Methods of Measuring 

Soil Moisture in the Field

By A. I. JOHNSON

CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES 

GEOLOGICAL SURVEY WATER-SUPPLY PAPER 1619-U

Evaluates methods for measuring soil 

moisture and describes the equipment used

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1962
U.S. DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, Jr., Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

First printing 1962 

Second printing 1963 

Third printing 1992

Any use of trade, product, or firm names in this publication is for descriptive 

purposes only and does not imply endorsement by the U.S. Government

For sale by the Books and Open-File Reports Section 

U.S. Geological Survey, Federal Center, Box 25425, Denver, CO 80225
CONTENTS

	Page 

Abstract__ _ _____________________________________________________ U-1

Introduction..____________________________________________________ 1

Methods.________________________________________________________ 2

Gravimetric __ _______________________________________________ 2

Sample collection.___...___..___._.._.__._.____.__._..__.__ 2

Sampling augers_________________________________________ 3

Sampling tubes or core barrels______________________________ 4

Porter piston sampler._________________________________ 4

Pomona open-drive sampler_.___________________________ 5

Johnson open-drive sampler.... ......................... 5

Electrical-resistance. ___________________________________________ 6

Heat-diffusion,__._____________________________________________ 9

Absorption ___________________________________________________ 10

Tensiometric .................................................. 10

Penetration._ _________________________________________________ 12

Radioactive. _____-_____---______.__--____--_-_________-_____-_ 12

Summary _________________________________________________________ 14

Selected references.....-- ______ ____________________________________ 15

A. Gravimetric methods...------------.----------------------- 15

B. Electrical-resistance methods......._____. .____.-______-_____ 16

C. Heat-diffusion methods-_---_---_-----__-------------------- 19

D. Absorption methods_______________________________________ 19

E. Tensiometric methods.----.--------.----------------------- 20

F. Penetration methods.______________________________________ 21

G. Radioactive methods__-_-------_--------_--------_--_------ 21

H. General methods__--_-_---_-----------_-----------_---_--- 24

ILLUSTRATIONS

Page

FIGURE 1. Orchard auger.________________________________________ U-3

2. Porter piston sampler.__________________________________ 4

3. Pomona open-drive sampler....__________________________ 5

4. Johnson open-drive sampler...._-.__-__-.__-___----___-_- 6

5. Electrical-resistance soil-moisture blocks and meters._______ 8

6. Tensiometers _ _________________________________________ 11

7. Radioactive soil-moisture meter-.------..-------_---._--- 13

in
CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES

METHODS OF MEASURING SOIL MOISTURE IN THE

FIELD

By A. I. JOHNSON

ABSTRACT

For centuries, the amount of moisture in the soil has been of interest in 

agriculture. The subject of soil moisture is also of great importance to the 

hydrologist, forester, and soils engineer.

Much equipment and many methods have been developed to measure soil 

moisture under field conditions. This report discusses and evaluates the various 

methods for measurement of soil moisture and describes the equipment needed 

for each method. The advantages and disadvantages of each method are 

discussed and an extensive list of references is provided for those desiring to 

study the subject in more detail.

The gravimetric method is concluded to be the most satisfactory method for 

most problems requiring onetime moisture-content data. The radioactive 

method is normally best for obtaining repeated measurements of soil moisture 

in place. It is concluded that all methods have some limitations and t^at the 

ideal method for measurement of soil moisture under field conditions has yet 

to be perfected.

INTRODUCTION

The subject of soil moisture has long; been of interest in agriculture. 

For centuries the farmer has picked up and felt a handful of soil 

to determine the best time to plow his fields. The amount of moisture 

in the soil is also of great importance in hydrology, forestry, and soilmechanics engineering. Consequently, much effort has been expended 

in the last 50 years in developing methods and equipment for measuring soil moisture under field conditions.

Determination of soil moisture is one of the most difficult measurements required in the field of hydrology. Measurement of soil moisture ranges from the method of feeling the soil to the use of 

complicated electronic equipment using radioactive substances. The 

development of equipment has been directed primarily toward 

instruments that continuously measure changes in moisture content 

at a single sampling point.

U-l
U-2 CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES

METHODS

GRAVIMETRIC

The gravimetric method involves collecting a soil samp1 ^ weighing 

the sample before and after drying it, and calculating its original 

moisture content. The gravimetric method is the oldest (other than 

the ancient method of feeling the soil) but still continues to be the 

most widely used method for obtaining data on soil moisture. 

Because it is the only direct way of measuring soil moisture, it is 

required for calibrating the equipment used in the oth°sr methods.

Russell (II, 1950) 1 reporting on work completed in 1843 and 

Whitney (A, 1894), describe some of the first scientific investigations 

of soil moisture using gravimetric methods. The Kirg tube, for 

collecting drive-core samples, was developed in 1890 and vus modified 

and improved by Veihmeyer (A, 1929). Since that time many types 

of sampling equipment, as well as special drying ovens and balances, 

have been developed for use with the gravimetric methoc1 .

The disadvantage of the gravimetric method is the tirr e and effort 

required to obtain data. It is time-consuming work tc collect the 

samples, especially from depths greater than a few feet, and to oven 

dry and weigh the many samples required for most projects. For 

many problems, such as the study of evapotranspiration by grasses, 

the sampling procedure alters the area of experiment owing to 

trampling of the vegetation or the making of numerous hcles. Under 

these conditions, the sampling may have to be done from platforms, 

and the holes may have to be refilled and packed. Soils r.re normally 

variable within an experimental area and, as two samples cannot be 

collected from the same point, slight variations of moisture content 

may be noticed.

SAMPLE COLLECTION

For the best samples, the soil should be homogeneous-, just moist 

enough to permit easy cutting by the sampling equipment, and free 

from roots, organic matter, and stones. Seldom are all these conditions met.

The technique and equipment used for sample collection should be 

such that the samples do not lose or gain moisture, or otherwise become 

altered or contaminated, during sampling and transportation. In 

sampling through a wet layer into a dry layer, care must, be taken to 

keep the sampling equipment as dry as possible and to prevent water

1 Letters refer to the center beads In "Selected References," as follows: A, Gravimetric 

method ; B, Electrical-resistance method ; C, Heat-diffusion method ; D, Absorption method ; 

B, Tenslometric method; P, Penetration method; G, Radioactive method; H, General 

method.
METHODS OF MEASURING SOIL MOISTURE IN THE FIELD U-3

from running down the hole into the drier material. If there is free 

water in the soil, the moisture content as measured probably will be 

less than the correct value because some water will drip off as the sample is removed from the ground, or some may be squeezed out by 

compaction during sampling.

When dry hard fine-textured sediments are encountered, it is difficult to drive the core barrels or to rotate the augers. When dry, 

coarse-textured sediments are sampled, the sample may slide out the 

end of the core barrel or auger as it is withdrawn. Stony soils are 

very difficult to sample, especially volumetrically, owing to the danger 

of hitting a stone with the cutting edges of the equipment and 

because representative samples must be large. Soils that contain 

a considerable amount of roots and organic matter also present 

difficulty.

In soil-moisture sampling, it is essential that all sampling operations the transfer of samples to moisture cans and the weighing of 

the moist samples be done as rapidly as possible to prevent undue 

moisture losses. Many difficulties in the use of sampling equipment, 

whether augers or core samplers, may be overcome if all equipment 

is kept clean that is, free of moisture, oil, rust, and flirt.

SAMPLING AUGERS

The simplest equipment for soil-moisture sampling is the hand auger. 

Hand augers, with shaft extensions of aluminum pipe, have been used 

in sampling to depths as great as 55 feet.

One of the most useful types of hand augers is the Orchard auger 

(fig. 1). It consists of a cylinder 3 inches in diameter and 9 inches

FIGURE 1. Orchard auger.

long having a 4i/£>-foot extension pipe on the top and two curved 

cutting teeth on the bottom. Because the barrel is a solid cylinder, the 

sample is not as likely to become contaminated from the side of the 

test hole as with the lowan or the pesthole auger. Thus, a good 

representative, but disturbed, sample is obtained by use of this equip-
U-4 CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES

nient. For ease in sampling at depths greater than 5 feet, 3-foot 

extensions of %-inch aluminum pipe are added as needed.

To obtain a sample by the hand-auger method, the auger is turned 

by its handle and forced downward into the material to be sampled. 

