Soils 4
Chp 5 Soil Water
Critical Relationship Between Soil and Water
Water is essential for life on earth.
Adhesion Water – water that is held in thin films around soil particles by electrical attractions between the
water molecule and the soil molecule. Adhesion water is essentially unavailable to plants because of the
strong electrical bond.
The Cohesive tendencies of water draw other molecules to the surface film, as these molecules
agglomerate on the soil particle the attractive forces holding the water in place grow weaker in successive
layers…. The attraction decreases logrythmically with distance from the particle; about 14 layers of water
are possible to be held in place in this way. This water is held in place because of the cohesive tendencies
of water and is call cohesion water. Cohesion water is mobile and is available to plants.
As plants absorb the cohesion water the remaining water becomes more difficult to absorb as successibve
layers of cohesion water are drawn away the remaining water is increasingly under the influence of the
electrical attractions.
At the point where water is moving so slowly from the soil particle to the root, plants will wilt. The point
at which a plant will wilt and fail to recover in a humid chamber is known as the permanent wilting point.
Gravitational Water
Water that exists in soil pores and I removed by gravity is called gravitational water. Saturated soils have
no air in pore spaces. Gravitational water is to considered as available to plants since it drains away
quicky.. the water that is left after gravitational water drains in known field capacity (adhesion + cohesion
water)
Note the Summary Statement on pg 56 –
1.
2.
3.
4.
The forces that affect the energy level of soil water, and the mobility and availability of
water to plant are adhesion, cohesion and gravity.
A water saturated soil contains water with a widely varying energy content resulting in an
energy continuum between the oven dry state and saturation.
The mobility and availability of water for plant use parallel the energy continuum. As
dry soil wets over time, the soil water increases in energy, mobility, and availability and
likewise as the soil dries energy, mobility and availability decrease.
Plants wilt when soil water becomes so immobile that water movement to the plant roots
is too slow to meet transpiration needs.
ENERGY AND PRESSURE
Energy in water is usually characterized as pressure. (P=force/area)
Capillarity pg 58 –
Soil Water Potential – the amount of work needed to move water from a reference pool to another pool.
Comprised of the Gravitational Potential, Matric Potential and Osmotic Potential.
Gravitational potential – the higher above the reference point the greater the gravitational water potential.
Matric Potential – adhesion and cohesion are affected by the soil structure and texture (soil matrix), the
matric water potential is the interaction between soil water and the soil matrix. The drier the soil is the
greater the tendency of the soil to wet and the greater the release of energy when it becomes wetted.
Osmotic Potential – the osmotic gradient , the difference in salt concentrations allows bases to flow from
a place of lesser concentration (soils) to a place of greater concentration (plants). Osmotic potential has
little effect on water movement within the soil matrix but greatly effects the movement of soil water into
plants.
SOIL WATER MOVEMENT
Water moves from places of higher energy to places of lower. The driving force is the movement from
high energy to lower energy (water runs downhill). The water potential gradient is the potential difference
bewteen two points divided by the distance between the two points. The rate of flow is directly related to
the water potential difference and inversely related to the flow distance.
The velocity of water is affected by the soils ability to transmit water or its hydraulic conductivity (k).
Water will flow through large pores faster than it will through small pores and flow is fastest when
conductivity is greatest. V=kf
Water flow in a pipe is directly related to the fourth power of the radius of the pipe. Water can flow
through a pore having a radius 10x larger than a smaller pore at a rate 10,000 x faster! Water moves
through sands much faster than through clays. Therefore the larger pores of a soil are emptied first and
than succeeding pores from largest to smallest…as the amount of water decreases so does the hydraulic
conductivity.
Water movement in a saturated soil
In a saturated soil the difference between water potentials is the head (h). The gradient is inversely related
to the distance of flow through the soil (d), that is the farther the flow path through the soil the lower the
gradient., there fore f=h/d.
OR V = k x (h/d) that is Velocity is equal to the hydraulic conductivity times the hydraulic gradient
(h/d)
Darcy’s Law Q = k(h/d)At or quantity of water from a pipe is equal to the hydraulic conductivity
times the hydraulic gradient times the area of the opening times the time of the flow.
Water movement in an unsaturated soil
Water drains from large pores first than to smaller pores. Gravity always works. As drainage procees and
water movemernt occurs in smaller and smaller pores the hydraulic ocnductivity of a soil decreases and the
rate of drainage slows. Once field capacity is attained, gravitry is unable to overcome the cohesive and
adhesive forces that keep water present in the soil.
Water movement in a Stratified Soil
Because stratified soils are comprised of different layers, each with different properties of texture and
porosity, as water moves down through a stratified soil it may encounter different rates of saturation. Soil
ater moves through as infiltration or movement along the soil surfaces is limited by small pores, downward
movement of water is not as saturated flow but as unsaturated flow through the micropores, while the
macropores stay fillled with air. The water progresses slowly through the soil in wahat is known as a
wetting front.
Note the paragraph on page 64 regarding the progress of water through a loam soil and what happens as it
encounters a sandy soil.
Plant and Soil Relationships
The amount of water a soil can hold is related to texture and structure (dependent on soil pores and surfaces
– soil matrix). Note figure 5.9 silt loams have the highest available water capacity. (clays hold more water
in total but it is not all available , look at the amount of water still in clay soils at the permanent wilting
point!)
Read about the Diurnal Pattern of Water Uptake and Soil Water Regime pg 68-72