Soil Water
AGS 105 Soils

Soil Water: Characteristics and Behavior

 

Watch this video! Click this link to view a short video produced by the USDA/NRCS over the movement of water in soil. It's a good review of how soil structure can affect water movement through soil.

Water participates directly in dozens of soil and plant reactions and indirectly affects many others.  Water's polar structure with its hydrogen (positive) end and hydroxyl (negative) end is responsible for the attraction of water molecules, first for each other (cohesion) and then for solid surfaces (adhesion or adsorption), especially if these surfaces have either positive or negative charges.  Cations and anions likewise attract water molecules, being surrounded by a swarm of these as the ions move in the soil.  The attraction of water for soil solids accounts for capillary movement in soils and for the soils' ability to hold water against the pull of gravity and plant roots. 

water_molecule.jpg

Two-dimensional representation of a water molecule showing a large oxygen atom and two much smaller hydrogen atoms. The HOH angle of 105˚ results in an asymmetrical arrangement. One side of the water molecule (that with the two hydrogens) is electropositive; the other is electronegative. This accounts for the polarity of water

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The forces of cohesion (between water molecules) and adhesion (between water and solid surface) in a soil–water system. The forces are largely a result of H-bonding, shown as broken lines. The adhesive or adsorptive force diminishes rapidly with distance from the solid surface. The cohesion of one water molecule to another results in water molecules forming temporary clusters that are constantly changing in size and shape as individual water molecules break free or join up with others. The cohesion between water molecules also allows the solid to indirectly restrict the freedom of water for some distance beyond the solid–liquid interface.

Energy Relations

The retention and movement of water in soils, its uptake and translocation in plants, and its loss to the atmosphere are all energy phenomena.  In fact, the difference in energy levels of water at one site or condition (e.g., wet soil) from that at another site or condition (e.g., dry) determines the direction and rate of water movement in soils and plants, the movement always being from a zone of high energy level to one of low energy level.  Scientists identify the energy status of a soil in relation to that of pure water outside the soil at standard conditions (temperature, pressure, elevation).  They use the term soil water potential to describe the soils' energy status. 

The components of the overall soil water potential  that is due to gravitational forces is called gravitational potential, that due to osmotic forces associated with the presence of salts, osmotic potential, and that related to the attraction forces of the soil solids (the soil matrix) matrix potential.

Soil water potential is commonly expressed in terms of standard atmospheric pressure at sea level (bar) or in terms of Newtons/m2 expressed as the Pascal (Pa) or kilopascal (kPa) in the international system of units (SI) used in this text.

Soil Moisture Levels

Soil moisture levels are generally correlated with soil water potential.  In wet soils, the water molecules are loosely held, their energy status being relatively high compared to that of pure water.  In dry soils, the water is held tightly by the soils solids and its energy status (water potential) is much lower than that of pure water. 

Soil texture influences the amount of water held at a given soil water potential level.  Clay soils with numerous small capillary pores have high water holding capacities; while sandy soils with fewer micropores have low ones.  Likewise, a well-granulated soil with high total pore space generally holds more water than a poorly structured soil, especially if the latter has been compacted. 

Soil water content is measured directly by weighing a sample of soil before and after drying in an oven (100-110 EC).  Other indirect means of estimating soil moisture contents take advantage of the correlation between soil water levels and characteristics such as a) the electrical resistance of a gypsum  or nylon block in equilibrium with the soil, b) the tension of water due to attraction of soil solids, c) the scattering of neutrons by the hydrogen molecule in water, and d) the time it takes for an electromagnetic pulse to move a few centimeters in soil.

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This figure shows the instrumental measurement of soil water content using time domain reflectometry (TDR). The instrument sends a pulse of electromagnetic energy down the two parallel metal rods of a waveguide that the soil scientist is pushing into the soil (inset photo). The TDR instrument makes precise picosecond measurements of the speed at which the pulse travels down the rods, a speed influenced by the nature of the surrounding soil. Microprocessors in the instrument analyze the wave patterns generated and calculate the apparent dielectric constant of the soil. Since the dielectric constant of a soil is mainly influenced by its water content, the instrument can accurately convert its measurements into volumetric water content of the soil.

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Tensiometer used to determine water potential in the field. The side view (right) shows the entire instrument. The tube is filled with water through the screw-off top. Once the instrument is tightly sealed, the white porous tip and the lower part of the plastic tube is inserted into a snug-fitting hole in the soil. The vacuum gauge (close-up, left) will directly indicate the tension or negative potential generated as the soil draws the water out (curved arrows) through the porous tip. Note the scale goes up to only 100 centibars ( 100 kPa) tension at the driest.

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A cutaway view of a commercial gypsum electrical resistance block installed about 45 cm below the soil surface. Thin wires lead from the block to the surface, where they can be connected to a special resistance meter. In the inset, another gypsum block has been broken open to reveal two concentric metal screen cylinders that serve as the electrodes between which moistened gypsum conducts a small electric current. The resistance to current flow is inversely proportional to the wetness of the gypsum block.