Usually about 3 inches of the material may be penetrated before the 

cylinder barrel is filled. The auger is then raised to the surface, and 

the sample is jarred loose from the auger barrel by hitting the barrel 

with a rubber hammer.

SAMPLING TUBES OR CORE BARRELS

A soil-sampling tube, core barrel, or drive sampler of some type 

offers an advantage in soil-moisture sampling because volumetric samples can be obtained for calculating moisture content by volume. The 

King tube was developed in 1890, and Veihmeyer developed modified 

soil tubes in 1929. Since that time many people, including the writer, 

have developed variations of this type of sampling equipment. (See 

Ilvorslev, A, 1949.)

Core samplers provide uncontaminated samples if the equipment is 

kept clean. Oil should never be used on the samplers, and they should 

be kept free of dirt, rust, and moisture. A two-man crew is normally 

recommended for deep sampling. Depths to 65 feet may be sampled.

PORTER PISTON SAMPLER

The Porter piston sampler (fig. 2) is one type of sampling equip-

** lEt* ** * ^<^

FIGURE 2. Porter piston sampler.

ment that has been very useful, especially for sampling through loose 

or wet materials that tend to slough into the hole. This sampler is the 

retractable-piston drive-sampler type. By means of a hand-operated 

25-pound drop hammer, a plugged sample section may be driven to 

the required depth by the addition of 5-foot extension rods. The 

plug is then retracted, the sampler is driven a maximum of 2 feet, 

and the soil core is retained in brass insert liners contained in the 

4-foot sample section. Further retraction of the plug helps retain 

the sample by forming a partial vacuum above it. After extraction, 

the liners with the soil cores are removed from the sample tube and 

capped and sealed for laboratory testing. The liners are brass
METHODS OF MEASURING SOIL MOISTURE IN THE FIELD TJ-5

cylinders, 1 inch in diameter and 6 inches in length. Two types of 

hardened-steel cutting points are available for use in different soils. 

This sampler is made from high-strength steel and is rugged and 

dependable. However, reasonable care must be taken in its use and 

storage to insure efficient operation and long life.

POMONA OPEN-DRIVE SAMPLER

The Pomona open-drive sampler (fig. 3) consists of a core barrel 2

FIGURE 3. Pomona open-arlve sampler.

inches in inside diameter and 4 inches long, with extension tubes 1 inch 

in diameter and 5 feet long for sampling at depth. Brass cylinder 

liners, 2 inches in length, are used to retain the "undisturbed" core 

samples. The samples are removed from the core barrel by pushing 

a plunger.

A light drill rod or 1-inch pipe may be used for extensions. The 

cores are collected by use of a 25-pound drop hammer as described for 

the Porter sampler.

JOHNSON OPEN-DRIVE SAMPLER

A simple and economical sampler for obtaining volumetric core 

samples from shallow depths was designed by the writer in 1952. The 

sampler is easily constructed, and consists of a thin-walled brass tube 

2 inches in diameter and 6 inches long mounted on the end of a 3-foot 

T-handle of 94-inch pipe (fig. 4). Samples are collected by a downward thrust on the handle and are then pushed out of the core barrel 

by the central plunger. Because the inside diameter and area of the 

core barrel are known, volumetric samples may be easily obtained by 

cutting off a predetermined length of the core as it is removed from 

the sampler.
U-6 CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES

FIGURE 4. Johnson open-drive sampler. 

ELECTRICAL-RESISTANCE

The principle of electrical measurement of soil moisture was first 

reported by Whitney and others (B, 1897). However, many years 

passed before truly successful electrical units were developed by 

Bouyoucos and Mick (B, 1940a), Colman (B, 1946), Bouyoucos (B, 

1949), and Youker andDreibelbis (B, 1951).

The electrical-resistance "blocks" developed by those named above 

operate on the principle that resistance to the passage of an electrical 

current between two electrodes buried in the soil will depend upon 

the moisture content of the soil. Nylon or Fiberglas fabric or plaster 

of paris surrounding the electrodes permits uniform contact with the 

soil moisture. When buried in the soil, the porous material of the 

block readily absorbs moisture or gives it up so that the moisture 

content of the block tends to stay in equilibrium with the moisture 

content of the soil. These moisture-content changes cause changes 

in electrical resistance which are measured by a meter at the surface. 

The resistance read on the meter is converted to moisture-content 

values by means of a calibration chart. The calibration chart is prepared by correlation, either in the field or in the laboratory, of gravimetric moisture-content values and resistance readings for the soil 

in which the blocks are buried. Laboratory calibration consists of 

drying and intermittently weighing soil cores in which blocks have 

been inserted. Field calibration consists of taking gravimetric samples
METHODS OF MEASURING SOIL MOISTURE IN THE FIELD U-7

as close as possible to blocks that have been buried in the field, and 

relating the moisture content of the sample to the measured resistance.

Two main types of blocks are in current use, the Fiberglas unit 

developed by Colman and Hendrix (B, 1949) and the plaster-of-paris 

or gypsum-block unit devejoped by Bouyoucos and Mick (B, 1940a). 

The Colman Fiberglas block is also available with an integrally installed thermistor so that soil temperatures may be measured, and 

resistance can be corrected to a common temperature. (Corrections 

for temperature are necessary for resistance blocks if accurate results 

are required.) Two general types of meters are used for reading the 

resistance values of the moisture blocks the Colman meter (fig. 5 A) 

and the Bouyoucos bridge (fig. 5B). The blocks may also be wired 

to a recorder for obtaining a continuous record (Korty and Kohnke, 

B,1953).

The accuracy claimed by the developers of soil-moisture blocks is 

at best 1 percent by weight. All the types of blocks respond with 

equal rapidity to changes in soil moisture. Soil-moisture blocks are 

generally considered most dependable in the low-moisture-content 

range, below field capacity. Under these conditions, the Fiberglas 

blocks normally have a greater range of operation than the gypsum 

blocks. At higher moisture contents, between field capacity and saturation, the change in resistance per unit change in moisture content 

is small, thus reducing the sensitivity of the units. However, some 

of the apparent inaccuracies at the higher moisture contents may be 

due also to the loss of free water in sampling during the calibration.
U-8 CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES

8? .1fe

g S S fe
METHODS OF MEASURING SOIL MOISTURE IN THE FIELD U-9

The salt concentration in the soil moisture will materially affect 

results obtained by use of soil-moisture blocks. A drop in resistance 

correlates with an increase in salt concentration, but the changes in 

salt concentration at any single site are generally negligible under 

most conditions.

Blocks require relatively little effort to install and can be speedily 

read. The Fiberglas blocks are easier to install because they are 

thinner and thus may be inserted into the soil without disturbing it 

much. The speed of taking readings is greatest with the Colman 

meter. In well-drained soils the blocks are fairly durable, and have 

a life of 3 to 4 years.

HEAT-DIFFUSION

The basic theory of heat-diffusion blocks or cells was reported by 

Patten (C, 1909). The design for a cell was suggested by SharT and 

Baver (C, 1939a). Kersten (C, 1948), Momin (C, 1947), and Aldous, 

Lawton, and Mainfort (C, 1952) tested several modifications of the 

design suggested by Shaw and Baver.

The heat-diffusion method is based upon the principle that the heat 

conductivity of a soil varies with its moisture content. The teirperature rise caused by an electrically activated heat source installed in 

the soil is measured by a sensitive temperature-measuring device and 

is correlated with moisture content. Wet soil will conduct heat rapidly away from the heat source in the cell and will thus have a smaller 

temperature rise than dry soil.

To date, three general types of heat-diffusion cells have been developed. These are described by Aldous, Lawton, and Mainfort (C, 

1952) as a porous-block type in which the electrical elements are imbedded in a porous medium, a direct-contact type in which the electrical elements are directly in contact with the soil, and a modified 

direct-contact type (thermal-conductivity cell) in which the Hater 

and the temperature-measuring elements are in contact with, but separated by, a portion of the soil being tested.

