Electrical Resistance 'Gypsum Blocks
For Scheduling Irrigations
Cooperative Extension Service
Oregon State University
Corvallis
Extension Bulletin 810
March 1968
SE' r
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Figure 1. Electrical resistance soil moisture measuring equipment used in Oregon: (1) Coleman, (2)
Irrigage (Rader meter not shown is similar), (3) Buoycous (converted), (4) Delmhorst, (5) adapter to
single block units, (6) single block units, (7) block units assembled on Jones plug. A switch on meters
1, 2, and 3 can be used to select individual block units to be read when they are assembled on a
Jones plug.
COVER: Cliff Hess, Douglas County farmer, explains his soil moisturemeasuring program at an irrigation meeting.
Cooperative Extension Work in Agriculture and Home Economics, F. E. Price. Director, Oreand disgon State University and the U. S. Department of Agriculture, cooperating. Printed
3-63 --6M.
tributed in furtherance of the Acts of Congress of May 8 and June 14, 1914.
2
Electrical Resistance Gypsum Blocks
For Scheduling Irrigations
MARVIN N. SHEARER
Extension Irrigation Specialist
Oregon State University
Electrical resistance gypsum blocks
have been used by farmers in Oregon
since 1952. They have proved an effective aid to efficient irrigation on farms
when used according to recommended
mercially in quantity in Oregon at the
time of writing. For this reason, only
limited reference has been made to
other types which have been developed
through the years. A wide variety of
procedures. Oregon State University meters and equipment can be used with
has conducted extensive research to these moisture-measuring blocks. It has
determine what characteristics make been necessary to use trade names
equipment convenient and reliable throughout this bulletin, but no enunder Oregon conditions. The Delm- dorsement is made of one brand over
horst gypsum blocks referred to in this another, nor is criticism intended of
circular were evaluated by OSU in
1962 and were the only type sold corn-
equipment not mentioned.
Equipment
Figure 2. Cut-away of Delmhorst blocks showing
concentric electrode arrangement.
The measuring equipment consists
of a portable meter and gypsum blocks
(Figure 1, page 2).
The blocks are a casting of a special
gypsum mix around two stainless steel
coils (Figure 2) that are attached to
wire leads extending above the ground.
When a reading is made, the meter is
attached to the leads and the amount
of current flowing through the block
at a specified voltage is measured to
determine the amount of resistance
present.
The amount of water in the gypsum
block will increase or decrease as the
amount of water in the soil surrounding the block increases or decreases.
This results in resistance changes within the block, which can be interpreted
in terms of moisture content of the
soil.
Calibration
Figure 3
is a calibration of the
Delmhorst cylindrical gypsum block.
3
I00000
80,000
60,000
40,000
30,000
20,000
Resistence
in
ohms
10,000
8,000
6,000
4,000
3,000
2,000
1 2000
800
600
400
300
200
100
.2
.3 .4
.6 .8 1.0
2
34
6 8 10
15
Bars tension
Figure 3. Calibration of Delmhorst cylindrical gypsum block.
pressed in terms of moisture tension
with coarse sand. It is quite useful;
however, it is not as accurate as the
(a measure of the availability of water
to plants) to make them applicable
across a range of soil textures.
Another type of calibration is shown
slightly. However, for the extremes
Irrigation recommendations are ex-
in Figure 4. This calibration can be
used on nearly all medium to fine
textured soils which are not underlain
4
type of calibration shown in Figure 3.
Seasonal soil temperature changes
may cause the calibration to shift
occurring in Oregon at the depths the
blocks are installed, this shift will have
little practical significance.
200
180
160
II
I'Irri gage" 140
or
"Coleman "120
meter
reading
100
80
60
40
20
0.
o
10
20 30 40 50 60 70 80 90 100
Approximate percent of available
water in the soil.
Figure 4. Another type of calibration of Delmhorst cylindrical gypsum blocks.
Limitations
Number of installations required
Movement of water in and out of the
Gypsum blocks measure moisture
content only in the soil immediately
the soil and the block. This is fre- surrounding them. This measurement
quently difficult to maintain in coarse is projected over larger areas of the
sandy soil, or in fine adobe soil which field. It is important, therefore, that
shrinks and swells considerably be- the location of the blocks be "repreblock requires good contact between
tween irrigations. These soils are found
only in isolated areas in Oregon.
sentative" of the larger areas both as to
Soils high in salt also may cause
root concentration.
A minimum installation for a sprinkler irrigated area requiring more than
erratic resistance readings; however,
from a practical standpoint, the blocks
will work satisfactorily in any soil that
does not exhibit saline or alkaline
problems.
moisture-holding capacity and crop-
four days to irrigate and having uniform soil is four strings of four blocks
each (total 16 blocks) usually installed
5
12, 18, and 24 inches deep. Make
installations in pairs as shown in
Figure 5. The two at the start indicate
when to begin irrigation; those at the
end indicate how dry it got before
irrigation was completed.
6,
In the case of surface irrigation when
corrugations or rills are used, install the
blocks between the corrugations or rills
one fourth of the way from the top and
This corresponds to 5,500 ohms resistance (85 on the chart in Figure 4).
Some crops (fresh market for example) will respond to wetter treatment
while others (forage) will tolerate
drier treatments.
Active roots are indicated whenever
readings drop. By plotting all the
depths of one installation on a single
chart, the depth of rooting is immedi-
bottom of the run as shown in Figure
ately apparentproviding the blocks
6. Make duplicate installations between
are deeper than the roots. In Figure 7,
on July 10 roots were taking moisture
at the 6- and 12-inch depths but not at
other corrugations or rills 5 to 10 feet
away.
