Soil News 09

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INSIDE

2009

Complexitiesof Soil MoistureData

Two probes buried at thesame depth in the samefield give different watercontent readings. Atanother site, a probe inbone dry soil measuresnear-saturation watercontent. Find out whyyou might not get theinformation you expectfrom your soil moisturesensors and figure outwhat they’re really saying.

Five Optionsfor EffectiveProfiling

Evaluate strategies formonitoring the moisturein soil profiles. Discoverhow a little digging canproduce an array ofsingle sensors thatmimic—and evenimprove on—measurements fromexpensive all-in-oneprofile probes.

SixInstallationTips

Save time, reducedigging, outsmartrodents, and make yourtime in the field a littleeasier with these tipscontributed by every dayusers. Also find a link tocontribute a useful tip ofyour own. If it's new tous, we'll publish it in thenext newsletter and sendyou a free sensor.

$20,000 Worthof InstrumentsAwarded

On January 15th, theG.A. Harris ResearchInstruments Grantcommittee awarded over$20,000 worth ofinstruments to threegraduate students. Readabout the winningprojects and find outhow you or a graduatestudent you know canapply for next year'sFellowship.

DDECAGONDEVICES

“When the well is dry, welearn the wor th of water.”

—Benjamin Franklin

With 60% of the world's fresh water used forirrigation, soil moisture research willultimately impact how well this thirstyplanet deals with its impending water crisis.

SOIL MOISTURE APPLICATIONSElements

go to page 2

go to page 7go to page 11 go to page 12

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e sometimes get calls fromcustomers (even soil scientists)

concerned that their ECH2O probes arenot accurate because probes at different

locations in the same field have differentwater content readings. In contrast, onecustomer, a retired engineer who had justinstalled probes in his centralWashington vineyard, was delighted tosee the variation in water content acrosshis land. “Now I can tell where thosepockets of sand are,” he said. Evenwithout formal soil science training, herealized that spatial variation in watercontent represents valuable informationabout soil texture, watering patterns, andwater use.

It’s helpful to distinguish variation in thevertical from variation in the horizontal.Most people expect strong verticalvariation due to wetting and dryingpatterns, soil horizonation, andcompaction. Water content can varydrastically over distances of only a fewcentimeters, especially near the soil

Surprise! The Probe is Right

surface. Horizontal variation is typicallyless pronounced—in a bare or uniformlyplanted field at a given depth, it mightbe quite small. But surprisingly largevariations can exist, indicating isolatedpatches of sand or clay or differences intopography. The retired engineer noticeda few sensors indicating low watercontent after a heavy rain that haduniformly wetted his vineyard. Knowingthat sand has a low field capacity watercontent, he surmised (correctly) that hehad found the sandy areas in hisvineyard.

Because properly installed dielectric soilmoisture sensors lie in undisturbed (andtherefore unanalyzed) soil, theysometimes measure unexpected things.

FindingExplanations for

DisagreementbetweenSensors

Surprisingly large variations in soil moisturerepresent valuable information.

SPATIAL VARIATION INSOIL WATER CONTENT READINGS

WSOIL MOISTUREAPPLICATIONS

Elements

SensorsDiscover theUnexpected

A retiredengineer wasdelighted to seethe variation inwater contentacross his land."Now I can tellwhere thosepockets ofsand are!"



One researcher buried a probe in whatappeared to be a very dry location andwas startled to measure 25 to 30%volumetric water content. Those readingsmade the soil appear saturated, butobviously it wasn’t. She dug down to thesensor and found a pocket of clay. As shediscovered, it is impossible to get muchinformation from an absolute watercontent measurement without knowingwhat type of soil the sensor is in.

So, since we expect variation, how do weaccount for it? How many probes areneeded to adequately characterize thewater content in an application orexperiment? There is no simple answer tothis question. The answer will be affectedby your site, your goals, and how youplan to analyze your data. Here are somethings you might consider as you plan.

What information do you have when youknow a field’s volumetric water content?That number independently tells anirrigator very little. Soil moisture can beused like a gauge to show when a field isfull and when it needs to be refilled, butthe “full” and “empty” are only

SPATIALVARIATION INSOIL WATERCONTENT

Planning forVariation

How manysensors doyou need? meaningful in context. How far can you

go on a quarter of a tank of fuel? You’llonly know after you’ve driven the car fora while.

