FoG: T8 (PART 2) - Soil Formation

What is the key process in soil formation that is critical to existence on Earth?

Weathering is a key part of the process of soil formation.

Define 'soil' in the context of geology.

Soil is the material that includes organic matter, lies within the top few tens of centimetres of the surface, and is important in sustaining plant growth.

What are the three main components of soil and their approximate percentages?

Soil is a complex mixture of minerals (approx. 45%), organic matter (approx. 5%), and empty space (approx. 50%).

The empty space within soil is filled to varying degrees with what two substances?

The empty space is filled with air and water.

What two types of minerals typically dominate the mineral content of soils?

The mineral content of soils is typically dominated by clay minerals and quartz.

In a warm climate where chemical weathering is dominant, soils tend to be richer in what mineral type?

Soils in warm climates with dominant chemical weathering tend to be richer in clay.

Soil texture is described in terms of the relative proportions of what three components?

Soil texture is described by the relative proportions of sand, silt, and clay.

The sand and silt components of soil are primarily dominated by which mineral?

Sand and silt components are dominated by quartz.

The clay components of soil are dominated by what type of minerals?

The clay components are dominated by the clay minerals.

What are the two primary processes through which soil forms?

Soil forms through the accumulation and decay of organic matter, and through mechanical and chemical weathering processes.

What is the acronym used to remember the five key factors influencing the nature and formation rate of soil?

The acronym is CLORPT.

Who coined the acronym CLORPT in 1941 to describe the factors of soil formation?

Hans Jenny coined the acronym CLORPT.

In the CLORPT model of soil formation, what does 'CL' stand for?

'CL' stands for climate, especially average temperature and precipitation amounts.

In the CLORPT model of soil formation, what does 'O' stand for?

'O' stands for organisms, especially the types and intensity of vegetation.

In the CLORPT model of soil formation, what does 'R' stand for?

'R' stands for relief, which is the slope and aspect of the surface.

In the CLORPT model of soil formation, what does 'P' stand for?

'P' stands for the type of parent material.

In the CLORPT model of soil formation, what does 'T' stand for?

'T' stands for the amount of time available for formation.

Under what general climate conditions does soil form most readily?

Soil forms most readily under temperate to tropical conditions with moderate precipitation.

Why are chemical and biochemical weathering reactions fastest under warm conditions?

They are fastest under warm conditions because plant growth, which contributes to these reactions, is enhanced in warm climates.

What is a negative consequence of too much water in soil, such as in rainforests?

Too much water can lead to the leaching of important chemical nutrients, resulting in acidic soils.

In humid and poorly drained regions, swampy conditions may produce soil dominated by what material?

These conditions may produce soil dominated by organic matter.

What are two consequences of too little water for soil development, as seen in deserts?

Very limited downward chemical transportation and the accumulation of salts and carbonate minerals.

Why do soils in dry regions often suffer from a lack of organic material?

There is only sparse vegetation growing in the area, which limits the source of organic matter.

Soils cannot develop in areas where the rate of soil formation is less than the rate of _____.

erosion

Why do steep slopes tend to have little or no soil?

On steep slopes, the rate of erosion is often greater than the rate of soil formation, and surface materials are frequently moved away.

What are soils called if they develop on bedrock?

They are called residual soils.

What are soils called if they develop on transported material like glacial sediments?

They are called transported soils.

Parent material that is rich in quartz, such as granite or sandstone, tends to lead to the development of what type of soil?

It leads to the development of sandy soils.

Parent material that is poor in quartz, such as shale or basalt, generates soils with little sand and sometimes elevated levels of what?

It generates soils with elevated clay levels.

The mineral apatite in granitic rocks is an important source of which soil nutrient?

Apatite is a source of the important soil nutrient phosphorus.

Why does basaltic parent material tend to generate very fertile soils?

It provides phosphorus, along with significant amounts of iron, magnesium, and calcium.

Why do unconsolidated materials like river-flood deposits make for especially good soils?

They tend to be rich in clay minerals, which are good at holding nutrients.

How do clay minerals contribute to soil fertility?

They have large surface areas with negative charges that attract and hold positively charged nutrient elements like calcium, magnesium, and iron.

