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
What does the hydrometer measure?
grams of solids per litre left in suspension
After the 40 seconds and all sand has settled out, what does the hydrometer reading show?
grams of silt and clay left in 1 litre of suspension
% sand =
(sample size – 40s cor. hydro.)/(sample size) x 100
% clay =
8hr cor. hydro/sample size x 100
% silt =
100 – % sand – % clay
How much must you wet your soil sample when determining its texture by hand?
consistency of putty
Determining Texture by Hand: what does each soil type form?
- sand/sandy loam
- Loams
- Silt loams
- Clay loams
- Silty clay loams
- Clays
- a cast that is easily broken, have gritty feeling
- a cast upon squeezing that can be handled freely without breaking, considerable grittiness
- a cast that can be handled freely, moist soil has a soapy feeling
- the cast is quite malleable, resistant to breaking
- as in clay loams (quite malleable, resistant to breaking), but a soapier feel
- cast is plastic, sticky, and highly malleable
How does soil organic matter affect the plasticity of soils?
reduces it
Ah horizons normally have what structure?
granular - highly porous
What structure is considered best for most plants?
granular - highly porous
How are soil aggregates formed?
when primary particles are drawn together and cemented into clumps
What is the major stabilizing structure of soil aggregates?
colloidal fraction - act as binding agents for primary particles
What are the 3 main groups of colloidal matter that act as binding agents for primary particles? (most important to least)
- colloidal organic matter (including microbial gums)
- colloidal oxides of iron and aluminum
- clay minerals
Why are medium sized, water-stable soil aggregates important in fine and coarse-textured soils? (4)
- increase porosity
- aid in soil aeration
- facilitate downward movement of water in the profile
- increase the resistance of the soil to wind and water erosion
What did Tisdall and Oades develop?
a hierarchical model of how soil aggregates are constructed and stabilized
According to Tisdall and Oades, what are larger stable aggregates made of?
smaller aggregates
What size are the smaller aggregates (micro aggregates) that make up the larger ones?
2 um - 250 um (diameter)
How are the smaller aggregates that make up the larger aggregates stabilized?
humic compounds
what are humic compounds?
persistent organic materials from microbial processed plant and microbial debris which are resistant to further degradation
What are particles < 2 um diameter held together by?
electrostatic bonding and cation bridging
what happens to micro aggregates above 250 um diameter?
they are packed into macro-aggregates by network of fungal hyphae and fine roots
What does the stability of macro-aggregates depend on?how much time can it take to break this down? build back up?
fungal mycelium and fine roots
1 growing season, but can be quickly increased by establishment of plants producing abundant fine roots and mycorrhizal mycelium
What does the stability of micro-aggregates depend on? how much time can it take to break this down?
organic compounds and electrostatic forces
several decades
What is the most abundant fungi in soils?
mycorrhizal fungi
What is the function of mycelium? What produces them?
mycorrhizal fungi produces mycelium
function: form an extensive web entangling and tying together micro-aggregates on the top 10-15 cm soil layer
What is the most well known method of encouraging soil granulation?
add OM
What do bacteria and fungi produce as they decompose OM?
polysaccharide gums
What does the decomposition of plant roots produce?
microbial gums
Why does the action of earthworms have a large effect on soil aeration and structural stability?
because they secrete mucus on the walls of their tunnels as they burrow
reread plage 46
reread page 46
