Soils Chapter #5:
Water Storage in Soil
Water Retention & Capacity
The amount of soil water available to plants depends on:
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:
Soil water storage and plant available water is maximized if:
Water Content & Water Potential
Volumetric water content (q, theta)
Mass water content
Soil water content equivalent to Mass water content
Example Soil Moisture Calculation
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.
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 (%)
Hence: Water (%)
by volume =( 26 cc / 1000 cc) x 100 = 2.6 %
Water potential (Y psi)
Zero is high water potential. It means the water is loosely held.
Water always moves to a more negative water potential.
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 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 pathways cause water flow to be non-uniform because:
Pollutant Transport in Soils
Principles of Water Movement
Factors Affecting Water Potentials
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
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:
Actual Evapotranspiration -