Use of heat-diffusion cells has indicated that the blocks are sensitive 

to minor variations in construction. The cells are unsatisfactory 

when used in soils at moisture contents above field capacity; in highshrinkage soils, intimate contact between the cell and the soil is lost 

as the moisture content decreases and erratic results are obtained until 

the shrinkage limit is reached. The porous-block type of cell has 

been reported as entirely unsatisfactory because consistent correlation 

between soil moisture and cell measurements could not be obtained 

under different soil conditions. The thermal-conductivity cell has 

been the most satisfactory of the three types but needs further 

development.
U-10 CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES

Heat-diffusion cells require calibration for different soils and densities, but Shaw and Baver (C, 1939b) noted that salt concentrations 

from 100 to 10,000 ppm did not affect the readings. None of the types 

of cells can be easily installed at depths of more than 5 feet or in 

undisturbed soil. These cells have not received widespread use and 

are not presently available from commercial sources.

ABSORPTION

Livingston and Koketsu (D, 1920) developed porour points or 

blocks that would absorb moisture from the adjacent area when 

installed in the soil. The soil moisture was then estimated from the 

change in weight of the points or blocks. Wilson (D, 1927) and 

Stoeckeler (D, 1937) did additional work on the use of absorption 

blocks. Davis and Slater (D, 1942) used an absorption block consisting of a porous chamber that contained a close-fitting plug that could 

be removed for weighing. The plug overcame the disac? vantage of 

having to disturb the installations in the soil each time the blocks were 

to be weighed. Dimbleby (D, 1954) developed a pencil-type absorption block which is stuck into the soil; the moisture contents are estimated from the color changes of the "pencil."

This method is more qualitative than quantitative and has considerable inherent error; it has never been used extensively.

TENSIOMETRIC

A tensiometer consists of a porous point or cup (usually ceramic) 

connected through a tube to a pressure-measuring device. The system 

is filled with water and the water in the point or cup comes into 

equilibrium with the moisture in the surrounding soil, "^ater flows 

out of the point as the soil dries and creates greater tens'on, or back 

into the point as the soil becomes wetter and has less tension. These 

changes in pressure, or tension, are indicated on a measuring device, 

usually a Bourdon-tube vacuum gage or a mercury manometer. (See 

fig. 6.) The tensiometer may also be attached to a pressure recorder 

(Richards and Gardner, E, 1936) or to an electonic pressure transducer to maintain a continuous record of tension changes Tensiometers are available in lengths of 6 inches to 4 feet, but probably could 

be manufactured in longer lengths if desired. Specially constructed 

tensiometers have been installed to depths of 15 feet (Kiel <vrds, L. A., 

E, 1942). Multiple tensiometers, for determining tension data at 

several depths by use of a single probe, were developed by L. A. Richards (E, 1954).'

Tensiometers were probably most fully developed by L. A. Richards 

(E, 1942). They are most useful for measuring moisture content
METHODS OF MEASURING SOIL MOISTURE IN THE FIELD U-ll

FIGURE 6. Tensiometers. Top, vacuum-gage type. Bottom, mercury-manometer type.

of tensions below approximately 0.9 atmosphere. Such tensions will, 

on the average, correspond to a range in moisture content from 

slightly below field capacity to saturation. At the higher tensions 

found in drier soils, tensiometers become inoperative because air 

enters the system through the porous point. To determine the moisture content with a tensiometer, the relation between moisture tension 

and moisture content must be known. This relation may be found in 

the laboratory from a moisture-tension curve constructed by means 

of a pressure-membrane or porous-plate apparatus or by collecting 

soil samples in the area surrounding a tensiometer installation and 

relating the moisture content of the samples to the tensiometer reading 

obtained concurrently. L. A. Richards (E, 1949) noted that the 

vacuum-gage type of instrument will generally provide an accuracy 

within 2 percent, and the mercury-manometer type is even more 

accurate.

Tensiometers are affected by temperature. The temperature 

gradients between the porous point of the tensiometer and the soil 

may cause variations in the tension readings.

The salt concentration in the soil or in the pore water seems to affect 

tensiometric methods less than electrical methods. S. J. Richards 

(E, 1938) pointed out that tensiometers exhibit considerable hysteresis effect; they tend to give a higher soil-moisture tension during 

soil drying than during soil wetting. In 1949, L. A. Richards noted 

that this effect is not too serious a disadvantage because the wetting 

cycle is usually rather short in comparison with the drying cycle. 

Ewart and Baver (B, 1950) reported a serious disadvantage because 

of the timelag in response to soil-moisture changes. Tensiometers 

have exhibited lags of half an hour to many hours in indicating 

changes in tension caused by changes in moisture content. Recent
U-12 CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES

studies by the hydrologic laboratory of the U.S. Geological Survey, 

Denver, Colo., showed that semipermeable plastic points provide much 

faster response than ceramic points.

The tensiometer is probably the easiest to install and the most 

rapidly read of all soil-moisture measuring equipment. However, 

at present, tensiometers are not suitable for installation at depths 

greater than about 20 feet.

PENETRATION

Moisture content may be estimated by relating it to the force 

required to push an instrument through the soil. Allyn and Work 

(F, 1941a) developed an instrument they called the "availameter" that 

measured the force required to drive a pair of needles into a soil 

core. Allyn (F, 1942) reported a newly developed soil probe with 

which he found moisture-content estimation possible within 0.5 percent. Many others, especially in the Netherlands, have developed 

equipment for measuring penetration resistance (Hvorslev, F, 1949).

Penetration equipment must be calibrated for each type of soil to 

obtain the relation between penetration resistance and moisture content. The method is very fast, although the equipment is difficult 

to use in gravelly or stony soils.

Penetration equipment designed by the Waterways Experiment 

Station, Corps of Engineers, Vicksburg, Miss., consists of a pipe with 

a point at the bottom and a T-handle containing a pressure-indicating 

device at the top. Depth of penetration is limited by the amount of 

force available.

RADIOACTIVE

Belcher, Cuykendall, and Sack (G, 1950) apparently introduced 

the radioactive method of measuring soil moisture in 1950. This 

method is based on the principle of measuring the slowing of neutrons 

emitted into .the soil from a fast-neutron source. The energy loss is 

much greater in neutron collisions with atoms of low atomic weight 

and is proportional to the number of such atoms present in the soil. 

The effect of such collisions is to change a fast neutron to a slow neutron. Hydrogen, which is the principal element of low atomic weight 

found in the soil, is largely contained in the molecules of the water 

in the soil. The number of slow neutrons detected by a counter 

tube after emission of fast neutrons from a radioactive source tube 

is electronically indicated on a sealer.

The radioactive method indicates the amount of water per unit 

volume of soil. The dry density of the soil must be determined if 

moisture content in percent by weight is desired. The U.S. Corps 

of Engineers (G, 1955b) stated that the soil volume measured by this
METHODS OF MEASURING SOIL MOISTURE EST THE FIELD U-13

method is bulb shaped and has a radius of 6 to 15 inches, according 

to the moisture content and density of the soil.

One type of equipment presently available is shown in figure 7.

FIGURE 7. Radioactive soil-moisture meter.

It consists of: a portable battery-powered sealer having five glowtube decade counters that can accumulate as much as 99,999 counts, 

and a spring-wound timer that has a time-counting range of y2 to 

5 minutes and weighs approximately 35 pounds; and a depth moisture 

probe having a 5-millicurie fast-neutron source of radium 226 and 

finely ground beryllium (half-life, 1,620 years) within a probe 15 

inches in length and iy2 inches in diameter and having a weight of 

45 pounds when complete with a lead and paraffin shield 6 inches 

in diameter and 8 inches long. These meters have been used with 

as much as 200 feet of cable.

Most investigators have reported an accuracy within 1 to 2 percent 

by volume. However, to obtain this accuracy, it is recommended 

that the probe be calibrated in the type of soil to be tested and the 

type of casing into which the probe is to be lowered.

Salt concentration in the soil moisture does not materially affect 

the data obtained by the radioactive method. Temperature usually 

has been considered ineffective in the radioactive method, but there
U-14 CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES

is some evidence of a temperature effect. Readings close to the surface are affected by the position of the probe with respect to the airsoil interface; proximity of the interface causes lower counts than 

are characteristic for a particular moisture content at greater depth. 

Timing errors may be kept to a minimum by using a standard-count 

timing cycle of 2 minutes. Access tubes must be kept free of excess 

moisture, or erroneous readings will result. One must remember 

that the type and size of casing and the method of installation of the 

access tubes have a considerable effect on the readings, and new 

calibration curves should be obtained for each type of installation. 

Calibration may be done by means of gravimetric sampling in the 

area surrounding an access tube, by using large samples in the laboratory, and by use of boric-acid solutions of various concentrations.

