R2102 1.1 – 1.4 Properties of Soil

Question 1

How is soil formed?

Why is it important?

Answer 1

Formation:

(1) Weathering of parent rock into sand, silt, clay; proportion of these three gives the texture, which influences the structure
(2) Addition of organic materials

Importance:

Interface between biosphere (life), hydrosphere (water), lithosphere (earth), air (atmosphere)

Question 2

Name the 3 ways rock weathers

Answer 2

Physical – heat, cold, rain etc

Chemical – water dissolves minerals

Biological – roots create fissures and dislodge

Minerals also leave channels where water can travel and erode more rock.

Question 3

What is soil texture?

What are the particle sizes of sand, silt, clay?

Answer 3

The relative proportion of the different-sized mineral particles; sand, silt and clay in a particular soil; the look and feel of a soil.

Gravel: > 2mm

Sand: 0.06mm – 2mm

Silt: 0.002mm – 0.06mm

Clay: < 0.002mm

Question 4

Describe the characteristics of sandy loam.

Answer 4

Feels gritty

Good drainage

Little water retention capability

Little nutrient retention capability

Quick to heat up

No electrical charge

Question 5

Describe the characteristics of a silty loam.

Answer 5

Feels silky and soapy when wet

Fair drainage

No electrical charge

Question 6

Describe the characteristics of a clay loam.

Answer 6

Feels sticky when wet and hard when dry

Poor drainage

Good water retention

Good nutrient retention

Slow to heat up

Negative electric charge

Question 7

Describe key things crumb structure allows.

Answer 7

Free water movement

Gaseous exchange

Thorough root exploration

Question 8

How can root environment be improved and protected?

Answer 8

Cultivation – single and double digging, forking, raking, rotavating

Addition of organic and inorganic material

Managing soil water content – drainage and irrigation

Question 9

What are advantages and disadvantages of soil cultivation?

Answer 9

Advantages:

Prepares for planting

Improves structure

Exposes clods to winter weathering

Breaks up pans

Allows fertilisers to be incorporated

Buries crop remains and weeds

Disadvantages:

Disturbs natural structure – earthworms etc

Damages soil structure

Dormant seeds may be brought to the surface

Moisture may be lost from soil

Hard work

Question 10

What are symptoms of poor drainage?

Answer 10

Poor plant growth

Water collecting on surface

Soil constantly wet

Mosses

Blue/black colouring

Smell

Question 11

How can one increase drainage in any soil?

Answer 11

Add organic matter

Add lime

Question 12

Why does adding lime (raising pH) improve drainage?

Answer 12

Lime causes flocculation of clay particles – clay particles group together to form larger particles, improving structure of soil and making it less dense.

Question 13

What is erosion?

Answer 13

The breakdown of rocks by means of transportation e.g. by movement by wind, waves, streams, rivers, or glaciers

Question 14

What is weathering?

Answer 14

The break-down of rocks by mechanical, chemical or biological means

Question 15

What are primary colonisers?

Describe the symbiotic relationship between algae and fungi.

Answer 15

Certain species of bacteria, fungi and plants that have evolved to live in water- and nutrient-limited environments. (Includes lichens.)

Algae can obtain carbon and nitrogen from the atmosphere, sharing it with the fungi, which can access nutrients from the rock by dissolving its surface with acid, and share these with the algae. Hence the symbiotic relationship.

Question 16

Why are soil organisms important?

Why is humus important?

Answer 16

They transform the plant and animal residues of soil into humus.

Humus:

• holds nutrients in forms that break down slowly and can be made available to plants
• helps bind soil together

Question 17

What is the CAC (cation exchange capacity)?

Answer 17

Cation-exchange capacity is a measure of how many cations can be retained on soil particle surfaces. Negative charges on the surfaces of soil particles bind positively-charged atoms or molecules, but allow these to exchange with other positively charged particles in the surrounding soil water.

• Each soil will have a different capacity to hold these cations and this is called the cation exchange capacity.
• This is a measure of soil quality and increases with higher clay and organic matter content.

Question 18

What are soil horizons?

Answer 18

O – Organic – organic matter that has not yet been added into the soil, e.g. surface mulch; may not be present when bed newly created

A – Topsoil – Darker topmost layer, mostly mineral material mixed with organic matter from O horizon

Greater number of soil organisms, where most biological activity occurs

Plants concentrate their roots in, and obtain most of their nutrients

(E – Alluviation – sand, silt)

B – Subsoil (E, B and C horizons) – Usually lighter in colour than topsoil

Minerals and nutrients pushed down

Where finer materials accumulate and where deposition occurs.

Higher levels of clay, salt and lime than topsoil.

