unit 10: soil water properties chapter 3. objectives properties of soil/water that help w/ water...
TRANSCRIPT
Objectives Properties of soil/water that help w/ water
retention Measurement of soil water Amounts of water held, why is/not held Characteristics of soil water flow Effects of saturated, unsaturated soils Environmental affects Improving water-use efficiency
Introduction Most common limit of plant growth Irrigation has made more land productive Many roles for water in the soil
Water Chemistry Peculiar properties of water
Molecule so small, it should be a gas Highest vaporization temp Solid phase less dense than liquid High surface tension Greatest solute, solvent
Water held in soil due to H bonds Bonding of water to solid particles = adhesion Bonding of water to water = cohesion
Water Chemistry Strong adhesion/cohesion forces cause water
films in soils to be held on soil particles More surface area of a soil > water held
Soil Water Content Measuring Water Content
Gravimetric method – measure mass water content
Sample – weigh – dry sample – weight again Time depends on equipment Measures mass water content
Can also measure soil water w/ volumetric water content
Soil Water Content Gains & Losses of Water
Measuring soil water volume can help in determining:
Amount of irrigation water needed Amount of water evaporated Depth that rainfall/irrigation water will wet soil
Soil Water Potential & AvailabilityFree energy – energy to do work
Soil water has less potential to do work than water molecules in a pool of water
Can’t transport as many materials
Soil Water Potential – work the water can do as it moves from its present state to the reference state, which is the energy state of a pool of pure water at an elevation defined to be zero
Soil Water Potential & Availability Water Potential Gradient & Water Flow
Soil water moves in response to water potential gradient
Water flows from areas of higher water potential (wetter areas) to areas of lower water potential (dryer areas) = unsaturated flow
Explains water’s ability to move upward w/ capillary action from a water table
Soil Water Potential & Availability Water movement after rainfall or irrigation moves
into & through a saturated soil by gravity Overrides ability of water to adsorb to soil Called saturated flow
Soil Water Classification for Water Management Gravitational water – water that drains freely
through the soil by force of gravity
Soil Water Potential & Availability Field Capacity – measure of the greatest amount
of water a soil can store under conditions of complete wetting followed by free drainage
Full saturation minus water lost to drainage Difficult to determine average field capacity in field
situations because water continues to drain & redistribute through soil following rain/irrigation
Soil Water Potential & Availability Permanent wilting point – water held at PWP
held so tight that plants not able to extract it fast enough to meet their needs
Partially explains temporary wilting (rolling) of corn – recovery at night when water transpiration slows
In conditions of true PWP – plant probably won’t recover, unless additional water added
Soil Water Potential & Availability Plants, Wilting Point, & Available Water
Plants vary in their abilities to extract soil water PWP - ~40-50% of field capacity Available water capacity – amount of water that
would be available to plants, if the soil were at field capacity
Difference between FC & PWP
Soil Water Potential & Availability Capillary water & Saturation Percentage
Capillary water – held tightly in small capillary pores by H bonding
Water in minute tubes that will rise through soil matrix to needed areas
Height of capillary rise inversely related to radius of the tube
Smaller pore diameter, greater the movement
Soil Water Potential & Availability Saturation percentage – water content of the soil
when all pores are filled with water ~ Double the amount of water at field capacity
Soils as Water Reservoirs Water held as films on particle surfaces Large soil pores – allow water to drain by
gravity flow (sands, large aggregate soils) Small soil pores – retain water by capillary
action >clay & humus % >water storage ability Water held in clay soils, held very tightly
Hold large amounts of water at FC & PWP
Soils as Water Reservoirs Medium textured soils – unique
combination of have pores that hold large amounts of water, but not so tight that plants can’t get it Largest available water capacity found in silt
loams & other loamy soils
Soil organic matter, compaction, types of clay affect available water capacity
Methods of Determining Water Content or Potential
Porous Blocks Can be used in the field to help w/ soil water
measurement Bury at various depths Electrodes attached Assists w/ irrigation needs
Capacitance Probes Neutron Probes Time Domain Reflectometry
Methods of Determining Water Content or Potential
Tensiometers Thermocouple Psychrometers
All can perform specific soil water measurements
Predict irrigation needs
Water Flow Into & Through Soils Saturated Flow
Water flow caused by gravity Infiltration – water entering soil
Rapid into large, continuous pores Reduced by anything w/ reduction in pore size
Percolation – water moving through the soil Can carry away dissolved nutrients & salts
Leaching – removal of soluble compounds in percolating water
Water Flow Into & Through Soils Rate of water movement controls
% of sand, silt, clay Which will infiltrate faster? Which will percolate slower? Which has highest leaching potential?
