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CHAPTER 7. ISOTOPES METHODS AND USES IN SOIL PHYSICS RESEARCH

CHAPTER 7. ISOTOPES METHODS AND USES IN SOIL PHYSICS RESEARCH

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322



DON KlRKHAM A h 9 RAYMOND J. W N Z E



also of nonradioactive isotopes. The isotopes which will be discussed are:

H2, H3, C14, N16, 0 l 8 , P32,S35, C136,K42, Ca45,Cow, Rbs6, SrgO,Il3I, C S I ~ ~ ,

and radium-beryllium.

Recent reviews on the use of radioisotopes, including some uses in

soil physics, have been prepared by Fried et al. (1958) and by Raney

and Thorne (1957). A recent general paper on radioisotopes and agronomy is that of Norman (1959). Most of the work reviewed on uses of

isotopes in soil physics has been concerned with moisture determination

by neutron scattering and with density determination by gamma-ray

transmission or scattering. Very little other work on nuclear methods in

soil physics has, at the time of this writing ( 196l), been reported. What

work is known to the authors, including recent work in neutron and

gamma ray scattering, will be reported here.

This paper will include, primarily at the end, suggestions for future

work.

11. Soil Water



A. MEASUREMENT

BY THE KEIJTRONMETHODOF WATERRETAINED

IN SOIL

The most important information generally needed about soil water is

the amount of it contained in the soil rooting depth. This amount, in

humid regions where irrigation is not commonly practiced, will vary

with the following seven factors: (1) rainfall, ( 2 ) evaporation, ( 3 )

transpiration, ( 4 ) runoff, ( 5 ) deep seepage, ( 6 ) condensation, and ( 7 )

rising ground water level. In the above list, for arid regions, irrigation

must be considered with or in place of rainfall. With these many factors

and their interaction with various soils, and without prior moisture

tests of the soils, it is impossible to predict the soil moisture content; it

must be measured. The most useful method we know for measuring

soil moisture is the neutron-scattering method. We at Iowa State

University have also tried nuclear magnetic resonance, a method which,

with its present costly equipment ( $25,000 from the Varian Associates ) ,

does not seem practical. This technique was also tried by others ( Andreev

and Martens, 1960). The neutron method appears much more practical

and is becoming fairly well known to soil physicists.



1. How the Neutron Meter Works

Early work on the neutron meter for use in agriculture was done by

Gardner and Kirkham (1952). Even earlier Belcher et al. (1950) had

worked on the problem and had developed equipment for soil tests in

airport runway design. The Gardner-Kirkham equipment was not

portable. Stone et aE. (1955) and van Bavel et al. (1956) developed

portable meters. Commercial meters are now available. One commercial



323



ISOTOPES IN SOIL PHYSICS RESEARCH



model including surface and depth gauges is described by Kuranz

(1960). Other papers dealing with the calibration and the use of

commercial models are those of Burn ( 1961), Neville and van Zelst

( 1961), and Gnaedinger (1961).

The device works as follows: A source of fast neutrons, (about

1,000,000electron volts) usually from a mixture of radium and beryllium,

is lowered into a pipe (called an access pipe) driven into the soil. With

the neutron source at the desired depth the fast neutrons penetrate into

the soil, bouncing about from soil atom to soil atom, some of them

returning to the access pipe where a detector of slowed neutrons has also

been located with the neutron source. Essentially, the only slowing of the

neutrons is that resulting from collisions with hydrogen atoms. The other

soil atoms, all much heavier than hydrogen, do not slow the neutrons

appreciably. So the detector measures essentially the density of hydrogen

atoms in the soil; and since the hydrogen atoms in soil are almost all

associated with soil water, the number of slowed neutrons detected per

second is a measure of the soil water per unit volume of the soil.

