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III. Soil Density and Soil Structure

III. Soil Density and Soil Structure

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intensities could conceivably result in an increase in o with an increase

in density. Vomocil found the edge effect of the gamma beam to be

negligible; thus the size of a calibration chamber was not critical.

Bernhard et al. (1956) presented a statistical technique for analyzing

data obtained by the gamma-ray transmission method. The main purpose

in their study was to determine parameters of the equation which govern

the “best fit” lines, that is, correlate soil density with transmitted radiation

intensity, and distance between the radiation source and the detector.

They observed measurement deviations from supposedly correct density

values of _t 2.3 per cent.

The use of only primary radiation for measuring soil density by the

transmission technique was emphasized by van Bavel et al. (1957). The

inclusion of secondary radiation, they found, resulted in unwanted

complications. They described a technique whereby a separation of

primary and secondary radiation could be achieved by scintillation

counting and electronic discrimination. Van Bavel (1959) described a

gamma transmission technique for field measurement of soil density.

This technique necessarily required that the source and the detector be

placed some distance apart. In field measurements van Bavel obtained

bulk density values the precision of which was of the order of 0.01 g. per

cubic centimeter and for which the resolution was one-half inch. Moisture

content had been taken into account to obtain dry-bulk density values.

Volarovych and Churaev (1960) used gamma-ray transmission to

measure soil density in peat soils. They also used other isotopes methods

to study peat. ( S35 was used in a water movement study.) Their 200-page

book came to our hands too recently for an adequate review. The book

contains 314 mainly Russian references. About half of the references are

on isotopes work and these isotopes references are largely from nonagronomic sources.

2. Back-Scattering Measurements

The back-scattering principle involves the absorption and backscattering of gamma rays by the outer orbit electrons of all atoms present

in the soil, atoms of water molecules included. Here again as with the

transmission technique, in order to obtain dry bulk density, one must

know the volumetric moisture content and subtract it, expressed as grams

per cubic centimeter, from the wet-bulk density. Kuranz (1960) described

the use and calibration of a commercial depth density gauge and a

commercial surface density gauge. He pointed out that the sensitive volume

of measurement is up to about one cubic foot of soil for the depth

gauge. The radius of penetration for the density gauges is 3 to 8 inches,

decreasing with an increase in wet density. Depth density measurements



FIG.8. Radiation equipment for soil density determination: left, density unit; middle, counter; right, moisture

unit. The central unit is 12 inches wide. (From Phillips et al., 1960.)



may be made from 1 to 200 feet. Calibrations methods and evaluations,

among many others, of the same commercial soil density gauges described

by Kuranz (1960) are given by Mintzer ( 1961),Carey et al. (1961),

and Carlton (1961).

Phillips et al. (1960) used a commercial gamma-ray density (backscattering) meter for determining soil density at different levels of

artificially applied compaction. The equipment ( Fig. 8 ) included two

surface probes, one to measure the moisture density and one to measure

FIG.9. Yield of corn (maize) versus bulk density on a dry soil basis, where the

density is determined by radiation equipment, and also, for comparison, by cores.

( A ) no fertilizer added to soil; ( B ) fertilizer added. (From Phillips et al., 1960.)

the moisture-plus-solids density. Subtraction of the former density from

the latter gave the soil-solids density. In Fig. 9 one sees how corn

(maize) growth depends on the density. The density measurements plus

root studies indicated that root impedance developed by the compaction

and reflected by the bulk density was the factor which caused yield


Trouse and Humbert ( 1961 ) , working with radioactive rubidium,

found that soils compacted to different densities showed decreasing

rooting efficiency for Hawaiian sugar cane as bulk density increased.



Critical bulk densities for the rooting of sugar cane were empirically

established for the principal cane sugar soils of Hawaii.



A few papers on use of isotopes in soil structure investigations have

been published. They deal with aggregate stability of the macrostructure

and bonding and kinetic effects of the macrostructure.

1 . Aggregate Stability

Toth and Alderfer (1960a) described a procedure for tagging waterstable soil aggregates with Corn. Their data indicated that with their

techniques it is possible to tag different-sized aggregates uniformly

throughout the aggregate. Tagged water-stable aggregates have been

kept in distilled water for over a year without releasing Corn. In later

work Toth and Alderfer (1960b) used their tagging technique for

studying the formation and breakdown of water-stable soil aggregates.

The most important findings from an incubation and greenhouse study

\yere: “( 1) the physical composition of water-stable soil aggregates is

constantly changing; ( 2 ) during incubation the contribution of smallsized tagged aggregates to the formation of larger ones decreased as the

size of the tagged aggregates was reduced; ( 3 ) aggregates of all size

ranges examined contained fragments of the original tagged aggregates;

and ( 4 ) under bluegrass sod, the contribution of the tagged aggregates

of small-sized ranges to the formation of larger one was greater than that

noted under incubation.”

