Dry Soils: Sponge Model

 

"Dry" Soils Infiltration and Runoff Wetting and Saturation
Water Holding Capacity Percolation and Drainage Engineering and Bearing Capacity

 

Air Dry

Dry vs Air-Dry vs Oven-Dry

The sponge on the left is moist, but no more water can be removed by squeezing, so in one sense, it is "dry". Similarly, when plants use all the water they can extract from the soil, it is "dry" to them, yet it still contains water.

The sponge on the right is air-dried. 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, air-dried soils contain water. The definition of dry soil is soil that has been dried at 105º C until it reaches a constant weight, often 24 hours.

Air-dried sands retain the least water, while air-dried clays retain the most relative to oven drying.

 

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. The water remaining in the sponge is 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 that identifies when plants consider the soil dry, when they are no longer able to extract more water from the soil. 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. When using this example in a classroom setting, have two (or more) different individuals squeeze water from the sponge, preferably from the one with the least 

Sponge

grip strength to the one with the greatest grip strength. This demonstrates the principle that plants vary in the ability to extract water from the soil just as people vary in their ability to extract water from the sponge.

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.

 

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.