– Steve Orloff, Blaine Hanson & Dan Putnam –

Soil-MoistureMonitoring

A Simple Method to ImproveAlfalfa and Pasture Irrigation Management

Soil-MoistureMonitoring

Soil-MoistureMonitoring

A Simple Method to ImproveAlfalfa and Pasture Irrigation Management

Irrigation water is essential for profitable cropproduction in most of the arid West. Proper irrigationmanagement is key for high yields and to avoid stressfrom too much or too little water. Improper irrigationmanagement limits yields more often and to a greaterdegree than any other production factor. Perhaps thereason why irrigation practices often fall short ofoptimum is that nearly all the action occurs inthe soil out of our view.

Determining when to irrigate and how much waterto apply are not simple tasks. How can you assesswhether irrigation practices are correct? Thispublication provides information on a relativelysimple and effective method for managing irrigationon alfalfa and irrigated pasture.

What’s the BestStrategy for IrrigationScheduling?

The decision of when to irrigateis usually based on past experiences,weather-based information (cropevapotranspiration data), or soil-basedmeasurements. Past experiences maynot be correct and are often not adjustedfor annual changes in weather.

Scheduling irrigations based on cropevapotranspiration can be difficultbecause, unlike other crops with asingle harvest per season, the multipleharvests of alfalfa and pasture confoundthe process. Irrigation water cannot beapplied too close to a cutting, and fieldsobviously cannot be irrigated while thecrop is curing. Therefore, there istypically a 6- to even 20-day periodduring which fields cannot be irrigated.This can make irrigation schedulingusing weather-based informationproblematic. In addition, it may bedifficult to obtain accurate data forsome locations and, even when data areavailable, the task of keeping track ofevapotranspiration data for individualfields can be time consuming. Becauseof the difficulties and shortcomings ofthese methods, soil-based irrigationscheduling may be the preferredtechnique.

Soil-based measurements may befar more practical and easy-to-use foralfalfa and pasture producers. Soilmoisture content often goes unchecked.If soil moisture is monitored, it isusually only done using a shovel or soilauger. While better than nothing, usinga shovel or auger is imprecise and isonly useful for a gross evaluation of thesoil moisture content in the upper foot,or less, of soil. Several inexpensivetechnologies may help growers monitorsoil moisture.

Funding provided by:U.S. Department of the Interior, Fish and Wildlife Service under the authority of theKlamath Act and Klamath River Basin Conservation Area Restoration Program,administered and implemented by: The Siskiyou Resource Conservation District.

In cooperation with:University of California Cooperative ExtensionScott River Watershed Council

Published by:University of California Cooperative Extension, Siskiyou County1655 S. Main Street, Yreka, California 96097

Authors:S. Orloff, UC Cooperative Extension 1655 S. Main St., Yreka, CA 96097;B. Hanson, Dept. Land, Air, and Water Resources; and D. Putnam, Dept. Agronomyand Range Science, University of California, One Shields Ave., Davis, CA 95616

Additional Information:For additional information and an Excel Spreadsheet to graph data see:http://alfalfa.ucdavis.edu.

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IntroductionIntroduction

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Expressing SoilMoisture Content

Soil moisture levels can be expressedin different ways, depending largely onthe instrument used. Soil moisturecontent is often expressed as a percent(the weight of the water in the soildivided by the weight of oven-dry soilx 100). Other soil moisture monitoringdevices use soil moisture tension toindicate soil moisture levels. Soilmoisture tension refers to how stronglywater is held on soil particles—thehigher the tension the more difficult itis for plant roots to extract water from

➤ When do I startirrigating?

This is often a difficult decision.The soil profile is often filled fromwinter rains. But after the cropresumes growth in spring and theweather warms, you must decidewhen the soil moisture is depletedenough to require irrigation. Mostirrigation systems do not have thecapacity to “catch up” when the soilprofile has been excessively depletedin spring.Answer: The first irrigation shouldoccur when soil moisture tensionreaches the recommended value foryour soil type (see table on nextpage).

➤ Do I irrigate againbefore harvest?