Soil structure Organic matter – improves soil structure,
increases #/size of pores Depth of the soil to impervious layers Amount of water in the soil – if soil is already
wet/dry
Water Flow Into & Through Soils Soil temp – warm > cold Compaction – can reduce pore space, decrease
infiltration Permeability – the amount of saturation in the
root zone (top 60”) that will affect the amount of water flowing through the soil profile
Limited by least permeable layer in the soil Major factor in productivity of soil/suitability for
development
Water Flow Into & Through Soils Hydraulic conductivity – commonly used
indicator of permeability Permeability rates:
Impermeable - <.0015”/hr Very slow - .0015 - .06”/hr Slow - .06 - .2”/hr
These soils limited for campsites, playgrounds, tillage of ag fields
Moderately slow - .2 - .6”/hr Soils < moderately slow considered insufficient for
septic tank fields & irrigation
Water Flow Into & Through Soils Moderate - .6 – 2.0”/hr Moderately rapid – 2-6”/hr
Soils > moderately rapid also not favorable for septic tank fields, wastewater irrigation – doesn’t filter well
Rapid – 6-20”/hr Very rapid - >20”/hr
Unsaturated flow Water moves naturally from wetter – drier areas Movement may not be downward
Water Uptake by Plants Water Absorption Mechanisms of Plants
Passive absorption – caused by constant pull of water moving through plants
Plant water lost by transpiration Drier air exerts more atmospheric pull on water,
increases transpiration rates Root extension – expansion/extension of roots
into new areas in the soil ability to absorb new water as it is encountered
Water Uptake by Plants Active absorption – plant expends energy to
absorb water Plant selects specific solubles to absorb
Helps equalize osmotic potential Accounts for very small part of total water
absorbed Absorption through leaf stomata – plants can
take in water from fog, rain, dew
Water Uptake by Plants Depths of Water Extraction
Most plant water extracted from shallow depths Depends on:
Saturation of the soil Soil texture Plants
Trees will go deep Grasses remain shallow
Want to encourage roots to get water from deep soils – more drought tolerant
Water Uptake by Plants When Plants Need Water Most
Visible symptoms of wilt – damage already done Especially during critical growth periods (flowering
to fertilization), rapid size increase Plants can wilt even when soils are sufficiently
wet – if climate is so hot that evapotranspiration rate > absorption rate
Consumptive Use & Water Efficiency
Evapotranspiration (ET) – water lost by evaporation from soil & transpired through plants Occurs in dry, windy, warm conditions, soil
surface moist Can involve a large amount of water
Consumptive Use & Water Efficiency Water Use Efficiency (WUE)
WUE – transpiration + plant growth + evap from soil + drainage loss (to produce a unit dry plant wt)
Ex. – soybeans may use ~.5”/d Want to encourage plant available water to maximize
growth by reducing evap losses, excessive drainage losses
Evap loss – keep soil canopied (soybeans) Drainage loss – proper drainage through fields, waterways,
terracing, etc.
Reducing Water Loss Reducing Evapotranspiration
Mulches Straw, peat, gravel, etc. Barriers to moisture moving out of soil Keep soil temp cooler Long dry periods – doesn’t necessarily decrease
amount of water lost (can actually increase if mulch wicks moisture from ground)
Reducing Water Loss Fallow
Common in dryland farming Leave land unplanted in alternating years to
accumulate extra soil water Amount of water saved is small, but enough to
justify Ex - ~4” water needed to produce wheat from
seed to maturity Each additional 1” available water increase yield 4-7
bu/ac
Reducing Water Loss Reducing Waste & Runoff
Plant selection should carefully match soil’s water characteristics or conserve soil water
Some research into converting brushland to grasslands to help conserve soil water
Grasses root less deeply than brush Grasses go dormant earlier in fall Grasses intercept less precipitation, more water
infiltrates soil
Reducing Water Loss More protection from soil erosion Found to conserve >2” more water/yr
Forests transpire much water Also intercept rain that’s allowed to evaporate
before it can reach soil Still can’t clear-cut all forests
What consequences would there be?
Reducing Water Loss Improved irrigation
Closely manage irrigation systems w/ better water controls
Drip irrigation – most efficient use of water, sprinkler irrigation least
Reuse of Wastewater Municipal treatment plants, industry, irrigation
tailwater Can be high in salts/sediment Much can be available
Reducing Water Loss Conservation terraces
Slow water runoff Catch basins to collect water
Soil organic matter Positive impact on PWP Increased organic matter %, increases ability of
water to store water