Soils 205- General Soils

Lecture 9- Description of soil water

I.  Soil water can be described in terms of its energy status or percentage within the soil.

    a.  Soil water content is expressed either on a weight or volume basis.

    Volumetric water content = qv

            qv =  [volume of water (ml)/total volume of soil (cm³)]*100%

   Gravimetric water content = qg 

           qg = [grams of water/grams of oven dried soil] *100%

To convert between gravimetric and volumetric water content, we either multiply or divide by the bulk density of soil.

II.  Soil water energy- recall that the difference in energy states between two contiguous sites cause the development of a potential.  There are three main potentials that we will deal with in this course.  For your own knowledge, I have included formal definitions of these potentials below.

y_m - the amount of work that a unit quantity of water in an equilibrium soil-water system, in all respects identical to water in the reference state except that it is in the presence of a matrix, is capable of doing relative to water in the reference state.

y_g - The amount of work that a unit quantity of water in an equilibrium soil-water system at some arbitrary level is capable of doing when it moves to another equilibrium system identical in all respects except that it is at a reference level.

y_o - The amount of work that a unit quantity of water in an equilibrium soil-water system is capable of doing when it moves to another equilibrium system identical in all respects except that there are no solutes.

III.  Relation between soil water content and potential is expressed through the use of  "soil moisture characteristic curves" shown below.

There are several points on this curve that are commonly used to characterize the soil water energy.  They are:

    a.  Saturation

        Saturation is a state of high free energy where qv = % pore space.  The potential = 0.

    b.  Field capacity

        Field capacity is reached when the downward drainage of water caused by gravity ceases.  This typically occurs 2-3 days after saturation.  The soil water potential at this time is approximately -10 to -30 kPa.  In the laboratory we recreate this condition using a tension of -33 kPa (-1/3 bar).

    Water held against gravity will then be used by plants, evaporate or move to areas of lower potential.  Plants will continue to consume the water until the attraction for the water by the soil forces is greater than the attraction exerted by the plants.  This water content is called the permanent wilting point.

    c.  Permanent wilting point (PWP)

        At this point, most crop plants can no longer pull the water out of a drying soil.  The soil water potential at this moisture content is about -1500 kPa (-15 bars).

    d.  The last point we will consider is called the Hydroscopic coefficient.  This refers to an air dry soil where the pores are empty and the only water left is adsorbed tightly to the soil particles.  Soil water potential at this point is about -3100kPa (-31 bars).

    e.  Soil texture and water energy

III.  Plant available water (PAW)