Alfalfa is most sensitive to waterstress after cutting when the plantsstart to regrow. Resistance blockreadings help assess whether thesoil-moisture content is sufficient toavoid water stress through theduration of the harvest period untilirrigation can be resumed.Answer: Track soil moisture topredict the rate of soil moisture

the soil. Therefore, low soil moisturetension indicates moist soil and highsoil moisture tension indicates drysoil. Soil moisture tension is usuallyexpressed in centibars. For some typesof soil moisture resistance blocks, theinstrument readings must be convertedto soil moisture tension using appropri-ate calibration relationships.

Resistance Blocks –A Useful Tool

More than six years of field studiesin the Scott Valley (a high-elevation

valley in northern California) demon-strated the usefulness of soil moisturemonitoring in alfalfa and irrigatedpasture fields. These studies showedthat significant improvements inirrigation management were possiblein many fields by monitoring soilmoisture levels and adjusting irrigationpractices accordingly. Electricalresistance blocks provided a reliableindication of soil moisture levels andare a cost-effective tool for effectiveirrigation management.

depletion. By graphing the data andextending the line through the harvestperiod, you can anticipate soil moistureloss during harvest. If anticipated soilmoisture levels fall well below therecommended values for your soil type(see table on next page), an irrigationor partial irrigation may be desirablebefore cutting.

➤ Did I fill the profile?Soil moisture sensors are very useful toassess the moisture status at the lowerend of the root zone—especially for adeep-rooted crop like alfalfa. The lowerhalf of the root zone supplies moisturereserves to draw upon when needed andshould not be excessively depleted.Answer: If the soil profile is filled afterirrigation, soil moisture readings at alldepths should return to less than 10centibars for a sandy soil and less thanabout 30 centibars for medium/fine-textured soil. If the sensors do notrespond after irrigation the water didnot penetrate to the depth of the sensor.Monitor soil moisture after eachirrigation to determine how manyirrigations are needed to refill theprofile.

➤ Should I change myirrigation practices?

Soil moisture monitoring is helpfulto verify that current irrigationpractices satisfy, but do not exceed,the needs of the crop. A graph ofsoil moisture readings over theseason provides a sound basis foraltering and fine-tuning irrigationpractices.Answer: Plot soil moisture readingson a graph (see example graphs onpage 5). The lines on your graphshould oscillate as they do in Figure1. The highest soil moisture tensionreached should be the values whereirrigation is recommended (see tableon next page). Then, followingirrigations, the values should returnto less than 10 for a sandy soil andless than about 30 for a medium/fine-textured soil. If values exceedthe recommended range, the soil isbecoming excessively dry betweenirrigations and the field should beirrigated more frequently or withmore water per irrigation. If valuesare low (indicating adequatemoisture), irrigation can be skippedor delayed until soil moisturesensors indicate irrigation is needed.

Ask Questions AboutWater Management!Ask Questions AboutWater Management!

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Resistance BlocksElectrical resistance blocks (also

called gypsum blocks or resistanceblocks) are not new, but recent advanceshave improved their accuracy and ease-of-use. Resistance blocks evaluate soilmoisture tension by measuring theelectrical resistance between twoelectrodes. The blocks take up andrelease moisture as the soil wets anddries. The higher the water content ofthe blocks, the lower the electricalresistance. The electrical resistance ismeasured with a portable meter that isconnected to wire leads coming fromthe moisture sensors (Figure 4).Because there is a known and consistentcalibration between electrical resistanceand soil moisture for the Watermark®

block (a resistance block made byIrrometer Company, Inc.), it can closelyestimate soil moisture tension incentibars.

Interpreting the resis-tance block readings

An important point to understandwhen using the Watermark® sensors isthat the lower the reading the higher thesoil moisture content, and conversely,the higher the reading the lower themoisture content. When the soil issaturated after a rainfall or irrigation(air spaces are mostly filled with water)the Watermark® reading is low, typicallyless than 5 to 10 centibars. As evapora-tion from the soil surface and transpira-tion by the crop dry the soil, themoisture sensor readings graduallyincrease until they indicate need foran irrigation.