There is some radioactive hazard from use of this equipment. The 

danger from exposure is proportional to the distance between the 

source and the operator and to the length of time of exposure. Thus, 

most of the danger can be minimized by proper handling of the 

equipment.

The radioactive method is time consuming, especially if the time 

required for calibration is considered. The equipment is heavy and 

delicate and equipment failures are likely. Considerable time is 

lost in repairs and in the recalibration needed after most repairs. 

The repair of the sealer may require the services of an electronics 

specialist.

SUMMARY

Of the methods described, only the gravimetric, electrical-resistance, 

tensiometric, and radioactive methods are used commonly. All 

methods have their disadvantages, as well as advantages, and the ideal 

method that will give accurate, reliable, and rapid measurements in 

place has yet to be developed.

The gravimetric method is considered the best for most soilmoisture-measurement problems requiring one-time moisture-content 

data. Because the gravimetric method provides data directly, the 

effort and possibility of error associated with the conversion of indirect readings (electrical-resistance, radioactive, and tensiometric) 

to moisture content are avoided. The gravimetric method requires 

less experience than the indirect methods, but also requires more effort 

under many conditions. Considerable gravimetric sampling is required for calibration even if one of the indirect methods is used. 

Some indirect method may have to be used if continuous or frequent 

moisture-content readings are necessary.

Radioactive methods are probably the best for obtaining repeated 

measurements of soil moisture in the field. Because of the delicate
METHODS OF MEASURING SOIL MOISTURE IN THE FIELD U-15

equipment and the work of calibration, this method requires sufficient 

effort and expense that one must consider seriously whether gravimetric sampling would not be a satisfactory substitute.

SELECTED REFERENCES 

A. GBAVTMETBIC METHODS

Bethlahmy, Nedavia, 1952, A method for approximating the water content of

soils : Am. Geophys. Union Trans., v. 33, pt. 1, p. 699-706.

Bidwell, G. L., and Sterling, W. F., 1925, Preliminary notes on the direct determination of moisture: Assoc. Official Agr. Chemists Jour., v. 8, p. 295-301. 

Broatlfoot, W. M., 1954a, Core vs. bulk samples in soil-moisture tension aralysis:

U.S. Dept. Agriculture, Southern Forest Expt. Sta. Occasional Par^r 135,

p. 22-25. 

 1954b, Procedures and equipment for determining soil bulk density: U.S.

Dept. Agriculture, Southern Forest Expt. Sta. Occasional Paper 135, p. 2-11. 

Carter, C. E., 1938, A field method for determining soil moisture: Australian

Forestry, v. 3, p. 15-16.

Cline, M. G., 1944, Principles of soil sampling: Soil Sci., v. 58, p. 275-288. 

Coile, T. S., 1936, Soil samplers : Soil Sci., v. 42, p. 139-142. 

Davisson, B. S., and Sivaslian, G. K., 1918, The determination of moisfure in

soils : Am. Soc. Agronomy Jour., v. I'O, p. 198-204.

Ferguson, E. R., and Duke, "W. B., 19.54, Devices to facilitate King-tube soilmoisture sampling: U.S. Dept. Agriculture, Southern Forest Expt. Sta.

Occasional Paper 135, p. 26-29. 

Hehrie, J. O., 1950, A direct method of computing and correcting the equivalent

depth of water: Agr. Eng., v. 31, p. 290-291. 

Hoover, M. D., Olson, D. F., Jr., and Metz, L. J., 1954, Soil sampling for pore

space and percolation: U.S. Dept. Agriculture, Southeastern Fores* Expt.

Sta. Occasional Paper 42. 

Hvorslev, M. J., 1949, Subsurface exploration and sampling of soils for civil

engineering purposes Am. Soc. Civil Engineers Research Project Rept.:

U.S. Corps Engineers, Waterways Expt. Sta., 521 p. 

Lutz, H. J., 1944, Determination of certain physical properties of forest soils,

pt. 1, Methods utilizing samples collected in metal cylinders: Soil Sci., v.

57, p. 475-487. 

Noyes, H. A., and Trost, J. F., 1920, Determination of moisture in field samples

of soil: Assoc. Official Agr. Chemists Jour., v. 4, p. 95-97. 

Reed, J. F., and Rigney, J. A., 1947, Soil sampling from fields of uniform and

nonuniform appearance and soil types: Am. Soc. Agronomy Jour., v. 39,

p. 26-^0. 

Reinhart, K. G., 1954, Relation of soil bulk density to moisture contert as it

affects soil-moisture records: U.S. Dept. Agriculture, Southern Forest Expt.

Sta. Occasional Paper 135, p. 12-21. 

U.S. Bureau of Plant Industry, Soils, and Agricultural Engineering, 19P1, Soil

Survey manual: U.S. Dept. Agriculture Handb. 18. p. 327-338. 

Van Bavel, C. H. M., and Gilbert, M. J., 1954, Discussion of A method for rnproximating the water content of soils, by Nedavia Bethlahmy: Am. Geophys.

Union Trans., v. 35, p. 168-169. 

Veihmeyer, F. J., 1929, An improved soil-sampling tube: Soil Sci., v. 27, p.

147-152.
U-16 CONTRIBUTIONS TO THE HYDROLOGY OP THE UNITED STATES

Whitney, M. A., 1894, Instructions for taking samples of soil for moisture determination : U.S. Dept. Agriculture, Div. Soils Circ. 2.

 1896, Methods of the mechanical analysis of soils and of t>e determination of the amount of moisture in soils in the field: U.S. Dept. Agriculture, 

Div. Soils Bull. 4, 24 p.

B. ELECTRICAL-RESISTANCE METHODS

Anderson, A. B. C., 1943, A method of determining soil-moisture content based 

on the variation of the electrical capacitance of soil, at a low frequency, 

with moisture content: Soil Sci., v. 56, p. 29-41.

Anderson, A. B. C., and Edlefsen, N. E., 1942a, Laboratory study of the response 

of 2- and 4-electrode plaster of paris blocks as soil-moisture content indicators : Soil Sci., v. 53, p. 413-428.

 I942b, The electrical capacity of the 2-electrode plaster of paris block as 

an indicator of soil-moisture content: Soil Sci., v. 54, p. 35-4**

Bethlahmy, Nedavia, 1951, An alinement chart for use with the Tiberglas soilmoisture instrument: Soil Sci., v. 71, p. 377-380.

Bouyoucos, G. J., 1949, Nylon electrical resistance unit for continrous measurement of soil moisture in the field: Soil Sci., v. 67, p. 319-330.

 1950, A practical soil moisture meter as a scientific guide to irrigation 

practices: Agronomy Jour., v. 42, p. 104-107.

 1951, The effect of fertilizers on the plaster of paris electrical resistance 

method for measuring soil moisture in the field: Agronomy Jour., v. 43, p. 

508-511.

 1952, Improvements in the nylon method of measuring soil moisture in 

the field: Agronomy Jour., v. 44, p. 311-314.

 1953, More durable plaster of paris moisture blocks: Soil Sci., v. 76, p. 

447-451.

 1954a, Electrical resistance methods as finally perfected for making continuous measurement of soil moisture content under field cond'tions: Michigan Agr. Expt. Sta. Quart. Bull., v. 37, p. 132-149.

 1954b, New type electrode for plaster of paris moisture blocks: Soil Sci.,

v. 78, p. 339-342. 

Bouyoucos, G. J., and Crabb, G. A., Jr., 1949, Measurement of soil moisture by

the electrical resistance method: Agr. Eng., v. 30, p. 581-583. 

Bouyoucos, G. J., and Mick, A. H., 1939, A method for obtaining a continuous

measurement of soil moisture under field conditions: Science, v. 89, p. 252.

 1940a, An electrical resistance method for the continuous measurement 

of soil moisture under field conditions: Michigan Agr. Expt. Sta. Tech. 

Bull. 172, 38 p.

 1940b, Comparison of absorbent materials employed in the electrical resistance method of making a continuous measurement of soil moisture under 

field conditions: Soil Sci. Soc. America Proc., v. 5, p. 77-79.

 1947, Improvements in the plaster of paris absorption block electrical resistance method for measuring soil moisture under field conditions: Soil 

Sci., v. 63, p. 455-465.

 1948a, A comparison of electrical resistance units for making continuous 

measurement of soil moisture under field conditions: Plant Physiology, v. 

