By Jim Bilskie, Ph.D.Campbell Scientific, Inc.

he state of water in soil is describedin terms of the amount of water and

the energy associated with the forceswhich hold the water in the soil. Theamount of water is defined by water con-tent and the energy state of the water isthe water potential. Plant growth, soiltemperature, chemical transport, andground water recharge are all dependenton the state of water in the soil. Whilethere is a unique relationship betweenwater content and water potential for aparticular soil, these physical propertiesdescribe the state of the water in soil indistinctly different manners. It is impor-tant to understand the distinction whenchoosing a soil water measuringinstrument.

Water contentSoil water content is expressed on a

gravimetric or volumetric basis.Gravimetric water content (θg) is themass of water per mass of dry soil. It ismeasured by weighing a soil sample(mwet), drying the sample to remove thewater, then weighing the dried soil(mdry).

Volumetric water content (θv) is thevolume of liquid water per volume of soil.Volume is the ratio of mass to density (ρ)which gives:

Soil bulk density (ρbulk) is used forρsoil and is the ratio of soil dry mass tosample volume. The density of water isclose to 1 and often ignored. Anotheruseful property, soil porosity (ε), is relatedto soil bulk density as shown by the fol-lowing expression.

The term ρsolid is the density of thesoil solid fraction and is approximated bythe value 2.6 g cm-3.

A simple data set is given at the top ofthe following column as an example. Asample of known volume was weighedbefore and after oven drying at 105°C for24 hours.

The porosity of 0.50 defines the maxi-mum possible volumetric water content.The measured θv value of 0.267 indicatesthe pore space is just over half-full ofwater. If the sample is from a 30-cmdepth profile, there are 8 cm of water inthe profile.

Water content measurement methodsinclude gravimetric, as described above,neutron probe, time-domain reflectometry(TDR) and other dielectric permittivitysensitive devices such as the CS615.

Water content indicates how muchwater is present in the soil. It can be usedto estimate the amount of stored water ina profile or how much irrigation isrequired to reach a desired amount ofwater.

Water potentialWater flux—the movement of water—

occurs within the soil profile, between thesoil and plant roots, and between the soiland the atmosphere. As in all natural sys-tems, movement of a material is depend-ent on energy gradients. Soil waterpotential is an expression of the energystate of water in soil and must be knownor estimated to describe water flux.

Water molecules in a soil matrix aresubject to numerous forces. If no adhe-sive forces were present, the water mole-cules would move through the soil at thesame velocity as in free air minus delaysfrom collisions with the solid matter—much like sand through a sieve. Soilwater potential accounts for adhesive andcohesive forces and describes the energystatus of soil water.

The fundamental forces acting on soilwater are gravitational, matric, and osmot-ic. Water molecules have energy byvirtue of position in the gravitational forcefield just as all matter has potential ener-gy. This energy component is describedby the gravitational potential componentof the total water potential. The influenceof gravitational potential is easily seenwhen attractive forces between water andsoil are less than the gravitational forcesacting on the water molecule and watermoves downward.

The matrix arrangement of soil solidparticles results in capillary and electro-static forces and determines the soil watermatric potential. The magnitude of theforces depends on texture and the physi-cal-chemical properties of the soil solidmatter. Most methods for measuring soilwater potential are sensitive only to thematric potential.

Soil water is a solution. The polarnature of the water molecule results ininteraction with other electrostatic polespresent in the solution as free ions. Thiscomponent of the energy status is theosmotic potential.

Methods for measuring soil watermatric potential include tensiometry, ther-mocouple psychrometry, electrical con-duction (generically Buoyucous blocks),and heat dissipation such as the CampbellScientific sensor model 229.

There is a unique relationship betweenwater content and water potential for eachsoil. The soil water characteristic curvesfor three soils are depicted below. For agiven water potential, the finer the soiltexture the more water held in the soil.Coarse texture soils like sand consistsmostly of large pores which empty ofwater when a relatively small force isapplied. Fine texture soils have a broaderpore size distribution and larger particlesurface area. Consequently, a largerchange in water potential is required toremove the same amount of water.Greater surface area means more water isadsorbed via electrostatic forces.

θgwater

soil

wet dry

dry

m

m

m m

m= =

−

θρ

ρ

θ ρρv

water

soil

water

water

soil

soil

g soil

water

volume

volume

m

m= = =

∗

ερρ

= −1 bulk

solid

θg

water

soil

m

m

g g

gg g= =

−= −94 78

780 205 1.

Soil water status: content and potential

Tmwet 94 g

mdry 78 g

sample volume 60 cm3

ρbulk

drym

volume

g

cmg cm= = = −78

60133

3.

θθ ρ

ρvg soil

water

cm cm=∗

= −0 267 3 3.

ερρ

= − = − =−

−1 113

2 6050

3

3bulk

solid

g cm

g cm

.

..

Soil Water Characteristic Curves

mat

ric p

oten

tial

volumetric water content

sand

loam clay

Copyright © 2001 Campbell Scientific, Inc. 1815 W. 1800 N., Logan, UT 84321-1784 (435) 753-2342 App. Note: 2S-I