. It is good for aeration.

—Silt is medium in size.

—Clay is microscopic, sticky when wet, holds a lot of water but does not drain easily

Question 7

What is a good soil?What does it allow?

Answer 7

loam soil that contains an ideal combination of sand, silt, and clay, which allows good drainage, water storage, and aeration

Question 8

Topsoil

Answer 8

Most roots are found in the topsoil.

The topsoil contains soil mineral particles of different sizes (< 2 mm in diameter), humus, and living organisms (worms, insects…).

The topsoil is characterized by their relative amount of sand, silt and clay.

Soil composition determines the degree of water and nutrient binding to the soil particles.

Question 9

Which minerals are available for uptake by the roots?

Answer 9

Only minerals that are dissolved in water, found in spaces or pores between soil particles,

Question 10

What charge do both minerals and organic soil particles tend to have?

Answer 10

negative charge
they attract positive ions.

Question 11

Mineral availability

Answer 11

*The negatively charged anions remain in solution
*The anions create a charge gradient (voltage) between soil and root cells.
*Consequently, positive ions move out of root cells
*Roots have proton (H+) pump: they actively pump H+ out and into soil.
*Creating a strong potential/electrochemical gradient.
*The gradient causes K+ and other positive ions to enter the roots via ion channels.
*Some anions enter roots (actively) via co-transporters

Question 12

What affects root transport of minerals?

Answer 12

Membrane potential maintained by the root, water potential difference inside and outside the root.

Question 13

Pores in soil

Answer 13

*About half of the soil volume is occupied by pores
*Pores may be filled with air or water.
*Some of this water is unavailable because it drains immediately due to gravity
*However, water that is held in small pores is readily available to plants.
*A balance of air and water in the soil is essential for root growth.

Question 14

Acid soils

Answer 14

*The pH of a soil affects the release of minerals from weathering rock.
*For example, at low pH, aluminum, which is toxic to many plants and stunts (inhibits) root growth, is released.
(Example: most soils in the tropical Americas (ex. Columbia) are acidic; and aluminum-toxicity reduces corn yield).
*Most plants grow best at a slightly acidic pH.
*About 26% of the world’s arable soil is considerably more acidic than optimal pH for plant growth.
*Efforts are underway to breed more (aluminum) tolerant plants and to clean up polluted soil

Question 15

Saline soils

Answer 15

–Accumulation of salt ions, usually Na+ and Cl-, changes the water potential in soil.

—Leading to a loss of water and turgor in plants.

—Saline soils most commonly occur in dry areas where salts are introduced through irrigation; and rain is not sufficient to wash the salts down beyond root area; so water evaporates, leaving salts behind in topsoil.

—Also saline soils can occur when overwatering pulls salts from the lower soil levels into the topsoil.

—In such areas, rain is not sufficient to remove the salts, which gradually accumulate in the soil

Question 16

Desertification

Answer 16

One of the most dramatic examples of soil salinity occurred in Southern Iraq.
Desertification was accelerated in 1990s, when most of 20,000km2
of marshlands was drained by using dams to redirect water flow.
This turned the marshes into a salty desert, and left fishing boats sitting on dry, cracked soil.

Question 17

What determines the availability of nutrients to plants?

Answer 17

soil composition, pH, water and salinity

Question 18

Plant nutrients

Answer 18

—Photosynthesis is the major source of nutrition for plants, via the fixation of CO2 into sugars using solar energy.

—In addition, plants need inorganic nutrients:
a. Macronutrients–needed in large amounts.
9 = C, O, H, N, K, Ca, Mg, P, and S.
b. Micronutrients–needed in minute amounts.
7 = Cl, Fe, Mn, Zn, B, Cu, and Mo.

—Deficiency of any one can have severe effects on plant growth

Question 19

How can mineral deficiencies be expressed?

Answer 19

differences in color of plant leaves

Question 20

How are nutritional requirements assesed?

Answer 20

by growing plants in hydroponic cultures

Question 21

Hydroponics

Answer 21

—Plant roots are suspended in aerated water containing nutrients.
*First, a plant seedling is grown in a complete nutrient solution.
*Then, the seedling is transplanted to a solution lacking one suspected essential nutrient.
*Growth of the seedling is monitored for presence of abnormal symptoms →the deficiency symptoms of that particular (omitted) nutrient is studied.

Question 22

What can be replaced in hydroponic systems to maximize growth?