23, p. 532-543.

 1948b, A fabric absorption unit for continuous measurement of soil

moisture in the field: Soil Sci., v. 66, p. 217-232.
METHODS OF MEASURING SOIL MOISTURE IN THE FIELD U-17

Briggs, L. J., 1899, Electrical instruments for determining the moisture, teirnerature, and soluble salt content of soils: U.S. Dept. Agriculture, Div. Soils

Bull. 15, p. 1-35. 

Carlson, C. A., 1953, Moisture equilibration in natural cores during laboratory

calibration of Fiberglas soil-moisture units: U.S. Dept. Agriculture, Southern

Forest Expt. Sta. Occasional Paper 128, p. 31-39.

 1954, Comparison of laboratory and field calibration of Fiberglas moisture 

units: U.S. Dept. Agriculture, Southern Forest Expt. Sta. Occasional Paper 

135, p. 34-42.

Colman, E. A., 1946, The place of electrical soil-moisture meters in hydrologic 

research : Am. Geophys. Union Trans., v. 27, p. 847-853.

 1950, Manual of instructions for use of the Fiberglas soil-moisture instrument : U.S. Dept. Agriculture, California Forest Range Expt. Sta., 20 p. 

Colman, E. A., and Hendrix, T. M., 1949, The Fiberglas electrical soil-moisture

instrument: Soil Sci., v. 67, p. 425-438.

Doss, B. D., and Broadfoot, W. M., 1954, Terminal panel for electrical soilmoisture instruments: U.S. Dept. Agriculture, Southern Forest Expt. Sta.

Occasional Paper 135, p. 30-31. 

Dreibelbis, F. R., and Youker, R. E., 1951, An improved soil-moisture measuring

unit for hydrologic studies: Am. Geophys. Union Trans., v. 32, p. 447-449. 

Edlefsen, N. E., 1933, A glass wool cell for measuring aqueous vapor pressure of

soils : Rev. Sci. Instruments, v. 4, p. 345-346. 

Edlefsen, N. E., and Anderson, A. B. C., 1941, The four-electrode resistance

method for measuring soil-moisture content under field conditions: Soil

Sci., v. 51, p. 367-376. 

Edlefsen, N. E., Anderson, A. B. C., and Marcum, W. B., 1942, Field study of the

response of the electrical resistance of 2- and 4-electrode plaster of paris

blocks to variations on soil moisture: Soil Sci., v. 54, p. 275-279. 

Ewart, G. Y., and Baver, L. D., 1950, Salinity effects on soil moisture-electrical

resistance relationships: Soil Sci. Soc. America Proc., v. 15, sec. 1, p. 56-63. 

Fletcher, J. E., 1939, A dielectric method for determining soil moisture: Soil

Sci. Soc. America Proc., v. 4, sec. 1, p. 84-88. 

Gardner, F. D., 1898, The electrical method of moisture determination in soils:

U.S. Dept. Agriculture, Div. Soils Bull. 12. 

Graf, F. H., and Johnson, R. H., 1930, Study of methods for determining

moisture in sand: Am. Soc. Testing Materials Proc., pt. 1, p. 578. 

Haise, H. R., and Kelley, O. J., 1946, Relation of moisture tension to heat transfer

and electrical resistance in plastor of paris blocks: Soil Sci., v. 61, p. 411-422. 

Hendrix, T. M., and Colman, E. A., 1951, Calibration of Fiberglas soil-moisture

units: Soil Sci., v. 71, p. 419-427. 

Horton, J. S., 1955, Use of electrical soil-moisture units in mountain soils:

Western Snow Conf. Proc., 23d Ann. Mtg., Portland, Oreg., 1955, p. 20-26 

Johnson, W. R., 1929, Comparison of methods of determining moisture in sands:

Am. Concrete Inst. Proc., v. 25, p. 261. 

Kelley, O. J., 1944, A rapid method of calibrating various instruments for

measuring soil moisture in situ: Soil Sci., v. 58, p. 433-440. 

Kirkham, Don, and Taylor, G. S., 1949, Some tests of a four-electrode probe for

soil moisture measurement: Soil Sci. Soc. America Proc., v. 14, p. 42-46. 

Korty, B. L., and Kohnke, Helmut, 1953, Recording soil moisture automatically:

Soil Sci. Soc. America Proc., v. 17, p. 307-310.
U-18 CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES

LeOompte, S. B., Jr., 1952, An instrument for measuring soil moisture: New

Jersey State Horticultural Soe., Horticultural News, v. 33, p. 2563, 2568,

2572. 

Mackaness, F. G., and Rowse, R. W., 1954, Equipment for installing gypsum

moisture blocks: Agr. Eng., v. 35, p. 337. 

Mather, Bryant, 1945, Determination of surface moisture in fine aggregate:

Eng. News-Record, v. 134, p. 402-403. 

McCorkle, W. H., 1931, Determination of soil moisture by the method of

multiple electrodes: Texas Agr. Expt. Sta. Bull. 426.

Musgrave, G. W., 1942, A modified resistance block for soil moisture measurement : Agronomy Jour., v. 34, p. 284-285. 

Palpant, E. H., 1953, An inserter for Fiberglas soil-moisture urits: U.S. Dept.

Agriculture, Southern Forest Expt. Sta. Occasional Paper ir^ p. 16-20. 

Palpant, E. H., and Lull, H. W.» 1953, Comparison of four types of electrical

resistance instruments for measuring soil moisture: U.S. Dent. Agriculture,

Southern Forest Expt. Sta. Occasional Paper 128, p. 2-15. 

Palpant, E. H., Thames, J. L., and Helmers, A. E., 1953, Switch shelters for use

with soil-moisture units: U.S. Dept. Agriculture, Southern Forest Expt

Sta. Occasional Paper 128, p. 21-30.

Reinhart, K. G., 1953, Installation and field calibration of Fiberglas soilmoisture units: U.S. Dept. Agriculture, Southern Forest Expt. Sta.

Occasional Paper 128, p. 40-48. 

Remson, Irwin, and Fox, G. S., 1955, The displacement of calibration curves

for electrical soil-moisture units: Am. Geophys. Union Trans., v. 36,

p. 821-826. 

Rowland, E. F., Fagan, T. D., and Crabb, G. A., Jr., 1954, A slide rule for. soil

moisture determinations: Am. Soc. Agr. Engineers Jour., v. 35, p. 163-164. 

Slater, C. S., 1942, A modified resistance block for soil moisture measurement:

Agronomy Jour., v. 34, p. 284-285. 

Southern Forest Experiment Station, 1953, Soilj-moisture measurement with the

Fiberglas instrument: U.S. Dept. Agriculture, Southern Forest Expt. Sta.

Occasional Paper 128, 48 p. 

Stackhouse, J. M., and Youker, R. E., 1954, Evaluation of the accuracy of

Fiberglas-gypsum blocks for measuring soil moisture changes: Agronomy

Jour., v. 46, p. 405-407. 

Tanner, C. B., Abrams, E., and Zubriski, J. O., 1949, Gypsuir. moisture-block

calibration based on electrical conductivity in distilled water: Soil ScL

Soc. America Proc., v. 13, p. 62-65. 

Tanner, C. B., and Hanks, R. J., 1953, Moisture hysteresis in rvpsum moisture

blocks: Soil Sci. Soc. America Proc., v. 16, p. 48-51. 

U.S. Corps of Engineers, 1958, Evaluation of electric meters for determining

moisture content of fine aggregate: Waterways Expt. S'^A. Tech. Rept

6-476, 22 p. 

U.S. Weather Bureau, 1949, Installation and operation of a soil-noisture station:

U.S. Dept. Commence, Weather Bur., provisional, 17 p.

Wallihan, E. F., 1945, Studies of the dielectric method of measuring soil moisture : Soil Sci. Soc. America Proc., v. 10, p. 39-40. 

Waters, C. R., 1944, New method to determine moisture contert in sand: Eng.

News-Record, v. 132, p. 341. 

Weaver, H. A., and Jamison, V. C., 1951, Limitations in the use of electrical

resistance soil moisture units: Agronomy Jour., v. 43, p. 602-605.
METHODS OF MEASURING SOIL MOISTURE IN THE FIELD U-19

Whitney, Milton, Gardner, F. D., and Briggs, L. J., 1897, An electrical method 

of determining the moisture content of arable soils: U.S. Dept. Agriculture, 

Div. Soils Bull. 6, 26 p.

