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Unit 10: Soil Water Properties. 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

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  • 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
  • Most common limit of plant growth
  • Irrigation has made more land productive
  • Many roles for water in the soil
water chemistry
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 chemistry1
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
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 content1
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 availability
Soil Water Potential & Availability

Free 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 availability1
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 availability2
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 availability3
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 availability4
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 availability5
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 availability6
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 availability7
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
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 reservoirs1
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
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 potential1
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
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 soils1
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 soils2
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 soils3
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 soils4
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 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 plants1
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 plants2
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 plants3
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
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 efficiency1
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 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 loss1
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 loss2
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 loss3
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 loss4
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 loss5
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