SS105 Soil Texture

What is soil texture?

The relative abundance of mineral particles of different sizes.

What aspects of soil does texture effect?

The rate at which water and air can enter soil, the ability of soil to retain water despite gravity, the ability of soil to prevent nutrient leaching, resistance of soil to changes in pH, the ability of soil to provide good root anchorage, ease of tillage, accumulation and maintenance of organic matter.

What things are ignored when determining a soil's texture?

-Particles with diameters over 2mm (start by sieving out all "coarse fragments")

-Organic matter (burn out OM by boiling sample in 30% hydrogen peroxide)

What things are considered when determining a soil's texture?

Mineral (inorganic) particles smaller than 2 mm (sand, silt, and clay)

What are the size categories for mineral particles?

sand, silt, and clay

How big are sand silt and clay?

-Coarse fragments: >2mm

-Sand: 0.05-2 mm

-Silt: 0.002mm-0.05 mm

-Clay:

What are primary minerals?

Originally formed from cooling magma, exactly like the minerals in some kinds of igneous rock, chemically inert (does not readily react with other substances)

What are secondary minerals?

Formed slowly in the biosphere from ingredients originally present in primary minerals, chemically reacts.

Which soil texture in particular has lots of built-in negative charge?

Clays

Which particle size has the highest specific surface area?

Clay

What are soil textural classes based on?

Percentages of sand, silt, and clay

What is the ideal textural class?

Loam

What percentages of clay, silt, and sand is loam?

40% sand

40% silt

20% clay

What is the name of a soil with almost perfect texture but a bit too much silt?

silt loam

What do you need to determine a soil's textural class?

Its percentages of sand, silt, and clay by weight.

What is stoke's law?

Formula that expresses the settling velocities of soil particles in water

What is the more soil-oriented version of Stoke's law?

V=kd^2

What are the steps of determining soil texture with a sample?

1) Put through a 2mm sieve and oxidize away organic matter in 30% H2O2 (hydrogen peroxide)

2) Shake it up in a volumetric cylinder

3) Let particles settle

4) After 44s, sand will be below 10 cm

5) After 7.72 h, silt will be below 10 cm

6) At each time, take sample with hydrometer

What ribbon does a sandy loam produce?

gritty, non-cohesive, short

What ribbon does a silt loam produce?

smooth, dull, crumbly

What ribbon does a clay produce?

smooth, shiny, long, flexible