Soil science exam # 2 updated

Hydration of cations

Refers to how cations interact with water, influencing their effective radii and susceptibility to exchange in soil.

Effective radius

The size of a hydrated ion, which can affect its reactivity and exchangeability in soil.

Acid saturation

A measure of the amount of acidic cations in the soil, affecting its overall pH.

Justus von Liebig's Law of the Minimum

States that plant growth is limited by the most deficient nutrient, even if all other nutrients are adequate.

pH of soil

A measure of the acidity or alkalinity of soil, influencing nutrient availability and plant growth.

Aluminum toxicity

Occurs when Al3+ ions become available in acidic soils, competing with essential nutrients and potentially harming plant growth.

Liming

The practice of adding lime to soil to neutralize acidity and increase pH.

Sodic soil

Soil that contains high levels of sodium, leading to poor soil structure and reduced permeability.

Total Dissolved Solids (TDS)

A bulk measurement of all dissolved salts in water, often used to assess salinity.

Electrical conductivity (EC)

A measure of the soil's ability to conduct electricity, which increases with the concentration of dissolved salts.

Soil structure

The arrangement of soil particles and the pore spaces between them, influencing water movement and root growth.

Alkaline soil

Soil with a pH greater than 7, often found in arid regions with limited biological activity.

Salinity

The concentration of soluble salts in the soil, which can negatively impact plant growth.

Aluminum (Al3+)

A cation that can become toxic to plants at low pH levels, competing with vital nutrients.

Exchangeable Sodium Percentage (ESP)

A measure of the proportion of sodium in relation to other cations that affect soil sodicity.

Cation Exchange Capacity (CEC)

The ability of soil to hold and exchange positively charged ions, crucial for nutrient availability.

At soil pH levels below 6, crops will experience growth-limiting levels of potassium availability.  True or false

False

The best descriptor of aluminum toxicity is:

an increasing solubility of aluminum in low pH soils interferes with nutrient cation retention in CEC

Alkaline soils:

could be calcareous soils

Sodic soils (choose the best answer):

can be identified from their ESP or SAR

Salts entering the root zone in saline soils occurs through

additions of slightly saline irrigation water, or upward movement of saline groundwater

Sodium interferes with soil structure because

it creates a thick ionic swarm between soil colloids, interfering with their ability to attach to each other

Reclamation of sodic soils focuses on:

replacing sodium with calcium in exchange matrix, while creating a soluble salt that can be leached away

Alkaline soils are more common in humid regions because:

n/a, alkaline soils are more common in arid regions

Water balance equation (SS)

SS = P - ET - D, where P is precipitation, ET is evapotranspiration, and D is drainage.

Evapotranspiration

The sum of evaporation and transpiration from the land to the atmosphere.

Transpiration

Loss of water from plants, particularly trees and grasses.

Evaporation

Loss of water from water bodies and land surface into the atmosphere.

Potential Evapotranspiration (PET)

The amount of water that would be lost through evapotranspiration if a well-watered, densely vegetated system is present.

Actual Evapotranspiration (AET)

The actual amount of water lost due to evapotranspiration under real conditions.

Soil Aeration

The process by which air in the soil is replaced with air from the atmosphere.

Water-filled pore space

The percentage of soil pores filled with water; soil is saturated when this equals 100%.

Microbial activity

The process by which microorganisms in the soil consume oxygen and produce carbon dioxide.

Aerobic soil

Soil that contains oxygen.

Anaerobic soil

Soil that lacks oxygen.

Wetland soils

Soils that are water-saturated for prolonged periods, supporting growth of plants and microbes under anaerobic conditions.

Hydric soils

Wetland soils that exhibit specific characteristics due to water saturation.

Aerenchyma

Large intracellular air spaces in plants that allow for gas transport and storage to submerged roots.

Soil Colloids

Organic and inorganic matter with very small particle size and large surface area per unit of mass.

Cation Exchange Capacity (CEC)

The ability of soil colloids to hold and exchange cations.

Isomorphic substitution

The process of replacing one structural cation for another of similar size, leading to net charge differences in soil.

pH dependent charge

The charge on soil colloids that varies with the soil pH.

Buffering capacity

The ability of soil to resist changes in pH when acids or bases are added.

Acidic organic material

Organic matter that contributes to soil acidity, released during decomposition processes.

Active acidity

The acidity present in soil solution at any given time.

Exchangeable acidity

The acidity held near colloid surfaces that can be exchanged.

Residual acidity

Acidity that is tightly bound to colloid surfaces.

Albedo

The proportion of radiation that is reflected by a surface; high albedo means more reflection.

Specific heat capacity

The amount of energy required to raise the temperature of a unit of a substance by 1 degree Celsius.