2. W h y Neutrons Are Slowed Down by Hydrogen Atoms

To show the basic principle of the neutron moisture meter, which is

that hydrogen atoms slow up neutrons rapidly whereas other soil atoms

do not, a simple experiment may be performed. As a preliminary to such

an experiment, Table I is presented which gives the masses and relative

abundances of different soil atoms. The table shows that atoms comprising

TABLE I

Masses and Percentages of Elements in Soil@

Element



H



Mass



1



0 (not in water form)

Si

A1

Fe

Ca

Mg

Na



K

Other



16

28

27

56

40

24

23

39



Per cent in soil

by weight

(Depends on water

content of soil)

46.9

32.3

7.6

4.0

1.1

0.7

1.0

1.7

4.7



100.0b



a



b



Calculated from data of Thompson (1957, p. 7 ) .

Hydrogen of soil water is not considered as a part of the soil.



324



DON KIRKHAhL A S 9 FIAYMOND J. KUNZE



the dry soil substance are all heavy (massy) compared to hydrogen.

With the table in mind one can now perform the experiment.

Take two glass marbles, an orange one and a blue one, say, each

one weighing 5 g. These are standard-sized marbles used by boys. Take

also a third ball about the same size made of steel and weighing about

45 g., 9 times as much as the glass balls.

Now suppose that the blue glass ball represents a neutron, one such

as would be shot from the radium-beryllium neutron source. Suppose

also that the orange glass ball represents a hydrogen atom in soil. Let

the steel ball represent a heavy atom in the soil. Now, in a plastic

V-groove trough strike the ‘leavy atom” with the “neutron.” The

“neutron” will bounce back from the “heavy atom” (steel ball) with

very little loss of velocity, that is, energy. Now strike the “hydrogen

atom” (orange glass ball) with the “neutron” (blue glass ball). Observe

that the neutron stops; the hydrogen atom takes all the neutron’s velocity.

In soil, of course, the blows are not head-on as in the V-groove trough,

but the principle is the same. The hydrogen atoms slow up the neutrons.

The more hydrogen, and hence water, the more slowing down.



3. Adoantages and Disadvantages of the Neutron Meter



The neutron meter has some outstanding advantages. Some of these

are:

1. The method is rapid, about one-half minute being required for a

determination at a single depth in the soil. The detector is easily moved

up and down in the access pipe to get the soil moisture at any desired

depth. Movement of the equipment between access pipes may be

facilitated by a gocart ( Ameniya and Namken, 1960).

2. Fewer samples are required than for most other methods. A

neutron determination is as good as six (Shaw et al., 1959) or seven

(Stone et al., 1960) gravity determinations.

3. The neutron meter equilibrates instantaneously with the soiI

moisture. Some other type instruments require 2 to 3 weeks to equilibrate.

4. One access pipe may be used the year round to determine

moisture content in the soil. A small test plot need not be dug up for

getting moisture samples. The method is nondestructive.

5. The meter gives the soil moisture on a soil bulk volume basis.

Moisture on a volume basis is what is generally desired. Soil moisture

detenninations by oven drying are on a weight basis. To get them on a

volume basis the bulk density of the soil sample must be determined,

Bulk density determinations may introduce large errors (Stone et al.,

1960).

6 . The meter gives the average moisture of about a 6-inch diameter



ISOTOPES IN SOIL PHYSICS RESEARCH



325



sphere. Ordinarily this is an advantage, as variations over very small

distances are not desired. In some cases a smaller distance is desired.

Mortier et al. (1959,1960) has shown how a paraffin sphere surrounding

the radium-beryllium source may be used to slow up the neutrons and

reduce the sphere of influence,

7 . The meter is insensitive to changes in salt concentration in the

soil. Different rates of fertilizer application will not upset the moisture

reading.

8. The type of material of which the access pipe is made is not

critical. Neutrons pass through steel tubes and aluminum tubes easily.