 1898, An electrical method of determining the moisture content of arable

soils: U.S. Dept. Agriculture, Div. Soils Bull. 12. 

Woods, Frank, and Hopkins, Walt, 1954, Phone-jack terminals for soil-moisture

units: U.S. Dept. Agriculture, Southern Forest Expt. Sta. Occasional Paper

135, p. 32-33. 

Youker, R. E., and Dreibelbis, F. R., 1951, An improved soil-moisture measuring

unit for hydrologic studies: Am. Geophys. Union Trans., v. 32, p. 4*7-449.

C. HEAT-DIFFUSION METHODS

Aldous, W. M., Lawton, W. L., and Mainfort, R. C., 1952, The measurement of

soil moisture by heat diffusion: U.S. Civil Aeronautics Adm. Tech. Devel.

Rept. 165,17 p. 

Johnston, C. N., 1942, Water-permeable jacketed thermal radiators as ind'~ators

of field capacity and permanent wilting percentage in soils: Soil Sci., v.

54, p. 123-126. 

Kersten, M. S., 1948, The thermal conductivity of soils: Highway Research

Board Proc., v. 28, p. 391-409. 

Momin, A. U., 1947, A new simple method of estimating the moisture content of

soil in situ: Indian Jour. Agri. Sci., v. 17, pt. 2, p. 81-85. 

Patten, H. E., 1909, Heat transference in soils: U.S. Dept. Agriculture, Div.

Soils Bull. 59, p. 3-54. 

Shaw, B. T., and Baver, L. D., 1939a, An electrothermal method for following

moisture changes of the soil in situ: Soil Sci. Soc. America Proc., v. 4, p.

78-83.

 1939b, Heat conductivity as an index of soil moisture: Agronomy Jour., 

v. 31, p. 886-891.

D. ABSORPTION METHODS

Davidson, A. L. C., and Schofield, R. K., 1953, Measurement of the suction of

soil water by Portland stone absorbers calibrated by a new method for

determining vapour pressures near to saturation: Jour. Agr. Sci., v. 32,

pt. 4, p. 413-427. 

Davis, W. E., and Slater, C. S., 1942, A direct weighing method for sequent

measurements of soil moisture under field conditions: Agronomy Jour.,

v. 34, p. 285-287. 

Dimbleby, G. W., 1954, A simple method for the comparative estimation of

soil water: Plant and Soil, v. 5, p. 143-154. 

Geffen, T. M., and Gladfelter, R. E., 1952, A note on the x-ray absorption

method of determining fluid saturation in cores: Am. Inst. Mining Hetall.

Engineers, Petroleum Branch, Trans., v. 195, p. 322-323. 

Livingston, B. E., and Koketsu, Riichiro, 1920, The water-supplying pover of

the soil as related to the wilting of plants: Soil Sci., v. 9, p. 469-485. 

Livingston, B. E., and Wilson, J. D., 1932, Wilting and withering of grasses in

greenhouse cultures as related to water-supplying power of the soil: Plant

Physiology, v. 7, p. 1-35. 

Richards, L. A., and Weaver, L. R., 1943, The sorption block soil moisture meter

and hysteresis effects related to its operation: Agronomy Jour., v. 35,

p. 1002-1011.
U-20 CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES

Stoeckeler, J. H., 1937, Measuring soil moisture in the forest nursery: U.S.

Dept. Agriculture, Forest Service, Lake States Forest Expt. Sta. mimeo.

rept., 4 p. 

Wilson, J. D., 1927, The measurement and interpretation of the water-supplying

power of the soil with special references to lawn grasses and some other

plants: Plant Physiology, v. 2, p. 385-440.

E. TENSIOMETBIC METHODS

Ashcroft, G., and Taylor, S. A., 1953, Soil moisture tension as a measure of water

removal rate from soil and its relation to weather factors: Soil Sci. Soc.

America Proc., v. 17, p. 171-174.

Boynton, Damon, 1939, Capillary tension as a measure of the po~e space unoccupied by water in some dense orchard soils: Soil Sci., v. 47, p. 347-352. 

Colman, E. A., Hanawalt, W. B., and Burck, C. R., 1946, Some improvements

in tensiometer design: Agronomy Jour., v. 38, p. 455-458. 

Edlefsen, N. E., Hendrickson, A. H., and Veihmeyer, F. J., 1943, Use of tensio

meters in measuring availability of water to plants: Plant Physiology,

v. 18, p. 66-78. 

Haise, H. R., and Kelley, O. J., 1950, Causes of diurnal fluctuations of tensiometers : Soil Sci., v. 70, p. 301-313. 

Hunter, A. S., and Kelley, O. J., 1946, Changes in construction o* soil moisture

tensiometers for field use: Soil Sci., v. 61, p. 215-217. 

Kenworthy, A. L., 1945, Cup conductance, field and laboratory calibration of

tensiometers employing inexpensive porous cups: Soil Sci., v. 59, p. 397^404. 

Miller, R. D., 1951, A technique for measuring soil-moisture tensions in rapidly

changing systems : Soil Sci., v. 72, p. 291-301. 

Neal, O. R., Richards, L. A., and Russell, M. B., 1937, Observations on moisture

conditions in lysimeters: Soil -6ci. Soc. America Proc., v. 2, p. 35^44. 

Richards, L. A., 1942, Soil moisture tensiometer materials anc1 construction:

Soil Sci., v. 53, p. 241-248.

 1949, Methods of measuring soil moisture tension: Soil Sci., v. 68, p. 

95-112.

 1954, Multiple tensiometer for determining the vertical component of the

hydraulic gradient in soil: Soil Sci. Soc. America Proc., v. 18, p. 7-10. 

Richards, L. A., and Gardner, Willard, 1936, Tensiometers for measuring the

capillary tension of soil water: Agronomy Jour., v. 28, p 352-358. 

Richards, L. A., and Huberty, M. R., 1941, Moisture studies under citrus using

tensiometers: Am. Soc. Horticultural Sci. Proc., v. 39, p. 73-79. 

Richards, L. A., and Neal, O. R., 1936, Some field observations wit^r tensiometers:

Soil Sci. Soc. America Proc., v. 1, p. 71-91. 

Richards, L. A., Neal, O. R., and Russell, M. B., 1939, Observations on moisture

conditions in lysimeters: Soil Sci. Soc. America Proc., v. 4, sec. 1, p. 55-59. 

Richards, L. A., and Pearson, R. W., 1939, A soil moisture tensiometer with a

compact manometer: Agronomy Jour., v. 31, p. 986-989. 

Richards, L. A., and Wadleigh, C. H., 1952, Soil water and p^nt growth, in

Shaw, B. T., ed., Soil physical conditions and plant growth: New York,

Academic Press, p. 73-252. 

Richards, L. A., Russell, M. B., and Neal, O. R., 1937, Further developments on

apparatus for field moisture studies: Soil Sci. Soc. America Proc., v. 2,

p. 55-64. 

Richards, S. J., 1938, Soil moisture content calculations from capillary tension

records : Soil Sci. Soc. America Proc., v. 3, sec. 1, p. 57-64.
METHODS OF MEASURING SOIL MOISTURE IN THE FIELD U-21

Richards, S. J., and Hagan, R. M., 1958, Soil moisture tensiometer: California

Agr. Expt. Sta. Leaflet 100. 

Richards, S. J., and Lamb, John, Jr., 1937, Field measurements of capillary

tension : Agronomy Jour., v. 29, p. 772-780. 

Rogers, W. S., 1935. A soil moisture meter depending on the capillary pull of the

soil: Agr. Sci. Jour., v. 25, p. 326-343. 

Russell, M. B., Davis, F. E., and Blair, R. A., 1940, The use of tensiometers for

following soil .moisture conditions under corn: Agronomy Jour., v. 32,

p. 922-930. 

Schofleld, R. K., 1935, The pF of the water in soil: Internal. Cong. Soil Sci.,

3d., Trails., v. 2, p. 37-^8. 

Scofield, C. S., 1945, The measurement of soil water: Agr. Research Jour., v. 71,

p. 375-402. 

Stoeckeler, J. H., and Aamodt, Binar, 1940, Use of tensiometers in regulating

watering in forest nurseries : Plant Physiology, v. 15, p. 589-607. 

