Soils Chapter #5:
Water in the Soil-Plant System


Water Storage in Soil

Water Retention & Capacity

The amount of soil water available to plants depends on:

  • precipitation
  • infiltration
  • percolation to groundwater table
  • soil water holding capacity






Soils retain water because water molecules cohere to one another (hydrogen bonding) and adhere to wettable surfaces of soil minerals and organic matter




Soil water storage capacity and water's freedom to move depends on two soil properties:

  • the colloidal content
  • the pore properties



Soil water storage and plant available water is maximized if:

  • the soil is deep
  • the roots exploit a large volume of soil
  • a large fraction of the water is held loosely enough to move freely to roots




Water Content & Water Potential

Volumetric water content (q, theta)

Mass water content

Soil water content equivalent to Mass water content


Example Soil Moisture Calculation

Calculation of soil moisture content by mass and volume

A soil sample weighed 230 g in a moisture box. The mass of the moisture box was 78 g.

After drying at 105 degrees C to a constant mass, the soil and box weighed 204 g. The soil sample filled a 1000 cc container as it was taken from the field.

Find the moisture percentage in the soil by mass and by volume.

1. Water (%) by mass:
= Wet mass of soil = (wet mass of soil + box)-(mass of box) = 230 - 78 = 152 g

Dry mass of soil = (dry mass of soil + box)-(mass of box) = 204 - 78 = 126 g

Water (%) by mass = (wet mass - dry mass / dry mass) x 100 = (26 / 126) x 100 = 21 %

2. Water (%) by volume:
= (vol. of water / bulk vol. of soil) x 100 Vol. of water = mass of water / density of water = 26 g / 1 g per cc = 26 cc

Hence: Water (%) by volume =( 26 cc / 1000 cc) x 100 = 2.6 %

So, the answers are:
1. Water by mass = 21 %
2. Water by volume = 2.6 %



Water potential (Y psi)

Water potential






Zero is high water potential. It means the water is loosely held.

Water always moves to a more negative water potential.




**** Water Potential Video **** Osmosis ****



Water retention curve - relationship between the water content (q theta) and soil water potential (Y psi)

The matric potential is used to account for the reduction in free energy of water when it exists as a thin surface layer, one or two molecules thick, adsorbed onto the surface of relatively dry soil particles, cell walls, and other materials.






Water Movement


Water vapor moves fastest from soil to the atmosphere when the relative humidity (RH) is low and winds are fastest


Water flow rates depend on differences in water potential at different locations.


These various hindrances affect the soil hydraulic conductivity; the ease with which water moves through soil


Water flows faster in..........




Preferential Flow


Preferential pathways cause water flow to be non-uniform because:

  • macropore flow
  • finger flow or wetting front instability





Finger flow........





Pollutant Transport in Soils



Principles of Water Movement


Factors Affecting Water Potentials

Water Content





Time Changes












Conductivity in Soil-Plant Systems



Factors that can limit leaf transpiration........





Phloem & Xylem

Phloem: Transportation of food.

Xylem: Water and mineral transport. 




Water Stress in Plants




Plants survive drought with the help of adaptations to conserve water and to get more water from the soil

  • anatomical features for preventing water loss
    • sunken stomates
    • leaf hairs
    • narrow leaf form
    • ability to drop leaves
    • ability to tolerate water loss
  • reflect sunlight
  • turn their leaf edges to the sun
  • root modifications
    • close root spacing reduces the distance water must move
    • deep roots

Evapotranspiration (EVT) - the sum of evaporation and transpiration

Potential Evapotranspiration (PET) - amount of EVT that were to occur if there is no shortage of available water.

Amount of PET depend on:

  • surface and air temperatures
  • wind


Actual Evapotranspiration -