Are you familiar with capillary tubes, the very thin rods with very narrow bores? When placed into water, the water is pulled into and up the tube, against gravity. This is capillary action. And the narrower the bore, the higher the column of water will rise.
The smallest pores and channels in soil also act like capillaries. You can see this in action when water drips onto dry soil and spreads sideways across the soil. Similarly, should you place a small piece of soil on a drop of water, you’ll see how easily that piece soaks up the water.
An amount of soil that has been saturated with water and allowed to drain will still hold some water. (As does a sponge.) Just as water clings to the insides of narrow capillary tubes, or to the sponge particles that surround the pores, so too does water cling to soil particles surrounding the narrowest (smallest) pores in the soil. These pores are in fact often called the capillary pores because of this. The water that did drain was the water that was not held as tightly by the larger, non-capillary, pores.
Because of this attraction between water and soil particles, water can act as a glue and is one reason wet soils can be harder to work than their dry counterparts. Clayey soils especially become very sticky when wet.
When a wetted soil has been given time to drain and no more water leaves it — this could be several days after rain or irrigation — it is at field capacity. This is the amount of water the soil could theoretically retain in its capillary pores indefinitely, assuming no losses through transpiration from plants or evaporation from the surface. A soil at field capacity is moist, crumbles easily and is easiest to work in this state or a little drier. Water in a soil at field capacity fills all pores of up to 0.03 mm diameter, and clings as a thin film to soil particles surrounding the larger pores, something like this:
Field capacity will vary depending on soil texture and structure. With regards to texture, heavily sandy soils, with their large minerals and pores cannot retain as much water as the loams and clays with their smaller particles and pores. But it’s structure that is more important than texture — a sandy loam without organic matter will have a lower field capacity than one with organic matter. Likewise, a well-structured clay will have a higher field capacity than a poorly-structured clay.
In a following post we will cover how water is removed from soil.
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