The energy content of water is dependent on the energy input and output of a system. When we add energy to water, we continue to excite the water molecules until we "blow" them out of the water as steam. Water in the atmosphere is called water vapor or relative humidity.
Relative Humidity: is the measure of the amount of water vapor in the air at a particular temperature, compared with the total amount of water vapor the air can hold at that temperature. Relative humidity is a percentage that tells how much of the available space in air has been filled by water vapor. For example, if the relative humidity is 50 percent, the air is holding only 50 percent of the water vapor it is capable of holding at its current temperature. We can use a similar arrangement to measure the water content of a soil but, we have to know if we are measuring the % water content compared to total water holding capacity or to the plant available water holding capacity of the soil.
If we reduce the water molecules energy level or movement, we would
eventually turn them into ice as the molecules move slower and slower. In this
diagram the left soil has no water. When a small amount of water is added, the
water is reduced in its energy state or they move slower, (it gives this energy
off as heat or it does work) since the water molecules are tightly attracted to
the soil particles by adhesive forces.
The amount of energy the water molecules lose upon becoming attracted to the soil particles is directly related to the amount of energy required to remove them from the soil later by plants roots. The addition of more water creates cohesive force attraction between water molecules. The cohesive forces are not as large--the water molecules do not lose much of their potential energy, and the water molecules are, therefore, more easily removed again from the soil by roots.
Often when we add water to an oven dry soil, we can feel the soil warm. The heat being released is from the energy lost by water molecules not being able to move about as they did before. If we add water to a moist soil, this heat of wetting does not occur because the water molecules lose only a small amount of their movement potential or energy.
Water held in soil is expressed by water potentials (bars), a negative number. The positive number for water potential is called tension. The water held in the soil by adhesion and cohesion is called matric potential.
Eventually, if we keep adding water, there will come a point when cohesive forces are not large enough to hold onto the water; it remains free and moves by gravity downward. This is the gravitational potential of the total water potential. It is significant only when soils are saturated.
Plant available water is the portion of stored soil water that can be absorbed fast enough by plant roots to sustain life. This plant available water is held loose enough so that the energy available to plant roots is large enough to absorb the water molecules.
Over 90% of the total water plants absorb is by passive absorption: water moves from the soil into the plant and out through the leaves by transpiration. All the water molecules are connected by cohesive forces, and the amount of water entering the root is dependent on the amount of water leaving the stomata of the leaves.
The energy of the area around the leaf (heat, wind, & humidity) determines the forces (or energy) available to bring water from the roots. Root extension moves roots into new areas of the soil where more soil water can be encountered. Root extension is important because capillary water flow in soils is slow and becomes slower as the soil dries.
Active absorption takes place because of the accumulation of salts in the plant roots, which causes the water to enter the roots by the osmotic potential (10%).
Soil Water Classification Chapter 3
Soil Water Chapters
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