2. Bonding and Kinetic Effects

Katz (1960), in work that is pertinent to the use of isotopes in soil

structure studies, investigated the change in the physical and chemical

properties of deuteriated compounds as opposed to normal compounds

when subjected to certain tests. He pointed out that not only are

equilibrium properties of deuteriated compounds frequently different

from those of the corresponding protiated (normal hydrogen) compounds, but the rate at which deuteriated compounds undergo chemical

reaction also may be different. These reaction rate differences are a

function of the more stable bonds-to-deuterium as compared to bonds-tohydrogen. Since the masses, hydrogen and deuterium, are different, the

vibrational frequencies of bonds-to-deuterium will be at slightly lower

frequencies, and these bonds will thus be slightly more stable than

corresponding bonds-to-hydrogen. Since type and strength of bonds is

apparently a factor that cannot be neglected in soil structure studies,

these techniques of exchanging deuterium for hydrogen and observing



the resulting change of the physical properties in the soil material appear

to justify some investigation.

The deuterium-hydrogen exchange phenomenon in soils has been

investigated. Faucher and Thomas (1955) found that the exchange of

deuterium for hydrogen in montmorillonite is very rapid for 75 per cent

of the total water associated with the clay minerals; and that very little

further exchange occurred even after contact periods of 120 hours.

Romo (1956) observed by infrared measurements the actual presence

of deuterium in the clay lattice. He concluded that the rate of exchange

appears to be characterized by two steps: one in which the exchange

takes place predominantly on the surface hydroxyls, and the other one

in which a process of diffusion takes place to influence exchange of the

intralattice hydroxyls.

No work seems to have been done on how the physical properties of

deuteriated soils or clays may be changed as opposed to those soils or

clays saturated with normal water. This could open up a completely new

field of soil physics research, if it seemed warranted for an understanding

of binding forces in soil. In place of deuterium, tritium could be used.

The latter, being radioactive, would probably simplify the analysis


IV. Soil Aeration

So far as the authors know, work with oxygen isotopes for studying

soil aeration has been confined to the use of the nonradioactive isotope

Ols. The radioactive isotopes of oxygen have half-lives too short for uses

that may be envisioned. This is seen from the following tabulation:




76.5 seconds

127 seconds

27 seconds





Jensen (1961) used Ol8 in studying oxygen diffusion through soil

cores and plant roots growing in the soil cores. The diffusion of oxygen

increased with increasing numbers of roots. His evidence indicated that

this increase took place almost entirely in the soil, rather than inside

the root. Although Jensen did obtain these and some other soil-plant

measurements, his work dealt largely with the development of tagging

and sampling techniques suitable for 0ls mass spectrometry.

Danielson and Russell (1957) measured Rbss absorption by corn

seedlings as affected by moisture and aeration. Their work indicates that

the absorption of rubidium ions was not significantly influenced by

oxygen concentration above 10 per cent when the flow rate through these



samples was about 1liter per hour. The critical oxygen level for rubidium

absorption decreased with decreasing soil moisture content. It is difficult

to ascertain whether the critical oxygen level at various moisture contents

was controlled by respirational activity or by the rate of diffusion of

oxygen through moisture films. Their data indicate that both factors are


Self-diffusion of radioactive carbon dioxide as a means of relating

diffusion to porosity in porous media was studied by Rust et al. (1957).

Diff usion-porosity relations, which were in good agreement with those

obtained by other workers, were determined for air-dry nonsoil materials.

Absorption of carbon dioxide precluded an evaluation of the diffusionporosity relationship of moist soils. Another experiment using C1*labeled CO, in a related area of interest was that of Rhykerd et uZ.

(1959), who measured the uptake of COP by alfalfa, red clover, and

birds-foot trefoil seedlings during a 15-minute exposure to natural light

immediately after these plants had been exposed to various light treatments for 30 days. Alfalfa and red clover fixed significantly greater

quantities of CO, under all light treatments than did birds-foot trefoil.

The effect of various light treatments, but all of the same energy for

any particular species, was found to be highly significant with the

amount of CO, fixed.

V. Soil Temperature

The use of isotopes methods in connection with the study of soil

temperature is relatively unexplored. The authors, using deuteriated

water as a tracer and a split root technique, presently are determining

rates of water absorption by plant roots when roots from one plant are

under two different temperature environments ( unpublished data). With

this technique half the roots are kept in one environment of untagged

moist soil and half in another where the moist soil is tagged. To detect

from which soil environment the water is being absorbed it is necessary

to analyze the transpired water for its deuterium content. In preliminary

experiments, with oats growing in nutrient culture and with temperature

regimes of 60" and 90"F.,it was found that the ratio of absorption from

the two differently heated soils was initially proportional to the reciprocal

ratio of the viscosity. As growth continued, more roots developed on the

cooler side and the' proportionality did not hold.

VI. Soil Particle Movement

Smith and Eakins (1958) have discussed various methods of marking

sand grains or pebbles that are to be used in investigations of littoral

drift. Approximately ten different radioisotopes suitable for tracing particle



movements are listed in their paper, choice of which will depend upon

length of the experiment. These are shown in Table 111.


