Chapter 3

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
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Highest vaporization temp
Solid phase less dense than liquid
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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
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More surface area of a soil > water held

Soil Water Content
 Measuring Water Content
 Gravimetric method – measure mass water content
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Sample – weigh – dry sample – weight again
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Time depends on equipment
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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:
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Amount of irrigation water needed
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Amount of water evaporated
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Depth that rainfall/irrigation water will wet soil

Soil Water Potential & Availability
Free energy – energy to do work
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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
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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
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Overrides ability of water to adsorb to soil
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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
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Full saturation minus water lost to drainage
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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
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Partially explains temporary wilting (rolling) of corn
– recovery at night when water transpiration slows
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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
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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
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Difference between FC & PWP

Soil Water Potential & Availability
 Capillary water & Saturation Percentage
 Capillary water – held tightly in small capillary pores by H bonding
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Water in minute tubes that will rise through soil matrix to needed areas
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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
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~ 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
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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

 Porous Blocks
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Can be used in the field to help w/ soil water measurement
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Bury at various depths
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Electrodes attached
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Assists w/ irrigation needs
 Capacitance Probes
 Neutron Probes
 Time Domain Reflectometry

 Tensiometers
 Thermocouple Psychrometers
 All can perform specific soil water measurements
 Predict irrigation needs

Water Flow Into & Through Soils
 Saturated Flow
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Water flow caused by gravity
Infiltration – water entering soil
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Rapid into large, continuous pores
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Reduced by anything w/ reduction in pore size
Percolation – water moving through the soil
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Can carry away dissolved nutrients & salts
Leaching – removal of soluble compounds in percolating water

Water Flow Into & Through Soils
 Rate of water movement controls
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% of sand, silt, clay
 Which will infiltrate faster?
 Which will percolate slower?
 Which has highest leaching potential?
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Soil structure
Organic matter – improves soil structure, increases #/size of pores
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Depth of the soil to impervious layers
Amount of water in the soil – if soil is already wet/dry

Water Flow Into & Through Soils
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Soil temp – warm > cold
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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
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Limited by least permeable layer in the soil
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Major factor in productivity of soil/suitability for development

Water Flow Into & Through Soils
 Hydraulic conductivity – commonly used indicator of permeability
 Permeability rates:
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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
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Moderate - .6 – 2.0”/hr
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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
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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
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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
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Plant selects specific solubles to absorb
 Helps equalize osmotic potential
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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:
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Saturation of the soil
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Soil texture
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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
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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

 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

 Water Use Efficiency (WUE)
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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
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Evap loss – keep soil canopied (soybeans)
Drainage loss – proper drainage through fields, waterways, terracing, etc.

Reducing Water Loss
 Reducing Evapotranspiration
 Mulches
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Straw, peat, gravel, etc.
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Barriers to moisture moving out of soil
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Keep soil temp cooler
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Long dry periods – doesn’t necessarily decrease amount of water lost (can actually increase if mulch wicks moisture from ground)

Reducing Water Loss
 Fallow
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Common in dryland farming
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Leave land unplanted in alternating years to accumulate extra soil water
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Amount of water saved is small, but enough to justify
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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
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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
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Grasses root less deeply than brush
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Grasses go dormant earlier in fall
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Grasses intercept less precipitation, more water infiltrates soil

Reducing Water Loss
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More protection from soil erosion
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Found to conserve >2” more water/yr
 Forests transpire much water
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Also intercept rain that’s allowed to evaporate before it can reach soil
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Still can’t clear-cut all forests
 What consequences would there be?

Reducing Water Loss
 Improved irrigation
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Closely manage irrigation systems w/ better water controls
Drip irrigation – most efficient use of water, sprinkler irrigation least
 Reuse of Wastewater
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Municipal treatment plants, industry, irrigation tailwater
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Can be high in salts/sediment
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Much can be available

Reducing Water Loss
 Conservation terraces
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Slow water runoff
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Catch basins to collect water
 Soil organic matter
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Positive impact on PWP
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Increased organic matter %, increases ability of water to store water

Assignment