Install them in the area of highest
root concentration (in the row) and
the 18-inch depth.
on knolls rather than in swales. Adapt
the depths the blocks are buried to the
rooting depth of the crop. Make extra
installations when there are wide variations in soil texture and the areas involved are important.
(Figure 8), it is possible to predict
By extending the slope of the lines
when an irrigation should be applied
days before it is actually needed. On
July 31 a reading of 50 could be predicted for the 6-inch depth and 120 for
the 18-inch depth on August 7. Extreme hot weather will steepen the
Installation procedure
Figure 7 shows the six steps in- slope and cool, cloudy weather will
volved in the installation of gypsum flatten it out.
Whenever the soil moisture changes,
blocks. Install them when the soil is
moist if at all possible. It is easier, and
the blocks will reach equilibrium with
the soil in less time.
Reading schedules and records
Read blocks every two days for best
results. Plot the readings on charts
(Figure 8) and place charts at a convenient,
frequently visited location.
Charts are available from county Extension offices.
Have one person responsible for the
reading. High school students can do
a very satisfactory job. In some counties, farmers have organized and hired
the meter readings change. By observing the change in meter readings at different soil depths, the adequacy of an
irrigation becomes apparent. On July
20 the irrigation wet the 6-inch depth
but not the 12. It, therefore, did not
refill the soil profile. On August 8
all depths were refilled.
When the available moisture-holding
capacity of a soil is known, a chart
similar to Figure 9 can be used to predict the amount of moisture required
to fill the root zone. Charts for specific
soils can be obtained from the Soil
Physics Laboratory, OSU, by request
one person to take and chart all of through a county Extension agent. To
their readings.
Interpretation of readings
As a general irrigation recommendation, one-half of the root system should
be kept wetter than 2 bars tension.
6
use the chart, note that readings made
with an "Irrigage" meter on August 8
in Figure 8 were 6" = 50, 12" -=-- 90,
18"= 120, 24"= 190. Then use the
procedure shown on pages 10 and 11.
Figure 5.
Location of gypsum block installations in a uniform field under sprinklers.
Figure 6. Location of gypsum block installations in
a uniform field under corrugations.
7
Figure 7. Left(1) Soak the blocks in water for two to three minutes; (2) d:g a hole with a Vs" soil
probe; (3) make a soil and water slurry of creamy consistency and place one or two tablespoons of
slurry in the hole. Right(4) Push block to the bottom of the hole, forcing slurry to come up around
block in each hole and
it; (5) backfill and tamp two or three inches at a time; and (6) install only one
fasten the leads to a stake.
8
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APRIL
MAY
JUNE
JULY
AUGUST
SEPTEMBER
OCT
IN
2 IN
I
IN
20
Stoke No
Farm
Crop
25
30
0
5
20
Year
Soil Typ.e.
County
25
962
31
1
5
10
15
20
RAINFALL
25.
30
5
10
IRRIGATION
15
CI
20
25
Scale
31
5
10
1/2" Vertical
15
20
25
31
5
10
I" Moisture
Figure 8. Typical soil moisture chart. Actual charts are twice this size.
15
20
25
30
5
o
200
"
310
570
890
This example is for
180
soils storing 2.4 inches
1,240
of available wafer
per foot of depth
Rader" 160
" lrrigage" 140
2,100
or
3,200
"Coleman" 120
meter
reading 100
4,800
80
7,000
60
11,000
40
18,200
20
37,000
55,000
0
Resistance
in
ohms
.2
.3 .4
.5
.6 .7
Inches of water required
.8
to
.9 1.0
11
12 13
refill six inches of soil
Figure 9. Converting meter readings to irrigation requirements.
Procedure for determining amount of water to apply:
1. Take meter reading (Irrigage) for depth measured and move across chart to
right until callibration curve is reached.
9. Drop straight down and read amount of water required to bring this depth
back, to field capacity.
3. Do the same for each depth.
4. Then :
Add 0.4" (for the 0" to 3" of soil depth).
Total these moisture requirements.
.-c. Increase the total amount by A to allow for evaporation, wind distortion
of sprinkler pattern, etc.
5. The total is the approximate amount of water required to bring the soil profile
back to field capacity.
10
Example of calculations
Depth of
unit
6"
12"
18"
24"
Plus
Soil depth
measured
Meter
reading
Inches H20
0" - 3"
9,,
9" - 15"
15" - 21"
21" - 27"
Not measured
0.4
0.8
0.6
0.5
0.0
50
90
120
190
required
for application losses -=
2.3
0.7
=
3.0
Total irrigation required
If, after applying the calculated irri- blocks at similar depths located near
gation requirement, the soil does not each other, the cause should be deterreturn to field capacity at all depths, it mined. The difference may be caused
may be because of uneven distribution by poor distribution of water, differfrom the sprinklers, or the moisture- ences in root concentrations surroundholding capacity of the soil may differ ing the installations, or soil differences.
from that indicated in the chart. If When the cause and importance of the
caused by using wrong moisture- problem is determined, a decision reholding-capacity information, adjust- garding corrective action may be taken.
ments can be made by evaluating block This may involve using the driest inreadings following an irrigation.
stallation as the guide for scheduling,
If there are differences of more than correcting sprinkler distribution, or re25 points between meter readings of locating the block installation.
11