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 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

Assignment