Lab 5 Flashcards
Soil texture refers to
size of inorganic soil primary particles
what are many chemical reactions influenced by?
particle size range of the soil
What gives rise to soil structures?
formation of aggregates
What are the 2 main textural classes (based on particle size)
- Coarse particles or non-earth fraction (stone and gravel) - diameter > 2mm
- Fine earth (sand, silt, clay) – diameters < 2mm
What is changed about the nomenclature if particles > 2mm are present in significant amounts?
gravelly / stony
sand diameter
2.00 - 0.05
silt diameter
0.05 - 0.002
clay diameter
< 0.002
sand characteristics
Gritty feeling. Not plastic or sticky when moist. Promotes aeration in soils
silt characteristics
Smooth, powdery when dry. Not sticky when moist. Provides poor physical structure, rapid capillary rise of water.
clay characteristics
Smooth, sticky, and plastic when moist. Very hard clods when dry. Absorbs water and nutrients for biological organisms in the soil
How can the sand, silt and clay % composition of each separate be determined without physically separating each one out?
via Particle-Size Analysis by the Hydrometer Method
What is Particle-Size Analysis by the Hydrometer Method?
when mineral particles of a soil are suspended in water, they settle at different rates depending on their surface area per unit volume & their density. Clay = high surface area so settle slowly. Sand = small surface area so settle faster.
How can the rate at which particles settle in water be calculated?
stoke’s law
what is stoke’s law?
a law which governs the rate of settling of spherical particles in a viscous medium
what are the 2 steps in Particle-Size Analysis by the Hydrometer Method
- separation of all particles from each other
2. determination of the amounts of sand/silt/clay present in the sample
What are the 2 types of dispersion methods used in Particle-Size Analysis by the Hydrometer Method?
- Chemical: add sodium hexametaphosphate. This overcomes the flocculating cations (Ca, Mg) in soil
- Physical: rapid stirring in a milkshake machine cup
After placing soil + water in milkshake cup, what method did we do?
hydrometer method
Where is the hydrometer placed and what does it read?
placed in soil suspension and reads the average density of the soil + particles.
What was the hydrometer calibrated to read?
grams of suspended solid material per litre of solution
When should the hydrometer be used?
when sand particles (2-0.05 mm) fall below it in 40 seconds and silt particles within 8 hours
What happens from the moment the milkshake machine is done stirring to the moment you use the hydrometer at 40 seconds?
Pour soil solution into 1000 mL cylindrical tube. Add distilled water until solution reaches 1000 mL mark. A stirring plunger is pushed up and down 20 times to get all soil particles in suspension. Place hydrometer in suspensions 20 seconds before reading needs to take place
What should you add to the soil solution if there is foam at the top after using the plunger?
iso-amyl alcohol
Why is it important to record the temperature of the soil mixture as you read the hydrometer? What must you do if the temperature changed?
viscosity and density of water changes with temperature
for every degree above 20C add 0.36 g/L
for every degree below 20C subtract 0.36 g/L
