water dornbush (set 1) - uwgb

11/9/2008

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Dr. Dornbush

LS 464

[email protected]

Soil Water

• A. Critical to plant growth, water quality,

groundwater, rivers, etc.

– Too much problems

– Too little problems

• B. Soil is a leaky water reservoir

– Excess water:

• Runoffs

• Deep percolation (leaching,

groundwater recharge)2

C. Questions to consider:

– Why does soil retain some water yet allow

part to drain deeper?

– What forces hold water in soil?

– What mechanisms are important to water

flow in soil?

– What limits water flow?

– How do soil texture & structure affect

water flow and retention?

– How do plants interact with soil water?3

D. Water is unique

– Polar – uneven charge

• Hydrogen bonds

• High surface tension

(attraction to other water

molecule>attract to air)

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How does this relate

to soil?

E. Soil water is described by it’s energy state

(Potential; or psi)

• Always moves from high potential low potential

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1. Matric forces or potentials (or matric

tension) m

• Negative pressure or tension due to Adhesion

and Cohesion (from Hydrogen bonding)

• More surface area more adsorbed water

–Clay vs. sand

• Less water (drier soil) stronger bond to

soil (Q: Why?)

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F. Forces affecting the potential energy of

water: In other words, what affects where

water moves in soil?

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• h = 0.15/r; h = ht water will rise; r =

radius of tube or pores; (units of cm)

• h is inversely proportional to r

• The wider the radius, the less water

directly interacting with glass wall

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How does capillarity relate to wet vs dry soil

question?2. Gravitational forces (gravitational

potential) g

• negative or positive; depends on ref. point

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F. Forces acting on water in soil:

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3. Osmotic (Solute) Potential (ψo or ψs):

• Relative to pure water, this has a negative

energy potential

• Diffusion favors water movement from

lower to higher solute concentration, but

only when separated by a semipermeable

membrane

• Not that important within soil but can be

very important at soil-root interface – Saline

soils

4. Submergence potential (saturated soils) p

• Hydrostatic pressure (positive)

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G. Soil Water Potential (ψt):

• Just simple addition of the above factors

• ψt is stated relative to a reference

• Reference – the energy state (ability to do work) of pure, free water at a specified elevation (the soil surface); (ψt = 0).

• ψt = ψm + ψg + ψo + ψp

• ψt - amount of energy required to move a quantity of pure water from point A to point B; assumes both points have equal temperatures, pressures, and osmotic potentials.

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• The units for ψt:

• MPa (SI pressure) or J m-3 (SI energy)

– Mpa = 1000 kPa

• Also see bars (non-SI pressure)

• 1 MPa = 106 J m-3 = 10 bars = 10 atmospheres

Water always moves from areas of higher to lower water potential

• So in practice, you can predict where water will move by comparing the soil water potential at two different locations


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• Bench marks:

-0.03 MPa = field capacity (-33 kPa)

-1.5 MPa = permanent wilting point (-1500 kPa)

• Field capacity – tension at which remaining

water cannot be removed by force of gravity

• Permanent wilting point – tension at which

remaining water is held too tightly to be removed

by plants


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Plant Available Water:

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I. How do we measure soil moisture?

1. Soil Moisture Content: (Water Amount)

a. Gravimetric Moisture (θw):

– (g H2O/ g 105oC soil)*100

– Soil Dry Mass - drying to constant mass at

105oC (24 to 48 hrs)

– Easiest to measure, very common, but

destructive

θw = 100*(Soil Wet wt. – 105oC wt.)/ (105oC wt)

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1. Soil Moisture Content con’t.

b. Volumetric Content (θv):

• (cm3 H2O/ cm3 soil)

– (recall 1 g H2O = 1 cm3 H2O)

• Very common method, preferred over θw

because it reports H2O quantity/soil volume

• Can be measured directly or using Time

Domain Reflectometry (TDR)

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TDR Basics

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TDR Basics

• Basically the machine sends a high-frequency

electromagnetic pulse down the probe

• The speed at which the wave propagates

along the probe depends on the soil moisture

• This rate is calibrated with θw to determine

θv

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Pro’s:

• Easy to measure

• Allows for site water budgets (quantity of water per unit soil volume )

Con’s:

• They don’t tell us squat about how available the water actually is for plants or drainage!

• Cross site comparisons are difficult because soil composition differs (recall the plant available water figure)

1. Soil Moisture Content

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Plant Available Water:

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Soil Moisture Content (with Soil Properties):

c. Water Filled Pore Space (WFPS):

Also called percent saturation

• (cm3 H2O/ cm3 soil pore space)*100

• Common and easy to measure

• Use θv with soil bulk density (g/cm3 soil

volume) and particle density (g/cm3 particles)

– Particle density is ~ constant at 2.65 g/cm3

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c. Water Filled Pore Space con’t.

• Bulk density – mass of dry soil per unit bulk

soil volume (volume includes both soil

particles and pores)

• Particle density – mass of soil particle

divided by volume of soil particles

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• Soil Pore Space (%) =

100 * [1 – (Bulk Density/ Particle Density)]

~ 50% for good loamy soils

• Sample WFPS (%) =

100 * [(cm3 H2O/ cm3 soil) / (cm3 pore/ cm3 soil)]

OR simplified

100 * [(θv) / Soil Pore Space]

c. Water Filled Pore Space con’t.

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What additional information is provided by

WFPS?

• Provides more biologically relevant

information about soil environment

– gas and nutrient diffusion or microbial

movement

• For example, 60% WFPS is ideal for microbial

activity (Linn and Doran 1984)

• Aids in cross-soil comparisons, i.e. 60% WFPS

is the 60% WFPS

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What additional information is provided by

WFPS?

• Problems:

– Still not a good measure of water

availability (see slide 20)

– NEED A MEASURE OF WATER

ENERGY STATUS!

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Soil H2O Potential: (Energy State)

• To get soil water potential (ψt), just like our

equation (i.e. in pressure or energy units)

you need to apply a tension (suction) to a

volume of soil.

• Field measurement is often done with a

tensiometer

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Why is a tension measure the best?

• Measure of work needed:

• Convertible into units of photosynthesis (C)

• Convertible into solar energy, oil, etc

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But wasn’t WFPS just as good? Isn’t 1 cm3

H2O per 1 cm3 pore space equal for all soils?

• Not for water availability - soil tension

varies

1. Differences in surface area (SA)

2. Differences in pore width

Soil Particles Diameter (mm)

Clay < 0.002

Silt 0.002 to 0.05

Sand 0.05 to 2.0

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Q1: Does a bucket of clay or a bucket of

marbles have a greater SA?

Q2: How does pore distribution differ?

Lambers et al. 1998

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Q3: Does a sandy or clay rich soil have a more negative ψt

potential at the same water content?

Q4: Which would retain more of its water two days

following a saturating rain?

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Figure 5.24 Water content–matric potential curve of a loam soil as related to different terms used to describe

water in soils. The wavy lines in the diagram to the right suggest that measurements such as field capacity are

only approximations. The gradual change in potential with soil moisture change discourages the concept of

different “forms” of water in soils. At the same time, such terms as gravitational and available assist in the

qualitative description of moisture utilization in soils.

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Conclusion

• Water quantity is useful for budgets, and some

biology, but energy is the unit of the universe.

• With the quantity of water set equal, ψt is

primarily a function of soil particle size (and

soil organic matter content)

• The same basic factors also apply to water

movement through the soil

water dornbush (set 1) - uwgb

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