sponge as soil water model

Soil Water and Plant Availability Concepts
From WTClass, use the back button to return.	*Go to* Clay Robinson, Ph.D., CPSSc	Back to Dr. Dirt
Models are used throughout education to simplify complex concepts to improve student understanding. In this case, a household sponge will be used as a model to demonstrate soil water holding characteristics.
Air-Dry Soil vs Dry Soil Air-dried sponge - Though the sponge is dry to the touch, it is not "dry". It contains water. If the sponge is placed in an oven at 105º C, it will lose water, and therefore lose mass. Similarly, the definition of dry soil is soil that has been dried at 105º C until it reaches a constant weight, often 24 hours.	Saturation Soaking sponge - water is added to the sponge to get it essentially saturated, that is, so that all the pores are filled with water. In truth, it is very difficult to remove all the air from the sponge. The same is true for soil. Even soil that has been saturated for long time periods usually has air still remaining in up to 10% of the pores (voids) between soil particles.

Gravitational water This is the water which drains through the soil under the influence of gravity. The next series of images will address drainage of the water from the large pores in the sponge.
Draining sponge, held flat - When the sponge is removed from the water, and held flat, water drains from it. This occurs rapidly at first, then slows to a stop.	Draining sponge, short vertical axis - If the sponge is turned from horizontal to vertical, the drainage begins again. As before, drainage will be rapid for a time, then it will slow to a stop.	Draining sponge, long vertical axis - If the sponge is turned one more time, the drainage will begin again, then slow to a stop. Careful observation of the side of the sponge from top to bottom will allow you to see which pores retain water, and which ones drain.

The only thing that changed as the sponge was turned is the distance over which gravity could work. Upon careful observation, you noticed that water drained from the large pores in the sponge, but was retained in the smaller pores. Field Capacity - When all the water that can has drained from the soil by gravity, the soil is at field capacity. A field definition is the amount of water in the soil 2 to 3 days after a soaking rainfall or irrigation when evaporation from the surface has been prevented. A laboratory estimate places a saturated soil sample on a porous ceramic plate and applies 33 kPA (1/3 bar or 5 psi) of pressure to force some water through the soil pores, into the plate and out of the system. Clays hold the most water at field capacity, while sands hold the least. However, field capacity is affected strongly by soil structure. The amount of aggregation in the soil determines the amount of small pores available to hold water against gravity.
Capillary Water - This is the water held against gravity in the small soil pores, or capillaries. This water can be extracted by the plants, just as the next series of activities with the wet sponge indicates.
Water extraction, wet sponge - Beginning with the sponge at field capacity, when all gravitational water had drained, begin to squeeze the sponge. Af first it takes little energy to extract water.	As the sponge becomes progressively drier, more energy is required to extract water.	When the sponge is nearly dry, much more energy is required to extract just a little bit of water.

Likewise, not all of the water held in the soil is equally available to the plants. Plants are able to extract water easily from soils that are near field capacity water contents. As the soil dries, the plants must work progressively harder to extract water, until finally, the soil is so dry that the plant can no longer expend enough energy to extract any more water. This is the concept of wilting point.
Squeeze-dried sponge - It is important to note that, although no more water can be extracted from the sponge by squeezing it, it is not "dry". If the sponge is allowed to sit on the counter for a week, it will become drier as water evaporates from it. In the same way, bare soil in a field, especially after a tillage event, can become drier than wilting point.
Wilting Point, plant unavailable water - When no more water could be squeezed from the sponge, was it dry to the touch? The answer is no, it still retains water, no longer absorbed in the pores between the sponge fibers, but adsorbed (or adhered) to the fibers of the sponge. The same is true in soil. All soils retain water that plants are unable to extract; clays hold the most unavailable water while sands hold the least. Wilting point is a water content. Sometimes it is measured in the field at the end of the growing season, but more commonly is estimated in the lab using a pressure plate. Saturated soils are subjected to 1500 kPa (15 bar or 225 psi) pressure until water stops flowing out of the soils and through the plate. It should be noted wilting point is both a concept and an approximation. All plants do not have equal ability to extract water from the soil. Grain sorghum, sunflowers, and some fieldbeans and fieldpeas have been documented to dry the soil below -1500 kPa, while lettuce, cole crops, and others, cannot extract water to even -1200 kPa. In the example of the sponge above, the first group of plants listed above are stronger, and can squeeze the sponge harder. The latter group are weaker, and cannot get as much water from the soil. Every good example has a weakness, and there is a difference between extracting water from the sponge and the soil. The sponge is squeezed to extract water. Plants must pull water away from the soil. Picture a continuous tug of war battle between the soil and the plants. The soil is holding on to the water (pulling) while the plants are trying to take water away (tugging or pulling). Wilting point is reached when the plants can no longer pull water away from the soil. Wilting point is primarily a function of specific surface area, and thus of soil texture (clay content and clay mineralogy).
Hygroscopic coefficient - This is the amount of water held in an air-dried soil in a humid environment, it is equal to about 3100 kPa. Note, if the humidity is low, the soil can become even drier. Another comment is important to soil management in dry areas. Tillage air-dries the soil to the depth of tillage. In regions where long droughts are common, and where fallowing (not growing a crop during a growing season) is practiced for water conservation, some of the precipitation is used to bring the soil from air-dried to wilting point before much water becomes available to plants. This is the reason that several rains are required to "break" a drought. The first rains bring the soil back to wilting point, and subsequent rains are required to recharge the plant available water.
Plant Available Water - This is the water that is considered available for plant use. As noted above it varies for different types of plants and for different types of soils. Though clays hold the most water at both field capacity and wilting point, silt loams hold the most plant available water.
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