The centibar reading at whichirrigation is necessary depends on soiltype (see table). Sandy soils retain farless water than soils with a high clayor organic matter content, so irrigationon sandy soils should occur morefrequently and at a lower soil moisturetension value. Soil moisture sensorsmay not be useful for very sandy soilswith extremely low water holdingcapacity, as the sensors might not

Using Soil Moisture Data

Recommended Values at Which to IrrigateAlfalfa and Pasture for Different Soil Types

Soil Type Moisture Reading (centibars)

Sand or loamy sand 40–50*Sandy loam 50–70Loam 60–90Clay loam or clay 90–120

*Caution: Soil moisture sensors may not be useful for very sandy soils withextremely low water holding capacity, as the sensors may not respondquickly enough to the rapid decline in soil moisture.

Note: These values were based on 50% depletion of available soilmoisture for different soil types.

respond quickly enough to the rapiddecline in soil moisture.

After the field is irrigated thecentibar readings typically return tothe teens or single digit values. Thesewetting and drying cycles continuethroughout the season as the crop isirrigated and then the soil dries withcrop water use. The key to properirrigation management using soilmoisture sensors is to monitor thesensors regularly, track the soil mois-ture level, and irrigate when thecentibar readings are in the desiredrange for your soil type (See Figure 1).Irrigating when the soil moisturereadings exceed the desired range mayresult in crop stress and yield loss.Irrigation before the readings reach thedesired range may result in excessiveirrigation, water wastage or runoff.

Using and interpretinga soil-moisture graph

The best way to use the soil-moisturemeasurements is to plot them on agraph. The plotted data present apicture of the soil moisture status andindicate how fast the soil is drying.After a few points are plotted, you canestimate approximately how many daysit will take for the soil to dry beforeirrigation is needed. Plot the datafrom each Watermark® sensor with a

different color line so that the variousdepths can be easily distinguished.An Excel-based spreadsheet templatefor entering field data is availablefor free from the authors or fromhttp://alfalfa.ucdavis.edu.

We prefer inverting the y-axis (thecentibar readings) so that zero is at thetop of the graph instead of the bottom(Figure 1). This simplifies interpreta-tion. Using this orientation, the line onthe graph drops as the soil dries andrises when the soil is wet. This patternis more consistent with how oneconceptually thinks of changes in soilmoisture content.

The graph in Figure 1 is an exampleof effective irrigation management andillustrates how readings typicallyfluctuate from spring through the firstalfalfa cutting. Soil moisture at theuppermost sensor normally declinesfirst, as there is greater root activity inthe upper portion of the soil profile thanat deeper depths. There was also farmore fluctuation in soil moisture at theshallow depths (1 and 2 feet), as notedby the oscillation in the lines, than atthe deepest depth (4 feet). Again, this isbecause of more extensive root activityin this zone. Figure 1 illustrateseffective irrigation management—irrigations occurred when the1-foot sensor readings reached theappropriate range for this soil type(60–90 centibars). Figures 2 and 3illustrate inadequate and excessiveirrigation examples, repectively.

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IrrigationFigure 1.Proper IrrigationManagement.Shaded area indicates where irrigationsshould occur. At the start of the season thesoil is moist from winter and spring rains—the readings are less than 20 centibars (A).Gradually the soil dries and the readingsincrease, beginning with the sensor locatedat the one-foot depth followed by thedeeper depths. When the soil moisturereading dropped to near 80 in early May (B),irrigation water was applied and the centibarreadings at all three depths went to below20, indicating the soil profile had beenrefilled. The drying cycle resumed until apartial irrigation occurred in late May (D).A partial irrigation was needed to replenishenough soil moisture to carry the cropthrough the harvest period without excessivesoil moisture depletion and crop stress.The first cutting occurred in early June[note point on graph when soil moisturecontent was lowest; (E)]. Following cutting,irrigation resumed until the soil moisturecontent at all three depths was restored(all readings below 20 centibars at point F).

Figure 2.Under-irrigation example.Soil moisture tension levels for an irrigatedalfalfa/grass field. The first irrigationoccurred too late, and soil moistureremained extremely low for much of theseason. Not until late August was the soilprofile refilled—too late for much benefit tothe crop. This field would have benefittedfrom careful monitoring of soil moistureand timely early irrigation.

Figure 3.Over-irrigation example.Soil moisture levels for an irrigated alfalfafield. Soil moisture tension never exceeded50 centibars and remained below 30centibars for most of the season. (Irrigationshould occur at 60–90 centibars; shadedarea). One or two irrigations could havebeen eliminated or less water applied perirrigation.

