Descriptions of soil moisture can often times be confusing. Sometimes soil moisture is described in terms of actual or total soil moisture content. That is, how much total water is actually in the soil. Other times, soil moisture will be described in terms of available water. This is soil moisture that is above the permanent wilting point, available to the crop.
Another way to describe soil moisture is to talk in terms of the "soil moisture depletion" or "soil moisture deficit". Remember that the upper limit of available soil moisture is field capacity. Adding water to the soil in excess of field capacity will just result in deep percolation. The amount of water required to take the soil from its current soil moisture to field capacity is termed the soil moisture depletion (or deficit), the SMD.
The figures below will help to explain. For example, assume that a soil has a field capacity of 2.0 inches/foot. The permanent wilting point is at .5 inches/foot. The current soil moisture reading is 1.7 inches/foot total water. Thus, . . .
There are 1.7 inches/foot total water in the soil. There are 1.2 inches/foot available water in the soil, 1.7 total minus the .5 inches below permanent wilting point which won't be used by the crop. The soil moisture deficit is .3 inches/foot, that is, adding .3 inches/foot to the current 1.7 inches/foot will take the soil to its 2.0 inches/foot field capacity.
All of the above describe soil moisture "volumetrically". They are "volumetric" measurements of soil water. The measure the actual soil water content (or what it takes to refill the soil to field capacity). The measurement standard of "inches of water held per foot of soil" is a standard for volumetric measure.
There is another way to describe soil moisture. This is in terms of the soil's water-holding forces, termed "soil moisture tension."
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Soil Moisture Tension
We have been talking of soil moisture in volumetric terms. That is, how much water is physically in the soil, the inches of water held per foot of soil.
However, the plant doesn't really care about the actual amount of water in the soil. It cares about the soil's water-holding forces, how hard the soil is holding on to that water. For example, a clay loam may have an available water content of 1.2 inches/foot. And a sandy loam may have the same available water content of 1.2 inches/foot. A plant in the clay loam is probably feeling much more stress than the plant in the sandy loam. Finer soils will hold more water than coarse soils. But they will also hold on to it much tighter for any given water level.
The term used for describing the water-holding force is "soil moisture tension". Soil (or plant) moisture "tension" is another way that moisture can be measured and described.
Soil/plant moisture tension is measured in terms of pressure, most commonly as centibars (1 bar = 100 centibars = 1 atmosphere = 14.7 PSI for agricultural work.
To a plant, all other things being equal, it doesn't care if it is growing in beach sand or black clay. If the moisture tension is equal, it feels the same amount of stress, regardless of the actual amount of water present. And it will develop at the same rate.
Last updated September 2000