2102 - Physical 7 Chemical Properties of Soil

Question 1

What is Soil?

Answer 1

Naturally occurring material on the earth’s terrestrial surface.

Made up of: Rock Minerals, Water, Air, Organic Matter and living organisms

Question 2

Average Soil Composition

Answer 2

45% - Mineral Particles
5% - Organic Matter
25% - Water
25% - Air

Question 3

Soil Formation - Weathering

Answer 3

Weathering is the action of Chemical or Physical forces on or near the surface of the rock.

(No rock movement associated with Weathering)

Question 4

Soil Formation - Erosion

Answer 4

Rock particles moving - ie landslide.

Erosion is the movement of rock fragments and soils.

Usually due to air, water or gravity.

Question 5

Mineral Soils

Answer 5

Consisting of mainly inorganic particles (rock)

Question 6

Organic Soils

Answer 6

Consisting of mainly organic matter (Peat)

Question 7

Physical Weathering

Answer 7

Expansion and Contraction of rock due to temperature.
Hot - rock expands while attached to cooler rocks.
Cold - Freeze / Thaw Cycles (heating and cooling)

Water - Carries abrasive particles which wear down rock. Larger/faster water can carry larger minerals.

Wind - similarly wind can carry abrasive particles that ‘sandblast’ exposed rock.

Glacial - Glaciers carry large amounts of rock, which gets churned up inside and also grinds exposed rock while passing.

Question 8

Chemical Weathering

Answer 8

Carbonic acids from plants roots or Sulphuric acid from rainwater gradually dissolving rock.
Oxygen can also directly react with chemicals in the rock such as Iron to form oxides, which results in mineral disintegration or dissolving in water.

Question 9

Biological Weathering

Answer 9

The results of Animals or Plant life (Organisms) physically breaking up the rock.
(Roots in cracks)

Question 10

Erosion cont…

Answer 10

Erosion is Rock being broken down by rivers, glaciers, wind and sea and deposited a distance away.

Soils formed by erosion are called Transported soils. Often have a different composition than the bedrock.

Alluvial soils - movement by - Rivers
Glacial soils - movement by - Glaciers
Aeolian Soils -movement by - Wind

Question 11

Mineral Particle Size

Answer 11

Soil Mineral Particles determine the Soil Texture and Structure.

• Sand 2mm - 0.06mm
• Silt 0.06mm - 0.002mm
• Clay - under 0.002mm

Question 12

Soil Texture

Answer 12

Soil Texture is the relative proportion of Sand, Silt & Clay in a given soil.

Different proportions determine the texture, which allows us to predict certain Soil Characteristics, useful for horticulture.

Determination may be performed by a lab or by hand in the field.

Question 13

Hand Texturing

Answer 13

An approximate idea of soil texture determined by Hand Texturing.
Small amount of moistened soil is worked between the fingers.

Sandy Soils - gritty. Pencil/worm will not bend.

Silty soils - silky / soapy. Will form into worm, possibly crack.

Clays - sticky. Malleable, will form into worm and bend around finger.

Question 14

Loams

Answer 14

Soil with a texture intermediate between two components, which appear to blend into each other.
Loams cane Sandy, Silty, Clay or any combination.
Eg. Sandy Silty Loam.

Changing soil texture is possible by incorporating mineral matter of the required size. But is prohibitively expensive except on a small scale.

Question 15

Soil Textural Characteristics - Clay

Answer 15

Low Drainage rate, High nutrient holding capacity. Slow to cool in Autumn, but slow to warm in Spring.

Question 16

Soil Textural Characteristics - Silty Loam

Answer 16

Available Water capacity is high, with a medium drainage rate. Medium nutrient holding capacity. Medium to warm and cool in Spring/Autumn,
High tendency to surface capping.

Question 17

Soil Textural Characteristics - Sandy Loam

Answer 17

Low available water capacity, and high drainage rate. Poor nutrient holding capacity.
Fast to warm, but fast to cool in Spring/Autumn.
Medium high chance of surface capping.

Question 18

Soil Structure definition

Answer 18

The arrangement of particles in the soil. The soil ‘Architecture’.

Question 19

Soil Structure particles…

Answer 19

Mineral Particles are grouped in AGGREGATES which vary in size, and have a significant effect on how plants grow. (chemical and physical forces holding them together).

Large Aggregates - (Peds) can be difficult to work and have limited air/water movement within them. They can often be compacted, which discourages root penetration.

Smaller Aggregates - (Crumbs) have a good variety of pore sizes within and between them. Good for root penetration and good air/water movement.

Fine Particles - Soils with no aggregate structure, tend to form caps on the surface, which prevent water and air penetration.
Can also be subject to wind erosion.

Plants need air and water equally if they are to survive. Thus the best balance of these will allow optimal growth.