The goals of irrigation are to keep rootzone water within prescribed limits andto minimize deep drainage.Understanding and monitoring thevertical variation lets you correlate a real-time graph of water use data with above-ground field conditions and plant waterneeds. It makes sense to place probesboth within and below theroot zone.

Soil moisture can be used like a gauge to show when a field is fulland when it needs to be refilled.

Volumetricwatercontentreadingsare onlymeaningfulin context.

Using SoilMoisture as a

Gauge



By contrast,measuringhorizontalvariation—placing

sensors at different spots in the field—isnot very helpful. If a field will be irrigatedas a unit, it should be monitored as aunit at one representative spot. Becausethere’s no way to adjust waterapplication in specific spots, there’s nobenefit to quantifying spatial variation inthe horizontal. Like a float in a fuel tank,a set of soil moisture sensors in the rightspot will adequately represent thechanging soil moisture condition of thewhole field. We recommend a singleprobe location in each irrigation zonewith a minimum of one probe in the rootzone and one probe below it. Additionalprobes at that site, within and below theroot zone, will increase the reliability ofthe information for the irrigationmanager, at minimal additional cost.

In some research projects, it will beimportant to account for horizontalvariation. So, how variable is the watercontent across a field? We conducted anexperiment in which we set out atransect across a field of bare, tilled soil.

Crop StudiesRepresentingVariation in a

HomogeneousEnvironment

Like a float in a fueltank, a set of soilmoisture sensors inthe right spot canrepresent changingconditions in thewhole field.

SOIL MOISTUREAPPLICATIONS

Elements

N = number of observations requireds = standard deviationd = magnitude from the difference from the meanwhen a/ = 0.05, 0.10, 0.50then xa/ = 1.960, 1.645, and 0.842, respectively

N = a/x2 s2

d2

You can determinehow many samplesare necessary tocharacterize ahomogeneous areain about an hourusing an EC-5and a ProCheck

Figure 1



Using a Decagon EC-5 soil moistureprobe connected to a Procheck meter, wesampled water content at one meterintervals over a 58 meter distance. Theindividual readings are shown in Figure 1.In this data set, the samples are notspatially correlated as shown by theapparent variation between theindividual samples. The mean watercontent of the data set is 0.198 m3/m3

with a standard deviation of 0.023 m3/m3

and a coefficient of variation of 12%.

Using some simple geostatistics (seeequation on page 4), we determined thatthree carefully placed sites wouldadequately represent the variationpresent in this very homogeneousenvironment. Of course, in someenvironments, samples will not beindependent. If a semivariogramindicates that some underlying spatialfactor influences soil moisture variability,you will have to consider that in yourexperimental design.

On a forested hillside, horizontalvariation in soil moisture will obviouslybe significant. Determining how manysensors to use and where to place them

Comparing Datafrom DifferentSites or Strata

is not at all trivial.Stratified sampling—

systematically sampling from moreuniform subgroups of a heterogeneouspopulation—may be a better way to dealwith this kind of variety. The researcherclassifies the site into strata (eg. forestedcanopy, brush, hillside, valley), andevaluates the number of samples neededto statistically represent the variationpresent within each stratum. Manypeople allow for the variation in soilmoisture values that come from slope,orientation, vegetation, and canopycover. Some fail to consider theimportant soil-level variations that comefrom soil type and density.

By taking into account the major relevantsources of soil moisture variation, youcan plan enough sampling locations todraw reasonable conclusions from yourdata. Choose too few locations, and yourun the risk of missing the patterns thatwill lead to higher level understanding.Choose too many, and not only will yoube unable to afford your experiment, youmay miss the patterns altogether as yourexperiment overflows with randomabundance.

Monitor importantsoil-level variationsthat come fromdiversity in soil typeand density.


ElementsSPATIALVARIATION INSOIL WATERCONTENT

Ecology StudiesUsing Stratified

Sampling inHeterogeneous

Environments



Comparing absolute water contentnumbers can give confusing results. Bothmeasurements are volumetric watercontent, but 35% here vs. 15% thereactually tells us very little. Was the site insand or clay, or something in between? Ifconditions at the two sites are virtuallyidentical, the comparison may makesome sense. But often, researchers wantto compare dissimilar sites. Waterpotential measurements determined byconverting absolute volumetric watercontent to soil water potential using amoisture characteristic curve specific to

Next: Can you sample the profilewithout a profile probe? æ

Use relative

values—quantites

of water used in

centimeters, for

example-to

compare

moisture content

at dissimilar

sites.