Less organic matter and fewer soil organisms.

Pore spaces smaller and particles will compact more easily.

Should not be mixed with topsoil.

Only the deeper roots will reach this layer.

C – Parent rock (‘parent material’)

First layer of weathered bedrock (unconsolidated).

Little affected by soil-forming processes, often containing lumps or shelves of split or cracked bedrock.

Retains very similar traits to the original parent material. For example, ‘ghost’ rock structure may be present in the C horizon.

Question 19

What are the benefits of organic matter?

Answer 19

Results in a greater number of soil organisms

Tilth (ease of tillage)

Structure (large pore spaces)

Water infiltration

Water-holding capacity

Question 20

What makes a good top soil?

Answer 20

Good tilth (0.5 – 5mm aggregates; inside these crumbs are predominantly small pores which hold water, and between the crumbs are macropores which contain air when soil is at field capacity)

Well spaced peds – allowing good water-holding capacity and room for root growth

Warms up quicker

Contains essential elements necessary for healthy growth

Question 21

Describe soil formation under headings “Physical”, “Chemical” and “Biological”.

Answer 21

Physical – Freeze/thaw cycles. The cracks widen – breakdown of rock particles. Heat – expansion and contraction of rock – breaks apart. Abrasion – rocks brushing against each other down a mountainside or in rivers where the rock is gradually worn away.

Chemical – Carbon dioxide in the air reacts with water to form a weak carbonic acid – erodes rock. Oxygen in the air can also directly react with chemicals in the rock, e.g. iron to form oxides which results in the mineral disintegrating or dissolving in water.

Biological – Plant roots and seeds can grow deep within the cracks in rocks and cause them to widen. The growth of moss and lichen on the surface of rocks can also cause cracks etc. The action of animals trampling/burrowing can cause rocks to break into smaller pieces.

Question 22

What is good soil structure?

What are the advantages?

Answer 22

Large macropores to allow movement of air and water. This “ideal” structure is called granular, or crumbly.

The particles of sand and silt are held together in aggregates (small clumps) by clay, humus and calcium. The small empty spaces (micropores) hold the water the plants need.

Advantages:

• holds water and nutrients well
• good drainage
• good aeration
• good plant-root-system development
• soil is easy to work
• soil warms up quickly in spring
• good biological activity in the soil
• soil resists erosion and compaction

Question 23

How can soil structure be improved?

Answer 23

Sandy soils:

• Organic matter – compost or composted manure
• Early spring, because working sandy soils in the fall promotes erosion
• Adding basalt improves their ability to retain water and minerals.

Clay soils:

• Organic matter in the form of compost or composted manure
• Late fall. Poorly drained clay soils can also be amended by adding sand.
• Some clay soils are very high in sodium, which prevents mineral particles from forming aggregates. In this case working in gypsum (if their pH is neutral or alkaline) or lime (if their pH is acidic).

Silty soils:

• Large amounts of organic matter in the form of compost or composted manure.
• Late fall

Also:

• Regularly add organic matter
• Encourage biological activity in the soil.
• Correct the pH as necessary.
• Avoid overworking the soil. Hoe the soil or turn it over lightly.
• Use mulch.

Question 24

How is humus created in the soil?

Answer 24

Microorganisms in the soil break down organic matter, quickly releasing nutrients (mineralization) and forming humus (humification).

Humification: The process by which organic matter decomposes to form earthy-smelling dark brown to black material, called humus. Humus is mineralized in turn, but very slowly. This makes small amounts of nutrients available to plants over a very long time.

Question 25

What are the characteristics of sand, silt and clay?

Answer 25

Sand:

Gives framework and stability but does not bind easily together so is easily eroded by wind and water

Has little nutrient capability

Large pore spaces allow water to enter and exit very freely, i.e. it is very free draining

Feels gritty, can be seen with the naked eye and when handled leaves no residue in the hand

Silt:

Behaves like sand but forms smaller pore spaces so water enters and exits more slowly

When dry it feels powdery and smooth; when wet it feels silky and slippery with no plastic or sticky feel

It can be seen with a hand lens or microscope. When handled it coats the hand but can be brushed off.

Clay:

Fine and sticky

Plate-shaped particles arranged in layers which means that water enters and exits slowly

Tiny particles or ‘colloids’ are positively and negatively charged and attract charged nutrient ions from humus in the soil, holding everything together

Effective at holding nutrients, making them freely available to plants

Has a hard dry feel when dry; when wet it feels sticky and plastic. It can be seen with an electron microscope. When handled it forms a thick film residue.

Question 26

What is affected by particle size affect in soil?