Suggested List of RadioisotoDes for Use in Studvine Sand Movement+





































Specific activity

in flux of 1011

n/cm.z/sec. for 1




(in Mev.)

15.0 hr.

38 mc/g


40.2 hr.

88 mc/g

1.60 and others

2.69 days

620 mc/g


12.8 days



1.60 and others

18.6 days

2.0 mc/g

1.08 (8.5%)

27.8 days

2.1 mc/g

0.32 (8%)

60 days

0.95 mc/g

0.6 to 2.1

74.4 days

175 mc/g

Up to 0.61

84 days

38 mc/g

0.89, 1.12

111 days

7 mc/g

u p to 1.2

245 days

100 Il.c/g

1.1 (45%)

270 days

320 pc/g

Up to 1.5

5.25 yr.

2.2 mc/g

1.17, 1.35

From Smith and Eakins (1958, Table I ) .

Inman and Chamberlain ( 1959), using irradiated quartz grains, have

traced the movement of beach sand under the influence of wave action.

The actual tracer isotope in the irradiated quartz grains was P32. The

investigators pointed out that, to be satisfactory, the natural tracer of

sand movements should be: "( 1) not a health hazard, ( 2 ) of the same

size, density, and shape as one of the major components under study,



( 3 ) easily and rapidly distinguishable from the sand mass, ( 4 ) inexpensive and available in relatively large amounts, and (5) able to

retain its distinguishing properties over time comparable to the time of

the fluid processes.” The above tracer (P32 in irradiated quartz) was

found to fulfill these requirements. Their experiment indicated a rapid

rate of dispersion of sand from the point of release, this dispersion being

most rapid in the offshore and onshore directions.

It should be obvious that these same techniques used in tracing beach

sands could also be applied to wind and water erosion of soil and to

the resulting formations of sediment ( Anonymous, 1961) . The techniques

appear to be equally suitable for studying results of various tillage

practices, such as the uniformity and mixing of soil. Main (1959) gives

some theoretical considerations for uniformity of mixing. The incorporation of small irradiated sand grains in aggregate stability studies should

prove to be interesting. Results might be compared with those of Toth

and Alderfer (196Ob).

VII. Transformation of Soil Materials from One Form to Another

Carbon in the form of soil organic matter may break down into

gaseous carbon dioxide. Oxygen may be taken from the soil air by

organisms and bound up into a chemical form. Nitrogen may be in a

plant-available form or a plant-unavailable form. Water may be in the

liquid phase or the vapor phase. Thus there are many ways in which

soil materials may change from one form to another. The rate of change

from one form to another is important, but this rate of change cannot

often be measured directly. With methods of mathematical physics,

however, measured quantities can be converted into the desired rates.

Details of the problem of determining the rate at which nitrogen goes

from mineral form to organic form and the rate at which the organic

form goes into the mineral form-both processes go on in the soil

simultaneously-has been presented in detail by Kirkham and Bartholomew (1954, 1955) and by Kirkham (1956). The latter paper is a

composite of the two former papers. In the work of Kirkham and

Bartholomew the mineralization rate and the so-called immobilization

rate of the soil nitrogen were desired from certain experimental data.

By mineralization rate is meant the rate at which plant-unavailable

nitrogen becomes plant available; by immobilization rate is meant the

rate at which plant-available nitrogen becomes plant-unavailable. Mineralization rate is sometimes referred to as mobilization rate; available

nitrogen, as mobile or mineral; unavailable nitrogen, as immobile. Soil

organic matter ordinarily contains most of the unavailable soil nitrogen.



Measured quantities from which m, mineralization rate, and i,

immobilization rate, are to be determined are ordinarily the concentrations of total and of tagged mineral nitrogen at the times when the

concentrations are measured. In the experimental work reported, tagging

was done with the heavy nitrogen W5.

The mathematics was simplified








t (doyr)





x (milligrams)


FIG.10. Theoretical (solid curves) and experimental data (circled points) of the

variation of x with time; and y with x; where x is the milligrams of available nitrogen

per 3 grams of soil mulch material and y is the milligrams of tagged available

nitrogen per 3 grams of the same mulch material (decomposing oat straw) (see

Kirkham, 1956)





3 0.5












FIG.11. Variation of mobilization 'and immobilization rates, m and i, of nitrogen

in soil mulch material (decaying oat straw), with time. Multiply the values of m and

i by 2/3 to obtain pounds of nitrogen mobilized or immobilized per day per ton of

the oat straw; values of m and i are derived from the values of x and y of Fig. 10

(see Kirkham, 1956).

when there was a large amount of unavailable nitrogen in the soil as

compared with available nitrogen. This situation was first analyzed by

Kirkham and Bartholomew (1954). The unrestricted case, where the

amounts of unavailable nitrogen and available nitrogen as well as other

materials involved were finite, was analyzed in the 1955 paper. The

complete details, which involve setting up differential equations and

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III. Soil Density and Soil Structure

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