ElementsSPATIALVARIATION INSOIL WATERCONTENT

each soil type can be used to compareresults across sites. Comparing relativevalues—quantities of water used incentimeters for example—can also beboth useful and valid.

An article in this issue (Sampling Moisturein the Soil Profile) details an experimentwhere water content measurements weremade over a growing season at 30, 60,90, 120, and 150 cm below a wheatcrop. The graph of soil moisture datashows how water is taken up fromsuccessively deeper layers. By subtractingone profile from another and summingover the layers where change occurs (forinstance, in Figure 2, subtract the far leftline from the far right line to see howmuch water was used from May 10th toAugust 21st), you can determine theamount of water used by the plants overa particular period. If similar data weretaken at different sites or in differentstrata, these relative values, in terms ofquantified water use, could form thebasis of solid comparison studies.

Figure 2

You can use soilmoisture profilesplotted at differenttime periods todetermine wateruse over time.

Figure 2



atterns of water replenishment anduse give rise to large spatialvariations in soil moisture over the

depth of the soil profile. Accuratemeasurements of profile water contentare therefore the basis of any waterbudget study. When monitoredaccurately, profile measurements showthe rates of water use, amounts of deeppercolation, and amounts of water storedfor plant use.

Figure 3 (next page) shows water contentmeasurement data collected at one of the

sites located onDecagon’sexperimental plotsat Cook Farm,Pullman,Washington. Thesemeasurementswere made using5TE probesinstalled one per

CommonObstacles to

AccuracyInstallation Pitfalls

to Avoid

hole at 30, 60, 90, 120, and 150 cmdepths below a winter wheat crop. Toinstall the sensors, we augured to thecorrect depth, pressed the probe (usingthe simple homemade installation toolshown on page 11) into the undisturbedsoil at the bottom of the hole, backfilledwith about six inches of native soilpacked to the approximate bulk densityof surrounding soil, sealed the hole with200–300 g of bentonite to prevent waterfrom flowing preferentially down thesensor cable, and refilled the hole withnative soil tamped to the approximatebulk density of surrounding soil.

Three common challenges to makinghigh-quality volumetric water contentmeasurements are: 1) making sure theprobe is installed in undisturbed soil,2) minimizing disturbance to roots andbiopores in the measurement volume,and 3) eliminating preferential waterflow to and around the probe. Alldielectric probes are most sensitive at thesurface of the probe. Any loss of contactbetween the probe and the soil orcompaction of soil at the probe surfacecan result in large measurement errors.

An ExampleInstallation

Profilemeasurements showthe rates of wateruse, amounts of deeppercolation, andamounts of waterstored for plant use.

PSOIL MOISTUREAPPLICATIONS

ElementsSAMPLING MOISTURE IN THE SOIL PROFILE

FIVE OPTIONS FOREFFECTIVE PROFILING





Figure 3. Soil water content in winter wheat measured at 30 cm increaments.

It’s easy todetermine exactlywhen the wheatstarted taking inwater at deeperdepths in a drylandfield.



Water ponding on thesurface and running inpreferential paths downprobe installation holescan also cause largemeasurement errors.

Installing soil moisturesensors will always involve some digging.How do you accurately sample the profilewhile disturbing the soil as little aspossible? Let’s consider the pros andcons of five different profile samplingstrategies.

Profile probes are a one-stop solution forprofile water content measurements. Oneprobe installed in a single hole can givereadings at many depths. Profile probescan work very well, but properinstallation can be tricky and thetolerances are tight. It’s hard to drill asingle, deep hole precisely enough toensure contact along the entire surface ofthe probe. Backfilling to improve contactresults in repacking and measurementerrors. The profile probe is also especiallysusceptible to preferential flow problemsdown the long surface of the accesstube.

AugerSide Wall

InstallationExcellent method,

installation tool notcommercially

available

Installing sensors at different depthsthrough the side wall of a trench is aneasy and precise method, but unless youown a backhoe, the actual digging of thetrench can be a lot of work. This methodputs the probes inundisturbed soilwithout packing orpreferential waterflow problems, butbecause it involvesexcavation, it’stypically only usedwhen the trench isbeing dug for otherreasons or when thesoil is so stony or gravelly that no othermethod will work. The excavated areashould be filled and repacked to aboutthe same density as the original soil toavoid undue edge effects.

