soil water

David Hershey dh321z at yahoo.com
Thu Oct 24 22:35:18 EST 2002


Tensiometers work best in a pot to measure soil matric
potential, which is the major component of soil water
potential except in very saline soils. Rather than use
the old vacuum gauge kind, you can get a portable
meter with a pressure transducer which can be used to
read many tensiometers. It has a needle you insert in
the rubber serum cap of the tensiometer. You can buy
the small tensiometers ready-made or to economize buy
the ceramic tips and make your own using PVC tubing. A
google.com search will turn up several tensiometer
companies, e.g. 

http://www.skyeinstruments.com/mini.htm
http://www.soilmoisture.com/
http://www.irrometer.com/

You can also estimate soil water content on a short
term basis using a balance. Unplanted pots of media
and a balance can be used to measure key parameters
such as container capacity (percent water by volume
held against gravity after drainage) and air-filled
porosity at container capacity (also percent by
volume). Container capacity and air-filled porosity
are often good predictors of plant performance when
media are maintained near container capacity via
frequent irrigation.

Do you intend to use field soil or potting media?
Water status of field soils are very different in pots
compared to in the field (Hershey 1990). The pot
height can greatly alter the container capacity and
air-filled porosity for the same growing medium.

My major professor had an interesting exercise that
involved tensiometers and using a wide variety of
potting materials under three different irrigation
frequencies, once a day, five times a day and when
needed. At that time we used inexpensive mercury
manometers made from plastic tubing rather than vacuum
gauges. Plants in media with low water-holding
capacity, such as sand or kitty litter, did well at
five times per day but poorly as needed. Good quality
media, such as peat-vermiculite, produced good plants
at all three irrigation frequencies. Media with low
water-holding capacity such as unaggregated clay or
sandy loam produced poor growth at daily or five times
daily but better growth when watered as needed. If you
are interested I can look up my notes and provide more
details.

Tensiometers are also very useful for irrigated
agriculture where the soil is kept fairly moist. They
work down to about -0.08 MPa. Electrical resistance
blocks or gypsum blocks have a wider range of -0.05 to
-1.5 MPa so are better for drier soils (Kramer 1969).

References

Tensiometer-based irrigation in highly-amended media:
http://lieth.ucdavis.edu/Research/tens/97/

Bunt, A.C. 1988. Media and mixes for container-grown
plants. Boston: Unwin Hyman.

Hershey, D.R. 1990. Container-soil physics and plant
growth. 40:685-686.

Kramer, P.J. 1969. Plant and Soil Water Relations: A
Modern Synthesis. New York: McGraw-Hill.


David R. Hershey




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