Electrical Resistance 'Gypsum BlocksFor Scheduling Irrigations

Cooperative Extension Service

Oregon State University

Corvallis

Extension Bulletin 810

March 1968

1.--HARV8IT2-1

H?a MIKA/10N NI

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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 aJones plug.

COVER: Cliff Hess, Douglas County farmer, explains his soil moisture-measuring program at an irrigation meeting.

Cooperative Extension Work in Agriculture and Home Economics, F. E. Price. Director, Ore-gon State University and the U. S. Department of Agriculture, cooperating. Printed and dis-tributed in furtherance of the Acts of Congress of May 8 and June 14, 1914. 3-63 --6M.

Electrical Resistance Gypsum BlocksFor Scheduling Irrigations

MARVIN N. SHEARERExtension Irrigation Specialist

Oregon State University

Electrical resistance gypsum blockshave been used by farmers in Oregonsince 1952. They have proved an effec-tive aid to efficient irrigation on farmswhen used according to recommendedprocedures. Oregon State Universityhas conducted extensive research todetermine what characteristics makeequipment convenient and reliableunder Oregon conditions. The Delm-horst gypsum blocks referred to in thiscircular were evaluated by OSU in1962 and were the only type sold corn-

Figure 2. Cut-away of Delmhorst blocks showingconcentric electrode arrangement.

mercially in quantity in Oregon at thetime of writing. For this reason, onlylimited reference has been made toother types which have been developedthrough the years. A wide variety ofmeters and equipment can be used withthese moisture-measuring blocks. It hasbeen necessary to use trade namesthroughout this bulletin, but no en-dorsement is made of one brand overanother, nor is criticism intended ofequipment not mentioned.

Equipment

The measuring equipment consistsof a portable meter and gypsum blocks(Figure 1, page 2).

The blocks are a casting of a specialgypsum mix around two stainless steelcoils (Figure 2) that are attached towire leads extending above the ground.When a reading is made, the meter isattached to the leads and the amountof current flowing through the blockat a specified voltage is measured todetermine the amount of resistancepresent.

The amount of water in the gypsumblock will increase or decrease as theamount of water in the soil surround-ing the block increases or decreases.This results in resistance changes with-in the block, which can be interpretedin terms of moisture content of thesoil.

CalibrationFigure 3 is a calibration of the

Delmhorst cylindrical gypsum block.

3

I0000080,00060,000

40,00030,000

20,000Resistence

inohms

10,0008,0006,000

4,0003,000

2,000

4

1 2000800600

400300

200

100.2 .3 .4 .6 .8 1.0 2 3 4 6 8 10 15

Bars tension

Figure 3. Calibration of Delmhorst cylindrical gypsum block.

Irrigation recommendations are ex-pressed in terms of moisture tension(a measure of the availability of waterto plants) to make them applicableacross a range of soil textures.

Another type of calibration is shownin Figure 4. This calibration can beused on nearly all medium to finetextured soils which are not underlain

with coarse sand. It is quite useful;however, it is not as accurate as thetype of calibration shown in Figure 3.

Seasonal soil temperature changesmay cause the calibration to shiftslightly. However, for the extremesoccurring in Oregon at the depths theblocks are installed, this shift will havelittle practical significance.

200

180

160II

140I'Irri gage"or

"120"Colemanmeter 100reading

80

60

40

20

0.o 10 20 30 40 50 60 70 80 90 100

Approximate percent of availablewater in the soil.

Figure 4. Another type of calibration of Delmhorst cylindrical gypsum blocks.

Limitations

Movement of water in and out of theblock requires good contact betweenthe soil and the block. This is fre-quently difficult to maintain in coarsesandy soil, or in fine adobe soil whichshrinks and swells considerably be-tween irrigations. These soils are foundonly in isolated areas in Oregon.

Soils high in salt also may causeerratic resistance readings; however,from a practical standpoint, the blockswill work satisfactorily in any soil thatdoes not exhibit saline or alkalineproblems.

Number of installations requiredGypsum blocks measure moisture

content only in the soil immediatelysurrounding them. This measurementis projected over larger areas of thefield. It is important, therefore, thatthe location of the blocks be "repre-sentative" of the larger areas both as tomoisture-holding capacity and crop-root concentration.

A minimum installation for a sprin-kler irrigated area requiring more thanfour days to irrigate and having uni-form soil is four strings of four blockseach (total 16 blocks) usually installed

5

6, 12, 18, and 24 inches deep. Makeinstallations in pairs as shown inFigure 5. The two at the start indicatewhen to begin irrigation; those at theend indicate how dry it got beforeirrigation was completed.

In the case of surface irrigation whencorrugations or rills are used, install theblocks between the corrugations or rillsone fourth of the way from the top andbottom of the run as shown in Figure6. Make duplicate installations betweenother corrugations or rills 5 to 10 feetaway.