What is the general timeframe required for soil to develop, even under ideal conditions?

Soil takes thousands of years to develop.

In the initial stage of soil formation (Step 1), what types of organisms are first present on the rock surface?

Lichen and moss are present on the rock surface.

In the second stage of soil formation (Step 2), what soil horizon begins to develop from weathering products and organic matter?

The C horizon soil has developed.

During the third stage of soil formation, what process involving the motion of ions in water is key to the evolution of the soil profile?

Chemical changes involving both downward and upward motion of ions in water.

In the fourth stage of soil formation, significant weathering of minerals like feldspar and amphibole produces what?

It produces clay minerals within the soil.

Besides living organisms and organic matter, what element is stored in large amounts in well-developed soil?

A great deal of carbon is stored in the soil.

The downward movement of clay, water, and dissolved ions in soil formation leads to the development of chemically and texturally different layers known as _____.

soil horizons

What is the O horizon in a soil profile composed of?

The O horizon is composed of organic matter.

The A horizon of soil is a mixture of partially decayed organic matter and what other material?

The A horizon is a mixture of organic matter and mineral material.

What is the E horizon, and why does it often have a pale color?

The E horizon is the eluviated (leached) layer; it is pale because clay and iron have been removed.

The B horizon is known as the zone of _____.

accumulation

What materials accumulate in the B horizon of a soil profile?

The B horizon is where clay, iron, and other elements from the overlying soil accumulate.

What defines the C horizon in a soil profile?

The C horizon consists of incompletely weathered material that grades down into unaltered parent material.

Why do the O and A horizons of soil tend to be dark?

They are dark because of their abundant organic material.

The B horizon's color can range from yellow-brown to dark red depending on the presence of clay minerals and what specific oxides?

The color depends on the presence of iron oxides.

What does a red soil color in well-drained soils typically indicate?

A red color indicates excellent oxidizing conditions, where iron is oxidized.

What does a yellow soil color often indicate about the soil's drainage?

A yellow color indicates the soil is wet and poorly drained

The arrangement of individual soil particles into larger groupings is known as soil _____.

structure

What are the larger groupings or aggregates of soil particles called?

They are called peds or aggregates.

What is the term for the ability of a soil to resist deformation?

Soil strength is the ability of a soil to resist deformation.

What is cohesion in the context of soil strength?

Cohesion is a measure of the ability of very small silt and clay soil particles to stick together.

What property measures changes in soil strength resulting from disturbances like vibrations or excavations?

Soil sensitivity measures these changes.

Which soil types are the least sensitive to disturbances?

Sand and gravel soils with no clay are the least sensitive.

What is the term for a soil's tendency to consolidate or decrease in volume?

This property is known as soil compressibility.

Excessive settling due to soil compressibility can cause what kind of damage to structures?

It can cause cracks in foundations and walls.

Which soil materials, fine-grained/organic or coarse materials, have a higher compressibility?

Fine-grained and organic soils have a high compressibility.

What does soil 'erodibility' refer to?

Erodibility refers to the ease with which soil materials can be removed by wind or water.

Soils with more than 20% clay are described as _____ soils in the context of erodibility.

cohesive

What is 'hydraulic conductivity' a measure of?

It is a measure of the ease with which water moves through a material.

What is 'corrosion potential' in soils?

It is a slow weathering or chemical decomposition that proceeds from the surface into the ground, affecting buried objects.

What does 'ease of excavation' pertain to in soil science?

It pertains to the procedures and equipment required to remove soils during construction.

What is the term for excavation accomplished with an earth mover or backhoe without prior scraping?

This is known as common excavation.

What type of excavation is required for a tightly compacted or cemented soil?

Rippable excavation, which requires breaking up the soil first, is needed.

What property refers to the tendency of a soil to gain or lose water, causing changes in volume?

This property is the shrink-swell potential.

Soils that tend to increase or decrease in volume with changes in water content are called _____ soils.

expansive

What causes the swelling in expansive soils?

Swelling is caused by the chemical attraction of water molecules to the flat particles of certain clay minerals.

What are the three distinct parts, or phases, of soil located above the water table?

The three phases are solid material, liquid (water), and gas (air or carbon dioxide).