Soils 2 Flashcards
(33 cards)
The components of soil texture and how to differentiate between textures in the field
(Define)
- Soil texture is a description of proportions of sand, silt & clay
The components of soil texture and how to differentiate between textures in the field
(Soil texture influences) [5]
- The supply of air
- The availability & movement of water & nutrients
- Ease of root growth
- Erosion potential
- Organic matter level
The components of soil texture and how to differentiate between textures in the field
(Clay soils) [3]
- Retain more moisture & nutrients than sands, but a greater proportion of water is not available to plants (small pore size and distribution)
- More prone to water logging in wet years
- More compaction from livestock and machinery
The components of soil texture and how to differentiate between textures in the field
(Sandy soils) [5]
- Poor ability to retain moisture & nutrients, but allow plants to extract these with little effort (more free drainage)
- Prone to compaction
- Can be very hard setting, but can still be cultivated
- Prone to nutrient leaching & retention of nutrients can be improve by building up organic matter & incorporation of clay
- Prone to the development of water repellency (addition of clay overcomes this)
A normal soil profile [4]
- Soil cannot be judged from the top cm’s
- Can describe each horizon based on colour, texture, nutrients etc. to identify limiting factors for crop growth
- Crops may not be able to move into the horizon below, thus not be able to access water and nutrients
- To obtain full story need to examine horizons
A normal soil profile
Top soil or A Horizon
- Most of the available plant nutrients & soil organisms, the upper part is often dark cos of higher organic matter content
A normal soil profile
Sub-soil or B Horizon
Contains materials leached from the A horizon, often contains much more clay from leaching. Depth & water holding capacity of B horizon greatly affects value of soil
A normal soil profile
Parent material or C Horizon
Either rock or partly decomposed sand or clay deposited thousands of yrs ago, ability of roots & water to travel through C horizon can greatly affect plant growth
Some factors that influence soil colour
[4]
- Organic matter
- Mineral matter
- Form & abundance of iron
- Moisture content
Importance of soil structure and some consequences of reduced soil structure [4]
- How soils components are arranged into aggregates
- A certain % of pores must always be air filled
= Transmission pores
= Water storage pores - As clay content increases, bond between particles increases & structure improves
- Good structure results in abundance of soil pores (movement of water, air, microbes)
Transmission pores
-Large/macro pores, allow good free drainage within the soil profile
Good porosity [3]
- Indicates a good soil structure
- Good amount of transmission pores
- Good proportion of water storage pores
Greater porosity and low density
Means there will likely be more pores for nutrients & air
The impact soil texture has on field capacity and wilting point
(Water is held in the soil in several ways) [3]
- Gravitational water (gravitational potential)
- Capillary water (matric potential)
- Chemical water (matric potential)
The impact soil texture has on field capacity and wilting point
(field capacity and wilting point) [2]
- After saturation and all gravitational water has drained out, the soil is at field capacity
- When plants have drawn all accessible water from the soil, it is then at wilting point
The impact soil texture has on field capacity and wilting point
(Soil texture and structure influence the amount of water that) [2]
- Is held in the soil
- Growing plants can extract from the soil
The impact soil texture has on field capacity and wilting point
(Soil texture) [2]
- Smaller clay particles fit together more tightly than larger sand particles, only small pores for air and water
- Smaller clay particles have proportionally a much larger surface area than larger sand particles (10,000x), the amount of water absorbed by the soil increases as the surface area increases (less available to plants)
The impact soil texture has on field capacity and wilting point
(Plant available water)
- Plant available water is the difference between field capacity and wilting point
Factors leading to and consequences of water repellent and waterlogged soils
(water repellent soils) [4]
- Some soils become water repellent due to hydrophobic material coating soil particles
- Some soils more susceptible (low clay %)
= sandy soil classes are more likely to be affected by water repellence - Amount of polar and non-polar waxes coating the sand grains can cause water repellence, high polar and low non-polar is more likely to be water repellent
- Hydrophobic organic matter is produced by the decomposition of plants,
some plants produce more dry matter than others (continuous blue lupins and permanent annual pasture)
Factors leading to and consequences of water repellent and waterlogged soils
(consequences of water repellence) [4]
- Patchy pasture germination, favours aerial seeders
- Sub-clover at a disadvantage as seed is buried and may not be wet by light rains
- End up with established and young plants in the same pasture making chemical use difficult
- Increased risk of erosion from summer rainfall events
Factors leading to and consequences of water repellent and waterlogged soils
(waterlogged soils) [4]
- Roots require oxygen for respiration
- In well drained soils, 10-60% of soil volume is air
- In saturated soils, these pores are filled with water, reduce gas diffusion 10,000 fold
- Results in anaerobic conditions in the soil
The impact soil texture has on field capacity and wilting point
- Gravitational water (gravitational potential)
- Water held in large soil pores and rapidly drains out of the soil under gravity, plants can only use it while it’s there
The impact soil texture has on field capacity and wilting point
- Capillary water (matric potential)
- Water held in pores small enough to hold water against gravity, the smaller the pore, the harder it is to remove water
The impact soil texture has on field capacity and wilting point
- Chemical water (matric potential)
- Water tightly held by electrostatic forces to clay surfaces, unavailable to plants
