1.
Topic 3.4
Introduction to Soil System

2.
• SIGNIFICANT IDEAS
– The soil system is a dynamic ecosystem that has
inputs, outputs, storages and flows
– The quality of soil influences the primary
productivity of an area.

3.
• KNOWLEDGE & UNDERSTANDING
– Soil profile, a layered structure
(horizons)
– Inputs:
• Organic matter
– Leaf litter &
• inorganic matter from parental
material
• Precipitation
• Energy
– Storages:
• organic matter
• Organisms
• Nutrients
• Minerals
• Air & water
– Flows: transfers of material
within the soil
• Biological mixing & leaching
– Contribute to the
organization of the soil
– Outputs
• Uptake by plants
• Soil erosion
– Transformation:
• decomposition, weathering &
nutrient cycling
– Structure & Properties
• Sand, Clay & Loam
– Mineral content
– Drainage
– Water logging capacity
– Air, biota & organic matter
– Primary productivity
– Soil Texture triangle

4.
• APPLICATION & SKILLS
– Outline:
• of transfers, transformations, inputs, outputs, flows & storages within
SS.
– Explanation:
• Soil can be viewed as an ecosystem.
• GUIDANCE/NOTE
• Studies of specific soil profiles viz podsol, not required
• Familiarity with the soil texture triangle diagram used for soil
classification on the % of sand, silt & clay in soil is required.
• INTERNATIONAL-MINDENESS
• Significant differences exist in arable soil availability around the world.
• These differences have socio-political, economic & economic
influences
• TOK
– The SS may be represented by a soil profile- since a model is,
strictly speaking, not real,
– How can it lead to knowledge?

5.
Soils
• Naturally occurring unconsolidated material on surface of Earth, influenced by
– parent material from rock below
– Climate, macro/micro organisms & relief.
• The interface between:
– Atmosphere
– Lithosphere
– Biosphere
– Hydrosphere
• Comprised of:
– Regolith (weathered bedrock)
– Organic matter (living and non-living)
– Air
– Water
• Exist in all 3 states
– Solid (organic and inorganic matter)
– Liquid (water from precipitation, seepage and groundwater)
– Gas (volatiles in atmosphere and within pores)
• Soils take so long to develop that they are generally considered to be a non-
renewable resource

6.
– Extra:
• Inorganic: minerals ex P & K
– Lithosphere: solid, rocky
– InoTopography
• Organic: minerals ex N & S
– Death & decay of flora & fauna
• Shape of landscape
– Considerable effect
» Amt of soil present in an area
» Ex steep slopes, struggle to hold soil.
• Soil: wide range of animal life, top 30 cm, 1 ha, 25
tonnes of soil org
– 10 tonnes of bacteria
– 10 tonnes of fungi
– 4 tonnes of earthworms
– 1 tonnes- spring tails, mites, isopods, spiders, snails, mice etc
» Earthworms: 50-60% of total wt of fauna

7.
Soil Profiles
Horizon Characteristics
Organic horizon (O) undecomposed litter
partly decomposed litter
well-decomposed humus
Mixed mineral-
organic horizon (A)
humus
ploughed
gleyed or waterlogged
Eluvial or leached
horizon (E)
strongly leached
weakly leached
Illuvial or deposited
horizon (B)
iron deposits
clay deposits
humus deposits
Bedrock or parent
material (C/R)
rock
unconsolidated loose deposits
The boundaries between horizons are often blurred due to earthworm activity
soil horizons

8.
How Are Soils Formed?
• They are considered to be open systems in steady-state
equilibrium
• The main processes of formation are:
– Weathering
– Translocation (movement of substances) *
– Organic changes (largely near the surface)
– Gleying (waterlogging)
• At the surface, humus is created (humification) and eventually
decomposed completely (mineralisation) – they always occur
together
– Humus: a dark crumbly substance, very fertile for plant growth. Avg
soil is approx half mineral & half water and air.
• Human activity is having severe effects on soil formation
* Translocation usually occurs downwards due to the movement of water and dissolved
substances. However in arid environments movement is upwards due to evaporation
soil formation animation

9.
Using Soils
• The main human use of soils is for
cultivation (also peat extraction to a
lesser extent)
• For cultivation, the ideal soil has a
good balance between water-
retention and drainage (porosity)
and aeration
• These properties are based on soil
texture
• Texture depends on the proportions
of different sized particles
(sand/silt/clay)
• The ideal balance of particle size is
achieved in loamy soils

10.
Porosity vs Surface Area
• Pore size determines the rate of drainage of
water and how easily it is aerated
• Particle size/ surface area determines how easily
water and dissolved nutrients are retained
(against gravity)
– Light soils (> 80% sand) – coarse texture, easily
drained; low primary productivity
– Heavy soils (> 25% clay) – fine texture, small pores (<
0.001mm), water and nutrient retentive, chemically
active, not easily worked (ploughed)*; low primary
productivity
– Medium soils – somewhere in between (loam); high
primary productivity

