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Chapter 4

Chapter 4: Soils and Agriculture

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4.1 Soil as a system
• soil: complex plant-supporting system consisted of disintegrated rock, organic matter, wastes,
gases, nutrients, and microorganisms
• is fundamental to life and provision of food for growing human population
• renewable if managed carefully - in risk in many locations in the world
• consists of half mineral matter with the rest of the space taken up by air, water, soil gases
• organic matter in soil includes living and dead microorganisms as well as decaying
material derived from plants and animals
• single teaspoon of soil can contain 100 million bacteria, 500,000 fungi, 100,000 algae,
50,000 protists
• soil itself meets definition of ecosystem because is composed of living and nonliving
components that interact in complex ways
4.1.1 Soil formation is slow and complex
• formation of soil plays key role in terrestrial primary succession which begins when
lithosphere’s parent material is exposed to effects of atmosphere, hydrosphere, and biosphere
• parent material: base geological material in particular location, can include lava or volcanic
ask; rock or sediment deposited by glaciers or rivers/lakes/oceans, wind-blown dunes
• processes most responsible for soil formation: weathering, erosion, deposition, decomposition
of organic matter
• weathering: describes the physical, chemical, biological processes that break down rocks
and minerals, turning large particles into small particles called regolith which are the
precursors of soils
• physical weathering (or mechanical weathering): breaks down rock without triggering
chemical change in patent material
• i.e. wind and rain
• chemical weathering: when water substances chemically interact with parent material
• wet, warm conditions accelerate chemical weathering
• icidic groundwater, such as bogs
• biological weathering: occurs when living things break down parent material by
physical or chemical means
• i.e. a tree accelerating weathering through physical action of its roots as they grow
and rub against rock
• weathering is first step in soil formation
• another process often involved: erosion - movement of soil from one area to another
• may sometimes help form soil in one locality by depositing material it has depleted from
• transport process itself can promote physical weathering as transported particles collide
and scrape against one another
• erosion particularly prevalent when soil denuded of vegetation leaving surface exposed to
water and wind that may wash/blow it away

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• biological activity contributes to soil formation through the deposition, decomposition, and
accumulation of organic matter
• five primary factors that influence formation of soil:
• Climate: Soil forms faster in warm, wet climates. Heat speeds chemical reactions and
accelerates weathering, decomposition, and biological growth. Moisture is required for
many biological processes and can speed weathering.
• Organisms: Earthworms and other burrowing animals mix and aerate the soil, add organic
matter and facilitate microbial decomposition. Plants add organic matter and affect soil
• Topographical relief: Hills and valleys affect exposure to sun, wind, and water and they
influence how and where soil moves. Steeper slopes result in more runoff and erosion and
in less leaching, accumulation of organic matter, and differentiation of soil layers.
• Parent material: Chemical and physical attributes of the parent material influence
properties of the resulting soil
• Time: soil formation takes decades, centuries, or millennia - affected by factors above
4.1.2 A soil profile consists of layers known as horizons
• each layer of soil is called a horizon, the cross-section as a whole is a soil profile
• layers subdivided according to their characteristics and processes that take place within them
• five major horizons - O, A, B, C, R - few soil profiles contain all but all contain some
• O: organic matter
• A: topsoil - most nutritive for plants and therefore most vital to ecosystems and agriculture
• B: subsoil
• C: less weathered rock particles
• R: rock (parent material)
• some have W layer: permafrost (arctic)
4.1.3 Soil can be characterized by colour, texture, structure, an pH
• soil colour: can indicate composition and fertility
• red = high iron content
• brown, dark brown = rich in organic matter
• white, pale grey = low organic content
• soil texture: determined by size of particles and is basis on which soils are assigned to one of
three general categories: clay, silt, sand
• a mixture of the three is loam
• soil structure: measure of the organization or “clumpiness” of soil
• soil pH: degree of acidity or alkalinity influences a soil’s ability to support plant growth
4.1.4 Cation exchange is vital for plant growth
• characteristics of soil affect it’s ability to provide plants with nutrients - plants gain many
nutrients through process called cation exchange
• cation exchange: soil particle surfaces that are negatively charged hold cations or positively
charged ions such as calcium, magnesium, potassium
• in cation exchange, plant roots donate hydrogen ions to soil in exchange for these nutrient ions
which the soil particles then replenish by exchange with soil water
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