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InterpretingSoil MoistureData

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Why Monitor SoilMoisture?

Soil moisture tension data canhelp ‘train’ the manager or irrigatorto make wise irrigation decisions.Combined with observations of thecrop and other irrigation schedulingtechniques (e.g. the ‘checkbook’method), soil moisture blocks canhelp you know what is happening inthe soil, and to ‘ground truth’ yourdecisions. It can help answer questionssuch as:

When Do I StartIrrigating?

Figure 2 shows a real-life example ofa field where irrigation was delayed toolate in the spring. The first irrigation didnot occur until early June, when the soilwas way too dry (160 centibars at the6-inch sensor). It is likely that moisturestress occurred and yield was reduced—monitoring would have triggered anearlier irrigation decision.

Is There EnoughDeep Moisture?

It is important to have adequate soilmoisture in the lower half of the rootzone. Deep moisture serves as a reservefor use when needed, such as harvestperiods during the hot summer. Whilethe field in Figure 1 shows plenty ofsubsurface moisture, Figure 2 shows afield where moisture was excessivelydepleted at lower depths, stressing theplants. Information about a lack ofdeep moisture may enable a grower totake corrective action to refill theprofile.

Am I ApplyingEnough Water?

Under-irrigation is a common hazardof forage production. Figure 2 shows areal-life example of a field that waswatered too-infrequently throughoutmuch of the growing season. Many ofthe readings are above 160 centibars,indicating very dry soil. All but the last

irrigation was inadequate to wet theentire profile. Soil moisture monitoringindicates the grower should irrigatemore frequently to correct this situation.

Am I Watering atthe Wrong Time?

In some cases, water is applied at atime when it’s not needed by the crop.In Figures 2 and 3, the growers irrigatedlate in the season (August) to fill theprofile. However, irrigation was mostlikely unnecessary because the growingseason was nearly over, and the cropneeded less water. These irrigationsprobably could have been saved withoutreducing yield. Soil monitoring canhelp indicate when to stop watering,as well as when to start.

Am I WateringToo Much?

Figure 3 shows over-irrigation, ortoo-frequent irrigation. Soil moisturetension never exceeded 50 centibars,and for most of the season remainedbelow 30 centibars, indicating acontinually wet soil (the grower shouldhave waited until soil moisture tensionwas 60-90 centibars). Maintaining thesoil too wet resulted in higher thannecessary pumping costs and anincreased infestation of the summerannual weeds lambsquarters andpigweed. Based on this graph, one ortwo irrigations could have been omitted,or less water applied per irrigation,without reducing yield.

Match Irrigationto Crop Needs!

The principle behind irrigationmanagement is to irrigate only enoughto meet crop needs. Irrigation mistakesare easy to make, even for experiencedgrowers. Soil moisture sensors improvethe chances of making the rightirrigation decisions.

Importance of AvoidingOver- or Under-IrrigationOver-Irrigation

Yield lossDiseasesNutrient losses through leachingWeed encroachmentPower or water costsEnvironmental concerns

Under-IrrigationYield lossFrost injury increasedWeed encroachment

Using Soil Data to GuideIrrigation DecisionsUsing Soil Data to GuideIrrigation Decisions

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Importance of AvoidingOver- or Under-Irrigation

Site SelectionA critical part of installation is

selecting an appropriate site for thesensors. Since irrigation of the wholefield is based on the sensor readings,the sensors need to be in an area thatrepresents the field. Locate the sensorsin an area having the soil type typicalof the field, uniform crop growth, andin an area that receives full uniformsprinkler or flood-irrigation coverage.If the sensors are placed in an area thatis not representative, the resultscan be very misleading.

One sensor site per field is usuallysufficient; however, if the field isvariable, two sites are desirable. It maybe helpful to install one of the sensorsin a slightly sandier area of the fieldto use as an early indicator of whenirrigation may be required.

Sensor PlacementProper sensor placement is critical

for accurately representing soil mois-ture in the crop root zone. We recom-mend installing two or three sensors ateach evaluation site (Figure 5). One

Figure 4. The Watermark® soil moisture sensor is an exampleof an electrical resistance block. Resistance blocks evaluate soilmoisture by measuring the resistance between two electrodesimbedded in the sensor (A) using a calibrated portable meter (B).The blocks absorb moisture as the soil wets and release moisture asthe soil dries—the higher the water content, the lower the electricalresistance.