9. One calibration ordinarily serves for a number of soils (Stolzy

and Cahoon, 1957). If soils which contain large amounts of organic

matter are avoided (Gardner and Kirkham, 1952) and if certain clay

soils which bind up a great deal of water in the clay crystal lattice

(Mortier and De Boodt, 1956) are excluded, a high degree of accuracy

in determining soil moisture can be obtained. Stewart and Taylor (1957)

discuss the advantages of measuring soil moisture with a neutron meter

as opposed to resistance and tensiometer-type measurements. They point

out that the neutron meter is reliable in all types of inorganic soils of

medium texture regardless of salt content and soil type.

At the present time there are two marked disadvantages of the

neutron meter. The initial cost is high: the scalar costs $1500, and the

depth probe costs $1000. The other disadvantage is that skilled personnel

are needed to service the unit. Presently available commercial units

seem to get quickly out of repair.

For an article giving 91 references on the neutron meter and the

gamma-ray density meter (the latter to be discussed below) see a paper

by Kuranz (1960).



4. Uses of the Neutron Meter in Soil Moisture Studies

a. Field capacity measurements. Figure 1 shows curves of moisture

content versus time and field capacities, F.C., at a number of depths in

two soils. When the curves flatten, gravitational water is gone, and the

field capacities, as shown by arrows in the figure, become evident. The

neutron meter is especially useful in these field capacity determinations

since one can follow the moisture content with time at one spot;

variations in moisture content due to sampling at different sites do not

confound results.

b. Determination of water stored termporarily above the jield capacity.

The neutron meter responds essentially instantaneously to the instantaneous moisture content in the soil. This characteristic has been used by

Nielsen et al. (1959) to measure the water stored temporarily above the



326



DON KIRKHAM A N D RAYMOND J. WNZE



field capacity. In Fig. 2 profiles of temporarily stored water are seen

for a Monona silt loam, a loess. The thickness of the profiles represents

moisture in excess of the field capacity at the time printed above the

profiles. Zero time is the time when the profile is near water saturation

to about the $foot depth and when water application to the profile has



MARSHALL

SILT LOAM



SILT



IDA

LOAM



Horiz. arrows indicote

scole o f ordinate



36

34

32

34



Vert. arrows give



. 4EC = 25.6



32

30



-0



28



-2



7 26

28

26

m

27



L

c



25



f 23



;28



26

24

26

I

24



38

42



22



20

18

16

‘14

12

10



40

38

36



0



12



24

36

48

Time (Hr.)



60



72



I

0



+



8



16



I



I



24

32

Time (Hr.)



I



I



40



48



56



FIG. 1. Moisture vs. time curves for ( A ) Marshall silt loam and ( B ) Ida silt

loam obtained with a neutron meter. Field capacities are given at the right-hand

ends of the curves. (From Burrows and Kirkham, 1958.)



been discontinued. In Fig. 3 the storage profiles are for Webster clay

loam, a Wiesenboden soil formed on glacial till. The neutron meter

shows for the Webster, unlike the Monona, large amounts of water

stored temporarily (above the field capacity) in the surface 12 inches.

From these last two figures one sees that a considerable amount of soil

moisture may be stored above the field capacity even after 5 or more



ISOTOPFS IN SOIL PHYSICS RESEARCH



327



days’ time. In a later paper Nielsen et al. (1961) showed, using neutron

moisture meter data, that soil moisture profiles in the field can be

theoretically calculated.

Two other recent papers on more conventional use of the neutron

meter may be cited here. Stolzy et al. (1959) calibrated the meter

against the soil suction and various moisture contents so that the meter

TIME IHOURSI

75 10 IS



20



mBo



;?42



40



54



so

HCB%XNTAL S & s r



=



I O S R C E N T WATER



FIG.2. Moisture profiles, obtained by the neutron meter, of water stored temporarily above the field moisture capacity for a Monona silt loam, a loess. (From

Nielsen et al. 1959.)