Installing probes through the side wall ofa single auger hole has many of theadvantages of the trench methodwithout the heavy equipment. Thismethod was used by Bogena et al. withEC-5 probes. They made an apparatus(see photos on this page) to installprobes at several depths simultaneously.

TrenchInstallationGreat results,

requires heavyequipment

CommercialProfile Probes

One-stop solution,susceptible to

installation errors

Bogena et. al's side wallinstallation tool. SOIL MOISTURE

APPLICATIONS

Elements

SAMPLINGMOISTURE IN THESOIL PROFILE

OPTIONSBEYOND ACOMMERCIALPROFILEPROBE



As with trench installation, the holeshould be filled and repacked toapproximately the pre-sampling densityto avoid edge effects. (H.R. Bogena, J.A.Huisman, U. Rosenbaum and A. Weuthen. SoilNet – AZigBee based soil sensor network and firstapplications. To be submitted in Sensors, Special

Issue: Wireless SensorTechnologies andApplications).

The method weused at Cook Farm,digging a separateaccess hole foreach depth,ensures that eachprobe is installedinto undisturbedsoil at the bottomof its own hole. Aswith all methods,you must take careto assure thatthere is nopreferential waterflow into therefilled augurholes, but a failureon a single holedoesn’t jeopardize

Single HoleInstallation

Often used but notrecommended

Next: Outsmarting Rodents æ

all the data, as it would if all themeasurements were made in a single hole.The main drawback is that a hole must bedug for each depth in the profile (five holesfor five depths, in our case). The holes aresmall, however (just large enough for oneprobe), so they are usually easy to dig.

It is possible to measure profile moistureby auguring a single hole, installing onesensor at the bottom, and then repackingthe hole, installing sensors into therepacked soil at the desired depths asyou go. Because the re-compacted soilcan have a different bulk density than ithad in its undisturbed state, and becausethe profile has been completely altered asthe soil is excavated, mixed, andrepacked, this is the least desirable of themethods discussed. Still, single holeinstallation may be entirely satisfactoryfor some purposes. If the installation isallowed to re-equilibrate with thesurrounding soil and roots are allowed togrow into the soil, relative changes in thedisturbed soil should mirror those in thesurroundings.

Multiple HoleInstallation

Good results with alittle spade work



FLEXIBILITY AND ACCURACY FROMAN ARRAY OF SINGLE SENSORS

ECH2O probes are designed tobe installed at any depth in any orientation.



1. Play with instrumentation in thelab before going out to installsensors.

2. If you can, install the sensorsthe day after a good rain. It’salways easier to install sensors intoundisturbed soil when the soil isslightly moist.

3. Use a 1⁄2 inch (1.27cm)notched PVC pipe to pushthe sensor into thebottom of a deep augeredhole.

4. Run the cablesthrough a PVC pipe fromjust below the surface tothe data logger toprevent rodent damage.

Six ECH2O Installation Tips

6

Contribute anECH2O Tip

If it's new to us, we'll publish itin the next newsletter and

send you a free sensor.

To submit an entry,click this link:


Elements

Next: Winning $20,000Worth of Sensors æ

5. If your soils are very rocky, use asensor that measures waterpotential instead of one thatmeasures volumetric water content.Because water potentialmeasurements are independent ofsoil type, water potential sensorsdo not have to be installed inundisturbed soil. Therefore, waterpotential sensors can be installedby sifting the rocks from the soiland installing the sensor in apocket of rock-free soil.

6. If you need to remove the datalogger from your field area (forplanting or harvesting purposes,for example), install anunderground valve box to store thecables until you can put the loggerout again. This also makes thesensors easier to find.Inexpensive supplies from the

hardware store can be used to createan almost perfect installation tool.

http://www.DecagonNews.com/elements-info/





Elements

Title of ProposalCharacterizing canopy developmentin alternative spring cerealproduction systems.