11.
CLAY: <0.002 mm
SILT: 0.002 – 0.05 mm
V. FINE SAND: 0.05 – 0.10 mm
FINE SAND: 0.10 – 0.25 mm
MEDIUM SAND: 0.25 – 0.50 mm
COARSE SAND: 0.50 – 1.00 mm
V. COARSE SAND: 1.00 – 2.00 mm

12.
The best soil type: loam

13.
Porosity vs Surface Area
• A variety of pore sizes is required to allow root
growth, water drainage, aeration and water
storage
– Pores > 0.1 mm are needed for root growth
– Pores < 0.05 mm are needed for good water
storage
• Overall soil structure depends on:
– Soil texture (see the triangle)
– Amount of dead organic matter
– Earthworm activity

14.
Ch: 3.4
SOIL DEGRADATION AND
CONSERVATION

15.
Soil Degradation
• Reduction in quantity and quality of soil
• It may be caused by:
– Erosion (by wind and water)
– Biological degradation (loss of humus and living
material)
– Physical degradation (loss of structure and
accompanying changes in porosity)
– Chemical degradation (acidification, loss of
nutrients, changes in pH, changes in salinity
(salinisation)

16.
Human Activity
• Removal of woodland or pasture
– loss of root systems which bind soil together increase erosion rates
• Cultivation
– Bare soil is exposed after harvesting and before planting. Cultivation of
slopes causes rills and gullies. Irrigation in arid areas leads to
salinisation
• Grazing
– Reduces vegetation cover leading to increased erosion, animals also
trample existing vegetation and waste causes eutrophication
• Road building
– Reduced infiltration causes rills and gullies
• Mining
– Exposure of bare soil
– The effects are often most severe in LEDCs which are highly dependent
on agriculture
– 15% of the world’s soil is thought to be degraded

17.
Soil degradation
• Water erosion makes 60% of soil degradationErosion
• Overgrazing, overcropping, increasing erosionOveruse
• Slash and burn techniques leave nutrient poor soils
unproductive after several crop cycles, increasing erosionDeforestation
• Also called toxification, releases toxic metalsAcidification
• Minerals in the water concentrate in the soil in dry or coastal
areasSalinization
• Spreading of desert into once productive areasDesertification
• Increasing all the above problems due to change in land use
and hydrologyClimate change

18.
Deforestation

19.
Interaction of soil and water systems

20.
Salinization

21.
Erosion

22.
Soil conservation
Reduce water flow
• Contour ploughing
• Terracing
• Gullies and ravines fenced and planted with trees
Erosion control after harvest
• Keep crop cover as long as possible
• Keep stubble and root structure after harvest
• Plant a grass crop or crop rotation
Long term
• Smaller fields
• Grow a tree crop
• Wind barriers
• Stop use of marginal lands
• Use of lime or organic material to improve soil condition

23.
What issues can you see here?
Bad:
• Slope  increased
erosion
• Fields very large
• Monoculture –
crop rotation?
Good:
• Tree buffers that
can absorb surface
runoff, lower wind
erosion
• Harvest does not
allow soil to be
exposed
• Contour ploughing

24.
Soil management
Subsistence farm
• Only enough food for family or small community
• Labor intensive
• Linked with poverty
• Good for the environment
• no GMOs, polyculture, limited selective breeding

25.
Soil management
Commercial farm
• High technological input
• Low labour
• High yields
• Bad for the environment
• GMOs, monoculture, selective breeding

26.
Another way to describe farming:
Extensive farming – Farms that are large in
comparison to the money and labour put into
them eg. large cattle ranches
Intensive farming – Farms that are small but
have high output (due to capital and labour)
eg. feed lots for cattle

27.
Soil Conservation
• Prevention of salinisation
– Regular flushing soil to wash away salts
– Application of chemicals to remove sodium salts
– Reduction of evaporation losses to prevent
capillary action

28.
Questions
1. Create a flow diagram of the soil system. Try
to show links with the lithosphere,
atmosphere, hydrosphere and biosphere
2. Compare and contrast the structure and
primary productivity of sandy, clayey and
loamy soils
3. Outline the processes and effects of soil
degradation
4. Evaluate a number of soil conservation
measures

29.
The Universal Soil Loss Equation (USLE)
Factor Description
R Total rainfall, intensity and seasonal distribution. Maximum for regular,
high intensity storms. Greatest if rain occurs on newly ploughed soil.
Lower for gentle rain, if crop is established or soil is frozen
K Depends on infiltration capacity, structural stability and ability to
withstand rain splash
L and S Affect the movement and speed of water flow and its ability to transport
particles. Linked to erodibility (K)
C Crops, grass and forest cover provide protection against erosion. This is
greatest for plants with extensive root systems and greatest foliage.
Fallow land or exposed cropland provide little protection
E Soil conservation measures such as terracing can reduce erosion and slow
runoff

30.
The Universal Soil Loss Equation (USLE)
• A = predicted soil loss
• R = erosivity index
• K = soil erodibility
• L = slope length
• S = slope gradient
• C = cover and management
• P = erosion control practice
A = R K L S C P