Chapter 7: Soil

Soil is a Complex Material

  • Soil consists of mineral matter, organic matter, water, air, and other gases
  • Dead and living microorganisms and decaying material
  • Bacteria, algae, earthworms, insects, mammals, amphibians, reptiles
  • Soil can support the growth of a plant

Soil Formation Starts with Bedrock

  • Parent material - base geologic material of soil
  • Bedrock: continuous mass of solid rock and is part of the Earth’s crust). This bedrock gives birth to soil by being broken up into pieces (due to volcanic erosions, tectonic collisions), leading to broken up rocks called Regolith
  • Regolith: Broken-up rock
  • Rock is not something unique to earth, but soil is
  • A term with “lith” in it refers to geology (Ex. Rego “lith”)
  • These small broken rocks (Regoliths) give rise to soil
  • Soil is unique to earth because soil has an organic component to it, meaning that there has to be something living to create soil
  • Soil has an intimate interaction with both Abiotic and biotic components of earth since it has a mineral component (rock being broken down) and organic component (depending on something living to create soil)
  • Lunar Regolith: Found on moon surface. It is not considered a soil since it has no organic component to it

How Does Rock Break Down into Soils?

  • Weathering: Processes that break down rocks from parent material (rock) into smaller pieces of rocks
  • These processes involve physical (mechanical), chemical or biological weathering
  • Physical (mechanical): No chemical changes in the parent material. It’s a mechanical process and is defined by not being other two because it involves the breaking down of the rocks through mechanical process. (wind, rain, thermal expansion and contraction, water freezing)
  • Chemical: Substances chemically interact with parent material. When the chemical interacts with the rock and breaks it down chemically and applies chemical changes to it. (water and gases)
  • Biological: Organisms break down parent material. Since we are on Earth and we have living things, they can also affect rocks by breaking them down. (Tree roots and lichens). Ex. Lichense chemically interacts with the rocks and degrade them by breaking them down. Ex. Tree roots fracturing rocks
  • Pressure Release: When there is a lot of pressure on rocks (for example: if they are stacked up on one another), it leads to pressure release where eventually they break into slabs due to crumbles and such through the pressure
  • Karts: types of rocks that result from the dissolution of layers of bedrock

Soil Formation is Very Slow

  • Factors that affect soil formation:
  • Parent material composition (mineral content)- what kind of rock is being broken (some break down easily and others take a long time to break down)
  • Climate (temperature, precipitation)
  • Topography (steepness of slope)
  • Vegetation (root activity; protective cover)- the type of vegetation there in the specific environment
  • Biological activity (soil microbes and fauna; decomposition and accumulation of organic matter)
  • This is problematic for humans, because soils form slowly; we are doing a lot to degrade them to speed up the process, which results into the damaging of the soil and making it hard to recover
  • There are many factors that contribute to how quickly soils will replenish, however, soil formation is very slow
  • Ex. Soil formation can take up to 10,000 years or more to form a well-developed soil profile

A Soil Profile Consists of Horizons

  • Soil Profile: The cross-section of soil as a whole. A layer cake forming of soil
  • Horizon: Layer of soil
  • Layers: Differ in colour, texture, nutrient content/ chemical composition. Darker part of soils consist of more organic matters
  • Leaching: Dissolved particles move down through horizons in a soil profile

Humus

  • Stable, decomposed organic matter. Stable meaning when humus is broken down fully resulting in chemically form
  • Contributes significantly to soil structure; influences water maintenance (how much water stays in the soil), nutrient content, cation exchange capacity

Soil Colour

  • Colour indicates composition
  • Black or dark brown: Organic matter
  • Pale gray or white: Leaching soils
  • Red: Iron rich soil (The red soil of Prince Edward Island)
  • Soils in rainforests are actually very poor because they leach out a lot of substances and nutrients
  • Leaching is that downward movement of nutrients

Soil Texture

  • Proportion of sand, silt and clay particles
  • Particle sizes and pore sizes: influence how air and water pass through the soil
  • Sand is the big particle, silt the medium, and clay the tiniest
  • Loam: Even mixture of sand-silt-clay; best for plant growth and productivity

Soil Structure

  • How the sand, silt, and clay particles are held together
  • Organic matter interact with mineral particles to form aggregates of various shapes and sizes
  • Affect pore size & distribution; affects the ability to drain and provide adequate exposure to air
  • Good soil structure has tilth (which has good mix of particles and structures), which allows plants to establish extensive root systems

Plough Pan

  • Repeated tilling solidify soil; resists water infiltration and root penetration
  • Plough pan results when your soil structure degrades