First PlaceLauren KolbUniversity of Maine, Orono.Ph.D. Candidate in Ecology andEnvironmental Science

StudyThere are two innovative and opposingstrategies to improve weed managementin cereals grown with minimal or noherbicide inputs (1) enhancing cropcompetition, achieved by increasing plantpopulations and sowing in a moreuniform pattern and (2) enhancingphysical weed control, achieved bysowing in wider rows than usual topermit inter-row cultivation with sweeps,i.e. growing row crops. By measuringcrop canopy development over thegrowing season, I hope to characterizehow the dynamics of leaf area index (LAI)vary between planting strategies anddetermine how this correlates to weedsuppression.

$20,000Worth of

InstrumentsAwarded

Decagon wouldlike to congratulatethe winners of the2009 G.A. HarrisResearchInstrumentsFellowship.

Title of ProposalFog, Aerosols, and Nutrient Cyclingin the Namib Desert

Second PlaceKeir SoderbergUniversity of Virginia, Ph.D. student inDepartment of Environmental Science

StudyThe Namib Desert on the southwesterncoast of Africa is hyperarid in terms ofrainfall but experiences frequent coastalfog events. The fog has been suggestedto provide sufficient water to certainplants which are endemic to the Namib,some of which occur only in the fog zone(up to 60 km inland). The GA Harrisfellowship will be used to set up five fogmonitoring stations along a climategradient in the central Namib utilizingleaf wetness, air temperature and relativehumidity measurements along with solarradiation and soil parameters (moisture,temperature, and electrical conductivity).Stable isotope analysis of samples willalso be used to help quantify theamounts of fog, groundwater and soilwater that plants utilize.

Lauren Kolb wasawarded an

AccuPar LP-80 tomeasure leafarea index ofwheat fields

with twodifferent weed

treatments.

Keir Soderbergwas awardeddata loggers,soil moisturesensors, leaf

wetnesssensors, and

otherenvironmental

sensors toquantify fog

contributions.




Elements

Printed in USA©2009 DECAGON

DDECAGONDEVICES


Elements

HarrisFellowship

Send informationabout the 2010Harris Research

InstrumentFellowship to aneligible graduate

student. 2365 NE Hopkins CourtPullman, WA 99163

1-800-755-2751http://www.decagon.com

[email protected]

The G. A. HarrisResearchInstrumentsFellowship, awardedannually, provides$20,000 worth ofDecagon researchinstruments to agraduate studentstudying any aspectof environmentalscience. The grantcommemorates thegenerosity andenthusiasm of Dr.Grant A. Harris,former chairman ofthe Department ofForestry and RangeManagement atWashington StateUniversity andformer Chairman ofthe Board ofDirectors atDecagon.

For more informationclick this link:

Title of ProposalMeasuring Carbob SequestrationProcesses in Subalpine Forest UsingWireless Sensor Arrays

Third PlaceLynette LaffeaUniversity of Colorado, PhD student inDepartment of Environmental Biology

StudyI propose that scale matters in modelingsoil respiration rates. I propose to deploya suite of soil respiration andenvironmental sensors at the Niwot RidgeAmeriFlux research site to explore atwhat scales (temporal and spatial) driversof soil respiration affect the respiratoryflux of CO2 from the forest floor. Thisproject will test our capabilities tomeasure soil environmental dynamicsacross small spatial scales and at hightemporal frequencies. We will developnew strategies for sensor deploymentand the use of wireless technology tosustain high frequency data collectionand archiving in a remote location.

Sara Baguskas,UC Santa Babara,Ecological interactionsbetween epiphyticmacrolichen (Ramalinamensiesii) and fog onSanta Cruz Island,California.

Justin Becknell,University ofMinnesota, SoilMoisture and CarbonUptake in RestoredTropical Dry Forests.

Robert Keefe,University of Idaho,Model-basedoptimization of seedgermination timing.

Toni Smith,Boise State University,Spatiotemporalvariations in soilmoisture with elevationand aspect in a semi-arid watershed; apotential control on thesoil carbon pool.

James Parejko,Washington StateUniversity, Determiningthe ecology andbiogeography ofphenazine-producingfluorescentPseudomonas spp. inthe wheat rhizosphere.

Jongyun Kim,University of Georgia,Modeling Water andFertilizer Use ofGreenhouse Crops forEfficient Irrigation.

Honorable Mention

Lynette Laffeawas awardeddata loggers, soilmoisturesensors, andotherenvironmentalsensors toexplore CO2

respirationdrivers.

For more information click this link:http://www.DecagonNews.com/elements-info/

http://www.decagon.com/ag_research/micro/gaharris/

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