FIG.3. Same as Fig. 2, except that soil is a Webster silty clay loam, a Wiesenboden glacial till soil, with an obvious tight layer at 6 to 12 inches depth. (From

Nielsen et al., 1959.)



could be used for determining soil suction. Weeks and Stolzy (1958)

used the meter to measure soil moisture in columns of soil 3 feet in

diameter. Satisfactory agreement between the calculated changes of

total water content and the measured amounts of added water were

obtained for two soils, a sandy loam and a silt loam.

c. Water movement through soil profiles. Measurement of moving

water and stored water are closely related. Water which is moving may

be considered as stored instantaneously.



328



DON KIRKHAM A?;D



RAYMOND J. KU’NZE



In the paper by Nielsen et al. (1961) the movement of water in the

soil profile was determined by the neutron meter and the shape of the

moving moisture profile related to the theoretical profiles determined

on the basis of diffusion theory. In Monona-Ida loess soils the agreement

of the measured profiles with the theory was fairly good.

Kixon and Lawless (1960a, b ) measured soil moisture with a neutron

meter down to 20-foot depth under brush cover. They found that the

moisture content of various soil layers under their test plots varied with

time t according to the relation W = At - b.



5. Uses of the Neutron Meter in Studying Soil-Water-Plant

Relationships

a. Water auailability for crops. Shaw (1959) covered some plots with

plastic film; other comparison plots he left uncovered. He pierced

neutron access pipes through the plastic-covered plots and installed

access pipes also in the uncovered plots. In both plastic-covered and

uncovered plots, corn (maize) was planted, holes for the corn seed

being punched in the plastic covers. Irrigation water could be added

under the plastic-covered plots and also onto the uncovered plots.

There was no seepage loss. Since no evaporation in the plastic-covered

plots could occur and since there was no seepage loss, a loss of moisture

in the plastic-covered plots was due to transpiration alone. In the uncovered plots the moisture loss was due to both evaporation and

transpiration. By subtracting values of the moisture content found in

the uncovered plots from the moisture content found in the covered

plots, evaporated moisture could be and was obtained. Evaporation and transpiration could be followed accurately and conveniently

from day to day. At high sunlight intensities, such as 5% g.-~al./cm.~/day,

the evaporation loss from the soil was a larger percentage of evapotranspiration than at lower sunlight intensities. Shaw found that on the

average for the season there was about 50 per cent of the water evaporated from the soil; this 50 per cent the plants did not get-it was wasted.

(Actual average vaIues were 46 per cent evaporation, 54 per cent

transpiration. )

There was an interesting side effect in the experiment. Large amounts

of rain water were found to be intercepted by the corn plants. This

water, 2.21 inches between July 15 and September 16, was evaporated

from the leaves and probably served little useful purpose. The writers

of this review suggest that tritiated or deuteriated water might be

applied to corn leaves to see how much of “intercepted water,” as noted

by Shaw, is used by the plant.

In a later paper Fritschen and Shaw (1961) pointed out that it is



ISOTOPES IN SOIL PHYSICS RESEARCH



329



not accurate to substract transpiration on plastic-covered plots from

evapotranspiration on uncovered plots to obtain evaporation. A principal

reason is that the plastic cover changes the microclimate of the soil.

They gave some correction factors, which, when applied to their moisture

data of 1961, indicated that only 11 per cent of moisture loss was due to

evaporation, the other 89 per cent, to transpiration. Even without the

correction factors the 1961 work indicated 27 per cent loss by evaporation

(not about 50 as in 1959) and 73 per cent by transpiration. But the 1959

and 1961 data are not strictly comparable. The 1959 data were for

evaporation for the whole growing season; the 1961 data for the portion

of the growing season after the corn crop had reached its maximum

height. It appears that further work on measurement of evaporation and

transpiration on field plots is needed.

Denmead (1961)used the technique of the neutron meter and plastic

covers successfully in smaller “lysimeter” plots to study soil-plant-climate

relations, work which we will now discuss.

