6.3 - Critical Zone 2

Question 1

How do soils differ on topography?

Answer 1

• Steep terrain soils are shallow and have poorly differentiates profiles
• Soils at the bottom of a slope usually have better developed horizons
• Soils at depressions, are wetter and have more soil organic matter (SOM)

Some crops grow best on hillsides

Question 2

What does the slope aspect affect?

Answer 2

The slope aspect affects temp, moisture, SOM and weathering

Question 3

How does topography affect parent material?

Answer 3

Topography can determine the distribution of residual, colluvial, and alluvial parent material

Residual = upper slopes, well drained
Colluvium = covers the lower portion of the slope
Alluvium = tends to fill in the valley bottoms eg. Floodplain

Question 4

What is soil formation a function of?

Answer 4

Climate
Organisms
Relief
Parent material
Time

Question 5

How does climate affect weathering?

Answer 5

Climate = precipitation and temperature affects the nature and intensity of weathering over large geographic areas.

Latitudinal changes in relative precipitation and temperature affect depth of weathering

Question 6

How does latitude affect weathering?

Answer 6

Latitudinal changes in relative precipitation and temperature affect depth of weathering

As well as the primary soil type which accumulate at depth

Question 7

How does precipitation affect weathering?

Answer 7

H2O is essential for all major chemical weathering reactions.

The effectiveness of precipitation impacts the depth of penetration into the CZ.

Question 8

What does water do that causes chemical weathering?

Answer 8

Water transports dissolved and suspended materials from upper to lower layers

Water carries away dissolved materials as water drains out to the groundwater or rivers.

Movement of water stimulates other weathering reactions (oxidation, cation exchange) and promotes differentiation of soil horizons

Question 9

What happens when there is water deficiency?

Answer 9

It promotes the formation of salts (eg. Evaporites)

It promotes the accumulation of carbonates (calcrete)

There is limited chemical weathering which leads to very different chemical profiles.

Question 10

What happens with H2O leaching of soils

Answer 10

To fully promote soil development, water must not only enter the profile and participate in weathering reactions, but also percolate through and translocate soluble products.

Question 11

Where will more leaching occur?

Answer 11

Given changes in evaporation rate, the same amount of rain will cause more leaching/soil profile development in a cooler climate.

Question 12

What does a high effective annual precipitation generally lead to?

Answer 12

Increasing clay and organic matter contents

Higher soil acidity (lower pH)

Lower ratio of Si/AI (an indicator of greater weathering of silicate minerals)

Question 13

What happens to soil when temperatures increase?

Answer 13

• The rates of biochemical reactions double per increase in 10 degrees Celsius
• SOM (soil organic matter) content increases via plant growth and microbial decomposition
• High temps and moisture maximises rates of weathering, leaching and plant growth

There is a vegetation selection:
In warm and humid climates there are trees
In subhumid and semiarid regions there are grasses
In arid regions there are shrubs and brush

Question 14

What climate is there greater rock weathering?

Answer 14

Under humid conditions there is greater rock weathering (deeper weathering)

Rock weathering in arid conditions is shallower

Question 15

Forests vs grasslands

Answer 15

In forests, organic matter is primarily at or above the surface, while grassland soils have more soil organic matter.

In addition, conifer forests are more likely to be acidic, and therefore leached at depth, with more developed soil horizons

Question 16

How is there nutrient cycling by trees?

Answer 16

• Conifers take up and store very small amounts of Ca, Mg and K
• Conifers retain leaves (needles) and debris is released by very acidic resinous
• Conifer debris accumulates as thick O horizon
• Conifer soils typically have a low pH and are highly leached

Question 17

What is pyrite weathering?

Answer 17

Pyrite (FeS2) is a common mineral formed in hydrothermal systems with many associated valuable metals (eg. Ag, Cu, Au, REE)

When brought to the surface (by uplift or mining), pyrite under goes oxidative weathering.

