Soil physical properties

Question 1

sand group

Answer 1

loamy sand, loamy fine sand

• at least 70% sand
• <15% clay

Question 2

clay group

Answer 2

sandy clay, silty clay

- at least 30% clay

Question 3

loam group

Answer 3

• even mixture of sand, silt and clay

- best combination for physical (a chemical) properties

Question 4

Sand group advantages

Answer 4

• good drainage
• easy cultivation
• quick to warm up in the spring

Question 5

Sand group problems

Answer 5

• poor water retention for plant use
• poor nutrient supply
• poor ability to hold applied nutrients

Question 6

Clay group advantages

Answer 6

• good nutrient supply & retention

- medium water retention for plant use

Question 7

Clay group problems

Answer 7

• poor drainage
• cultivation restricted to a narrow range of water content and has high power requirement
• easily puddled when wet
• slow to warm up in he spring

Question 8

Loam group advantages

Answer 8

• well drained
• good water retention for plant use
• good nutrient supply
• easy to cultivate over a wide range of water content

Question 9

Loam group problems

Answer 9

some are structurally weak and prone to erosion

Question 10

soil structure determines

Answer 10

• size
• shape
• distribution of pores both within and between the aggregates

Question 11

soil structure develop through

Answer 11

• natural processes during soil formation, often by management

Question 12

soil strucure influences

Answer 12

• water movement
• aeration
• heat supply

Question 13

soils structure types

Answer 13

• plate like (platey)
• prism like (prismatic & columnar)
• block like (blocky & subangular blocky)
• spheroidal (granular & crumb)

Question 14

pores are need for …

Answer 14

• water storage
• drainage
• oxygen entry
• carbon dioxide removal
• root penetration
• mechanical strength

Question 15

Processes involved in the development of soil structure

Answer 15

development of cracks and channels along planes of weakness through:

• shrinkage and swelling, caused by drying/wetting and freezing/thawing cycles during soil formation
• root and soil animal actions

Question 16

microbial activity during soil structure development

Answer 16

formation of humus during the break down of plant and animal residues

Question 17

Micro-aggregate

Answer 17

• <250um
• flocculation of clay particles to form domains
• interactions between organic matter and clay domains

Question 18

Macro-aggregate

Answer 18

• > 250um
• coalescence of micro-aggragates
• forces exploiting planes of weakness

Question 19

transmission pores (macro)

Answer 19

> 50um

• drainage
• root penetration
• gas movement

Question 20

storage pores (micro)

Answer 20

• 50-0.5um

- water available for plant growth

Question 21

residual pores (micro)

Answer 21

• <0.5um
• water not available to plants
• controls mechanical strength of soils

Question 22

Porosity =

Answer 22

1 - (bulk density/particle density)

Question 23

Bulk density

Answer 23

mass of solid soil particles in a unit volume of field soil

- soils + pore space occupied by air and water

Question 24

Particle density

Answer 24

mass of soil soil particles in a unit volume of those solid particles

Question 25

unstable soil structure

Answer 25

• soils that lose structure easily when rain falls on the surface or water is help up in the soil
• soils dominated with single particle size class
• repeated or poorly timed cultivation

Question 26

consequences of unstable soil structure

Answer 26

• surface capping
• slaked and puddled structure
• under-consolidation
• compaction and erosion