Soil Water

Section 1 The types of soil water and measuring soil moisture content

The importance of soil water ：

•It is the major constituent of plant protoplasm.

• It is essential for photosynthesis and conversion of starches to sugar .

•It is the solvent in which nutrients move into and through plant parts.

What are the main components of soil?

• Mineral Matter

• Air

• Water

• Organic Matter

Classification of soil fractionsClassification System

USDA International (ISSS)

DIN, BSI, MIT

Sand 2.0-0.05 mm 2.0-0.02 mm 2.0-0.063 mm

Silt 0.05-0.002 mm

0.02-0.002 mm

0.063-0.002 mm

Clay < 0.002 mm < 0.002 mm < 0.002 mm

USDA U.S. Department of Agriculture

ISSS International Soil Science Society

DIN German Standards

BSI British Standards Institute

MIT Massachusetts Institute of Technology

Soil Components (volume basis)

Spaces for Gas and Water• Partical size

effects spaces for gases and water.

• Water movement is dependent on the spaces

• Adhesion and cohesive forces

• This is called capillary water

一、 The types and available of soil moisture 1 、 Classification of soil water

• Adsorbed water• Capillary water• Gravitational water

（ 1 ） Soil adsorbed water ：

•held by strong electrical forces - low energy

•little movement- held tight by soil

•exists as a film

•unavailable to plants

•removed from soil by drying in an oven

Soil Water Adhesion Water- water attracted to solid

surfaces

• held by strong electrical forces - low energy

• little movement- held tight by soil

• exists as a film• unavailable to plants• removed from soil by

drying in an oven

Sketch map of adsorbed water

（ 3 ） Soil capillary water ：

Capillary water-The water held in the “capillary” or small pores of a soil,

Capillarity ：

0.1-1mm

Capillarity obvious

0.05-0.1mm

Capillarity strong

0.05-0.005mm

Capillarity very strong

〈 0.001mm

Capillarity disappears

Water is drawn up into the capillary tube

Soil particle

Capillary water sketch map

（ 4 ） Gravitational water

Gravitational water -Water which moves into, through, or out of the soil under the influence of gravity.

Water held in large pores

Available for crop use

CapillaryWater

GravitationalWater

FieldCapacity1/3 bar

Water adheres to soil particles

Water drains through soil profile

HydroscopicWater

WiltingPoint

15 bars

What is Field Capacity?

• when the soil contains the maximum amount of available water, the greatest amount of water it can hold against gravity

Section 4 、 Control of soil water

Available soil water-The amount of water released between in situ field capacity and the permanent wilting point (usually estimated by water content at soil matric potential of -1.5 MPa).

一、 Availability of soil water

Estimating water contents

• Gravimetric method: The soil sample is dried in an oven at 105°C and the mass of dry soil recorded.

• Water potential

•Neutron scattering method

Calculating Soil Moisture

• Gravimetric

Pw = (weight of wet soil – weight of oven dry soil) X 100weight of wet soil

Soil Water PotentialSoil Water Potential• DescriptionDescription

– Measure of the energy status of the soil waterMeasure of the energy status of the soil water– Important because it reflects how hard plants must work Important because it reflects how hard plants must work

to extract waterto extract water– Units of measure are normally bars or atmospheresUnits of measure are normally bars or atmospheres– Soil water potentials are negative pressures (tension or Soil water potentials are negative pressures (tension or

suction)suction)– Water flows from a higher (less negative) potential to a Water flows from a higher (less negative) potential to a

lower (more negative) potentiallower (more negative) potential

Components of Water Potential

• Pressure potential: pushing (positive pressure, like the hose) or sucking (negative pressure, like a straw) Major factor moving water through plants

• Osmotic, or Solute potential: reduction in water potential due to the presence of dissolved solutes – salty water has lower water potential (lower

concentration) than pure water • Matric potential: reduction in water potential due to the

presence of matric forces (tendency for water to adhere to surfaces) – pressure potential and Matric potential

dominates soil water

Soil water potential

• Total soil water potential = Matric potential + gravitational potential + Osmotic (salts)

• As the soil dries the water potential decreases or a larger negative number

• 00 -5 -8 -10 -15 -55 -100

• sat. wet - -------- dry------- > very dry

Tensiometer for Measuring Soil Water Tensiometer for Measuring Soil Water PotentialPotential

Porous Ceramic Tip

Vacuum GaugeVacuum Gauge

Water ReservoirWater Reservoir

Variable Tube Length (12 in- 48 in) Based on Root Zone Depth

Units of soil water potential:

bars MPa

-0.01 -0.001

-0.1 -0.01

-0.33 -0.033

-1 -0.1

-10 -1

-15 -1.5~PWP

~FC

Soil Water Classification- a way to quantitatively describe the water in

the soil.

• -0.3 = Field Capacity • -15 bar = wilt point• Between -0.3 & -15 is plant

available water (AWC)

0 bar -0.33 -15

Saturated Field Cap Wilt point

AWC

Water Moves through soil by bulk flow

• The rate of water flow depends on:– Size of the pressure gradient– Soil hydraulic conductivity (SHC)

• Measure of the ease in which water moves through soil

• SHC varies with water content and type of soil– Sandy soil high SHC

• Large spaces between particles

– Clay soil low SHC• Very small spaces between particles

Water moves from areas of high potential (wet soil : -2 or -4) to areas of lowpotential (dry soil -8)

SoilSoilRoot

-7

-2

-3-.4

-8

•Water secretion

Water equilibration method

Pressure Chamber

Root and root hairs

Root hair

Cortex cellsfilled withamyloplasts

Endodermiscell

Pericycle cell

Xylem vesselelements

Intercellularspace

(b) A close-up of the stele of the buttercup root.Note the solid core of vascular tissues.

Phloem cell

Fig. 6-4b, p. 116

Casparian strip

Root hairs increase surface area and make intimate contact with components of the soil.

Root Absorption of Water and Solutes

Symplast and Apoplast

Root Hairs –• increase absorption• develop in region of

maturation• extensions of

epidermal cells