Veihmeyer, F. J., Edlefsen, N. E., and Hendrickson, A. H., 1943, Use of tensiometers in measuring availability of water to plants: Plant Physiology,

v. 18, p. 66-78. 

Wallihan, E. F., 1939, Use of tensiometers for soil moisture measurement in

ecological research : Ecology, v. 20, p. 403-^412.

F. PENETRATION METHODS

Allyn, R. B., 1942, A calibrated soil probe for measuring field soil mo'sture:

Soil Sci., v. 53, p. 273-285. 

Allyn, R. B., and Work, R. A., 1941a, The availameter and its use in soil moisture

control, pt. 1, The instrument and its use : Soil Sci., v. 51, p. 307-319.

 1941b, The availameter and its use in soil moisture control, pt. 2, Calibration methods: Soil Sci., v. 51, p. 391-406.

Hvorslev, M. J., 1949, Subsurface exploration and sampling of soils fcr civil 

engineering purposes Am. Soc. Civil Engineers Research Project Rept.: 

U.S. Corps Engineers, Waterways Expt. Sta., p. 34-45.

G. RADIOACTIVE METHODS

American Society for Testing Materials, 1952, Symposium on the use of radioisotopes in soil mechanics: Am. Soc. Testing Materials Spec. Tecl . Pub. 

134, 34 p.

Belcher, D. J., Cuykendall, T. R., and Sack, H. S., 1950, The measurement of 

soil moisture and density by gamma ray scattering: U.S. Civil Aeronautics 

Adm. Tech. Devel. Rept. 127, 20 p.

 1952, Nuclear meters for measuring soil density and moisture in thin 

surface layers: U.S. Civil Aeronautics Adm. Tech. Devel. Rept. 161, 8 p.

 1953, Modifications and tests of radioactive probes for measuring soil

moisture and density: U.S. Civil Aeronautics Adm. Tech. Devel. Rept. 194, 

13 p.

Belcher, D. J., Herner, R. C., Cuykendall, T. R., and Sack, H. S., 1953, Use of 

radioactive material to measure soil moisture and density: Am. Soc. Testing 

Materials Spec. Tech. Pub. 134, p. 10-22.

Berdan, D., and Bernhard, R. K., 1950, Pilot studies of soil density measurements by means of X-rays: Am. Soc. Testing Materials Proc., v. 50, p. 

1328-1342.
U-22 CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES

Bernhard, R. K, and Chasek, M., 1955, Soil density determinate on by direct

transmission of gamma rays: Am. Soc. Testing Materials An">. Mtg., 58th,

Atlantic City 1955,18 p. 

Brocard, M. J., i955, Application des isotopes radioactifs a la mesure de la

density et de la teneur en eau des materiaux et des sols: Inst. Tech. du

Batiment et des Travaux Publics Ann., 8th, no. 89, p. 427-450. 

Burn, K. N 1960, Design and calibration of a neutron moistur? meter: Am.

Soc. Testing Materials Spec. Tech. Pub. 293, p. 14-26. 

Carey, W. N, Jr., and Reynolds, J. F., 1958, Some refinements in measurement

of surface density by gamma ray absorption: Highway Research Board

Spec. Rept. 38, p. 1-23. 

Carey, W. N, Jr., Shook, J. F., and Reynolds, J. F., 1960, Evaluation of nuclear

moisture-density testing equipment: Am. Soc. Testing Materials, Spec. Tech.

Pub. 293, p. 55-78. 

Carlton, P. F., 1957, The application of radioisotopes to the measurement of soil

moisture content and density, in Advances in nuclear engireering, v. 1:

Nuclear Eng. Sci. Conf., 2d, Philadelphia 1957, Proc., p. 403-411.

 1960, Application of nuclear soil meters to compaction control for airfield 

pavement construction: Am. Soc. for Testing Materials Spec. Tech. Pub. 

293, p. 27-35.

Church, G. R., and Smith, D. B., 1955, An instrument for the measurement of 

hydrogen content of soil and bulk material: Atomic Energy Research Establishment [Great Britain] Testing Rept., 8 p.

Davidson, J. M., Nielsen, D. R., and Perrier, E. R., 1959, Influence of temperature on soil moisture neutron probes: Soil Sci. Soc. Americr Proc., v. 23, 

p. 251-252.

Gardner, W. R., 1951, Determination of soil moisture by neutron scattering: 

Iowa State Coll., master's thesis, Ames, Iowa.

Gardner, Wilford, and Kirkham, Don, 1952, Determination of soil moisture by 

neutron scattering: Soil Sci., v. 73, p. 391-401.

Gnaedinger, J. P., 1960, Experiences with nuclear moisture and density surface 

probes on O'Hare Field project: Am. Soc. Testing Materials Spec. Tech. 

Pub. 293, p. 36-44.

Goldberg, Irving, Trescony, L. J., Campbell, J. S., Jr., and Whyte, G. J., 1954, 

Measurement of moisture content and density of soil masse? using radioactivity methods, in Clays and clay minerals: Natl. Conf. C'ays and Clay 

Minerals, 3d, Houston 1954, Proc., p. 516-548.

Gueron, Par J., 1954, Examples d'applications industrielles de la radioactivity: 

Jour. Physique le Radium, v. 15, p. 65A-73A.

Holmes, J. W., 1953, Methods of measuring soil water status: Australian Conf. 

Soil Sci., Adelaide, v. 2, p. 3.

 1956a, Calibration and field use of the neutron scattering method of 

measuring soil water content: Australian Jour. Appl. Sci., v. 7, p. 45-58.

 1956b, Measuring soil water content and evaporation by the neutron scattering method : Netherlands Jour. Agr. Sci. v. 4, p. 30-34.'

Hood, E. E., 1953, Determination of soil moisture content by measurement of 

neutron scattering: North Carolina State Coll., master's thesis, Raleigh, 

N.C., 50 p.

Horonjeff, Robert, ana Goldberg, Irving, 1953, Field measurements of soil moisture and density with radioactive materials: Highway Research Board 

Proc., 32d Ann. Mtg., Pub. 271,1953, p. 500-511.
METHODS OF MEASURING SOIL MOISTURE IX THE FIELD U-23

Horonjeff, Robert, and Javete, D. F.. 1955, Neutron and gamma-ray methods for 

measuring moisture content and density to control field compaction: Highway Research Board Bull. 122, p. 23-34.

Horonjeff, Robert, Goldberg, Irving, and Trescony, L. J., 1954, The use of radioactive material for the measurements of water content and density o* soil: 

California Univ. Street and Highway Conf., 6th Ann. Mtg., Berkeley 1954, 

Proc., p. 136-147.

Huyghe, G., and Mortier, P., 1952, Mesure de la densit£ du beton par absorption 

du rayonnement gamma: Inst. Interuniversitaire Sci. Nucleaires [Brussels] 

Ann. Rept., p. 93-100.

Jensen, W. E., 1953, Use of isotopes in soil testing: Military Engineer, v. 45, 

p. 275.

Knight, A. H, and Wright, T. W., 1954, Soil moisture determination by n?utron 

scattering: Oxford, 2d Radioisotope Conf. Proc., v. 2, p. 111-122.

Krueger, P. G., 1950, Soil density by gamma-ray scattering: Cornell Univ., 

master's thesis, Ithaca, X.Y., 74 p.

Kuhn, S. H., 1960, A new technique for compaction control: South African 

Council for Sci. and Indus. Research Trans., v. 2.

 1961, A comparison of nuclear and conventional methods for the determination of the moisture content and density of soils: Am. Assoc. State 

Highway Officials, Denver. Colo., October 10,11 p., 8 figs.

Lane, D. A., Torchinsky, B. B., and Spinks, J. W. T.. 1953, Determining soil 

moisture and density by nuclear radiations: Am. Soc. Testing Materials 

Spec. Tech. Pub. 134, p. 23-34.

McGuinness, J. L., Dreibelbis, F. R., and Harrold, L. L., 1961, Soil moisture 

measurements with the neutron method supplement weighing lysimeters: 

Soil Sci. Soc. America Proc., v. 25, p. 339-342.

Miles, M. E., 1953, Energy distribution of gamma rays scattered around a soil 

density probe: Cornell Univ., Eng. Physics Dept. Project Rept., 38 p.

