Exam Flashcards
Describe features of good soil structure
- Stable (resistant to compression maintaining below attributes)
- Many small aggregates (good aeration, drainage, large SA)
What makes a clay soil “reactive”
Clay has an electrical charge, mostly negative, which means that it can attract cations such as K+ and NH4+.
It also has a very small size so a huge surface area to volume ratio
Why do coarse textured soils have good drainage but smaller total pore volumes
Coarse-textured soils have smaller total pore volume as coarse-textured soils cannot pack as close together so form more macropores. Macropres are important for drainage as the capillary action is too weak to hold water within them.
What is permanent wilting point
The point where the soil water content decreases so low that the plant dies.
What is plastic limit
Plastic limit is the moisture content at which soil begins to behave as a plastic material.
What is mottling
Mottling is when iron and aluminium oxides become insoluble again, precipitating in the soil profile.
Describe a method for determining soil structure within the field
You need to see the number and stability of peds. Working aggregates within fingers to determine the resistance to compression. The distinctiveness of aggregates and their stability under stress indicates the degree of development.
What is wilting point
Wilting point is the water content in the soil when the plants have extracted all the water that they are capable of taking from the soil.
What is soil saturation
A soil is said to be saturated when all the pores are full of water (i.e all the air has been displaced).
What is field capacity
Field capacity is the water content of the soil after all the excess water has drained away
What is stress point
The drier a soil is relative to field capacity, the greater the force that plants must exert to extract water. If a soil dries to a point where the roots cannot extract water, at the rate demanded by the leaves then the stomata will close and the plant will be “under stress”.
What is infiltration rate
The speed that water enters the profile
What is hydraulic conductivity
The speed that water moves through the soil
What is drainage
The speed that water exits the soil
What are some factors that affect available water storage?
- Pore size
- Texture
- Structure
- Organic matter content
- Depth to which plant roots can penetrate
What is CEC?
CEC is the amount of negative charge which is accessible for the retention of positively charged cations.
What is PZNZ?
The pH are at which the amounts of negative and positive charges on the soil particle surface are the same resulting in zero net charge.
What happens to the soil when the pH is above PZNC?
When the pH is above PZNC, the soil is net negatively charged, resulting in more cation retention.
What happens to the soil when the pH is below PZNC?
When the pH is below PZNC, the soil is postively charges, resulting in anion retention. This is impractical in most nz soils
What does isomorphous substitution create?
It creates a permanent charge that is not affected by the pH of the soil.
What does dissociation of H+/OH- create?
It creates a variable charge that is pH dependent.
How does isomorphous substitution create negative charge?
Permanent negative charges are created by substitution of higher valency cations with lower valency cations.
What are some examples of normal siliceous rocks in NZ?
- Greywacke
- Argillite
- Schist
- Granite
What soils are classified as azonal soils?
Recent soils from alluvium, recent soils from volcanic ash and skeletal soils in steep land.