Soils: Composition & properties

Question 1

Soil forming factors

Answer 1

climate, 
parent material, 
topography, 
biology, 
time

Question 2

Soil water important for

Answer 2

• soil fauna and flora
• surface flora
• maintaining good soil structure
• influence on temperature
• transport
• medium for chemical reactions

Question 3

Volume of solution depends on

Answer 3

• total volume of pore space

- shape and size

Question 4

Saturation point

Answer 4

all size pores full

Question 5

Field capacity

Answer 5

pores 0.05mm and less are full

Question 6

permanent wilting point

Answer 6

pores 0.001mm and less are full

Question 7

Soil pores are created by:

Answer 7

• wetting and drying of soil
• freeze-thaw
• roots
• soil fauna
• spaces between sand, silt and clay and organic matter particles
• spaces within organic matter clay

Question 8

Soil solution

Answer 8

contains all 18 essential nutrients for plant growth

Question 9

Alkalinity and Acidity

Answer 9

• chemical and biological reactions dependent on pH levels

- influence solubility and availability of some essential nutrients

Question 10

Soil Air influences

Answer 10

• soil organisms
• biochemical activity and chemical reactions
• soil structure and temperature

Question 11

Water and air in soils is influenced by

Answer 11

• pore size
• pore shape
• connectivity between pores

Question 12

Leaching

Answer 12

• water infiltration > evapotranspiration
• physical movement of material down profile by water
• often associated with frequent wetting and drying
• movement of material in solution down profile
• deposition in lower horizons
• loss to groundwater

Question 13

Salinity

Answer 13

• accumulation of salts near surface
• salts form groundwater, irrigation water or soil
• irrigation raises ground water table and plants increase evapotranspiration
• salts

Question 14

Salts

Answer 14

• toxic to plants
• make water uptake difficult
• make soil very alkaline

Question 15

Sodicity

Answer 15

• often associated with salinity but different process
• if salts contain lots of sodium (Na)
• Na replaces Ca in between clay lamellae (plates)
• 2 Na for every 1 Ca
• Na attracts much more water. water flows between eh lamellae
• lamellae forced apart

Question 16

Sodicity 2

Answer 16

• soil weak and breaks down easily
• pores filled
• ‘concrete’ when dry ‘porridge’ when wet
• poor rooting, aeration and drainage
• crust at surface

Question 17

Gleying

Answer 17

• permanently/temp waterlogged
• produced gleyed soil
• blue/grey when wet, mottled when dry
• caused by reduction of iron oxides by bacteria to obtain oxygen
• mottles caused by concentration of oxides and oxidation

Question 18

Soil Air

Answer 18

• as soil dries out, pore space becomes occupied by soil air
• soil air may be in contact with the atmosphere or isolated by water
• soil organisms usually breath oxygen and produce carbon dioxide (roots)
• soil carbon dioxide concentration can be 100 times greater than in atmosphere

Question 19

Soil Air 2

Answer 19

• when soils become flooded, air gets trapped in the soil pores
• this leads to large increases in the amount of CO2 in the soil
• air can escape form flooded soil through diffusion, mass flow or bubbling
• carbon dioxide dissolved in water produces carbonic acid, lowering the soil pH