Soil Water Balance

Question 1

Hard setting soil definition

Answer 1

Have horizons that, when dried harden significantly, constituting a mass without structure

Question 2

Where are HS soils found

Answer 2

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

Question 3

Effects of HS

Answer 3

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

Question 4

Compacted soil vs Hard setting

Answer 4

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

Question 5

Process of Hard setting

Answer 5

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

Question 6

Development and increase in strength of hard setting soils is due to:

Answer 6

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

Question 7

HS BD

Answer 7

1.54-1.75g/cm as wetness decreases from 25-20%

Question 8

Characteristics and effects of

hardsetting soils

Answer 8

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

Question 9

Penetronmeter resistance

HS vs non HS

Answer 9

PR increases as soil dries

N HS = 3MPa resistance at PWP (1500kPa)

HS = 3Mpa at 100kpa

Question 10

Direct (biological effect of HS)

Answer 10

Seedling emergence

Root penetration

Timing of sowing

Question 11

Indirect (physical) effect of HS

Answer 11

Aeration

Rainfall infiltration

Increased run off

Question 12

Management of HS

Answer 12

Salinity removed by leaching, Sodicity not easily removed

Question 13

Objective of Sodic soil management

Answer 13

Improved water storage and transport in the root zone

Improved water use efficiency by improving structure

Question 14

Methods to fix HS soils

Answer 14

Inorganic Chemical Treatment

OM Management

Reduce Compaction

Subsoil Cultivation

Pasture Rotation

Conservative Tillage

Modifying soil texture

Manage the Whole rotation

Question 15

Inorganic Chemical Treatment

Answer 15

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

Question 16

How does gypsom reduce swelling and dispersion

Answer 16

Slightly increases salinity of soil and hence reduces swelling

Ca swaps with Na raising cation concentration of soil, reducing sodicity.
Increase floccculation and infiltration

Question 17

Murray and Murrumbidgee

Answer 17

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

Question 18

Management of OM

Answer 18

Crop residues improve physical characteristics

OM does:
increase cohesion
reduce swelling and dispersion
increase coagulation of clay particles
Stabilise aggregates

Question 19

Reduce compaction

Answer 19

increase tyre width or decrease tire pressure

Question 20

Sub soil cultivation

Answer 20

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

Question 21

Rotation - pasture

Answer 21

Perennial pasture improves soil structure

Winter grazing decreases porosity and infiltration and increase density

Question 22

Conservation Tillage

Answer 22

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

Question 23

Modifying soil Texture

Answer 23

Sand addition to HS top soil can decrease hardsetting

Often wehn deep top soil - DR doesnt bring up clay

WA

Question 24

Manage whole rotation

Answer 24

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

Question 25

Challenges emerging due to NT

Answer 25

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

Question 26

Effects of ST

Answer 26

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

Question 27

Factors to be considered in ST decision

Answer 27

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