SOIL MOISTURE MONITORINGAn Overview of Monitoring Methods and Devices

By Mike Risinger, USOA·SCS andKen Carver, High Plains Underground Water Conservation District No.1

Inches of Water Various Soil Types Will Hold AtField Capacity Which is Available 10 Plants

Soil WaterThere are three categories of soil water.

(1) "Gravity water" is that water in the soil whichwill drain into deeper soil layers as a result of theforces of gravity. After gravity drainage is com­plete, (2) "plant available water" is that waterloosely held by surface tension between the soilparticles. This water equals about one-half of the

The Soil

In order to monilor soil moisture accurately, itis necessary to know a few things about the soilitself, such as the type of soil on your farm and itswater-holding capacity.

Each type of soil has a different capacity tohold water, depending on its structure, texture,and other properties. The following chartdescribes four basic soil types and gives theirrange of water-holding capacity in inches of waterper fool of soil.

E10 ~

•••CI.yS.ndy Le.," ."d Cl.y Ie.,"Ie.., lilt Ie.,"

water-holding capacity of the soil and can be ex­tracted by plants. The other one-half of the water­holding capacity of the soil is, (3) "molecularwater." This water is strongly bound fa the soilparticles and is generally considered unob­tainable by most agricultural crops.

Even though the molecular water bound to thesoil particles is not generally available to plants,some of it can be removed from the upper part ofthe soil through evaporation. Evaporation lossesare increased by cultivations, especially thosewhich do not leave a protective surface mulch.Once removed, this water must be replacedbefore water which will be available to plants canaccumulate in the small pore spaces between thesoil particles.

The amount of water needed to refill the sur­face layer of soil dried by evaporation is almosttwice that needed to refill the deeper layers of thesoil which are not exposed to evaporation whereonly the water removed by plant use needs to bereplaced.

The total water·holding capacity of different soils, asillustrated above, reveals what percentage of the total isavailable to plants. However, also note that a goodpercentage of the total is generally considered unob­tainable by fietd crops.

METHODS OF MEASURING SOIL MOISTURE

FeelThe simplest method for measuring soil water

content is Ihe "feel" method. A person takes ahandful of soil and makes observations on the ap­pearance and feel of the sample. Soil samplesshould be taken from each foot of depth wheresoil moisture is to be considered.

A free pocket-sized soil moisture guide isavailable from the local offices of the High PlainsUnderground Water Conservation Dis1rict and theSoil Conservation Service for each of the 15 coun·ties served by the Water District. Each card iden·

"Clay

_ ".:- ~'~.------.:4~.~----~••;;0.51 50• •,

4.'~

•30~,20~,

; 0.4 f'::::'=::::="'F=~",.. 0.3+----••Bo.2••0°·1••".'" S.nd

1.4 - 1.9

1.5-2.3

Per Footof Soil

.80 - 1.25

1.2-1.5

.12 - .16

Per Inchof Soil

.06· .10

.10·1.25

Clay, clay loam or siltyclay loam .125 - .19

Dominant Texture

Fine sand and loamyfine sand

Fine sandy loam

Sandy clay loam andloam

HOW much waler do I need to apply with my pre­plant irrigation? Should I water now or wait

until next week? Can I skip this last watering andsave $15 per acre, or will it cost me more in re­duced yields?

These and many other questions go through afarmer's mind before and during each growingseason. Making the right decision in many ofthese instances can be the difference betweenmaking a profit and losing money. How can afarmer gather the information necessary to makeIhese important decisions? Soil moisturemonitoring is one way.

There are numerous soil moisture monitoringmethods and devices available which can helpproducers make informed water managementdecisions.

tifies the major soils in the county, with the tex­ture, depth and water-holding capacity. The cardalso provides a guide for estimating water contentby the feel method.

GUIDE FOR JUDGING HOW MUCH MOISTUREIS AVAILABLE FOR CROPS

Dominant T.~tur. Fine Sand and Fine Sandy loamloamy Fine Send

Inches ot water per loot soilwill hold atlleld cepaclty .60·1.25 1.2·1.5

Available 5011 moisture Feel or eppearance of soilremaining end moisture deficiency

Dry. loose. single Dry. loose. lIowso to 25 p.re.nt grained. flowS through fingef$.

through lingers

Inches per loot to be .dded: 1.25· .60 1.5 • 1.0

Appears 10 be dry. Appears to be ctry.25 to 50 percent will not lann a ball will not form a ball.

with pressure.Inches per loot to be added: .60· .45 1.0· .65

Appears to be dry, Tends to ball under50 to 15 percent will not lorm a ball pressure but seldom

with pressure. holds togetner.

Inches per fool to be .dcMd; .45 . .20 .65· .30

TendS to slick Forms weak ball.15 to 100 percent together slightly. breaks easily.

sometimes forms a will not stick.very weak ballunder pressure.

Inches per loot to be .dded: .20·00 .30·00

Upon squeezIng. no Upon sqlH!ezing, nofree water appears tree water apj)83rS

At field cepacity on so;i but wet on soil but wet(100 pereenl) outline 01 ball is outline 01 ball Is

lell on hand, lett on hand.

