Soil physical properties Flashcards
sand group
loamy sand, loamy fine sand
- at least 70% sand
- <15% clay
clay group
sandy clay, silty clay
- at least 30% clay
loam group
- even mixture of sand, silt and clay
- best combination for physical (a chemical) properties
Sand group advantages
- good drainage
- easy cultivation
- quick to warm up in the spring
Sand group problems
- poor water retention for plant use
- poor nutrient supply
- poor ability to hold applied nutrients
Clay group advantages
- good nutrient supply & retention
- medium water retention for plant use
Clay group problems
- 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
Loam group advantages
- well drained
- good water retention for plant use
- good nutrient supply
- easy to cultivate over a wide range of water content
Loam group problems
some are structurally weak and prone to erosion
soil structure determines
- size
- shape
- distribution of pores both within and between the aggregates
soil structure develop through
- natural processes during soil formation, often by management
soil strucure influences
- water movement
- aeration
- heat supply
soils structure types
- plate like (platey)
- prism like (prismatic & columnar)
- block like (blocky & subangular blocky)
- spheroidal (granular & crumb)
pores are need for …
- water storage
- drainage
- oxygen entry
- carbon dioxide removal
- root penetration
- mechanical strength
Processes involved in the development of soil structure
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
microbial activity during soil structure development
formation of humus during the break down of plant and animal residues
Micro-aggregate
- <250um
- flocculation of clay particles to form domains
- interactions between organic matter and clay domains
Macro-aggregate
- > 250um
- coalescence of micro-aggragates
- forces exploiting planes of weakness
transmission pores (macro)
> 50um
- drainage
- root penetration
- gas movement
storage pores (micro)
- 50-0.5um
- water available for plant growth
residual pores (micro)
- <0.5um
- water not available to plants
- controls mechanical strength of soils
Porosity =
1 - (bulk density/particle density)
Bulk density
mass of solid soil particles in a unit volume of field soil
- soils + pore space occupied by air and water
Particle density
mass of soil soil particles in a unit volume of those solid particles
unstable soil structure
- 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
consequences of unstable soil structure
- surface capping
- slaked and puddled structure
- under-consolidation
- compaction and erosion
