Notes on Soil Composition and Properties

Source material covers Topic 4.3: Soil Composition and Properties, Essential Knowledge, and related learning objectives. Includes the relationships between soil texture, structure, horizons, water holding capacity, productivity, and methods for soil testing.
Page context:
- Daystarter exercise asks to identify soil texture from given compositions (percent clay, sand, silt).
- Emphasis on soil texture triangle, texture classes, and loam as ideal soil.
- Distinctions among soil components (weathered parent material, organic matter, gases, liquids, organisms).
- Soil horizons and their characteristics (O, A, E, B, C, R).
- Soil tests and practical decisions (irrigation, fertilization, development).

Given textures are determined by the relative proportions of clay, silt, and sand:
- 25% clay, 61% sand, 14% silt → texture: Sandy clay loam (sand-dominated with some clay)
- 12% clay, 18% sand, 70% silt → texture: Silt loam (silt-dominated with some sand and clay)
- 33% clay, 33% sand, 34% silt → texture: Loam (balanced proportions; near the loam region on the texture triangle; could be treated as loam or a close variant such as a silty/clay loam depending on boundaries)

Soils are composed of:
- Weathered (broken-down) parent material (aka inorganic sediment)
- Organic matter
- Gases
- Liquids (mostly water)
- Organisms
Relative amounts vary greatly across environments (deserts, hardpan clays, black loams, etc.).
Key implication: texture and fertility depend on the relative amounts of these components.

Sediments are classified based on grain size; focus on:
- Sand
- Silt
- Clay
- Humus (organic matter, not sediment)
Texture is determined by the ratios of sand, silt, and clay.
Organic matter can vary widely within these textures, affecting fertility and water retention.

Sand:
- Large grain size → large pore sizes
- Drains easily; low water retention
Silt:
- Moderate grain size; foils to productive soils when in floodplains
- High water retention; medium drainage
- Overwatering can clog air pockets (anoxia, root rot) but overall good soils
Clay:
- Smallest grain size; can compact easily
- Forms hardpan clays; high water-holding capacity but poor drainage
- Very small pores; can hold water strongly (hygroscopic) and may be difficult for roots to access
Organic matter (humus) varies and contributes to fertility and color

Sand: large pores → excellent drainage; low water retention
Silt: intermediate pores → balanced drainage and water retention
Clay: very small pores → high water retention; poor drainage; can hold water more than plants can absorb
Implication: soils with more sand drain well but hold little water; soils with more clay/silt retain water but drain slowly; loam is often the best balance
Loam definition (typical): ~
ext{Loam}
ightarrow 40\u00A0 ext{egin{small%}sandegin{small%} }, 40 u00A0 ext{egin{small%}siltegin{small%} }, 20 u00A0 ext{egin{small%}clayegin{small%} }
Practical takeaway: loam is often the preferred soil for productivity due to balanced drainage and nutrient retention.

The best soil is loam because it combines desirable features from sand, silt, and clay.
Typical loam composition: roughly
ext{Loam}
ightarrow 40 ext{% sand}, ext{ } 40 ext{% silt}, ext{ } 20 ext{% clay}
Loam is often high in organic matter (humus) and tends to be darker in color, which indicates higher carbon content and fertility.
Darker soils generally indicate better productivity due to higher organic matter and nutrient availability.

WHC is linked to soil productivity because it determines how much water the soil can store for plant use.
Productivity varies with rainfall; different regions illustrate how WHC interacts with climate:
- Sandy soils drain well and do not hold water well; in arid regions (e.g., West Texas) they may be productive only when water is available; in boggy conditions (e.g., East Texas) they can be too wet.
- Silty soils can be very productive (e.g., Iowa River Valley) but can become waterlogged if poorly drained.
General pattern:
- Sand: drain well; low WHC
- Clay/Silty: poor drainage; high WHC
- Silt is typically more productive than clay due to balance of drainage and WHC
Expressed relation (from the slide):
ext{Water holding capacity} = ext{Infiltration (drainage)}
Overall rule of thumb:
- Sand drains; low WHC
- Clay and silt hold water; high WHC but may have drainage problems
- Loam offers a favorable balance

Tool to organize and identify soil types based on percentages of clay, silt, and sand.
Reading orientation (as per slide):
- The angle of the numbers indicates the direction to read the lines.
- Clay is read horizontally (straight across).
- Silt is read down and to the left.
- Sand is read up and to the left.
Example from slide: a soil with
35 ext{% clay}, ext{ } 25 ext{% silt}, ext{ } 40 ext{% sand}
is classified as Clay loam on the texture triangle.

Learn how to read the soil texture triangle via provided video resources:
- Learn How to Read Soil Texture Triangle
- YouTube links provided for practical understanding (notes reference only)

Distinct horizons can differ in material and grain size across a profile:
- O horizon: Primarily organic material
- A horizon: Rich in organic matter, variable particle size
- E horizon: Low organic matter; more sand and silt present
- B horizon: Low organic matter; higher clay content
- C horizon: Little organic matter; partially weathered bedrock pieces
- R horizon: Solid bedrock
These horizons influence nutrient availability, water movement, and root penetration.

Tests options include:
- Soil texture / water holding capacity
- Plant available nutrients
- Macronutrients: Nitrogen (N), Phosphorus (P), Potassium (K)
- Micronutrients: Sulfur (S), Calcium (Ca), Magnesium (Mg), Manganese (Mn), Zinc (Zn), Iron (Fe), etc.
- pH
- Soil contaminants / pollutants
- Stability (resistance to erosion)
How tests inform decisions:
- Irrigation requirements
- Fertilizer rates and timing
- Construction and development planning

Enduring Understanding (ERT-4): Earth's systems interact to reach a balance over time.
Learning Objective (ERT-4.C): Describe similarities and differences between properties of different soil types.
Essential Knowledge:
- ERT-4.C.1: Water holding capacity varies with soil type and influences land productivity and fertility.
- ERT-4.C.2: Particle size and horizon composition affect porosity, permeability, and fertility.
- ERT-4.C.3: Various methods test chemical, physical, and biological soil properties to aid irrigation and fertilizer decisions.
- ERT-4.C.4: A soil texture triangle identifies and compares soil types based on clay, silt, and sand percentages.

Texture determines porosity, permeability, drainage, and water retention.
Loam (≈40% sand, 40% silt, 20% clay) is typically the most productive texture.
Clay holds water well but can impede drainage; sand drains well but holds little water; silt offers a balance.
Soil horizons influence fertility and root growth; texture and composition vary by horizon.
A range of soil tests informs irrigation, fertilization, and development decisions.
Always relate texture to practical context (climate, rainfall, drainage) for productivity outcomes.