Esther,
Yes, field capacity is the water that's left. More exactly, it's the
field at
saturation minus the gravitational water. What all this means is the
amount
of water that the soil will hold, resisting what drains away through
gravitational pull (and what doesn't soak in and runs off).
Theoretical values can be assigned to each soil type based on the amount
and
type of clay content of the soil, but there are many more factors
involved.
The major ones are:
-tillage depth- determines how far the water will soak in at a rapid
rate. A
"tillage pan" develops in soils with clay content at the tillage depth.
This
is a result of water moving rapidly through the tilled area and carrying
clay
particles. When it hits the bottom of the tillage zone, the water slows
down
and the clay is deposited at this depth forming an impenetrable layer.
(Just
like putting clay in a pond to seal the bottom.) Then the water moves
horizontally, resulting in erosion.
-organic matter content- the higher the organic matter percent, the
greater
the water holding capacity of a soil. Organic matter can hold up to 8
times
it's weight in water. It will help a sandy soil retain water (slow down
the
drainage) and help a clay soil drain (accomplished by the open structure
built
by life in the soil).
The problem with a high clay soil is that this water holding capacity
excludes
high oxygen content atmospheric air resulting in aerobic soil life plant
death. Plants need oxygen at the root hair for nutrient absorption and
root
extension.
The problem with sandy soils is they don't hold water very well (though
water
is easier for plants to absorb) and the high pore space means faster
evaporation, as well as higher gravitational water.
Though clay soils hold more water (higher field capacity) than sandy
soils,
the water is not as available to plants. That is, plants cannot pull
the
water as easily away from clay particles. So, there is no advantage to
any
soil type, there are just plus and minuses for each. The advantage is in
knowing your particular soil texture and knowing how to manage it for
the best
soil and plant life response.
The whole thing is theoretical and instantaneous. Field capacity is
diminished right away by evaporation and plant absorption. But it does
give
us an idea of the amount of water a particular soil type will hold.
This is
valuable for determining when and how much irrigation will be needed by
a
particular crop.
I hope this has helped and has not further confused you. I wish I could
refer
you to some tome on soil moisture, but the above is a combination of
many
references. Please ask if you have more questions.
Alex McGregor
Walden Farm
Walden Ridge, TN
Esther Day wrote:
> I am sorry for cross posting, however, I have run into a small snag
> regarding soil field capacity.
>
> I was able to find the definition: The percentage of water remaining
> in the soil 2 or 3 days after the soil has been saturated and free
> drainage has practically ceased. The percentage may be expressed in
> terms of weight or volume.
>
> Ok, that sounds reasonable, but why can't I find anything on it?
> Nothing past this definition. Has anybody seen a methodology to
> calculate this or, better yet, a reference with default values for
> certain soil classes/subclasses? Any input would be much
appreciated.
>
> Thanks so much,
>
> Esther Day
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