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Water Quality Data
SOIL, WATER AND PLANT RELATIONSHIPS
There are limits on the amount of water that soil holds for crop use. The upper limit is termed "field capacity". During an irrigation, or whenever excess water is added to soil, water drains down through the soil due to the pull of gravity. At first, this internal drainage is relatively rapid. However, it soon slows to almost nothing. (The increasing soil water-holding forces finally start to counteract gravity.) At this point we would say the soil is at field capacity.
You can demonstrate field capacity using a visualization of a sponge (like soil, a porous material that will hold water). Using a pan of water, hold a sponge under water until it is saturated. Now, pull the sponge out of the water. It will immediately start to drip water, quickly at first, then slower and slower. At some point it will essentially stop dripping. The internal drainage has stopped and the sponge is at field capacity.
It is very important to note that you can soak more water into soil that is already at field capacity. There will be open soil pores that will take the water. However, the excess water will not be held. It will just drain down until the soil moisture returns to field capacity.
You can use the sponge again to demonstrate this important fact. With the sponge at "field capacity", use a cup to pour water on it. The water will soak in, there will be open pores in the sponge that will take in water. But you will see that the sponge starts dripping again as the excess water starts to drain off the bottom.
Because of this ability to hold water against the pull of gravity, soil does not act like a bathtub during irrigations. That is, irrigation water does not have to go to some "bottom" and then fill back up to the top. Rather soil fills to field capacity from the top down.
Field capacity is a soil-based concept. That is, it depends on the texture and structure of the soil as well as the physical conditions in the field. Coarse soils have lower field capacities than fine soils. If there is a high water table or severe stratification that would restrict drainage, the field capacity would be higher than normal.
We said there are limits to the amount of water that soil will hold available for crop use. The upper limit is field capacity, dependent on the soil texture and structure. What is the lower limit of available water? Is every bit of water in the soil available to the crop?
The plant, through its root system, will try to extract water from the soil. But as already noted, the soil holds onto water with greater and greater force as the soil moisture level drops. Sooner or later the water-holding forces generated by the soil will be so great that the plant will not be able to extract any more water. This lower limit of available soil moisture is termed the "permanent wilting point". (As the plant extracts more and more water from the soil, you will note temporary wilting during the hottest parts of the day. At night, the plant will recover. Sooner or later though, the plant will wilt and not recover- it has reached the permanent wilting point.)
Permanent wilting points depend on the soil, plant, and climate. Some plants are better able to withstand moisture stress than others.
Last updated September 2000