Lecture 5 - Soil science

Color depends on the amount of organic matter and pigments in minerals.
Soils rich in organic matter appear black or dark brown; more organic matter leads to darker color.
Silicate minerals are gray or colorless; iron/aluminum oxides can color soils red, brown, or yellow.
How to determine color:
- Collect a small soil sample from the soil or horizon; compare to a Munsell chart.
- The Munsell chart is a standardized color reference for soils and paints.
Munsell color determination (three steps):
- Hue: dominant color of the rainbow the soil resembles.
- Value: lightness or darkness of the color.
- Chroma: purity or intensity of the color (e.g., bright red vs dull red).

Soils contain inorganic substances including solid minerals, air, and water; solid minerals are sand, silt, and clay.
Particle size ranges:
- Sand: 0.05 \text{ to } 2 \text{ mm}
- Silt: 0.002 \text{ to } 0.5 \text{ mm}
- Clay: < 0.002 \text{ mm}
Texture depends on the proportions of sand, silt, and clay; described by the Soil Texture Triangle.
Textural effects on plants: nutrient retention, water holding, water movement, ease of tillage.
Field vs lab texture:
- Lab: determine proportions; Field: hand texturing by feel.
- Feel cues: Sand gritty; Silt floury/smooth; Clay sticky.
Example: lab classifies soil as silty loam; field recognition matches by feel.

Aggregates range in size from a few millimeters to over 10 cm.
Structure is the arrangement of aggregates and controls water/air flow in pores.
Texture relates to the size/shape of aggregates.
Horizon-specific structures: A horizon is granular; E horizon is platy or blocky; B horizon is prismatic, angular, or columnar.

Consistency = strength or firmness and resistance to deformation (e.g., by machinery).
Varies with texture and moisture content.
Sandy soils tend to be loose; cemented soils have high consistency.
Field evaluation across moisture levels: dry, moist, and wet.
- Dry: loose to very hard.
- Moist: loose, friable, firm, or very firm.
- Wet: sticky vs non-sticky; plastic vs non-plastic.
Laboratory determination: Atterberg Limits (moisture content for solid → plastic and plastic → liquid).