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w Forest Service ’ Research Note - Southern F

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Experiment tation

SO-285

July, 1983

Core Versus Nuclear Gauge Methods of

~Determining Soil Bulk Density and Moisture Content

Jacqu@ne G. Steele, Jerry L. Koger, Albert C. Trouse, Jr. and Donald L. Sirois

SUMMARY

and moisture content measurements were obt d using two nuclear gauge systo those obtained from soil

cores. The soil wassee sandy loam, a Lakeland

loyd clay, were free of organic

matter and uni in mechanical composition. The

ns developed for the nuclear

hase of the study failed to predict bulk density and moisture content in the vali-

‘dation phase. T he test results indicate that if

greater confidence is placed on the standard soil

core determinations, then regression equations

should be develo

soil condition an8

ed for the nuclear gauges in each

for each soil type.

INTRODUCTION

Until recently, ~soil bulk density was obtained by

removing “undis urbed” sample cores for weight,

moisture, and vo ume measurements. The developi

ment of nuclea gauges introduced a possible

method of deter ining bulk density and moisture

without removal lof soil cores. In addition, nuclear

gauges have the potential of increasing the speed

of measurement

I

, minimizing soil disturbance, and

allowing for con inual monitoring of the same soil

over a period oft me.

LITERATURE REVIEW

Early studies using the gamma-ray attentuation

technique were conducted by Vomocil (1954), Bernhard and Chasek (1955), and van Bavel et al. (1957).

Studies dealing with measurement and calibration

techniques have been done by Corey et al. (1971),

Mansell et al. (1973), Santo and Tsuji (1977), and

Soane and Henshall (1979). Vigier and Campbell

(1980) and Smajstrla and Clark (1981) discuss

methods to improve the accuracy of nuclear gauges.

Campbell (1973) and Hassan (1977) used nuclear

gauges to measure soil compaction and moisture

content on forest sites following timber harvesting

activities. Their results indicate a high degree of

variation in these measurements and a need for

correction factors. Soane (1974), in a review paper,

summarized the findings of several researchers

concerning the types of detectors, probe design

and spacing, spatial resolution, as well as gauge

accuracy and precision.

The purpose of this study was to compare the

accuracy and consistency of values for soil bulk

density and soil moisture content obtained with

two nuclear gauge systems with those obtained by

soil cores (gravimetric).

SOIL TREATMENTS

The soils used in the tests were the lower Ap

horizon of a Hiwassee sandy loam (clayey, kaolinitic, thermic, oxide, Typic Rhodudults), a Lakeland

loamy sand (thermic, coated, Typic, Quartzisamments), and a Lloyd clay (clayey, kaolinitic, thermic, Typic, Hapludults). These soil materials were

collected years ago and are confined in large outdoor bins for testing with full scale farm machinery.

The bins are located at the testing facilities of the

National Tillage Machinery Laboratory at Auburn,

Southsm Forest ~Expadmsnt StatbuT-102lO U.S. portrl Services Bldg., 701 Loyola Avenue, New Orleans, La. 70113

Ford SOWIU, U.S. Department of Agriculture.

Serving Alabama, Arkansas, Loulslana, Misslsslppi, E. Oklahoma, Rnnssaee, E. Texas, Puerto Rico, U.S. Virgin Islands

1
Alabama. Each it is generally stone free, uniform

in mechanical mposition throughout its depth,

aterial. Its uniformity makes

um for comparing nuclear gauge

isture and density to results from

Selected physical and chemical

8 soils have b88n reported by

pared with a deep plow pan

ow pan depth just prior to

niform soil mix above the pan.