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Movement in Soils

Water flows from sites with high soil water potential to those with lower potentials.  Three types of water movement are recognized:  a) liquid flow in soils saturated with water (saturated flow), b) liquid flow in unsaturated soils (unsaturated flow), and c) vapor flow.  Saturated flow takes place after a heavy rain or irrigation water application where all soil pores are filled with water.  It moves quite rapidly, its rate of movement being determined by the size and configuration of the soil pores and by the driving force (hydraulic gradient).  It is largely responsible for most of the ready drainage from soils including the downward movement of pesticides, organic wastes, and plant nutrients. 

Unsaturated flow is the dominant means of movement in upland soils.  It moves slowly, traveling only in the finer soil pores and being driven by the matric potential gradient.  Unsaturated flow is higher in the clay soils.

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This figure shows the comparative rates of irrigation water movement into a sandy loam and a clay loam. Note the much more rapid rate of movement in the sandy loam, especially in a downward direction.

The quantity of water moving in the form of vapor is relatively small, being of significance in meeting plant water needs only in arid and semi-arid areas.  However, water movement from the soil to the atmosphere is in the vapor form.

A stratified layer in the soil that differs sharply in texture from the layers below and above it acts as a moisture barrier, resisting water movement up or down the profile.  The same principle holds for a layer of coarse sand or gravel in the bottom of a flower pot. 

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The wetting front 24 hours after a 5 cm rainfall. Water removal by plant roots had dried the upper 70 to 80 cm of this humid-region (Alabama) profile during a previous three-week dry spell. The clearly visible boundary results from the rather abrupt change in soil water content at the wetting front between the dry, lighter-colored soil and the soil darkened by the percolating water. The wavy nature of the wetting front in this natural field soil is evidence of the heterogeneity of pore sizes. The graph (right) indicates how soil water content decreases sharply at the wetting front. Scale in 10-cmintervals.

Qualitative Description of Soil Wetness

There are no distinct forms of soil water.  However, certain qualitative terms are commonly used to describe practical aspects of the degree of soil wetness.  A soil is said to be at maximum retentive capacity when it is essentially water saturated; when gravitational water has drained away it is said to be at field capacity (water potential approximately -10 to -30 kPa).  Capillary water occupies the finer soil pores and moves according to the laws governing capillarity (water potential between -10 to -30 and -1500 kPa).  Hygroscopic water is held very tightly by soil colloids (water potential lower than -3100).   Available soil water is that held between the field capacity and the wilting coefficient (plants permanently wilted from lack of water, moisture potential is -1500 kPa); of this, the water held in the larger pores is the more easily available.

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The image above shows volumes of water and air associated with a 100 g slice of soil solids in a well-granulated silt loam at different moisture levels. The top bar shows the situation when a representative soil is completely saturated with water. This situation will usually occur for short periods of time during a rain or when the soil is being irrigated. Water will soon drain out of the larger pores (macropores). The soil is then said to be at the field capacity. Plants will remove water from the soil quite rapidly until they begin to wilt. When permanent wilting of the plants occurs, the soil water content is said to be at the wilting coefficient. There is still considerable water in the soil, but it is held too tightly to permit its absorption by plant roots. A further reduction in water content to the hygroscopic coefficient is illustrated in the bottom bar. At this point the water is held very tightly, mostly by the soil colloids.

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This figure demonstrates the water content–matric potential curve of a loam soil as related to different terms used to describe water in soils. The wavy lines in the diagram to the right suggest that measurements such as field capacity are only approximations. The gradual change in potential with soil moisture change discourages the concept of different "forms" of water in soils. At the same time, such terms as gravitational and available assist in the qualitative description of moisture utilization in soils.

The take home point of this graph is this: The tighter water is held to soil particles, the lower the soil moisture potential, or, the more negative the number. For example, if a soil is saturated with water, it will have a number greater than -10 kPa (in this example of a loam soil). If the same loam soil is less than completely saturated (ie. somewhere between field capacity and the wilting point, the moisture potential will be a number between -10 kPa and -3100kPa (the wilting point).

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This figure demonstrates the general relationship between soil water characteristics and soil texture. Note that the wilting coefficient increases as the texture becomes finer. The field capacity increases until we reach the silt loams, then levels off. Remember these are representative curves; individual soils would probably have values different from those shown.

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The effects of organic matter content on the field capacity and permanent wilting percentage of a number of silt loam soils. The differences between the two lines shown is the available soil moisture content, which was obviously greater in the soils with higher organic matter levels. What this graph demonstrates is that as you increase the amount of organic matter in a soil, you greatly increase the field capacity, or water holding capacity, of that soil.

Water Supplied to Plants

Water is supplied to plant roots by capillary movement of the water to root surfaces and by the growth of roots into the soil.  Since roots are in contact with less than 1% of the total soil surface area most of the water moves from the soil to the plant roots, although the actual distance of movement needed is very small.

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Cross-section of a root surrounded by soil. (a) During periods of adequate moisture and low plant moisture stress, the root completely fills the soil pores and is in close contact with the soil water films. (b) When the plant is under severe moisture stress, such as during hot, dry weather, the root shrinks (mainly in the cortical cells), significantly reducing root–soil contact. Such root shrinkage can occur on a hot day even if soil water content is high.