There has long been a controversy on whether or not water is equally

available to plants over the whole range of available water; that is, from

the field capacity to the wilting point. Veihmeyer and Henrickson (1955)

have supported the equal availability concept; Slatyer (1956) and

Richards and Wadleigh (1952) have disagreed. In his work Denmead

used the neutron meter to follow the moisture in 136 “lysimeters” in

which corn was grown. The lysimeters were 20-gallon garbage cans.

They were buried in the field to the surface of the soil and were

surrounded by growing corn. Evaporation was prevented by covering the

lysimeters with plastic covers through which the corn grew. It would

have been impracticable to attempt to weight this many lysimeters.

Denmead (1961) found that each of the two viewpoints on plant

available water were tenable. On overcast days in soils of high capillary

conductivity the corn plants would transpire at about the same rate at

moisture contents ranging all the way from the field capacity to the

wilting point; whereas, in soils of very low capillary conductivity and

on hot days, the soils would not supply enough water to the plant at

any moisture content between the field capacity and the “wilting point.”

Wilting would occur even at the field capacity.

The neutron meter already has been adapted in a variety of agronomic

moisture availability problems. Stolzy and Cahoon (1957) used the

neutron meter in measuring soil moisture under citrus orchards. Marston

(1958) used the meter in mountain soils for studying evapotranspiration.

Reproducibility of moisture values was a problem because of rocks.

Pits had to be dug for placing the access pipes. Merriam (1960) used the

neutron method in sampling “wild land soils.” He found that use of



330



DON KIRKHAM AND RAYMOND J. WNZE



herbicides resulted in a saving of 1 inch of water in 10-foot depth for

west-facing slopes and resulted in saving of about 3 inches of water in

10-foot depth on east-facing slopes. This was a sandy soil which contained

about 15 to 16 inches of water in 10 feet of profile.

b. Plant spacing and water utilization. Neutron meters have been used

by Yao and Shaw (unpublished data as of July, 1961) to test efficiency

of water use by corn at various rates of planting. Since Shaw’s work of

1959 (cited above) indicated that about 50 per cent of soil moisture

might be lost by evaporation, Yao and Shaw wondered if increased shade

at the soil surface would decrease this loss. The increased shade could

be obtained by planting the corn rows closer together or by planting

the rows in such directions as to obtain more shadow on the ground.

Table I1 shows the results for 21-inch spacing and 42-inch spacing. The

TABLE I1

Water Use by Corn (Maize) for Different Rates of Planting and Different

Directions of ROWS=

Distance apart of

corn plants

Number of corn plants

per acre



21 inches

14,000b



28,0006



Direction of corn row

EWandNS EWandNS

132

153

Yield, bushels corn per acre

Inches of water used

13.2

15.0

Bushels of corn produced

per inch of water

10.0

10.2



42 inches

14,000b



28,OOoC



E-W N-S

E-W N-S

120

124

148

143

16.1 15.3 17.4 18.8



7.4



8.1



8.3



8.5



0 Yao and Shaw, Iowa State University, Ames, Iowa, unpublished data; moisture

data collected with neutron meter.

0 One corn plant per hill.

c Two corn plants per hill.



table indicates that 1inch of water produces 10 bushels of corn per acre

at 21-inch spacing of rows; and about 7 to 8 bushels per acres at 42-inch

spacing of rows. Whether there were 14,000 or 28,000 plants per acre at

a given spacing seemed to make little difference in the yield per inch

of water. Also, the direction of the rows seemed to make little difference

in the yield per inch of water.

Bahrani and Taylor ( 1961) investigated the influence of soil moisture

potential and evaporative demand on the actual evapotranspiration in

an alfalfa field. Soil water was measured to a depth of 9 feet using a

neutron meter. The actual evapotranspiration and its ratio to “potential

evapotranspiration” ( calculated by Penman’s formula ) showed a curvilinear relation with the average moisture potential.



ISOTOPES IN SOIL PHYSICS RESEARCH



331



B. MEASUIIEMENT

OF MOVINGWATER

AND IONS

IN SOILBY

USEOF VAFUOUS

ISOTOPES

In Section 11, A above, the papers reviewed were largely concerned

with the static condition of water in soil. In this Section (B) papers on

moving water and ions are reviewed. The movements are considered

first for unsaturated flow and then for saturated flow.