wide, 1,950.5 kg) was used to

) before the test treatments

nt A was the control in which

ere applied to the soil. In the

four bins ret additional traffic treatments,

ays were made t0 form a t8St

band 90 cm wi m8aSUr8m8ntS were made in

athway to obtain relatively

Two treatments (B & C)

were obtained g 2 and 4 passes, respectively,

with a steel wh 30 cm wide, 106 cm in diameter,

and mass of 5 kg. Tr8atm8ntS D 81 E resulted

8 same wheel, but with a

different treatment com1 METHODS

determined at two depths, 5 cm

88 t8ChniqU8S in each treatment

selected sites in 8aCh treatment

At each site bulk density was

roxler Model 34111, a Campbell

-1, and a Cornelison soil core

t each site measurements were

first made usin single probe Troxler unit. Then

c unit used the existing hole

and a new hOl8 was mad8 for

(dual probe mode). The last

measurements 8 made on the soil cores taken

sampler from the zone between

‘The use of trade, fir’ or corporate names is for the information

and convenience o the reader. Such use does not constitute

an official evaluati n, conclusion, recommendation, endorsei

ment, or approval o any product or service to the exclusion of

others which may b suitable.

Table l.-Soil compaction treatments

Treatment Number

code of passes

Steel wheel

mass

Kg

A (control) 1 1950.5

B’ 2 581.8

C 4 581.8

D 2 1068.2

E 4 1088.2

‘Treatments B, C, D, 8 E also received treatment A

Soil cores

The Cornelison sampler removed approximately

150 cm of soil per sample. The soil was weighed,

dried at 105 degrees Celsius for 72 hours, and ra

weighed. The wet and dry bulk densities and the

moisture contents of the soil samples were calculated using th8 following equations.

Dry bulk density = oven dry weight of soil (g) (1)

volume of soil core (cm)

Wet bulk density = field weight of Soil (g) (2)

volume of soil core (cm)

(field soil weight-dry

Percent moisture = soil weight) (3)

(0.01 *dry soil weight)

Nuclear Gauges

The information supplied by the equipment

manufacturers, other sources such as Freitag (1971),

and the Soils Manual (1976) explain the theory and

use of nuclear gauges.

The gauges in this study used Cesium 137 as a

source of gamma radiation and Am8riCUm-241

Beryllium for th8 n8UtrOn source. The gauges were

used in the direct transmission mod8 and standard

counts were taken at the beginning and 8nd of each

day. The general calibration equation (Campb8ll Pacific Nuclear Corporation 1976) relating the standard count to the actual count is:

CR = A(e**-BD)-C (4)

where: CR = count ratio (actual count/standard

count)

A, C = constants dependent on gauge

g8Om8trY

Steele, formerly a R search Engineer with the Southern Forest Experiment Station, is now a Mechanical Engineer at the U.S. Army

Missile Command a Redstone Arsenal, Alabama. Trouse is a Soil Scientist, National Tillage Machinery Laboratory, Auburn, Alabama. Koger and

t

S i ois are Research Engineers, Forest Service-USDA, Auburn, Alabama.

2
B = constant dependent on attenuation

coefficient

D = soil wet bulk density

e = natural log, (base e,2.718282)

l * = exponent notation

Each nuclear gauge manufacturer supplied separate equations relating wet bulk density and

moisture content to the count ratio as follows:

using: 02 = wet bulk density (Ib/ft) at a depth

of 5 cm.

D8 = wet bulk density (Ib/ft) at a depth

of 20 cm.

R 2 = density count with the probe at a

depth of 5 cm.

R 8 = density count with the probe at a

depth of 20 cm.

S = standard soil density count

M C = moisture density (Ib/ft)

MCR = moisture count ratio (Imoisture

count/standard count)

L n = the natural log ((base e, 2.718282)

Troxler Electronic Laboratories, Inc. (1975):

132 = - Ln(( R2/S + 0.0486)/7.7988)/0.016 (5)

D8 = - Ln((R8/S-0.0141)/13.5525)/0.02637 (6)

MC = - 1.57+(57.08*MCR) (7)

Campbell Pacific Nuclear Corporation (7978):

(These equations were supplied for use in single

probe mode only; none were available for the double

probe mode)

D2 = 332.44-171.78*(R2/S)**0.348180 03)

08 = 444.41-339.18*(R8/S)**0.134620 (9)

MC = -1.42+(37.09*MCR) (10)

ANALYSIS

The mean (n- 10) of the wet soil bulk density

and moisture density meansurements as determined

by the Cornelison soil sampler, Troxler, and Camp

bell Pacific (single probe) is shown in table 2. There

were significant differences (t test, Ostle and Mensing 1975) in about 90 percent of the test combinations in the wet soil bulk density and percent

moisture means obtained by the soil core and the

Figure i.-Nuclear gauges and soil core sampler used in the study (lett to right: Campbell Pacific, Cornelison sampler & Troxler).

