Unit 3: Soil Dynamics

Question 1

4 purposes of soil

Answer 1

• medium for plant growth
• habitat for other organisms (besides plants)
• breaks down organic material and recycles nutrient
• filters and purifies water

Question 2

five factors that determine the properties of soil

Answer 2

1. Parent Material: the original source of weathered matter from which soil develops (typically the bottom layer of a soil profile)
2. Climate: Influences type of vegetation that grows and detritus that is left. in below freezing conditions, decomposition and water movement occur very slowly. In humid conditions decomposition occurs too quickly for nutrients to be absorbed by soil
3. Topography: the slope and arrangement of a landscape. Steeper places experience more erosion so soils at the top of a slope may be thin and coarse, as finer particles are often washed away, while soils at the bottom (like valley floors) tend to accumulate finer particles and organic matter, leading to deeper, more fertile soils. In general steeper = more erosion and flat = collections of things like organic matter and eroded material
4. Types of organisms: microorganisms like bacteria and fungi -> break down organic material and recycle nutrients. Detritivores like earthworms, snails, slugs -> help form organic matter (humus) via excretion and death.
5. Time: “Old” soil has had continual inputs of organic matter for thousands of years (become deep and fertile)

Remember using the accronym “PC triple T”

Question 3

What are the various components that make up individual soil horizons?

Answer 3

Soil horizons = soil layers
Composition of soil horizons depends on climate, vegetation, and parent material

O horizon: organic matter (humus) in various stages of decomposition

A horizon: organic material mixed with underlying mineral material

B horizon: (subsoil): zone of accumulation of metals and nutrients

C horizon: (subsoil): least-weathered portion of the soil profile, similar to the parent material

R Horizon: Bedrock which is the parent material

Question 4

Soil erodes due to which factors:

Answer 4

• water
  watering from agriculture leads to runoff in waterways. This runoff often contains fertilizers and pesticides, which contribute to water pollution.
• wind (movement of the soil overtime)
• gravity
• human factors
  lead to water contamination which affects algae growth. Excessive algae growth, caused by nutrient pollution (often from agricultural runoff with fertilizers) block sunlight and keep plants from growing in waterways. This causes more erosion because there is less roots holding onto the soil in those waterways.

Question 5

How do agricultural practices influence soil erosion?

Answer 5

Problem #1: Deforestation
- lack of roots holding down the soil
- often replaced with plants that worsen erosion like soybean and wheat

Problem #2: Overgrazing
- plastureland can lead to cattle overeating and the top layer of soil can erode with wind and rain

Problem #3: Pesticides & Fertilizers
- can change chemistry of soil and kill microorganisms in soil

Problem #4: Tillage
- tillage is the turning of soil to keep it full of oxygen
- turning and breaking up soil keeps top layer from accumulating organic material and roots
- roots keep soil in place

Question 6

What are the different ways soil erodes into water?

Answer 6

sheet erosion - surface layer is removed by water

rill erosion - small channels of water flow which can happen because of tillage

gully erosion - wider cuts from expanded rill erosion

Question 7

What are the solutions to the different ways soil erodes into water?

Answer 7

sheet erosion - maintain plant cover

rill erosion - strip cropping (planting regular crops and close-growing plants, such as hay or nitrogen-fixing legumes, in alternating rows or bands to help reduce depletion of soil nutrients) or contour plowing (practice of plowing or planting across a slope following its elevation contour lines)

gully erosion - divert water away from rills and gullies

Question 8

3 Properties of soil

Answer 8

Porosity: refers to the space between soil particles

Permeability: refers to how quickly air and water drain through soil

Water-Holding Capacity: ability of soil to retain water. informed by the sizes of the particles present in soil

Question 9

Silt

Answer 9

Second largest particle in soil. Less space between particles; porosity and permeability are less compared to sand but more compared to clay

Question 9

Sand

Answer 9

Largest particle in soil. Has the most space between individual particles; porosity and permeability are the highest; water holding capacity is low

Question 10

Clay

Answer 10

Smallest particle in soil. Least amount of space between particles; porosity and permeability are the lowest; water holding capacity is high

Question 11

Why is it important for healthy and productive soils to have (for the most part) a combination of all three particles?

Answer 11

Needs to have some permeability and porosity for water to reach plant roots or not be waterlogged (sits in a pool of water and has too much water) but needs to be able to retain water and nutrients.

Question 12

Soil Triangle

Answer 12

Used to determine textural classes of soil from the percentages of sand, silt, and clay present

Start with the percentage of clay first then move on to percentage of silt and sand that makes sense

Question 13

Chemical property of soil: Base saturation

Answer 13

• Proportion of soil bases to soil acids, expressed as a percentage
• Soil bases include: NH4+, CA2+, Mg2+, K+, Na+ (ammonium, calcium, magnesium, potassium, sodium ions)
• These are micronutrients that promote plant growth
• Want soil to be more basic, if it’s more basic it has a higher base saturation

Question 14

Chemical property of soil: Concentration of nitrate, phosphate, and potassium

Answer 14

• Nitrate: NO v3 ^-,
• Phosphate: PO v 4 ^3-
• Potassium: K+
• These nutrients are essential for plant growth
• These nutrients are the main ingredients in fertilizers (often referred to as N, P, K)
• plants sometimes can’t absorb these nutrients in acid soils. This is because in low pH (acidic) conditions, elements like phosphorus, calcium, and magnesium can react with other compounds.

Question 15

Cation Exchange Capacity (CEC)

Answer 15

• Indicates how well soil retains/holds onto positive ions (cations)
  *Referred to in terms of being HIGH or LOW
• The cation exchange capacity (CEC) of a soil determines how frequently and in what doses it should receive fertilizer

Question 16

How does the CEC work?

Answer 16

• cations are positively charged ions, with a + symbol because less negatively charged electrons
• anions are negatively charged particles
• soil is often negatively charged, cations (some nutrients) are positively charged, so they are attracted to each other
• some soils are positively charged so anions would be attracted to it, but not soils in the U.S.
• more electronegativity of soil and more surface area means more CEC
• Soil’s negative charge comes from the weathering of rocks and minerals and from the pressure of CLAY releases NEGATIVE ions in the soil
• It’s important that soil has a negative charge overall because It is more likely to “hold onto” or retain those micronutrients that are essential for growth
• more positively charged molecules have a stronger attraction to soil, so it’s harder to exchange
• s plant roots absorb cations from the soil, the balance of charges is slightly disrupted. To maintain electrical balance, the soil particles release other cations from their surface, replacing those taken up by plants. This is called “cation exchange” because the soil essentially “exchanges” one cation for another.
• When a plant root absorbs a cation, it often releases a hydrogen ion (H⁺) in exchange. The hydrogen ions increase the soil’s acidity around the root zone, which can release additional cations from the soil particles, making them available for the plant.

Question 17

how does the pH of the soil affect the CEC?

Answer 17

• A change in pH of the soil will affect the soil’s ability to hold these micronutrients
• happens when the soil is ACIDIFIED (too many hydrogen ions) H+ lowers pH (through acid rain or coal mining)
• H+ “kick out” micronutrients over time; the micronutrients end up leaching out of the soil to water bodies like rivers/lakes/streams. The soil is now nutrient deficient.
• the micronutrients are the positive cations that affect base saturation (NH4+, CA2+, Mg2+, K+, Na+)