Soil Water Balance Flashcards
Hard setting soil definition
Have horizons that, when dried harden significantly, constituting a mass without structure
Where are HS soils found
Low concentration of OM <2%
Clay minerals with low shrink-swell property (kaolinites) low CEC
Sols with dense packing particle size: sandy loam to clay loam (10-35%clay)
Sodic Soils - High ESP
They have a structure that is not water stable
Effects of HS
Irrigation or rainfall after sowing= surface collapse and drying causes surface to harden and prevent seedling emergence
Restrict root of established crop
Leads to associate soil degradation process (erosion, compaction, crusting
Restrictions on timing- when trafficable but not too hard
Compacted soil vs Hard setting
Increase bulk density or reduction in porosity occurs due to application of an external force
HS - increase DB or reduction in porosity due to slumping/collapse of aggregates created by cultivation without the application of external pressure
Process of Hard setting
Aggregates must be distrubted to provide fine material that matric potential can draw into bridges at points of contact
Aggregate distrubtion may occur by fast wetting , and spontaneous dispersion
Energy released causes slacking, leading to suspension of silt and clay sized particles
These are drawn by matric potential to form bridges between larger particles and draw ever closer as soil dries
Development and increase in strength of hard setting soils is due to:
CLay fraction of hardest soils are dispersive in nature - kaolinites
Initial wetting = slacking and dispersion of slay and silt
Dispersed material moves into retracting water film and lodges in the cavities of sand and aggregates assisting in bridge formation and cementatio of the ramaining more stable material
HS BD
1.54-1.75g/cm as wetness decreases from 25-20%
Characteristics and effects of
hardsetting soils
Difficault to cultivate until rewet
Reduces trafficability when wet
“sunday” “lunchtime” soils are small window to cultivate
By the time its dry enough to support traffic, results in cloddy tilth of high strength
Penetronmeter resistance
HS vs non HS
PR increases as soil dries
N HS = 3MPa resistance at PWP (1500kPa)
HS = 3Mpa at 100kpa
Direct (biological effect of HS)
Seedling emergence
Root penetration
Timing of sowing
Indirect (physical) effect of HS
Aeration
Rainfall infiltration
Increased run off
Management of HS
Salinity removed by leaching, Sodicity not easily removed
Objective of Sodic soil management
Improved water storage and transport in the root zone
Improved water use efficiency by improving structure
Methods to fix HS soils
Inorganic Chemical Treatment
OM Management
Reduce Compaction
Subsoil Cultivation
Pasture Rotation
Conservative Tillage
Modifying soil texture
Manage the Whole rotation
Inorganic Chemical Treatment
Ameliorate gypsom (CaSO4) combined with an improvement in soil OM
Replace exchangeable Na with Ca
Calcium sulphate a salt but isnt toxic to plants like sodium chloride
How does gypsom reduce swelling and dispersion
Slightly increases salinity of soil and hence reduces swelling
Ca swaps with Na raising cation concentration of soil, reducing sodicity.
Increase floccculation and infiltration
Murray and Murrumbidgee
Topsoil is non-sodic but sub soil is
Gypsom will have little effect on topsoil but will increase structure, aeration and permeability of the subsoil
Increase water storage and reduce water logging
Management of OM
Crop residues improve physical characteristics
OM does: increase cohesion reduce swelling and dispersion increase coagulation of clay particles Stabilise aggregates
Reduce compaction
increase tyre width or decrease tire pressure
Sub soil cultivation
DR greater than 20cm may help
Loosen soil giving plants access to deeper soil
Sodic soils should recieve gypsum before tillage
Makes it more friable adn less plastic
Wet soil DR = reduced yield due to smearing and further compaction
Dry = produce cracks
Rotation - pasture
Perennial pasture improves soil structure
Winter grazing decreases porosity and infiltration and increase density
Conservation Tillage
Non friable soils are not suitable for Direct drill
Intensively cultivated soil has lower friabilty vs direct drill
Improve soil structure and friability before attempting direct drill in HS using a pasture hase or RT stubble retention
Modifying soil Texture
Sand addition to HS top soil can decrease hardsetting
Often wehn deep top soil - DR doesnt bring up clay
WA
Manage whole rotation
When to till, when to crop, graze
HS soils are vulnerable to damage by cultivation outside a narro range of weather contents
OM braoden this
Challenges emerging due to NT
Herbicide resistance (summer grasses)
Build up of stubble borne disease
Stratification of nutrients and carbon (immobile nuts K and P)
Soil surface enrichment but need to be deeper down
Soil physical constraints (surface compaciton) -crusting can reduce infiltation
Application of lime or gypsum
Others- Enviro- chemicals ground water pollution Surface water pollution -P runoff Increase fert use
Effects of ST
Yeild better than NT
More costly than NT
Weed control
Soil physical quality
- Soil water storage
- Bulk density
- Aggregate stability
Soil Chemical properties
-vertical distribution of C,N,P
Factors to be considered in ST decision
Timing of tillage
- Close to sowing time = water loss
- After harvest = loss of soil cover
Optimum soil water content
-Best aggregate distribution
Machinery efficiency
Energy cost
