Soil components, Lecture 28 Flashcards
Soil is a three phase system, made up of
solid, liquid and gas
Pore space if made up of
and what does this depend on?
made up of water or air
and this depends on climate
What is soil made up of which isn’t pore space?
2
minerals and organic
Ideal percentage of organic matter in soil for crops?
5%
Which size pores are better for water?
smaller
Soil physical properties
7
- soil colour
- soil texture
- structure of mineral soils
- soil density
- pore space of mineral soils
- soil aggregates
- engineering properties
Three main factors determing soil colour
- carbon content, levels of organic matter
- water content, levels of moisture
- chemical makeup, oxygen levels
Inorganic components of soil
Sand/silt/clay,
minerals particles
primary minerals
secondary minerals
Inorganic components of soil
mineral particles
wide size and composition range
Inorganic components of soil
primary minerals
little changed from orginal rock
Inorganic components of soil
secondary minerals
altered by interacttion with air and water
Organic components of soil
3
living organisms
dead and decaying plant and animal remains
humus - humic and non-humic substances (OM)
Functions of organic matter in soil
5
- Supply of nutrients (release Ca, K, S, N, P)
- Maintaining structure (soil aggregation)
- Water storage (absorb water)
- Heat absorption (dark colour)
- Deactivation of chemicals (bind herbicides)
Decomposition in soil is a …
continuous process
Soil respiration is by
microbes
Importance of soil organic matter
2
- main source of energy for soil organisms, without this there would be no biochemical activity
- useful source of plant nutrients, helps to maintain soil fertility
Soil organic matter is comprised of
Partially decomposed plant and animal residues and organic compounds synthesised by soil microbes as the decomposition occurs.
Organic matter content of mineral soils percentage
0-10%
What does organic matter content of mineral soils depend on?
2
land use and climate
Organic matter content of organic soils percentage?
50% (on a volume basis)
Organic matter has a great influence on soil properties and therefore influences…
plant growth
Ways organic matter influences soil properties
4
- binds mineral particles into granules
- improves soil aeration
- increases soil capacity to hold water
- increases nutroent supplying capacity
Where is water found in soil?
pores
Soil solution
made up of
Water and soluble constituents, inc. nutrients
Soil solution
role
A critical medium for supplying nutrients and water to growing plants
How water transported through the soil?
2
(in lateral response to plants)
- upwards - evaporation
2. downwards - gravity
What is air in a given soil regulated by?
moisture content
Oxygen in soil needed for?
2
- respiration by plant roots
2. decomposition of organic matter by micro-organisms
Lack of oxygen in soils…
- will affect a number of biochemical reactionsm resulting in:
1. loss of certain nutrients, such as N and S
2. possible reduction in nutrient solubility
What characterises a well-aerated soil?
Rapid exchange between soil air and atmospheric air
A well-aerated soil is ideal for what?
plant growth
Soil horizons rich in organic matter colour/shade
darker
Water content affect on soil colour
moisture darkens colours
Colour of well-drained soil where oxygen levels are high?
why?
red,
presence of highly oxidised Fe - rust
Poorly drained soil colour with low oxygen levels
grey/blue
Soil texture
Combination of inorganic material in the soil.
Knowledge of the proportions of different-sized particles in a soil
What indicates soil texture?
sand, silt, clay
Which soil properties depend on soil clay content?
5
- shrink - swell capability
- plasticity
- water - holding capacity
- strength
- chemical adsorption
Types of sand
soil texture
fine and coarse
Clay in soil
Very high capacity to hold water and other substances and large specific surface area
Shape/form of clay particles
flakes or plates
AIr and water movement in clay
very slow due to tiny pores
Specific surface area
Total surface area of a material per unit mass (solids)
googled
Influence of soil tecture on soil properties (related to specific surface area (SSA))
5
- greater SSA means greater ability to retain water on the surface of soil particles
- greater SSA, greater capacity to retain nutrients and pollutants
- greater SSA, fast rate of weathering
- greater SSA, greater likelihood of forming a cohesive soil
- larger SSA, more microbial activity on particle surfaces
As particle size decreases, what happens to SSA?
it increases
Major soil textural classes are defined by
percentage sand, silt and clay
Determing the texture of a soil test
(like on 166 module)
form a ball, sausage, horseshoe
the more tests completed, the higher the clay content of the soil, therefore the smaller soil particles.
Granular soil (type of soil structure)
Looks like crumbs.
Soil found where roots have permeated the soil.
Crumb soil (type of soil structure)
very porous, the individual particles don’t fit together
What determines soil structure?
How individual soil granules clump or bind together and aggregate, and therefore, the arrangement of soil pores between them.
What does soil structure have a major influence on?
4
- water and air movement
- biological activity
- root growth
- seedling emergence
Soil struture (definition)
The arrangement of the solid parts of the soil and of the pores space located between them
What is soil structure dependent on?
5
- parent material
- environmental conditions during soil formation
- clay present
- organic materials present
- recent history of soil management
Most forms of cultivation will cause a decline in…
soil structure
What helps retain soil structure?
returns of organic matter
Platy soil (soil structure)
Form thin layers or horziontal planes
Blocky soil (soil structure)
Soil particles that cling together in nearly square or angular blocks having more or less sharp edges.
Soil particle density (Dp)
Mass per unit volume of soil solids
What determine the particle density of a mineral?
chemical composition and crystal structure
Does pore space affect particle density?
no,
Bulk density of soil (Db)
Weight of soil in a given volume
What affects bulk density of soil?
the packing arrangement of sand grains
Bulk density is generally lower if
the grains are mostly of one class size (well-sorted sand)
Characteristics of the highest density soils?
3
poor organic content, wide size range, tight packing of grains
Factors affecting bulk density
1 with 4 parts
Human activity
- agriculture
- forestry
- construction
- leisure
Where does the movement of fluids occur in soils?
through the pore spaces, in between the solid soil grains
Which two measurement help find total porosity of soil?
bulk denisty and particle densities
Are soil pores interconnected?
why?
They can be, it depends!
soil pores have a variety of shapes and sizes
What plays a large role in pore formation?
biota
Macropores of soil
large soil pores, greater than 0.08 mm in diameter
Biopores
Macropores which are created by earthworms, roots, moles, badgers, etc.
Which soil texture type is dominated by macropores?
sandy soils
Macropores, fluid movement
allows ready movement of fluids = air and water with associated dissolved substances
Micropores
Small soil pores,
Less than 0.08mm in diameter
Are macropores or micropores usually air filled?
macropores are usually air filled
Are macropores or micropores usually water filled?
micropores are usually water filled
Gas movement within micropores?
too small to allow significant gas movement
Water movement within micropores?
slow, not available to plants
Which soil texture type is mainly made up of micropores?
fine-textured soils, low organic content
Root development and microbial activity in soils which dominated by micropores?
why?
low levels,
due to poor aeration
What are vital for soil biota?
macropores