Answer 26

• Porosity: the size and construction of soil pores and therefore how much air and water is held in the soil. Sand has largest pores.
• Permeability: how water flows through the soil. Sand drains freely.
• Erodibility: silt is susceptible to wind and water
• Biological activity: ability to support soil microorganisms. Sand leaches nutrients, clay holds onto them
• Infiltration: the process by which rain or irrigation water seeps into the soil
• Shrink-swell: clay particles absorb water and swell when wet and shrink when dry. ‘Cracking’ in summer and ‘heavy’ physical properties in winter.
• Chemical reactions: affected by differing proportions of clay, silt and sand
• Water-holding capacity: the ability of soils to hold onto water. Sand is low, clay is high.
• Erodibility: the susceptibility of a soil to erosion through raindrop impact and runoff
• Soil cohesion: whether the soil crumbles or not, the sticking together of like molecules. A cohesive soil can be moulded easily when wet and is hard to break up when dry. The more clay particles, the more cohesive the soil.

Question 27

Advantages and disadvantages of different soil particles?

Answer 27

Sandy soils:
Sand drains easily = poor ability to retain moisture

Little chemical activity = little nutrient bonding

Need improving with OM to increase their water retention and nutrient content. Fertiliser inputs. Earlier crops as they warm up more quickly.

Silt soils:
Little chemical activity = little nutrient bonding

Poor air and water movement = may compact under heavy traffic

Need careful management to keep soil open and prevent drainage problems. They should not be worked when too wet. OM will keep the soil more open.

Clay soils:
Water adheres very well to clay = high ability to retain moisture (however, this water can be hard or impossible for the plant to use)

Very chemically active = good nutrient bonding

Question 28

How do soil structures form?

Answer 28

• Freeze/thaw (frost) surface layers – clays
• Shrinking/swelling (wetting and drying). Drying can affect the rooting depth. Cracks can open up heavier soils as the clay shrinks.
• Root penetration and soil microorganisms. Roots grow in the cracks of the soil structure and keep them open. They help create fracture lines, and bind soils together; as do fungal hyphae.
• Soil organisms such as earthworms are important in loosening soil and maintaining channels, thereby stabilising the soil structure.
• Soil cultivation by hand or machinery is used to make the soil suitable for plant growth.
• Additions of organic matter and their decomposition in the soil. Adds to the stability of soils.

Question 29

What can destroy crumb structure?

Answer 29

Overcultivation can lead to the formation of compacted areas (due to lack of vertical cracks) such as soil pans, and soil caps.

Cap is created when the surface is exposed to heavy rain and the crumbs collapse and individual particles fill the pore spaces. These caps interfere with gaseous exchange and prevent seedlings emerging. Can form a hard crust.

Cultivation pans caused by “smearing” of cultivators such as ploughs or rotovators or even feet (human and livestock) in wet soils.

Natural pans can be caused by accumulation of denser particles such as clays lower down in the soil profile. These can impede drainage.

Iron pans (cemented layers) can form in sandy soils rich in iron oxide (and these can move with fluctuating groundwater levels).

Waterlogging can also cause aggregates to collapse due to the formation of a slurried cap.

A decline in organic matter will impact soil structure.

Having large areas of bare earth subject to rainfall batter.

Question 30

How are soil peds bound together?

Answer 30

• Plant roots release exudates that are sticky which help bind soil particles.
• Fungal hyphae extend into the soil and help bind particles together.
• Oxides (such as iron oxides) help glue particles together.
• Soil microorganisms excrete glue-like substances that act like cement, helping to bind particles together.

Question 31

What is good crumb structure?

Answer 31

Sand, silt and clay held together by films of water and humus (the product of the breakdown of organic matter)

Will readily break down into smaller crumbs but not into dusty particles or remain a solid mass

Good porosity and crumbly

Effective nutrient exchange

•Tilth is the structure of the top 50mm of the soil. You want a crumb soil.

Question 32

What is saturation point?

Answer 32

Soil pores are full of water and no air.

Question 33

What is field capacity?

Answer 33

The amount of water remaining in the soil a few days after having been wetted, and after free drainage has ceased

The amount of water the soil can hold against the force of gravity

Question 34

What is the permanent wilting point?

Answer 34

The point when water can no longer be extracted from soil

The soil’s water content when a plant growing in it does not retain turgor overnight

Fine-textured soils have higher field capacity and a higher wilting point (because of increased surface area)

Question 35

What is the available water content?

Answer 35

The water that lies between field capacity and permanent wilting point

Question 36

What is soil moisture deficit?

Answer 36

The amount of water needed to bring the soil moisture content back to field capacity

Question 37

What is soil structure?

Answer 37

The arrangement of soil particles into aggregates/crumbs, within the presence of air, water and a small amount of organic matter