Answer 22

Soil provides nutrients and support, but both of these functions can be replaced in hydroponic systems to maximize growth

Question 23

identifying nutritional requirements

Answer 23

1) A seedling is grown in a solution with complete nutrients.
2) The seedling is then transplanted to a solution that lacks one of the suspected essential nutrients.
The growth of the seedling is studied to observe effect of the deficiency.
If growth is normal, then that specific nutrient is not essential.
If abnormal growth (yellowing of leaves or stunted growth), the tested mineral is essential for plant growth

Question 24

In some plants, scarce nutrients have been obtained through the evolution of what?

Answer 24

1.Mutualistic associations with N2 fixing bacteria
2.Mutualistic association with mycorrhizal fungi
3.Parasitism (plants with no chlorophyll)
4.Predation (insectivorous plants

Question 25

Nitrogen fixing bacteria

Answer 25

—Plants need ammonia (NH3) or nitrate (NO3-) to build amino acids.
—But most of the nitrogen in the atmosphere is in the form of gaseous nitrogen (N2).
*Plants lack the biochemical pathway (enzyme nitrogenase) to convert N2 to NH3.
*But some bacteria have this capacity: possess nitrogenase enzyme.
*A symbiotic mutualistic association evolved between some plant groups, mainly legume, and nitrogen-fixing bacteria.
*Some of these bacteria live in close association with the plant roots.
*In legumes (ex. peas/beans), the N2-fixing bacteria (mainly Rhizobium sp.) are housed in root nodules.

Question 26

In addition to protection, what does rhizobium gain from the plant?

Answer 26

Rhizobium gains carbohydrates and oxygen from the plant as it is expensive (in terms of ATP) to fix nitrogen.
There are complex signals between root hairs and bacteria to determine which bacteria should enter the roots

Question 27

Steps of nodule formation

Answer 27

step 1: Pea roots release flavonoids, which are recognized by rhizobial cells.
step 2:Rhizobia produce sugar-containing substances, called Nod factor.
step 3:Nod factors bind to root hair surface and signal the root hairs to grow and curl around the rhizobia.
4th: Rhizobia make an infection thread which grows inside root hair, then to root cortex.
Rhizobia take control of cell division in cortex and pericycle (root cells divide forming nodule).

5th: Rhizobia change shape: now called bacteroids.
They produce O2-binding heme group that combines with globin group in pea, making leghemoglobin: gives nodule a pinkish color.
Function of leghemoglobin is to bring O2 to bacteroids.

6th: Bacteroids produce nitrogenase enzyme and fix atmospheric N2.

Question 28

leghemoglobin

Answer 28

leghemoglobin: gives nodule a pinkish color.
Function of leghemoglobin is to bring O2 to bacteroids.

Question 29

MYCORRHYZAE

Answer 29

Mycorrhizae are symbiotic, mutualistic associations between plant roots and (mycorrhizal) fungi.
*
They are found in about 90% of vascular plants
*
The fungi allow the plants to draw more water and nutrients, mainly phosphorus, from the soil.
*
The fungi hyphae increase the surface area available for nutrient uptake.
*
The uptake of some micronutrients is also facilitated.
*
In return, the plants offer carbohydrates to the fungi
This is particularly important in uptake of phosphorus, one of the major nutrients required by plants.
When mycorrhizae are present, plants are less susceptible to water stress.

Question 30

Parasitic plants

Answer 30

May be photosynthetic or non-photosynthetic
*
(At least 3,000 types of parasitic plants)
*
They tap into the nutrient resources of other plants and steal carbohydrates
*
Adaptations include structures that are inserted into the vascular tissue of the host plant so that nutrients can be siphoned into the parasite.
*
Examples: dodder and Indian pipe
Indian pipe
Dodder

Question 31

Insectivorous plants

Answer 31

Often grow in acidic soils that lack nitrogen
*
Trap and digest small animals, primarily insects, to obtain adequate nitrogen supplies.
*
Have modified leaves adapted for luring and trapping prey
*
Prey is digested with enzymes secreted from specialized glands.
*
Ex. Pitcher plant; Venus flytrap; Sundews; Waterwheels

Question 32

Sundews

Answer 32

Sundew traps insects with sticky mucilage secretions that look like dew drops and then excretes digestive enzymes to obtain nutrients from the insect’s body.

Question 33

Waterwheels

Answer 33

This close relative to the Venus flytrap snaps shut to capture and digest small aquatic animals.
This is an aquatic plant.
(Its ancestor was a land dweller).

Question 34

Pitcher plants

Answer 34

Insects enter leaves; are trapped and digested.
Slippery or hairy sides prevent the escape of prey from the digestive liquid they fall into.
A canopy over the opening protects the pitcher from being flooded by rainwater