What is “Dry” ?

Air-Dry vs “Dry” vs Oven-Dry

Dry is a matter of perspective: 3 perspectives of dryness.

• Air-dry. The sponge on the left is air-dry – all the pores are filled with air – but there is still a minute amount of water adsorbed to (helds on) the fibers of the sponge. In the same way, water is adsorbed to soil particles in an air-dry soil. The amount of water is more for clay particles which have a lot of surface area than for sand particles that have very little.
• Oven-dry. Even that little bit of adsorbed water is lost when dried in an oven at 105 ̊C (just above boiling point) for 24 hrs or to a constant mass. This is the standard for all soil water calculations.
• “Dry” – Wilting Point. Plants have a different persepective on dry. To them a soil is dry when they can no longer extract water from it. This is called Wilting Point because when plants cannot obtain water from the soil, they wilt. The sponge on the right is moist – most of the pores are filled with air, but no more water can be removed by squeezing, so in one sense, it is “dry”. The remaining water is in very tiny pores and adsorbed to the fibers of the sponge. In the same way, at wilting point, the soil stilll contains water in very tiny pores and adsorbed to the soil particles. If this moist sponge is left on the counter for a week, it will lose water to evaporation.

Wetting and Saturation

• When a dry sponge is first submerged in water, it wets rapidly. This sponge is not submerged, only the bottom is in contact with the water. So the water is moving from the bottom of the sponge to the top of the sponge in response to capillarity and the differences in potential energy. As water moves into the sponge, it forces air out.
• The factor controlling water movement in nature (and thereby in soil), is that it moves from areas that have higher potential energy to areas that have lower potential energy. This is the reason water flows downhill: It discharges its energy as it moves downhill. This is also the reason water moves upward in the sponge – the water has high potential energy while the dry sponge has low potential energy.
• The capillary pores absorb water as water is adsorbed to the sponge fibers.

• Capillarity will not pull water into the large pores. While most of the small pores are nearly filled with water, the larger pores still hold air. Submerging the sponge begins filling the large pores.

• But the sponge still is not saturated. When the sponge is compressed, air is driven from the large pores, and even some of the smaller pores in which air became trapped during infiltration.
• In nature, soil is never truly saturated; some air is always trapped in it. Even soils that are always under water, such as swamps, only reach 80 to 95% saturation; some air always remains in the pores. One reason for this is microbial respiration releases carbon dioxide.

Water movement

Water runs downhill is a correct, but inaccurate, statement – it does not include the reason water on the surface flows downhill. The accurate statement allows water to move in other directions: Water always moves from areas of high potential energy to areas of lower potential energy. Gravity causes water at the top of a hill to have more potential energy than water at the bottom of a hill. But if you have ever used a paper towel, you have probably seen water moving “flowing” up.

• Infiltration. Infiltration is the movement of water from the surface into the sponge. In the same way, infiltration is when water on the soil surface – from precipitation, irrigation, snowmelt and runon – soaks into the soil through the pores.
• As long as the rate of water addition to the soil is slower than infiltration rate, water will move into the soil, and there will be no runoff.
• As the sponge wets, however, potential energy gradients develop in the sponge, resulting in downward movement of water. The gravitational (large) pores near the top of the sponge begin to fill. These pores are not able to retain water against gravity as the capillary pores can, so water percolates (moves through the sponge) due to the pull of gravity.

• The rate of water flow through the sponge is called the hydraulic conductivity. The hydraulic conductivity is faster in wet soils and soils with large pores, such as sands.
• The infiltration rate and hydraulic conductivity of soil is affected by many things, including texture, surface structure, surface cover (bare soil, growing plants or plant residues), hydraulic conductivity (rate of water movement through the soil), and the presence of limiting layers below the soil surface (compacted layers, bedrock, etc.).
• This sponge is nearing its holding capacity. As water reaches the bottom of the sponge, it flows out through the larger pores first, because the capillary effect holds water in the smaller pores.

• Drainage. Drainage is the loss of water out of the bottom of the soil profile, or in this case out of the sponge due to gravity.

Runoff

When water is added to the sponge (or soil) faster than the infiltration rate, runoff develops.

These sponges are wet and angled. Water poured on the sponge runs across the surface.

At the end of the sponge, the water running across the sponge surface is falling into the dish.

Water in Soil

Two forces are responsible for the way water is held and moves in soil: adhesion and cohesion. Adhesion is the attraction of water for soil particles; cohesion is the attraction of water for itself. Matric potential is the scientific term for the combined force (pull) of adhesion and cohesion that pull on water. These forces result in the capillary effect, so that water is held in small pores, but not in larger ones.

• Capillarity. Watch the water move upward into the dry sponge. Though gravity pulls down on the water, the capillary forces are stronger than gravity as they pull water upward into small pores.

• The wetted portion of the sponge is darker, but the water did not fill the large pores; they have no capillary effect.

• Gravitational water. This is the water which drains through the soil under the influence of gravity.

• When the sponge is removed from the water and held flat, water drains from it. Notice the drainage occurs rapidly at first, then slows to a stop. Such drainage occurs through large soil pores. Small soil pores have the ability to hold the water against the pull of gravity through the process of capillarity.

• Draining sponge, held flat. The image is enlarged to see the sponge is not equally wet from top to bottom. The top has more empty pores than the bottom.

Plant Available Water

Field capacity. When the water stops flowing by gravity, the remaining water is held in capillary pores, which range from those just barely able to hold water against gravity to vary small. The field definition is the water remaining in the soil two to three days after a soaking rainfall or irrigation when evaporation has been prevented. Soils in semiarid and arid regions may never attain field capacity through the rooting depth of plants. Laboratory estimates of field capacity place a saturated soil sample on a porous ceramic plate and apply 33 kPa (1/3 bar, 5 psi) of pressure to force some water through the pores, into the plate and out of the system.

• The water held in the soil between field capacity and wilting point is the plant available water, the water that could be available for plants roots to uptake.
• Laboratory estimates of wilting point place a saturated soil sample on a porous ceramic plate and apply 1500 kPa (15 bar, 225 psi) of pressure to force some water through the pores, into the plate and out of the system.

• Just as the energy required to squeeze water out of the sponge increases, not all “plant available water” is equally available. Wilting point is not the same for all types of plants.

• The sponge is squeezed in three cycles – each one represents the wilting point of different types of plants.
• A small amount of pressure is applied in the first cycle. This represents hydrophytes, wetland plants – plants that live in mostly saturated soils.
• In the second cycle, a little more pressure releases a little more water. This represents mesophytes, which include most agricultural, vineyard and orchard crops. Even agricultural crops vary in their ability to extract water. For example, sorghum, millet and cotton dry the soil more than corn and soybeans.
• In the third cycle, even more pressure releases even more water. This represents xerophytes, desert-like plants. They can extract water from really dry soils.
• All soils retain water that plants are unable to extract; clays hold the most unavailable water while sands hold the least. Though clays hold the most water at both field capacity and wilting point, silt loams hold the most plant available water.

Narrated videos – Modeling plant available water by mass and volume of water in sponges with large pores to represent sands and small pores to represent clays.

• By mass, 3 min. The sponge with large pores held 17.4 g of plant available water, while the sponge with small pores held 26.2 g of plant available water.
• By volume, 5 min. The sponge with large pores held about 18 ml of plant available water, while the sponge with small pores held about 26 ml of plant available water.
• Since the density of water is 1.0 g/cm³, both methods yielded the same result – within the precision of my measurements.