Soil pH

  • Soil pH (acidity) influences the soil’s ability to support plant growth
  • Affects bioavailability of nutrients because it interacts with ions such as magnesium, potassium etc which are chemical forms in nutrients
  • Affects toxicity of some compounds (e.g. Al3+)
  • Acidity is bad for plants because it changes the toxicity of aluminum, through which if aluminum is absorbed by plants, it can kill them
  • Changes in pH (acidity) will have significant influences on how easy and fast it is for the plants to pick up these nutrients

Soils Provide Plants With

  • A medium for roots
  • Water
  • Nutrients through adsorption & ion exchange
  • Nutrients through bacterial processes that exists through the organic compound of soils
  • Plants generally need soils for growth and to provide support
  • Plants need soils to absorb nutrients

Ion Exchange is Vital For Plant growth

  • Charged particles move between soil, water, and plants by ion exchange
  • Charged humus and clay particles in soil hold positively charged cations of Ca, Mg, K (adsorption)
  • We need a bit of clay in soils in order for humus and clay to form negative charges, through which they can hold and stick to positively charged cations
  • These negative charges on this clay-humus aggregate, will hold nutrients by sticking to the positively charged cations

Cation exchange capacity (CEC)

  • It’s the ability to hold cations, which protects from downward leaching
  • Greatest in fine soils
  • Increases nutrient availability to plants
  • Useful measure of soil richness (fertility)
  • Some negatively-charged plant nutrients (e.g.P) - anion exchange
  • Cation exchange capacity is mainly based on the texture of the soil, the structure of the soil (what organic material it is)
  • It can be measured on field to see the capacity of cation in soils
  • Cation: Positive charges
  • Anion: Negative charges

Nitrogen Fixation is Bacterially Mediated

  • N2 gas is inert (not in a usable form)
  • Nitrogen fixation and nitrification make N bioavailable to plants. They convert this N2 (gas form) into a form which can be available for plants
  • Denitrification returns it to the atmosphere

Soil Erosion

  • Transport and deposition (someplace else) of sediment
  • Soil erosion is the transport and deposition of soil from one place to another
  • It is a natural process which is often accelerated by human activities, particularly through agricultural activities
  • It is problematic at times because soil erosion occurs faster than new soil is formed, which takes a long time to form. Primary productivity depends on developed soil which takes a while, which is why soil erosion is problematic because it occurs fast and gets rid of the new soil formed.
  • Types of erosions: Aeolian (wind) and water erosion

Water Erosion Processes

  • Splash: Precipitation (rainfall) on bare soil causes splash erosion 
  • Sheet: When you have big flat open land with no vegetation, it can result to sheets of water over the land, and those sheets will transport sediments as they moves along the land
  • Rill: After sheets erode, they can lead to rills
  • Gully: Rills cut out and lead to gullies

Process of water erosion (Positive feedback loop): Water hitting bare surface and loosening up the sediment in which a whole lot of it can lead to a sheet, which starts to erode and lead to rills which cut out big gullies.

Accelerated soil erosion and land degradation are global problems:

  • We are losing billions of hectares and croplands worldwide, which suffer from accelerated or unsustainable erosion and soil degradation
  • Humans are the primary cause of erosion, even though it is a natural process
  • Soil degradation over the next 40 years in Africa could reduce crop yields by half
  • These are positive feedback loops, which is why they are problematic
  • The on-farm cost of agricultural land degradation in Canada is $670 million per year
  • Because we are losing a lot of soil and agricultural productivity, we are basically threatening our own food supplies, driving up food prices, through which it is leading to economical issues

Soil degradation has many causes:

  • Soil degradation results from deforestation (largely from cutting down trees, leading to the lost of roots which hold down soils in physical place), agriculture (we use agriculture on a massive scale, in which we use machines to tear and compact soil, leading to degradation), overgrazing, chemical contamination (which usually results from the chemicals we put onto the soils), etc.
  • Over the past 50 years, soil degradation has reduced global grain production by 13%
  • Grazing practices such as from cows walking on soil, can contribute to soil degradation (for example: Cows walking on soil can reduce the amount of air and water soil they consume. They compact soil) 

Effects of overgrazing can be striking:

  • After the effects of grazing, the grass land changes into a “shrub land”
  • When grass lands have been grazed, grass can no longer grow properly due to soil degradation. Through this we are losing livestock productions since cows cannot eat from the grazed lands
  • If we continue to destroy these lands, it will get harder to meet our food requirements as our population continues to grow

Irrigation (supplying dry land with water) boosts productivity, but can cause long-term problems and soil degradation:

  • Groundwater depletion (reduction) since water is being taken from another place in order to be used for something else, so in this case, for irrigation
  • Waterlogging of soils
  • Salinization: Build-up of salts in surface soil layers (when you are taking water from somewhere else in order to use onto the soils, the water is not PURE water since it contains salt, resulting in the build-up of salts in surface soil layers).

