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     Examples........     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   Macropores.......     Finger flow........         Pollutant Transport in Soils     Principles of Water Movement   Factors Affecting Water Potentials Water Content         Time Changes                       Conductivity in Soil-Plant Systems   Transpiration Factors that can limit leaf transpiration........         Phloem & Xylem Phloem: Transportation of food. Xylem: Water and mineral transport.        Water Stress in Plants   Desiccation.........   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 -