Mintzer, Sidney, 1960, Comparison of nuclear and sand-cone methods of density 

and moisture determinations for four New York State soils : Am. Soc. Testing 

Materials Spec. Tech. Pub. 293, p. 45-54.

Mortier, P., DeBoodt, M., and DeLeenheer, L., 1952, Mesure de la teneur en eau 

du sol par diffusion de neutrons: Inst. Interuniversitaire Sci. Nucleaires 

[Brussels] Ann. Rept., p. 100-105.

Mortier, P., and DeBoodt, M., 1956, Determination of soil moisture by neutron 

scattering: Netherlands Jour. Agr. Sci., v. 4, p. 111-113.

Neville, O. K., and Van Zelst, T. W. 1960, Design and application of the NuclearChicago d/M gauge: Am. Soc. Testing Materials Spec. Tech. Pub. 293, p. 3-13.

Nielsen, D. R., Kirkham, Don, and Burrows, W. C., 1958, Solids-water-air space 

relations of some Iowa soils: Iowa State Coll. Jour. Sci.. v. 33, p. 111-116.

Phillips, R. E., Jensen, C. R., and Kirkham, Don, 1959, Use of radiation equipment for plow-layer density and moisture: Soil Sci., v. 89, p. 2-7.

Pieper, G. F., Jr., 1949, The measurement of moisture content of soil by the 

slowing of neutrons: Cornell Univ., master's thesis. Ithaca, N.Y., 55 p.

Roy, S. E., 1956, Nuclear radiation methods for the determination of density and 

moisture content of soils: Princeton Univ. Civil Eng. Dept. Spec. B ept. 1.

Sharpe, J., 1953, Measurement of moisture content by neutron counting: British 

Jour. Appl. Physics, v. 4, p. 93-94.

Spinks, J. W. T., Lane, D. A., and Torchinsky, B. B., 1951, A new method of 

moisture determination in soil: Canadian Jour. Technology, v. 29, p. 

371-374.
U-24 CONTRIBUTIONS TO THE HYDROLOGY OP THE UNITED STATES

Stewart, G. L., and Taylor, S. A., 1957, Field experience with the reutron-scattering method of measuring soil moisture: Soil Sci., v. 83, p. If 1-158. 

Stone, J. F., 1955, A portable apparatus for measurement of soil moisture by

neutron scattering: Iowa State Coll., master's thesis, Ames, Iowa, 25 p. 

Stone, J. F., Kirkham, Don, and Read, A. A., 1955, Soil moisture determination by

a portable neutron scattering moisture meter: Soil Sci. Soc. America Proc.,

v. 19, p. 419-423. 

Swanson, R. W., 1954, Instrumentation of a field survey meter for soil moisture

determination: North Carolina State Coll., master's thesis, Raleigh, N.C.,

58 p. 

Timblin, L. O., Jr., 1955, Measurement of subsurface soil density by gamma ray

scattering: U.S. Bur. Reclamation, Chem. Eng. Lab. Kept. Sl-6, 12 p. 

Underwood, Newton, Van Bavel, C. H. M., and Swanson, R. W., 1954, A portable

slow neutron flux meter for measuring soil moisture: Soil Sci., v. 77, p.

339-340. 

U.S. Corps of Engineers, 1955a, Application of nuclear soil meters to the study

of seasonal variations in subgrade moisture and density at Lockbourne Air

Force Base, Ohio: Ohio River Div. Lab., Mariemont, Ohio Memo. Rept.,

15 p. 

 1955b, Field tests of nuclear instruments for the measurement of soil

moisture and density: U.S. Waterways Expt. Sta. Misc. Pap«r 3-117, 26 p. 

U.S. Department of Agriculture, 1958, Measurement of soil moisture content by

the neutron method: U.S. Dept. Agriculture, Agr. Research Service 41-24,

29 p. 

Van Bavel, C. H. M., 1956, Neutron and gamma radiation as applied to measuring

physical properties of soil in its natural state: Internat. Cong:. Soil Sci., 6th,

Paris, 1956, Trans., v. B, p. 355-360.

Van Bavel, C. H. M., Hood, E. E., and Underwood, Newton, 1954, ^ertical resolution in the neutron method for measuring soil moisture: Am. Geophys.

Union Trans., v. 35, p. 595-600. 

Van Bavel, C. H. M., Underwood, Newton, and Swanson, R. W., 1956, Soil

moisture measurement by neutron moderation: Soil Sci., v. 82, p. 29--41. 

Vomocil, J. A., 1954, In situ measurement of soil bulk density: Agr. Eng., v. 35,

p. 651-654. 

Weeks, L. V., and Stolzy, J. H., 1958, Neutrons in water-use stud'^s: California

Agriculture, p. 8, December. 

Yates, E. P., 1950, Soil moisture determination by neutron scattering: Cornell

Univ., master's thesis, Ithaca, N.Y., 45 p.

H. GENERAL METHODS

Baver, L. D., 1956, Soil water, in Soil physics, 3d .ed.: New York, John Wiley & 

Sons, p. 224-303.

Childs, E. C., 1940, Use of soil moisture characteristics in soil studies: Soil Sci., 

v. 50, p. 239-252.

Coile, T. S., 1953, Moisture content of small stones in soil: Soil Sci., v. 75, p. 

203-207.

Conrad, J. P., and Veihmeyer, F. J., 1929, Root development and soil moisture: 

Hilgardia, v. 4, p. 113-134.

Cummings, R. W., and Chandler, R. F., Jr., 1940, A field comparison of the electrothermal and gypsum block electrical resistance methods w'th the tensiometer method for estimating soil moisture in situ: Soil Sci. Soc. America 

Proc., v. 5, sec. 1, p. 80-85.
METHODS OF MEASURING SOIL MOISTURE IN THE FIELD U-25

Jackson, M. L., and Weldon, M. D., 1939, Determination of the weight of water 

in a soil or subsoil mass in which the moisture content increases with distance from a plant or group of plants: Agronomy Jour., v. 31, p. 11 -127.

Kelley, O. J., Hunter, A. S., Haise, H. R., and Hobbs, C. H., 1946, A comparison of 

methods of measuring soil moisture under field conditions: Agronomy Jour., 

v. 38, p. 759-784.

Kummer, F. A., and Cooper, A. W., 1945, Soil porosity determinations with the 

air pressure picnometer as compared with the tension method: Agr. Eng., 

v. 26, p. 21-23.

Lull, H. W., and Reinhart, K. G., 1955, Soil-moisture measurement: U.S. Dept. 

Agriculture, Southern Forest Expt. Sta. Occasional Paper 140, 56 p.

Nikiforoff, C. C., 1948, Stony soils and their classification: Soil Sci., v. 66, p. 

347-363.

Olson, D. F., Jr., and Hoover, M. D., 1954, Methods of soil moisture determination 

under field conditions: U.S. Dept. Agriculture, Southern Forest Expt. Sta. 

Paper 38, 28 p.

Page, J. B., 1948, Advantages of the pressure picnometer for measuring the pore 

space in soils : Soil Sci. Soc. America Proc., v. 12, p. 81-84.

Russell, E. J., 1950, Soil conditions and plant growth: Longmans, Green and Co., 

635 p.

Russell, J. C., 1939, The effect of surface cover on soil moisture losses by evaporation : Soil Sci. Soc. America Proc., v. 4, p. 65-70, November.

Russell, M. B., 1950, A simplified air-picnometer for field use: Soil Sci. Soc. 

America Proc., 1949, v. 14, p. 73-76.

Slater, C. S., and Bryant, J. C., 1946, Comparison of four methods of soil moisture 

measurement: Soil Sci., v. 61, p. 131-155.

Taylor, S. A., 1955, Field determinations of soil moisture: Agr. Eng., v. 36, p. 

654-659.

U.S. Department of Agriculture, 1954, Some field, laboratory, and office procedures for soil-moisture measurement: U.S. Dept. Agriculture, Southern 

Forest Expt. Sta. Occasional Paper 135, 47 p.

Van Bavel, C. H. M., Nielsen, D. R., and Davidson, J. M., 1961, Calibration and 

characteristics of two neutron moisture probes : Soil Sci. Soc. America Proc., 

v. 25, p. 329-334.

Woodruff, C. M., 1936, Linear changes in the Shelby loam profile as a function of 

soil moisture: Soil Sci. Soc, America Proc., v. 1, p. 65-70.

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