Figure 5. Twosensors at eachevaluation site areacceptable butthree are preferredbecause it providesa more completepicture of the soilmoisture status atdifferent depths,allowing theirrigator todetermine thedepth of the lastirrigation. Foralfalfa install sensorsat 1 ft., 2 ft., and 4ft. (9 inches, 1.5 ft.and 3 ft. in shallowsoils). For pastureinstall sensors at 6in., 1 ft. and 2 ft.

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sensor should be in the upper onequarter of the root zone. Another shouldbe toward the bottom of the root zone.When three sensors are used in analfalfa field, install them at 1 ft., 2 ft.,and 3.5 or 4 ft (depending on the depthof the soil). In cases where the rootingdepth is more restricted, installations at9 in., 1.5 ft., and 3 ft. are recommended.Because of shallower rooting inirrigated pastures, the sensors shouldbe installed at 6 in., 1 ft., and 2 ft.

Three sensors present a morecomplete soil moisture picture andallow better evaluation of the depthreached with each irrigation.

The uppermost sensor indicateswhen to irrigate. The second sensorhelps determine the depth of the lastirrigation and is a check for the firstsensor to make sure it is operatingproperly. The deepest sensor measuresmoisture reserves deep in the root zone.Maintain the deepest sensor within anacceptable range—not too dry (over90 centibars) or not too wet (saturatedconditions, readings less than 5 or10 centibars).

Installation and ManagementRecommendationsInstallation and ManagementRecommendations

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Photo by Suzanne Paisley

InstallationUse a probe or coring device to

create a hole slightly larger than thediameter of the sensor (See Figure 6).The sensor must be in direct contactwith the surrounding soil. To ensurecontact, prepare a small slurry (mixtureof the surrounding soil and water) topour into the hole before seating thesensors. As the sensor is pushed intothe hole, the slurry at the bottomsqueezes up around the sensor toprovide good contact between thesoil and sensor.

The wire leads come up from thesensors to the soil surface. Carefullyback fill the hole so as not to damagethe wires. A 4- to 6-inch length of2-inch PVC with a screw-on cap workswell and protects the wire leads froma swather or cattle. Dig a shallow (3- to6-inch) trench for the wire leads comingfrom each sensor to the PVC housing.You can color code the wires to keep

Photo by Clyde Elmore

track of the depth of the sensors bytying a small piece of colored wire toeach lead.

An alternative installation involvesgluing a section of PVC pipe ontoeach sensor and feeding the wiresthrough the pipe (Figure 7). Thisinstallation allows the sensors to beretrieved and reused. One-half inchClass 315 PVC has an inside diameter

Figure 6. This steel rod is pounded into thesoil to create a hole slightly larger than thediameter of the sensor. A coring device canalso be used. Direct contact between thesensor and the soil is essential for reliable soilmoisture readings.

Figure 7. Soil moisture sensors can beglued to the end of PVC pipe. Wire leadsfrom the sensor are connected to boltsprotruding from a PVC cap for easy accesswhile taking readings. This installationallows the sensors to be retrieved andreused.

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shown that soil moisture sensors arevery useful to diagnose changesneeded and to fine-tune irrigationpractices. Relatively minor adjustmentsin irrigation practices could paylarge dividends in increased yield orwater savings.

Photo by Suzanne Paisley

that fits exactly over the top of thesensor collar. Use PVC to ABScement to weld the sensor to thePVC. A cap can be fitted directly onthe pipe to house the wire leads andthe depth of the sensor engraved onthe cap.

How Often to TakeSoil Moisture Readings

How often you take soil moisturereadings depends on their intendeduse. Weekly readings should providean overall picture of the seasonalsoil-moisture status of the field forevaluating current irrigation practices.However, if the Watermark® readingsare used for irrigation scheduling,readings should be taken at least twicea week, especially immediately beforeand after an irrigation. Commercialdata logging devices are availablethat constantly record readings atpredetermined intervals.

ConclusionIrrigation management for

alfalfa and irrigated pastures canbe difficult—growers must schedulearound harvests, making it problematicto use many irrigation-schedulingtechniques. However, years ofresearch and field experience have