Nutrient availability and pH

The relationship between soil pH and nutrient uptake by plants, with variations impacting crop success.

Aluminum toxicity

At low pH, aluminum can compete with essential nutrients and become toxic to plants.

Primary processes that control soil aeration are best described as

soil water content, rate of O₂ consumption, and soil structure (macroporosity)

As a means of characterizing soil water content, percent water-filled pore space reflects

the proportional volume of pore space occupied by water, which cannot exceed 100%

True or false Soil temperatures are insensitive to soil color, since soil temperatures are so strongly influenced by water content. 

false, soil color affects heat absorption

Let's imagine that the average snow depth in Brownie's park for February 2025 is 18 inches. If the average snow depth for the same period in 2030 is zero, but air temperatures are similar, what can we expect with respect to soil temperature?

Soil temperature in 2030 will be cooler than in 2025

Your friend is studying soil science by trying to draw some of the figures from the lecture slides from memory, and they produce the image below. Help them to identify ways in which their diagram would not likely reflect true soil temperature dynamics in winter and summer in the same, temperate location. Select all reasons that apply (January temperatures are shown with the dashed line; July temperatures with the solid line; warmer temperatures to the right side.)

1	In July, surface soils should generally be warmer than deeper soil.
2	Temperatures throughout the soil profile should generally be cooler in January than in July.

Wetland plants overcome O₂ limitations by

developing aerenchyma, which are specialized tissues that allow for gas exchange between water and plant

T or F Due to uptake of CO₂ by plant roots, concentration of CO₂ in soil pore spaces is much lower than that of the atmosphere. 

false, Anerobic respiration causes CO2 to increase in pore spaces compared to atmosphere

You install some soil temperature sensors at 2 cm and 100 cm depths in your backyard in midsummer (hot days and cool nights). The sensor readings are as follows: 

midnight:

2 cm: 25 C 

100 cm: 27 C

midday:

2 cm: 25 C

100 cm: 38 C 

Are your soil temperature sensors installed properly?

No, the 100 cm fluctuation is unreasonably large compared to the 2 cm fluctuation. Sensors may have been switched.

The influence of tillage system on soil warming in the spring is important to consider because

crop seeds have a minimum temperature for germination

Evapotranspiration represents the difference between the two most important processes of water loss from land (evaporation and transpiration).

False, because evapotranspiration actually refers to the sum of evaporation and transpiration, meaning it represents the combined water loss from land through both processes, not the difference between them

The best definition of a soil colloid is:

Small soil particles with large surface area permass

What is the best description of a 2:1 silicate clay?

Layers of octahedral sheets, sandwiched by tetrahedral sheets, which can be either expanding or nonexpanding

Which of the processes below is an example of isomorphic substitution:

a structural aluminum cation in a dioctahedral sheet is replaced with a magnesium cation

True or false Higher pH conditions tend to lead to more negative charges in pH-dependent charge sites, which lead to greater cation exchange capacity. 

true

true or false Organic colloids have a high pH-dependent charge.

true

Hydration of cations influences:

their effective radii, and therefore how easily they are replaced in cation exchange, because bigger cations are not as tightly bonded

The three types of soil acidity are

active, exchangeable, and residual

True or false The influence of additions of H⁺ to soil on active acidity can be buffered by various soil processes, including protonation of soil colloids.

True

Two different soils both receive long-term fertilization, using ammonium-based fertilizers. One soil exhibits a greater reduction in pH than the other. This difference is likely due to:

all of the above, since all of the above may influence buffering capacity

The purpose of lime application to soil is to

Raise soil pH, because it neutralizes soil acidity due to the carbonate and hydroxide ions it releases The carbonate ions react with hydrogen ions in the soil solution, forming water and carbon dioxide gas. 

It's important to consider aluminum for soil pH because

Aluminum hydrolyzes in soil solution, releasing hydrogen ions and contributing to soil acidity.

What is a salt?

A compound that forms when an acid reacts with a base, typically dissolving into cations and anions in soil solution

true or false Irrigation water derived from ground water can contain dissolved salts.

True

How are cations vs. anions distinguished?

By their charge

true or false Once an ion has been absorbed to a soil surface, it can be exchanged with a different ion in soil solution. 

True

Which of the following best describes the concept of acidity?

Acidity is a measure of the concentration of H⁺ (protons) in a solution

Mass loss /decomposer activity increases with mesh size because?

The finer the mesh size, only the smallest organisms can get through the mesh size and the litter mass loss is less. As mesh size increases the things that can enter the bag are bigger and therefore the leaf loss is much quicker

Mollisols

Have mollic epipedon

mollic epipedon

High base saturation