1. Unsaturated Flow

a. Miscible displacement. When one solution moves into another

solution, both solutions being soluble in each other, and when movement

of the solutions occurs in a porous medium such as soil, the original

solution is moved by a process called miscible displacement. This term

is used because when the added solution is washed through the soil it

mixes with the solution which is already there while displacing it. To

study how applied soil water displaces water already in the soil, Nielsen

and Biggar (1961) originally displaced the soil solution with water

containing a small concentration of chloride ion. They wondered how

the tracing ion might influence the miscible displacement process; therefore they used, in some later experiments ( Biggar and Nielsen, 1!36l),

tritium and chloride ion. The results showed (Fig. 4) that the chloride

ions came ahead of the tritium ions. The movement of the chloride ions

was faster in the soil column than was the tritiated water, even though

the latter had a higher diffusion coefficient. In the unsaturated state the

tritiated water could also diffuse in a vapor phase, where movement is

faster than in the liquid phase. Thus under some unsaturated conditions,

one might expect that the tritiated water would be detected first.

b. Digusion flow of water. Moisture movement in soil using deuterium

as a tracer was studied by Kunze and Kirkham ( 1961). They directed

their attention to the determination of the self-diffusion coefficient of

soil water because of the need for this coefficient in order to separate

mass flow and diffusion flow-types of flow they had observed in an

earlier experiment. In this earlier experiment they used a soil core,

initially at constant moisture content and having its middle portion

tagged with deuterium. Letting the core dry from one end, they found

that a large part of the tagged moisture moved in a direction the reverse

of that of the drying gradient. Since mass flow of water would, by

definition of mass flow, have to move in the direction of the drying

gradient, they reasoned that some of the water motion was due to

molecular self-diffusion. They therefore turned their attention to the

measurement of the self-diffusion coefficient of soil water. It is emphasized here that the self-diffusion coefficient of water as determined by



332



D O S KIRKHAhf A33l M Y M O N D J. KUNZE



Kunze and Kirkham does not refer to the type of diffusion of water

spoken of in connection with capillary soil-moisture flow. This capillary

flow diffusion coefficient is defined as the product of the capillary

conductivity of moist soil and the rate of change of the soil water tension

with the soil moisture content. The ( molecular) self-diffusion coefficient

is a measure of the rate at which molecules of a material diffuse into

other molecules of the same material.

1.0



COLUMBIA SILT LOAM



0.8



0-0



0.6



A-



CHLORIDE

TRITIUM



@



f/

,p d



8 = 0.482



0



y 0.4

V



0.2



0



2



0.0



CL



+

z



1.0



$



0.8



W

V



w 0.6



L



2

=



0.4



f

W



0.2

0.0

0



400



800



1200



VOLUME OF EFFLUENT (ml.)



FIG. 4. Miscible displacement curves of soil solution being displaced by water

containing chloride ions and tritium, the displacing water moving into the soil

solution at a velocity V = 0.20 cm./hr. The moisture content 0 for the replicated

runs shown is about 0.48 cm."cm.3. C,, is the initial concentration of the ion or of

the tritium in the displacing solution; C is the concentration of the chloride ion or

tritium in the outflow solution (effluent) as measured in volumetric outflow incremants. (From Biggar and Nielsen, 1961.)



Figure 5 shows a curve used by Kunze and Kirkham in evaluating

the self-diffusion coefficient. In Fig. 5 on the y axis, a function F is

given which is essentially the concentration of deuterium atoms in a

soil core, the left half of which contained originally deuterium-tagged

soil water ( F = 1 ), and the right half, untagged water ( F = 0). The

test soil core in these experiments was kept completely enclosed to

prevent evaporation. The heavy line is a theoretical curve based on the

hypothesis that self-diffusion in soil water is a linear process. The circle



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