IncheS per 100110 be .ddecI: '" '"Dominant Textul'fl Sandy Clay loam Clay. Clay loam or

and loam Silty Clay loam

Inches oj waler per loot soitwill hold at lietd capacity 1.4·1.9 1.5·2.3

Available soil moisture Feel or appearance of soilremaining and moisture deliciency

Powdery dry, somo· Hard. baked.times slightly crusted cracked, sometimes

a to 25 percent 001 easily broken has loose crumbsdown into powdery on surlace.

conditiOn.Inches per loot to be edeled: 1.9 . 1.4 2.3 . 1.5

Somewhat Crumbly Somewhat pliable,25 to 50 plIrunt but holds togetl'ler will ball under

110m pressure pressure.Inches per loot 10 be added: 1.40 . 1.0 1.5 . 1.0

Forms iiIl ball some- Forms iiIl ball. ribbons50 to 15 percent what plastic. wilt out belwOOn thumb

sometimes slick and forefinger.slighlly with pressure

Inches per loot to be added: 1.0·0.5 1.10·.55

Forms a ball, Easily ribbonsis very pliable. slicks out belween

15 10 100 percent readily it relatively fingers. has slickhigh in clay. leeling.

Inches per tool to be added: 0.5·00 .55·00

Upon squeezing, no Upon squeezing, notree water appears Iree water appeals

Allield cepaelty on soil but wet on soil but wet(100 pen:enlJ outline 01 ball is outline 01 ball Is

lett on hand. lell on hand.

Inches per loot to be .dded: '" '"

Gravimetric:A soil sample is taken from the field, weighed,

dried by heating and reweighed. The difference inthe weight before and after drying is the "percentmoisture by weight." This method is accurate, butrequires a set of sensitive scales, a method of dry·ing, and a known or estimated bulk density valuefor converting the data from a weight to a volumebasis. It is destructive in the sense that iJ.requiresremoval of a soil sample. This makes it impossi·ble to make another measurement at a future dateat exactly the same point, eventually causing in·accuracy because of field variability from one siteto another,

Moisture Blocks and Resistance MetersMost soil moisture blocks are made by casting

gypsum around a pair of stainless steel elec­trodes with lead wires trailing.

Here the gypsum has been cut-away to expose twostainless steel electrodes which form the core of thesoil moisture block. Electrical resistance between theseelectrodes is an indication of soil moisture.

The blocks are buried in the soil at one foot in·tervals to a total depth of four feet with the leadwires extending above the land surface. Thesewires are then attached to a stake so that they canbe easily found. Resistance to an electrical cur­rent is measured with a resistance meter. Thisreading translates directly to soil moisture.

Resistance meters and gypsum blocks are inex·pensive, simple to use and measure a wide rangeof moisture content. The blocks are not always ac­curate when soils are near field capacity, but giveaccurate moisture readings when soils are lesswet.

Gypsum blocks work best when used with lesswater-sensitive crops such as cotton, grainsorghum and other small grains. The main disad­vantage of the gypsum block is that the calibra­tion changes gradually with time, and the "life" ofthe block is limited. New blocks should be install­ed each growing season, because the gypsumdeteriorates. Lead wires are also commonlydestroyed in tillage operations. On the whole,

however, they produce a level of accuracy suffi­cient for making irrigation water managementdecisions which require information on soilmoisture conditions.

The lead wires extending to iand surface tram theburied gypsum blocks are inserted into a hand-heldresistance meter when readings are taken. The meterreading indicates the soil moisture condition at thedepth of each biock.

TensiometersA tensiometer measures soil water suction,

which is similar to the process a plant root uses toobtain water from the soil. The instrument is awater-filled tube, with a special porou.s tip andvacuum gauge. Tensiometers are made in lengthsranging from 6 to 72 inches and can be installed inthe soil to the desired depth.

Three tensiometers commonty used, from top to bot·tom, measure 36. 24 and 12 inches in length. Installationof these three would provide soii moisture informationat the three, two and one foot depths.

This instrument requires maintenance in thefield. It is successfully used in determining theneed for irrigation when the soil moisture is keptabove 50 percent of field capacity. The range ofusefulness is dependent on the texture of the soilin which it is used.

In sandier soils, tensiometers will function pro­perly down to 40 percent of field capacity. In thissoil type, most crops, for maximum production,

need additional irrigation water before 50 percentof the soil moisture is depleted.

In clayey soils, tensiometers lose their effec­tiveness when soil moisture reaches 75 percent offield capacity. Clay soils hold more water undertensions greater than the tensiometer canmeasure. Therefore, contact with the soil is lostand lower moisture levels cannot be determined.

Tensiometers are reusable and relatively inex­pensive. They are simple to install and read, butperiodic refilling with fluid is required to avoid er­roneous readings caused by air entering the tube.

Neutron Moisture MetersThe neutron moisture meter uses a radioactive

source and state-of-the-art electronics to measuresoil water.

After a neutron probe is lowered into the soil throughan access tube the neutron moisture meter will displaythe percent moisture by volume in the soil.

While this instrument is comparatively expen­sive, it is reliable and accurate. Possession anduse require a license from the Texas Departmentof Health because the radioactive source couldcreate a health hazard. Its use requires specialstorage, transportation and handling procedures.

Use of this device is not normally consideredpractical for on-farm use by the individual ir­rigator. However, large operators could use itsatisfactorily and consultants often provide useof this instrument as a part of their service.

Further Information

More detailed information on any of the soilmoisture monitoring devices and techniquesdescribed in this brochure may be obtained fromthe local offices of the High Plains UndergroundWater Conservation District NO.1 and Soil Con­servation Service. The District office of the HighPlains Underground Water Conservation DistrictNO.1 is located at 2930 Avenue Q, Lubbock, Texas79405, or phone (806) 762-0181.