Question 20

Pores

Answer 20

Spaces between the Particles and Aggregates.

Macropores - Large free draining voids, common on heavy clays and stony soils. Drain under gravity.

Mesopores - Medium sized voids, within and between smaller aggregates that do not drain under gravity. Allows easy access to water for plant roots.

Micropores - Tiny pores within Aggregates, which hold onto water tightly and preventing soils form drying out, and allow clay spoils to retain soil fertility well.
Water in Micropores is unavailable to plants.

Question 21

Soil Structure can be afftected by…

Answer 21

Digging and Rotoavation
Surface cultivation
Incorporating Organic Matter
Regular Cropping
Earthworms and other organisms
Certain Crops can be beneficial while others can degrade soil structure.

Timing of these operations is also important and soil condition (especially moisture levels).

Question 22

How to Develop Good soil structure…

Answer 22

Avoid unnecessary cultivation (no-dig if possible)

Avoid digging to the same depth repeatedly (soil pans)

Do not work soil when wet

Reduce traffic over wet soils

Incorporating organic mater regularly

Avoiding Sprinklers with coarse sprays (Capping)

Question 23

Pores continued….

Answer 23

Macropores - Fills up with water via gravity, and is drained in the same manner. Pulling air into the pore as it is drained.

Mesopores - Hold water with Capillary Action. (plant can acmes this water).

Micropores - Water is held un electrostatic forces which are too strong for the plant to overcome. Therefore the water is unavailable to the plant. (Clays especially)

Question 24

Saturation Point

Answer 24

All pores are full of water. (waterlogged)

Question 25

Field Capacity

Answer 25

Water in the Macropores is absorbed into the Mesopores through Capillary Action. Occurs after 24/48 hours of drainage.

Macropores - empty/full of air
Mesopores - Full of water
Micropores - full of water

Optimal conditions for plant growth.

Question 26

Permanent Wilting Point

Answer 26

Soil has no available water for plants. Plant will become wilted through the day but not recover during the night.

Macropores - Empty/full of air
Mesopores - Empty/full of air
Micropores - remain full of water

Question 27

Air and Water in soil

Answer 27

Air is present in the soil where water is not.
It can be drawn into the soil pores from the surface.

Most soil organisms and plant roots use oxygen to respire. Deeper in the soil typically less oxygen will be found (more co2).

Less Oxygen = Less life/less organic matter present.

Soil horizons, top layer (topsoil) will typically be darker. This is the living layer with majority of organic matter and micro-organisms.

Fine particles are washed through the top soil to the subsoil below. Much lower levels of oxygen/air and thus a denser structure and lower fertility, with less organic matter.

Question 28

Sands and Sandy Loams - Aggregate and Pores

Answer 28

Larger mineral particle size so has little natural structure. Typically less pores too.

Fewer Macropores, more Mesopres, fewer Micropores

Question 29

Clay and Clay Loams - Aggregate and Pores

Answer 29

Much smaller particles, if the soil is well structured can have a good distribution of pore-sizes.

Naturally many micropores in Clays which hold water unavailable to plants. Also do not contribute to gaseous exchange.

Over-cultivated or compacted soils can lose their structure and the distribution of Macro/Mesopores will change affecting the water entry and drainage and thus the water holding capacity of the soil.

Question 30

Clays cont…

Answer 30

Well Structured Clays, will have plenty of water and air available to plants.
Poorly structured there will be less available water, with poor drainage and poor gas exchange.

Likelihood of aerobic soil conditions, which can become toxic to plant roots.
(shallow rooting, poor growth, loss of turgidity, wilting and disease may increase).

Question 31

Soil Particle Size again…

Answer 31

Sands - 2mm to 0.06mm
Silts - 0.06mm to 0.002mm
Clays - 0.002mm and below

Question 32

Cation Exchange Capacity (CEC) & Nutrient Buffering

Answer 32

Useful information for crop production, as will allow correct fertiliser application rates.

High Cation Exchange / High Nutrient Buffering

Soils have high capacity to hold onto nutrients (often clays)

Low Cation Exchange / Low Nutrient buffering
Soils have low capacity to hold onto nutrients, usually due to leaching. Will need to be fertilised more often with less fertiliser.

Question 33

Soil Profiles (layers/horizons)

Answer 33

Top

O horizon
Organic Materials - Organic litter tray

A horizon - Topsoil
High organic Content, Rich in Organic Life, high fertility, most root Content.

B horizon - Subsoil
Less organic content, lower fertility, high mineral content. Denser aggregates. Few Roots.

C horizon - Weathered or Decomposed Rock - no root penetration……..Solid Rock - Parent Rock

Question 34

Soil Capping

Answer 34

Soil Capping - The collapse of surface crumb structure. Occurs in soils with high silt content.