3
4 nuclear gauges. Therefore, regression equations

(table 3) were developed by soil type and depth

to relate the nuclear gauge values to those obtained

by the soil cores. “ ,

VALIDATION

Using the same soils, soil bins, and similar pretest

preparation techniques, a second series of co6

paction tests was conducted. Different levels of soil

compaction were obtained by using three skidder

tires under different conditions of dynamic load, inflation pressure, and travel reduction. Bulk density

and moisture measurements were made in the

center of the tire footprints at 1.5 m intervals along

the test strips at depths of 5 cm. The means (n=83)

obtained by the different methods are shown in table

4. The bulk density and moisture values shown for

the nuclear gauges are based on the regression

equations for the 3 cm depth by gauge and soil type.

The Campbell Pacific (single probe) and Troxler

were not used in the Lloyd clay.

There were significant statistical differences in

wet bulk density means between the soil core

method and the two nuclear gauges, except with

the Campbell Pacific (single probe) in the sandy

loam. The regres$ion equations developed for the

nuclear gauges gave lower wet bulk density values

than obtained by the soil core method. With percent

soil moisture there were statistical diffferences in

the means between the soil core method and the

gauges, except with the Troxler in the sandy loam.

In all but one case, the regression equations developed for the two nuclear gauges overestimated

percent soil moisture as compared to the soil core

method.

In the first phase of this study, regression equations were developed to improve the correlation between values obtained with nuclear gauges to those

obtained by the standard soil core method. The

soils involved in the tests were from the Lloyd

series, the Hiwassee series, and the Lakeland series.

Bulk density measurements were made at 5 cm

and 20 cm depths.

The wet bulk density means ranged from 1.19

g/cm to 2.00 g/cm for the soil core method and

from 1.48 g/cm to 2.09 g/cm with the nuclear

gauges. The nuclear gauges (using the factory

equations; equations 5,8,8 and 9) tended to overestimate the wet bulk density.

The molsture content ranged from 5.4 to 21.4

percent using the soil core method and from 3.8

to 23.2 percent with the nuclear gauges. Based on

Table 2.-Soil wet bulk dens/ty and percenf moisture means for the three methods of

measurement

Test Sol1 corn Troxler Campbell Pacific

code 5cm 2Ocm M' Scm 2Ocm M' 5cm 20cm M'

----g/cm---- % ----g/cm---- % ---g/cm--- %

alay

A 1.19 I.38 20.59 1.54 1.53 21.78 1.48 1.50 17.88

B I.57 I.49 21.39 1.88 1.88 22.91 1.81 1.87 19.27

C I.83 1.50 20.98 1.75 1.72 23.02 1.72 1.71 19.58

D I.85 I.58 21.42 I.73 I.75 23.17 1.70 1.75 19.58

E I.73 1.84 20.89 I.82 1.81 21.81 1.81 1.82 18.35

Sandy Loam

A 1.57 I.88 8.85 1.49 1.87 7.93 1.45 1.89 8.58

B 1.79 1.82 8.89 1.89 1.88 8.85 1.87 I.92 7.23

C 1.90 1.89 8.18 1.81 1.98 8.52 1.81 2.00 8.88

D I.89 1.94 8.88 1.77 1.99 8.90 1.78 2.02 7.39

E 1.98 2.00 8.10 1.55 2.05 8.13 I.55 2.09 7.22

LoamySand

A 1.71 I.85 5.40 1.71 1.72 4.89 1.70 1.73 3.81

B 1.77 1.70 5.59 1.78 1.79 4.98 1.79 1.82 4.30

C 1.82 1.71 5.47 1.82 I.82 4.91 I.83 I.85 4.12

D I.77 1.77 5.42 1.75 1.81 5.03 1.74 1.84 4.13

E 1.81 1.77 5.50 1.80 I.87 5.08 1.80 1.89 4.32

*M=90ilm0l8turein percent

4
Ta

-

c

-

Cl

P a

(S

TI

the factory suppl

lo), the Troxler 1

content while the (

estimate.