Install them in the area of highestroot concentration (in the row) andon knolls rather than in swales. Adaptthe depths the blocks are buried to therooting depth of the crop. Make extrainstallations when there are wide vari-ations in soil texture and the areas in-volved are important.

Installation procedureFigure 7 shows the six steps in-

volved in the installation of gypsumblocks. Install them when the soil ismoist if at all possible. It is easier, andthe blocks will reach equilibrium withthe soil in less time.

Reading schedules and recordsRead blocks every two days for best

results. Plot the readings on charts(Figure 8) and place charts at a con-venient, frequently visited location.Charts are available from county Ex-tension offices.

Have one person responsible for thereading. High school students can doa very satisfactory job. In some coun-ties, farmers have organized and hiredone person to take and chart all oftheir readings.

Interpretation of readingsAs a general irrigation recommenda-

tion, one-half of the root system shouldbe kept wetter than 2 bars tension.

6

This corresponds to 5,500 ohms re-sistance (85 on the chart in Figure 4).Some crops (fresh market for exam-ple) will respond to wetter treatmentwhile others (forage) will toleratedrier treatments.

Active roots are indicated wheneverreadings drop. By plotting all thedepths of one installation on a singlechart, the depth of rooting is immedi-ately apparentproviding the blocksare deeper than the roots. In Figure 7,

on July 10 roots were taking moistureat the 6- and 12-inch depths but not atthe 18-inch depth.

By extending the slope of the lines(Figure 8), it is possible to predictwhen an irrigation should be applieddays before it is actually needed. OnJuly 31 a reading of 50 could be pre-dicted for the 6-inch depth and 120 forthe 18-inch depth on August 7. Ex-treme hot weather will steepen theslope and cool, cloudy weather willflatten it out.

Whenever the soil moisture changes,the meter readings change. By observ-ing the change in meter readings at dif-ferent soil depths, the adequacy of anirrigation becomes apparent. On July20 the irrigation wet the 6-inch depthbut not the 12. It, therefore, did notrefill the soil profile. On August 8all depths were refilled.

When the available moisture-holdingcapacity of a soil is known, a chartsimilar to Figure 9 can be used to pre-dict the amount of moisture requiredto fill the root zone. Charts for specificsoils can be obtained from the SoilPhysics Laboratory, OSU, by requestthrough a county Extension agent. Touse the chart, note that readings madewith an "Irrigage" meter on August 8in Figure 8 were 6" = 50, 12" -=-- 90,18"= 120, 24"= 190. Then use theprocedure 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

8

Figure 7. Left(1) Soak the blocks in water for two to three minutes; (2) d:g a hole with a Vs" soilprobe; (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

it; (5) backfill and tamp two or three inches at a time; and (6) install only one block in each hole and

fasten the leads to a stake.

IN

2 IN

I IN

Stoke No Year 962Soil Typ.e.

Crop CountyFarm

1 5 10 15 20 25. 30 5 10 15 20 25 31 5 10 15 20 25 31 5 10 15 20 25 30 5

Figure 8. Typical soil moisture chart. Actual charts are twice this size.

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APRIL MAY

RAINFALL IRRIGATION CI Scale 1/2" Vertical I" Moisture

4

I

20 25 30 0 5 20 25 31

200

180

160" Rader"" lrrigage" 140

or"Coleman"

meterreading 100

10

80

60

40

20

0

This example is forsoils storing 2.4 inchesof available waferper foot of depth

.2 .3 .4 .5 .6 .7 .8 .9 1.0 11 12 13

Inches of water required to 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 toright until callibration curve is reached.

9. Drop straight down and read amount of water required to bring this depthback, 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 distortionof sprinkler pattern, etc.

5. The total is the approximate amount of water required to bring the soil profileback to field capacity.

310570890

1,240Resistance

2,100 inohms

3,200

4,800

7,000

11,000

18,200

37,00055,000

120

If, after applying the calculated irri-gation requirement, the soil does notreturn to field capacity at all depths, itmay be because of uneven distributionfrom the sprinklers, or the moisture-holding capacity of the soil may differfrom that indicated in the chart. Ifcaused by using wrong moisture-holding-capacity information, adjust-ments can be made by evaluating blockreadings following an irrigation.

If there are differences of more than25 points between meter readings of

blocks at similar depths located neareach other, the cause should be deter-mined. The difference may be causedby poor distribution of water, differ-ences in root concentrations surround-ing the installations, or soil differences.When the cause and importance of theproblem is determined, a decision re-garding corrective action may be taken.This may involve using the driest in-stallation as the guide for scheduling,correcting sprinkler distribution, or re-locating the block installation.

11

Example of calculations

Depth ofunit

Soil depthmeasured

Meterreading

Inches H20required

0" - 3" Not measured 0.46" 9,, 50 0.8

12" 9" - 15" 90 0.618" 15" - 21" 120 0.524" 21" - 27" 190 0.0

2.3Plus for application losses -= 0.7

Total irrigation required = 3.0