Occurs after heavy rain, heavy irrigation (sprinkler use) if the soil has a fine tilth.

Crumbs are pulverised by raindrops and dry to form a crusts on the surface which resists air and water movement, this seedling gemination and gaseous exchange. Will commonly re-form following further rain/irrigation.

To avoid Cap formation - Use copious amount fo organic matter to mulch the surface, avoid overworking the soil and avoid heavy/coarse irrigation.

Question 35

Soil Pan

Answer 35

Hard or Impervious layer the form directly beneath the final depth of cultivation.

Often the direct effect of repeated use of mechanical cultivation machines. Though can be formed by fine soils and particles which wash down through the soil and cement together.

Avoid Soil pans by cultivating to differing depths and avoiding cultivation when the soil is wet.

Alleviate Soil Pan - Double Digging, Sub Soil Plough, Mole Plough.

Question 36

Gley soil

Answer 36

Occurs with insufficient Oxygen in the soil horizon. Normally due to waterlogging. Microbes cannot respire, but other thrive. They act by changing the ferrous oxides (rust) to ferric form. Changing the soil colour, brown to grey/blue.
Often foul sulphurous smell.

anaerobic root environment is toxic to plant roots.

Question 37

Water Table

Answer 37

The level below which the ground is permanently saturated.
Varies season to season, and topography.

Perched Water Table - localised collection of water above the water table of surrounding areas. Can be present due to a Soil Pan, impeding the downward movement of water.

Question 38

Organic Matter

Answer 38

Soil Organic Matter is composed to living organisms, dead and decomposing matter and humus.

Question 39

Organic Matter cont…

Answer 39

Majority of Organic Matter is on or near the surfacer layer. Provides protection from weathering and drying.
Slowly incorporated into the soil as it is broken down.

Can help soil structure, by allowing water and air into the voids left as it decomposes.

Question 40

Humus

Answer 40

What is left over at the end of the decomposition process. The material cannot be broken down further.
Little energy or nutrient value.
Binds mineral particles together into aggregates, helping to give soil its structure and colour.

High Cation exchange capacity, which will help to increase the fertility of the soil.

Question 41

Living Organisims

Answer 41

Diverse population of Organisms (half billion per gram).

Majority present in the topsoil, eg Earthworms, Nematodes, Bacteria and fungi.

Question 42

Benefits of Good Soil Structure

Answer 42

Optimal balance between Aeration and Water Availability to the plant (approx 25% of each).

Make soil friable for better seed germination and root/shoot emergence and establishment.

Well Drained, Good storage and availability of air/water/

Firm Anchorage for the plant.

Resistance to structural degradation.

Question 43

Water/Nutrients/Leaching

Answer 43

Soil Solution - Fresh water mixes with pre-existing soil water and nutrients.

Leaching - Soil Water draining under gravity taking the nutrients with it.

Leaching is worse on Sandy, coarse soils with Low CEC or Nutrient buffering capacity.

Plants take up nutrients and water from the soil solution.

Some nutrients arrive form the atmosphere, or plants can fix them. Most in horticulture are added through the applications of fertilisers and composts.

Question 44

Water Loss in the Soil via…

Answer 44

Drainage - Gravitaitional loss down to the water-table or to rivers.

Evaporation - from the soil surface

Transpiration - through the plant and from the plant leaf surfaces.

Question 45

Effect of too little Water….

Answer 45

Less Available water to the plant resulting in Wilting (loss of turgidity), reduced Growth, Death.

Reduced Soil Microbial activity, resulting in less available soil nutrients.

Question 46

Effect of too much Water…

Answer 46

Damage to the soil structure, if worked or walked on.

Reduction of available Oxygen in the soil. Leading to reduced Soil Microbial activity, Gleyed soil. Reduced growth, potential root death and eventual plant death.

Increased chance of disease.

Question 47

Causes of too much Water in soil…

Answer 47

Poor Soil Structure - eg compaction.

High Watertable - Temporary or Permanent.

Surface run off - Water added beyond the capacity of the soil to drain it. Excessive rainfall, or from neighbouring hard-surfaces. (driveway, roof etc…)

Question 48

Methods to Deal with Excess Water in the soil

Answer 48

Primary cultivation (digging, adding organic matter)

Mole Ploughing, Sub Soiling. (create channel deep in the soil for water to run through and out).

Add a Soakaway

French Drain

Appropriate planting (right plant right conditions Marginal Plants, Lobelia cardinalis)

Raised bed

Question 49

Soil Moisture Deficit

Answer 49

The amount of water that needs to be applied to bring the soil back up to Field Capacity to its current moisture levels.

Question 50

Available Water Content

Answer 50

The Water held in the soil (mesopores) between Field Capacity and Permanent Wilting Point.