Yes, field capacity is the water that's left. More exactly, it's the
saturation minus the gravitational water. What all this means is the
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
type of clay content of the soil, but there are many more factors
The major ones are:
-tillage depth- determines how far the water will soak in at a rapid
"tillage pan" develops in soils with clay content at the tillage depth.
is a result of water moving rapidly through the tilled area and carrying
particles. When it hits the bottom of the tillage zone, the water slows
and the clay is deposited at this depth forming an impenetrable layer.
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
the water holding capacity of a soil. Organic matter can hold up to 8
it's weight in water. It will help a sandy soil retain water (slow down
drainage) and help a clay soil drain (accomplished by the open structure
by life in the soil).
The problem with a high clay soil is that this water holding capacity
high oxygen content atmospheric air resulting in aerobic soil life plant
death. Plants need oxygen at the root hair for nutrient absorption and
The problem with sandy soils is they don't hold water very well (though
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
the water is not as available to plants. That is, plants cannot pull
water as easily away from clay particles. So, there is no advantage to
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
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
us an idea of the amount of water a particular soil type will hold.
valuable for determining when and how much irrigation will be needed by
I hope this has helped and has not further confused you. I wish I could
you to some tome on soil moisture, but the above is a combination of
references. Please ask if you have more questions.
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
> Thanks so much,
> Esther Day
To Unsubscribe: Email firstname.lastname@example.org with the command
"unsubscribe sanet-mg". If you receive the digest format, use the command
To Subscribe to Digest: Email email@example.com with the command
All messages to sanet-mg are archived at:
This archive was generated by hypermail 2b29 : Mon Jul 03 2000 - 12:00:37 EDT