Problems with irrigation (supplying dry land with water): Positive feedback loops effects:

  • Expensive
  • Alters root morphology (surface, not deep roots)
  • Irrigation changes how plants access water
  • Increase fungal pathogens (when you have more water sitting around, It can have fungi which could damage crops etc.)
  • Can cause salinity problems
  • Subsidence (downward shift of surface)

Salinization is easier to prevent than to correct:

  • It is very costly and difficult to rehabilitate (restore) salinized soil (salted soils)

Remediating Soil Toxicity

  • Salinity (saltiness)
  • Soil pH (acidity of soil)
  • Heavy metal contamination (which correlates with acidity, because how much metals are available to plants depends on the acidity and saltiness of the soil) 

Phytoremediation

  • Refers to when you use plants, in order to fix the toxicity of soils
  • These plants can clean up the soils because they are hyperaccumulators, which can pick and absorb the toxic and metals within soils
  • Phytoextraction: Drawing chemicals up into plant biomass by action of plants that are hyperaccumulators
  • Ex. Hydrangea (which is a popular plant that people grow in their backyards) is a hyperaccumulator of aluminum (AL)
  • Phytoremediation can be used as a way of fixing toxics in soils

Desertification (The Loss of Soil Richness)

  • Loss of more than 10% productivity of arable land
  • Results from erosion, soil compaction, forest removal, overgrazing, salinization, climate change, depletion of water sources
  • Most at risk are semi-arid lands, which are lands that don’t get much precipitation (rainfall)
  • Overgrazing removes vegetation, causing erosion and loss of topsoil
  • Over-cultivation causes nutrient reduction
  • Toxicity: Improper irrigation practices result in salination

The Dust Bowl

  • Native grassland grasses originally held erosion- prone soils in place
  • 1879-1929: Widespread farming of wheat, and grazing of many thousands of cattle
  • Great Depression: brought a cycle of poverty and overly intensive agricultural practices
  • Dust storms travelled up to 2000 km which resulted in deaths
  • Lung irritation, dust pneumonia, grasshopper infestations

What was responsible for the Dust Bowl?

  • Rapid mechanization (The ability to turn up the soil using mechanical machines such as tractors)
  • Rapid conversion of grassland
  • Poor agricultural practices (deep tilling, bad harvesting techniques, bad irrigation techniques, etc.)
  • Weather conditions

The Dust Bowl was a monumental event in North American history because the dust bowl was basically when the soil had completely disappeared into massive dust storms.

Soil Conservation

Soil Conservation (protection) Council emerges from the experience of deficiency (Dust Bowl for example)

Soil Conservation Council of Canada

  • National plan
  • Communication among stakeholders
  • Develop policies and production methods

Protection Against Soil Degradation

  • Crop Rotation: Is when grown field of crops are alternated from season or year to the next. Crop rotation cover crops and protect soil
  • Contour Farming: Plowing furrows sideways across a hillside, perpendicular to its slope, to prevent rills and gullies (prevents water erosions)
  • Terracing: Level platforms are cut into steep hillsides, forming a “staircase” to contain water. Ex. Rice farming in Asia containing terracing
  • Intercropping: Planting different types of crops in alternating bands or other mixed arrangements to increase ground cover. The idea of planting different types of species amongst each other, which increase the amount of nutrients from these mix arrangements of species
  • Shelterbelts and Windbreaks: Rows of tall perennial plants are planted along the edges of fields to slow the wind, and prevent erosions. Shelterbelts work in small areas of land. Alley cropping: shelterbelts + intercropping
  • Reduced Tillage: This was a big problem in Dust Bowl. Tillage is how much you go down into the soil. Reduced tillage is when furrows are cut in the soil, a seed is dropped and the furrow is closed; shallow plough
  • Afforestation: Planting trees where they have not previously existed, in order to stabilize the soil.
  • Tree plantations are farms not forests - very artificial, and do not have the same biodiversity as actual existing forests

Plant Covers are Crucial for Reducing Erosion

  • Eroding land can be stabilized by plants that anchor soil
  • China - world’s largest tree planting program because there has been a lot of soil degradation in China
  • Agroforestry: mix of crops and trees

Conclusion

  • Soil is a complex material with both organic and inorganic components
  • Soil properties and processes are fundamentally important for plant growth and (therefore) agriculture
  • Soil loss, accelerated erosion, and land degradation are global problems but there are achievable solutions

Programs in Canada and worldwide have been successful in reducing topsoil erosion

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