Students will be able to… Describe the three basic soil particles Identify the major textural classes
Soil texture The way a soil feels A name given a textural group based on the relative proportions of each size of soil particle For example: sandy loam, silty loam, sandy clay loam Why is soil texture important?
Three Fractions of Mineral Matter Sand Silt Clay
Soil Particles Sand Largest of the soil particles Feels “gritty” Spaces between particles allow drainage and air entry Holds little water prone to drought Contains fewer nutrients than smaller particles How suitable are sandy soils for agriculture?
Soil Particles Silt Medium sized: Particle visible with microscope 0.05 mm to 0.002 mm Feel soft, silky, or “floury” Holds more nutrients than sand Retains water, slower drainage than sand Easily eroded
Soil Particles Clay Smallest particles <0.002 mm Feels “sticky” Easily formed into long ribbons High adsorption of water and chemicals Water is not readily available for plants Very small pore spaces Very slow movement of water and air
12 Soil Texture Classes Sand (S) Loamy Sand (LS) Sandy Loam (SL) Loam (L) Silt Loam (SIL) Silt (SI) Sandy Clay Loam (SCL) Silty Clay Loam (SICL) Clay Loam (CL) Sandy Clay (SC) Silty Clay (SIC) Clay (C) Compositions of each of the 12 texture classes is defined by the USDA Soil Triangle
What is loam? The Best of All Worlds A soil type that contains all three particle sizes. The most productive soil for farming crops Has good water holding capacity (from clay) Has good drainage (from sand) Can be nutritious (from silt)
Example: 60% silt 10% sand 30% clay = silty clay loam 60% silt 30% clay 10% sand What is a soil that is: 30% silt 0% sand 70% clay
Physical Properties of Soil Soil texture Soil structure Soil color Bulk density ** Remember These properties are used for classifying soil.
Definitions Aggregate Mass or cluster of soil particles such as a clod, crumb or granule Soil Structure The combination or arrangement of soil particles into aggregates
Factors that Affect Soil Structure Kind of clay Amount of organic matter Freezing and thawing Wetting and drying Action of burrowing organisms Growth of root systems of plants
Bulk Density Determined by dividing the weight of oven- dried soil in grams by its volume in cubic centimeters The variation in bulk density is due largely to the difference in total pore space Influences water movement, rooting depth of plants
Soil Color Indicator of different soil types Indicator of certain physical and chemical characteristics Due to humus content and chemical nature of the iron compounds present in the soil
Major Forms of Iron and Effect on Soil Color FormChemical FormulaColor Ferrous oxideFeOGray Ferric oxide (Hematite)Fe 2 O 3 Red Hydrated ferric oxide (Limonite)2Fe 2 O 3 3H 2 OYellow
Can you… Describe the three basic soil particles Identify the major textural classes