lecture 2 Flashcards
What the 3 phases we divide soils into to understand soil properties? List the soils these include.
- Solid phase: mineral and organic matter
- Liquid phase: soil water and soil solution
- Gaseous phase: soil has amount and composition
What are the three important aspects of the inorganic solid phase?
- Particle size distribution
- Soil structure
- Mineralogy
What are the 4 classes of particle size (in order of biggest to smallest)?
- Gravel
- Sand
- Silt
- Clay
How do we measure soil texture in the lab?
Using Stoke’s law
What are the 5 reasons why soil texture is important?
- Influences soil infiltration rate, and thus the generation of overland flow and soil erosion
- Influences soil permeability and therefore drainage
- Controls available water capacity of soil - ability to supply water to plants
- Influences soil structure, allowing root growth and aeration
Provides cation exchange capacity for nutrient supply to plants and buffering against acid rain
Rank the soil tecture class by surface area (smallest to largest):
- Coarse sand
- Fine sand
- Silt
- Clay
What are the characteristics of soil strucutre?
- Describes the spatial arrangements of particles to complex aggregates, pores, and channels
- Has a major influence on water and air movement as well as root growth
- Influences the movement of soil macro and meso fauna
What are the 6 major structural forms in soil?
- Prismatic
- Columnar
- Angular blocky
- Subangular blocky
- Platty
- Granular
What are the processes leading to the development of soil structure?
- Expansion/contraction upon wetting and drying
- Root penetration
- Earthworms and ingestion/excretion by other soil organisms
- Input and decomposition of soil organic matter
What are the properties maintaining soil strucuture?
- High organic matter content, especially input of fresh litter - promotes bonding between particles
- Clay minerals - large, reactive surfaces with positive and negative charges
- Presence of Fe and Al oxides and hydroxides as cementing agents
- Si coating on minerals - produces duripans in dry climates
- CaCO3 deposits - produces caliche in semi-arid soils
- Presence of flocculating cations (Ca2+ and Mg2+)
What are the processes leading to the destruction of soil structure?
- Loss of soil organic matter, especially fresh inputs
- Rain-drop impact on bare soils
- Reduction in root growth, especially in annual crops
- Trampling by stock and machinery
- Reduction in soil organisms (e.g. fewer earthworms)
- Loss of flocculating cations (Ca2+ and Mg2+) and replacement by dispersing cations (Na and K+)
What are the soil structural problems and its associated effects?
- Breakdown of weakened aggregates at soil surface
- Formation of pan at base of plough depth
- Compacted subsoil
- Erosion removes fine particles and o.m.
- Surface crusting do not infilitrate : water washes away, overflow, reduces seedling emergence
- Pan layers and compact subsoils impede root growth and water & air movement
What is the difference in soil composition between agricultural top soil by volume and organic soil by volume?
Organic soil by volume is mostly composed of water
What is the sweet spot soil texture for agricultural soil?
Balance of silt content and some clay
Describe whether sandy, loamy, and clayey soil is low, medium, or high in terms of:
1- water holding capacity
2- Aeration and drainage
3- Organic matter content
4- Decomposition rate
5- Nutrient holding capacity
6- Shrinkage
7- Compaction
1- low; medium; high
2- rapid; moderate; slow
3- loa; medium; high
4- fast; moderate; slow
5- small; moderate; high
6- none; small; high
7- resists; moderate; easily