• Lots of dissolved Fe2+ turns water bright red
• Lots of dissolved H+ turns water acidic

Question 18

What can grow in acidic soils?

Answer 18

Pine trees can grow in acidic soils

• They also create acidic soils

Question 19

How does soil biota influence soil formation?

Answer 19

1. Macro and microorganisms in soils enhance:
   - Organic matter accumulation
   - Chemical weathering
   - Profile mixing
   - Nutrient cycling
   - Aggregate stability
2. Vegetation cover reduces natural soil erosion rates
3. Organic acids released fro, plants and decay of plant litter
   - bring Fe and AI into solution by complexation
   - Accelerates Fe AI transport
   - Promotes accumulation of secondary minerals in B horizon

Question 20

Time to produce materials in soils:

Answer 20

SOM accumulation in A horizon = 1-2 decades

Incipient B horizon = 4+ decades

Well-structured B horizon = 100’s years

Silicate clay minerals in B horizon = 1000’s years

Deeply weathered mature profile = 100,000’s years

Young and mature soil usually refers less to age, and more to degree of profile development

Question 21

What causes changes to soils over time?

Answer 21

1. Additions = OM- plant debris, dust, salts, pollution
2. Losses = leaching and erosion
3. Transformations = mineral weathering and OM decay
4. Translocations = movement of material between horizons

Question 22

How do chronosequences help identify time-dependencies?

Answer 22

Soil changes slowly with respect to a soil scientist

Locations with similar biota, climate, parent material and topography help isolate the effect of time since formation

Question 23

What is stage 1 of development of a soil profile?

Answer 23

• No layering
• OM just beginning to accumulate
• Early colonisation by pioneer plants and microorganisms
• Weathering is getting going
• As OM accumulates, water and nutrients accumulate, allowing more/faster plant growth -> positive feedback

Question 24

What is stage 2 of development of a soil profile?

Answer 24

• Activity of earthworms, termites, ants, microbes and plant decay -> several cm of OM-rich soil
• Soluble ions (eg. Ca2+, K+, Mg2+) move down, clays forming

Question 25

What is stage 4 of development of a soil profile?

Answer 25

• More deeper OM, distinctive A
• B horizon is deeper and differentiating into sub-horizons

Question 26

What is soil development like in a grassland?

Answer 26

The fresh loess deposit, overtime, is turned into well-developed mollisol = over 10,000 years

Grasses have deep fibrous roots. Organic matter is accumulated. The relatively us weathered loess, develops into well developed mollisol after the accumulation of CaCO3 and/or CaSO4.

Soil profile development in a grassland is influenced by a range of factors, including climate, topography, vegetation, and parent material. In general, grassland soils tend to be characterized by a deep, dark topsoil layer, known as the A horizon, which is rich in organic matter and nutrients, and a lighter-colored subsoil layer, known as the B horizon, which contains less organic matter but is often clay-rich.

Over time, the development of a soil profile in a grassland will proceed through several stages, each characterized by different soil horizons. These stages can be broadly summarized as follows:

Initial stage: In the initial stage, soil formation begins with the weathering of parent material, such as rock or glacial till, to produce a layer of loose, unconsolidated material known as regolith. The regolith may contain some organic matter from decomposing plant material, but it is generally nutrient-poor and lacking in structure.

Early stage: In the early stage, grasses begin to establish themselves on the regolith, and their roots help to break up the soil and add organic matter. As the grasses grow and die back each year, they contribute to the accumulation of a layer of organic matter on the soil surface. This layer is known as the O horizon, and it provides a source of nutrients and a favorable environment for soil microorganisms.

Middle stage: In the middle stage, the accumulation of organic matter in the O horizon leads to the development of a dark, fertile layer known as the A horizon. The A horizon is typically several inches to several feet deep, depending on the length of time that soil formation has been occurring. It is characterized by a high content of organic matter, nutrients, and soil microorganisms.

