Soils Flashcards
Why is soil important?
- Plant growth
- Habitat for soil biota
- Nutrients for plant uptake
- Earth’s natural cycles (e.g. N)
What is soil fertility?
The ability of soil to sustain plant growth and optimise crop yield
What are the 5 components of soil?
- Mineral particles
- DOM
- Water
- Air
- Soil biota
What are the 3 types of soil minerals?
- Sand
- Silt
- Clay
Why is soil water content important for fertility?
- Essential for soil biota and plants
- Nutrients are dissolved within it for plant uptake
- Waterlogged soils are anaerobic
What nutrients do fertile soils contain?
- NPK in ionic form
- Iron and magnesium
Why is air content important for soil fertility?
- Aerobic soils increase the rate of decomposition
- Organisms and processes are mostly aerobic
Why is DOM content important for soil fertility?
- Provides nutrients through decomposition
- Increases soil water retention
- Binds the soil
Why is pH important for soil fertility?
Soils with a pH between 5.5 and 7.0 are in the range of tolerance for most plant species
What can acidic soils cause?
- Nutrient leaching
- Root hair cell damage
What can alkaline soils cause?
Phosphates become insoluble
Why are soil biota important for soil fertility?
- Detritivores break up DOM and release nutrients
- Increased drainage
- Nitrification and nitrogen fixation
What does soil texture refer to?
The proportions of sand, silt and clay
Which of the mineral particles are largest?
Sand (L)
Silt
Clay (S)
What is a loam soil?
A soil with a relatively even mix of mineral particles
What determines soil structure?
Soil particles form aggregates (peds) which are bound together
What are crumb peds?
Small and round aggregates which provide good drainage and aeration
What are platy peds?
Large and flat aggregates which provide poor drainage and aeration
Why is soil depth important for soil fertility?
- Deeper soils are less likely to become waterlogged
- Deeper soils provide anchorage for large root systems
How is drainage impacted by soil texture?
Large, spherical particles increase drainage as there are more pore spaces (sand)
How is water content impacted by soil texture?
Water stays closer to the surface in clay soils (capillary action)
How is aeration impacted by soil texture?
Large, spherical particles increase aeration are there are more pore spaces (sand)
How are nutrient levels impacted by soil texture?
Clay soils have higher nutrient levels as water remains close to the surface (ionic form)
How is root penetration impacted by soil texture?
Greater penetration in sandy soils
How is ease of cultivation impacted by soil texture?
Sandy soils are easier to turn and plough
Which type of soil is optimal for agriculture?
Loam
Which 5 human activities impact soil fertility?
- Aeration by ploughing and drainage
- Changing nutrient levels
- Irrigation
- Soil compaction
- pH control
How does aeration by ploughing and drainage impact soil fertility?
Creates aerobic conditions which increases N fixation, nitrification and decomposition
How can soil nutrient levels be increased?
- Inorganic NPK fertilisers
- Organic matter (mulch)
- Planting legumes to encourage N fixation
How can soil nutrient levels be decreased?
- Soil erosion
- Removal of biomass
- Increased nutrient leaching
- Aerobic soils encourage denitrification
How does irrigation impact soil fertility?
- Increases fertility where water is a limiting factor of growth
- Keeps stomata open for efficient gas exchange
- Water dissolves nutrients
How does compaction impact soil fertility?
- Heavy machinery and livestock farming compact the soil
- Reduced aeration creates anaerobic conditions and encourages denitrification
- Waterlogging is more likely
How can soil pH be controlled?
- Sulfur can be used to acidify soils (H2SO4)
- Agricultural limestone can be used to decrease acidity
What is wind erosion?
Wind energy lifts and moves soil particles to new locations (mostly impacts loose, sandy soils)
What are the 3 types of water erosion?
- Rainsplash
- Surface runoff
- Slumping
What is rainsplash erosion?
Soil particles are dislodged by raindrops
What is surface runoff erosion?
Soil particles are carried overland in runoff
What is slumping?
Soil becomes saturated which leads to reduced friction and mass movement such as landslides
How does vegetation reduce soil erosion?
- Roots have a binding effect
- Increased interception
- Dropped leaves provide a protective cover
- Increased OM binds the soil
- Root uptake reduces soil saturation
- Acts as a windbreak
How can agriculture accelerate soil erosion?
- Ploughing loosens soil
- Over-saturation from irrigation
- Harvesting leaves soil bare
- Overgrazing
- Compaction
- Agri-chemicals reduce biota
- Steep-slope cultivation
What are the 4 main problems caused by soil erosion?
- Reduced productivity
- Sedimentation
- Increased atmospheric particulates
- Desertification
What is accelerated soil erosion?
When the natural dynamic equilibrium between soil formation and erosion is disrupted
What are long-term crops?
Planting crops with longer growing periods to avoid frequent replanting and soil disturbance (e.g. permanent grassland, tea)
What is terracing?
Constructing retaining walls along contours of a slope to increase infiltration and reduce runoff
What is zero-tillage cultivation?
Drilling seeds directly into the ground to reduce soil disturbance and erosion
What is contour ploughing?
Ploughing along the contours of a field (soil particles are deposited in furrows as runoff speed is reduced)
What is tied ridging?
Dividing fields using intersecting ridges to increase water retention during heavy rainfall (used on flat fields)
What are rows of stones?
Laying stones along contours to reduce runoff speed on gentle gradients (disturbed soil is deposited)
What are windbreaks?
Construction hedgerows or rows of trees to reduce wind velocity and erosion
What is multi-cropping?
Harvesting and re-sowing different crops at different times to retain protection from the wind (can be strip cropping)
How can soil OM be increased?
Add a layer of mulch to protect from wind and rain splash erosion
What is livestock management?
Control of the stocking density of livestock to reduce soil compaction
What is an example of livestock management?
Controlling the movement of livestock (e.g. away from riverbanks to reduce sedimentation)
How can you determine soil texture?
Sedimentation method
- Separate soil particles
- Place in a container and shake
- Invert to mix
- Leave for at least two days to settle
How long does it take each of the mineral particles to settle?
Sand - 2 minutes
Silt - 2 hours
Clay - 2 days
Why does clay stay in suspension for longer?
Particles are charged
What is another method of determining soil texture?
Soil sieve method
- Separate dry soil particles
- Place in the stacked sieves and shake
- Smallest particles are collected in the last collection tray
How can you determine soil water content?
- Add soil to a pre-weighed crucible
- Heat at 100C
- Repeat heating and weighing until constant mass is reached
How can you determine soil air content?
- Add 250ml of soil to a 500ml container
- Add 250ml of water and stir
- Calculate loss of expected volume
How can you determine soil OM content?
- Add dry soil to a pre-weighed crucible
- Heat at 450C
- Repeat heating and weighing until contant mass is reached
Why does the temperature need to be below 550C when determining OM content?
Mass would be lost as mineral particles breakdown due to the extremely high temperature
How can you determine soil pH?
Use an electronic pH probe that has been calibrated using a buffer solution (of a known pH)
What is the optimum pH range for most plants?
5.5-7.0
What are 2 methods of sampling soil organisms?
- Soil flooding
- Soil pit extraction
What can be used to flood soil for earthworm extraction?
Water mixed with mustard powder
What are 3 methods of determining turbidity?
- Secchi disc
- Light penetration meter
- Turbidity bottle