The regression

bin prepared witl

conditions. The v;

the same soils w

techniques. The VA

moisture content 1

gression equation

obtained by the

agree. Based on tt

5

3.--Regression equations for determining wet bulk density (g/cm) and percent

moisture for the nuclear gauges

a6 I Soil Equation

3 . type

Regression equation R-SQ cv number

bell

C

e)

Ibell

P

sr

C

C

C

L

L

L

S

S

S

C

C

L

L

S

S

C

C

C

L

L

L

S

S

S

D2= 2.936- 0.706*X+ 0.052*X*X

DE= 2.965- 2.326*X+ 0.639*X*X

M = 7.004+ 46.644’Y- 41.725.Y.Y

D2= 2.590- 0.211*X- 0.061*X*X

D6= 2.956 2.252*X+ 0.963*X*X

M= 9.142- O.l97’Y- 9.621 ‘Y’Y

D2=-0.713+ 2.625*X- 0.761*X*X

D6= 2.565- 1.419*X+ 0.476*X*X

M= 2.650+ 16.941’Y+ 2.919’Y.Y

D2= 2.602- 0.969*X+ 0.196*X*X

D6= 2.464- 0.639*X+ 0.091 ‘X*X

D2= 2.530- 0.751*X+ 0.129*X*X

D6= 2.494- 0.532*X+ 0.059*X*X

D2= 1.462+ 0.653*X- 0.465*X*X

D6= 2.761- 0.697*X+ 0.176*X*X

D2= 2.673- 1.149*X+ 0.166*X*X

D6= 2.673- 2.463*X- 1.027*X*X

M= 12.965- 32.575’Y- 31.632’Y’Y

D2= 3.214- 1.343*X+ 0.234*X*X

D6= 2.624- 2.264*X+ 1.075*X*X

M= 19.469- 114.137’Y+292.295*Y’Y

D2= 0.633+ 2.406*X- 1.161 ‘X*X

D6= 2.130- 0.505*X- 0.097*X*X

M---5.502+151.099’Y- 500.524’Y’Y

0.92

0.94

0.35

1.79

1.51

1.51

(11)

(12)

(13)

0.69 2.74 (14)

0.97 1.14 (15)

0.09 4.63 (16)

0.60 1.05 (17)

0.65 1.66 (16)

0.66 3.57 (19)

0.96 1.36 (20)

0.96 1.19 (21)

0.90 2.56 (22)

0.96 1.24 (23)

0.77 1.13 (24)

0.67 1 .oo (25)

0.92 1.62 (26)

0.93 1.59 (27)

0.31 1.56 (26)

0.69 2.67

0.96 1.24

0.06 4.66

0.62 1.01

0.66 1.59

0.56 4.03

(29)

(30)

(31)

(32)

(33)

(34)

Jay, L=Loam, S=Sand, D2=Wet Bulk Density at 2 inch depth, DB=Wet Bulk Density

inch depth, M=Percent Soil Moisture, X=actual density count/standard density Count

auge type), Y=actual moisture count/standard moisture count (by gauge type)

I equations (equations 7 and

ded to overestimate moisture

npbell Pacific tended to underluations were challenged in a

different moisture and density

lation study consisted of using

slightly different compaction

bulk density (5 cm depth) and

asurements obtained using rewere compared to the values

I cqre method. They did not

res

v

ts of this study, regression

?

I

equations for the nuclear gauges would have to be

developed for each test condition by soil type and

depth. That is, the regression equations developed

in the initial tests did not give statistically good

results in the validation tests.

When using nuclear gauges to determine soil

bulk density and moisture content it is advisable

to collect soil core sub-samples in order to test for

variations in the two methods. If a greater confidence is placed on the standard soil core determinations, then regression equations should be developed for the nuclear gauges in each soil condition and for each soil depth.