Mid Term 2 Flashcards
Soil water functions
Essential for photosynthesis
Turgor pressure
Transports nutrients
Helps plant for temperature variations
What is hydrogen bonding
Weak interactions between the partial positive charge of hydrogen and with the partial negative charge of oxygen
What is cohesion
Sticking of water molecules to each other from hydrogen bonding. Responsible for surface tension
What is adhesion
Sticking of water molecules to other molecules
Enables water to “climb” up soil pores - capillary action
Forms of water in soil and what they are
Hygroscopic Water - films around clay particles, only form of water found in air dried condition
Capillary water - held on capillary pores (<1mm thick), main form of available water
Free water - drainage or free draining water found in the bigger pores and moves downward by gravity
Definition of field capacity
All macro pores have drained, only micro pores are filled. Plants can access this water
Definition of permanent wilting point
Any water still in soil is not available, plant wilts and dies
Infiltration definition
A measure of the rate at which rain fall or irrigation is absorbed into the soil (in/hr, mm/hr)
What are water movements through soil (4)
Gravitational flow
Matric flow
Capillary rise
Osmotic potential
What is gravitational flow
Movement of water through saturated soil
What Is matric flow
Movement of water through unsaturated soil ( wet to dry)
What is capillary rise
Water is pulled upward from water table in micropores by cohesive/adhesive forces
What is Osmotic potential
Determined by concentration of solutes in soil water (water flows from zones of low solutes to zones of high solutes) important for root-water interaction
Why is water movement important
Soil with rapid internal drainage have higher risk of leaching
Helps determine best drainage plan
Understanding how salinity works
Chemical properties of soil (5)
Anion-Cation exchange capacity Reaction (pH) Calcium carbonate content Salinity and solicits Soil organic matter
Cation exchange capacity
The total number of exchangeable cations a soil can hold
Anion exchange capacity
Relative amount of negatively charged anions which can be held by a given weight of soil
What is soil pH
Soil pH is a measure of acidity or alkalinity of soil
Seldom go beyond 4-9
Soils become acidic as rainfall leaches away basic cations such as (Ca, Mg, K, Na) while Al and H stay behind making the soil acidic
More rain = more acidic
Other factors that affect soil pH
Type of vegetation Type of parent material Amount +type of OM Amendments Salinity and sodicity
Why are saline and sodic soils alkaline
Not a lot of water movement through soil that causes leaching so basic cations stay in soil making it basic
Why do we use lime
Reduces Al and other metal toxicities Improves physical condition of soil Stimulates microbial activity Increases CEC in variable charge soils Increases availability of several nutrients Supplies Ca and Mg to plants Improves symbiotic N fixation in legumes Improve crop yields
How does salinity happen
Occurs when groundwater containing salt is able to reach the root zone or soil surface where it evaporates and leaves an accumulation of salt
Conditions required to create a soil salinity problem
- Soluble salts in subsoil, ground water or both
- High water tables to carry soluble salts into root zone by capillary action (<6 ft from surface)
- Plants susceptible to soil salinity (pulses, vegetables, oilseeds)
Salinization is a natural process controlled by
Hydrology
Geology
Biological processes
What is regional or artesian discharge salinity
Occurs when water within an underlying aquifer is under sufficient pressure to reach land surface. Then water evaporates at surface and salts are left behind
What is side hill seep salinity
Occurs where permeable glacial drift overlies impermeable materials such as clay or bedrock
Lateral movements eventually intersects a side hill surface
Groundwater discharges at this point and evaporation of water leave behind salt at surface
What is bathtub or evaporitic ring salinity
Occurs adjacent to wet depressions or sloughs
Lateral upward capillary flow from the water table concentrates salts adjacent to the slough, leaving salt in a concentric ring surrounding the slough
How do we test for salinity
Electrical conductivity-a measure of soluble salts within the soil
2 methods used to measure EC
1. Saturated paste method
- Soil:Water ratio method
Managing primary salinity
Strategic cropping choices to manage water
Reduce tillage
Increase soil OM
Managing secondary salinity
Strategic cropping choices to manage water
Drainage strategies to manage water and leach salts out of soil profile
Sodic soils
Characterized by: elevated sodium concentration relative to calcium and magnesium
pH usually above 8.5
May be able to manage with gypsum addition
Components of soil
45% mineral
25% water
25% air
5% OM
Components of OM
33-50% decomposing OM
33-50% stabilized OM
<10% fresh residue
<5% living organisms
What is OM
Generally considered to be plant residue in various stages of decomposition in the soil and…
Microflora-bacteria, fungi
Microfauna-Protozoa and nematodes
Macrofauna-earthworms and millipedes
What is decomposed residue called in OM
Humus
Responsible for dark colour in Ah horizon
How is humus determined
By measuring the amount of organic carbon in the soil
What happens when the C:N ratio is high
Immobilization of soil nitrogen
What happens when the C:N ratio is low
New mineralization of nitrogen
What is C:N ratio
Carbon to nitrogen C:N of plant residue depends on crop type 100:1 wide ratio 15:1 narrow ratio >25:1 net mobilization <25:1 net mineralization
C:N ratio numbers Cereal crop Legume residue Soil humus Microbes
Cereal 80:1
Legume 25:1
Soil humus 10:1
Microbes 6:1
Starting soil conditions
Modest levels of nitrate
Low c/n ratio
Low microbial activity
What happens when you return heavy amounts of cereal crop residue
Can cause a temporary shortage of mineral nitrogen for crops (why we burn residue in fields)
Removal of crop residue why
Common on heavy clay soils where cool wet conditions slow decomposition
Removal means loss of vital nutrients such as nitrogen and phosphorus will be lost
Factors affecting soil OM levels (7)
- Natural vegetation
- Temperature
- Soil moisture
- Soil texture
- Aeration
- Cropping and cultivation
- Landscape position