Late stage: In the late stage, the continued accumulation of clay and minerals in the subsoil layer leads to the development of a distinct, lighter-colored layer known as the B horizon. This layer is often clay-rich and contains fewer nutrients and less organic matter than the A horizon. In some cases, a C horizon may also develop below the B horizon, representing the unaltered parent material.

Overall, the development of a soil profile in a grassland is a slow and complex process that can take thousands of years. However, the presence of a deep, fertile topsoil layer is critical to the health and productivity of grassland ecosystems, as it supports the growth of a diverse array of plant and animal species.

Question 27

How is soil developed in forests?

Answer 27

Fresh in weathered rock is developed into well-developed Ultisol over 100,000’s years.

• Ultisols are acidic, clay-rich soils that form in humid or tropical climates over long periods of time. The transformation of fresh unweathered bedrock into a well-developed Ultisol involves a series of complex physical, chemical, and biological processes that occur over a very long period of time.
• The process of soil formation, known as pedogenesis, begins with the physical weathering of the bedrock. This can happen due to the impact of rain, wind, temperature changes, and other factors that cause the rock to break down into smaller particles. The chemical weathering of the bedrock also occurs due to the presence of water and other chemical agents that react with the minerals in the rock and cause them to break down further.
• As the bedrock breaks down, it forms a layer of unconsolidated material known as regolith. Over time, this regolith is subject to further physical and chemical weathering, as well as biological activity from plants, animals, and microorganisms. The regolith also accumulates organic matter from plant debris and other sources.
• As these processes continue, the regolith becomes more and more transformed into soil. The mineral particles become smaller and more rounded, and the organic matter content increases. The soil becomes more acidic as organic acids are produced by microbial activity and plant roots. Clay minerals also accumulate in the soil, giving it a characteristic reddish or yellowish color.

Ultimately, a well-developed Ultisol will have distinct layers, or horizons, each with its own set of characteristics that reflect the specific processes that have occurred over time. The top layer will be rich in organic matter and support plant growth, while the lower layers will be more clay-rich and have fewer nutrients.

Overall, the transformation of fresh unweathered bedrock into a well-developed Ultisol is a slow and complex process that can take thousands or even millions of years, depending on the local climate, geology, and other factors.

Question 28

Young grassland vs old forest

Answer 28

Young grasslands and old forests are two very different ecosystems that differ in many ways, including their soil profiles.

In a young grassland, the soil profile is likely to be relatively shallow and less developed than that of an old forest. This is because grassland ecosystems typically experience more frequent disturbances, such as fires or grazing by large herbivores, which can disrupt soil development and nutrient cycling. In addition, the roots of grasses tend to be shallower than those of trees, which can limit the depth of soil development.

The soil profile of a young grassland may consist of a thin layer of organic matter on top of a layer of unconsolidated soil or regolith. The organic matter is likely to be relatively low in nutrients and microbial activity, and the soil may be prone to erosion and nutrient loss.

In contrast, an old forest typically has a much deeper and more complex soil profile than a young grassland. This is because forests tend to be less disturbed than grasslands, allowing soil development to proceed over long periods of time. In addition, the roots of trees can penetrate deep into the soil, breaking up compacted layers and cycling nutrients to deeper layers.

The soil profile of an old forest may include several distinct horizons, each with its own set of characteristics. The topsoil layer, or A horizon, is likely to be deep and rich in organic matter, supporting the growth of a diverse array of soil microorganisms and plant species. Below the A horizon, the subsoil layers may become increasingly clay-rich, reflecting the accumulation of minerals over time. In some cases, a hardpan layer may develop in the subsoil, reflecting the compaction of soil particles over time.

Overall, the soil profiles of young grasslands and old forests differ in their depth, complexity, and nutrient content. These differences reflect the unique environmental conditions and ecological processes that have shaped each ecosystem over time.

Question 29

What explains most of the aspects of soil formation?

Answer 29

Variations in CLORPT explain most aspects of soil formation