Topic 1: Part 3 Flashcards
(22 cards)
What are soils?
Mixtures of organic matter, minerals, gases, and liquids that support life at the surface of the Earth.
What types of soils are there?
Organic soils: >20% OM
Mineral soils: <20% OM
How do soils form?
From the weathering of rocks/regolith and the accumulation of organic matter. Soil is just a regolith that supports plant growth. This process is called pedogenesis.
What does pedogenesis do?
It gradually increases the thickness of the soil and destroys original structures. It also produces soil layers called horizons.
What is a soil profile?
A soil profile comprises all of the horizons of a given soil.
What are some soil horizons in order from the top layer to the bottom layer, and what are their characteristics?
O horizon; humus. Dominated by OM (humus) of variable thickness, only present in organic soils, provides CO2 and organic compounds that make percolating water slightly acidic, forms coal layers in the rock record.
A horizon; topsoil. Consists of minerals and OM. Minerals dissolved and removed by percolating, slightly acidic water.
E horizon; bleached horizon. May or may not be present below A. Zone of “eluviation” or maximum leaching of clay/Fe/Al
B horizon; zone of illuviation. Illuviation is the addition of minerals from downward percolating water. If the soil is saturated, ions will generally stay in solution.
NOTE: Multiple B horizons may accumulate and can be numbered or given a suffix to describe their composition/character.
C horizon; bedrock. Weathered parent material, pedogenically unaltered. May transition into R horizon.
R horizon; unweathered parent rock
How do horizons form?
Over time, weathering can make surface horizons thicker and cause the C horizon to move downward.
What controls rates of chemical weathering?
- Climate (water availability and temperature)
- Relief of the land (where water infiltrates)
- Surface area
- Composition of parent rock
- Time
What minerals are most susceptible to weathering?
Those with higher crystallization temperatures, such as olivine and Ca rich plagioclase.
How do we determine the age of a soil?
It’s difficult. Soils are younger than the sediment they are built on, but its difficult to determine the age of the sediment. Carbon dating is often unreliable because OM is constantly added. An old soil could be poorly developed if rates of weathering slow and vice versa, so instead we describe soil maturity.
What is soil maturity defined by and what does a mature soil consist of?
Soil maturity is based on;
The number of horizons present
The thickness of horizons
How distinct horizons are from each other.
A mature soil has many thick and distinct horizons.
What types of soil exist?
Soils can be differentiated based on the type of parent material they develop in.
- Residual soil; weathering in situ: develop in place on underlying bedrock.
- Transported soil: Form on eroded materials, no R horizon, named for the erosional process that transported them (alluvial, eolian, glacial, volcanic)
What determines fertility of a soil?
The nutrient content. A fertile soil has greater nutrient content.
What is loess?
Windblown silt from glacial deposits and deserts, very fertile, can be excavated for housing.
What else can be used to differentiate soils?
Soil texture: Distribution of grain size (relative abundance sand, silt, clay)
Soil colour: Indicates humus and iron content
What soil colours are there and what do they mean?
Dark brown/black: High humus
Yellow: Iron compounds are hydrated (soil is generally poorly drained)
Gray-green: Iron compounds are reduced (soil is generally waterlogged and poorly drained, oxygen is absent)
Red: Iron is oxidized (soil is generally well-drained, pore spaces are filled with oxygen)
What is the basis of soil classification?
Environmental conditions cause certain set of soil processes which produce distinct horizons, which are the basis of soil classification.
What soils are classified in Canada?
Canadian Soil Classification
Chernozemic A grassland soil whose diagnostic horizon is formed by high levels of organic matter additions from the roots of grasses.
Solonetzic A grassland soil with high sodium levels in the B horizon; usually associated with a clay-rich B horizon and often with saline C horizon material.
Podzolic A forest soil normally associated with coniferous vegetation on igneous-rock derived parent materials. High acidity in the A horizon results in formation of a bleached Ae horizon and deposition of iron and aluminum in the B horizon.
Luvisolic A forest soil found in areas with parent materials derived from sedimentary rocks. Dominant process is eluviation of clay from the Ae horizon and its deposition in the Bt horizon.
Brunisolic A forest soil whose properties are not strongly enough developed to meet the criteria for the Luvisolic or Podzolic Orders.
Gleysolic Found throughout Canada wherever temporary or permanent water saturation cause formation of gleyed features in the profile.
Regosolic Found throughout Canada wherever pedogenic conditions prevent the formation of B horizons (unstable slopes, sand dunes, floodplains etc.).
Vertisolic Associated with high clay glacio-lacustrine landscapes; characterized by shrinking and swelling of clays.
Cryosolic A soil of arctic and tundra regions; characterized by presence of permafrost.
Organic Organic soils are associated with the accumulation of organic materials (peat) in water-saturated conditions. They are most commonly associated with Boreal Forest soils.
Which types of soil are “problematic”? Why?
Vertisols contain swelling clays, their expanding and shrinking can cause structural damage.
Oxisols are heavily leached and weathered, few nutrients can be derived. New nutrients from decay quickly consumed. Deforestation removes sources of nutrients, making the soil infertile. If you add fertilizer, exposure to sun dries it out and turns it into ironstone. Creates swamps because of impermeability.
Permafrost are unstable in summer, freezing and thawing can damage infrastructure, human activities can cause thawing.
Compacted soils can’t hold much water because pore spaces collapsed.
Desert soils are fragile.
What is soil erosion?
The removal of soil (a non-renewable resource) by wind, water, and human activities.
What are some soil erosion processes?
- Erosion caused by water
Sheet erosion: water removes thin layers, hard to detect, combat with cover crops.
Rill erosion (a): causes streamlets in soil
Gullying (b): rills become gullies - Erosion caused by wind
Wind removes soil from bare lands. Rates lower than water erosion except during drought. - Erosion caused by human activities
Any activities that remove vegetation increase rates of erosion.
Humans overwhelmingly responsible for soil loss.
What are some processes that can mitigate soil erosion?
- Terracing: Turning a slope into steps to limit erosion and runoff.
- Strip-cropping: groundcover and widely-spaced crops are sown in alternate strips.
- Crop rotation: Yearly alternation of crops so the soil does not become depleted in one set of nutrients, the ground is never left bare.
- Conservation tillage: Minimized plowing and retaining crop residues (stubble) limits erosion and increases water retention
- No till/minimum till: Seeds planted through crop residues and weeds controlled by chemicals. Requires special equipment and chemicals.
These practices are only minimally used in developing countries.
