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III. Soil Density and Soil Structure
ISOTOPES IN SOIL PHYSICS RESEARCH
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
DON KIRKI-IAM AND RAYMOND J. KUNZE
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.)
ISOTOPES IN SOIL PHYSICS RESEARCH
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.
DON MRKHAM AND RAYhlOND J. KUNZE
Critical bulk densities for the rooting of sugar cane were empirically
established for the principal cane sugar soils of Hawaii.
B. ISOTOPESLU SOIL STRUCTURE
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
ISOTOPES IN SOIL PHYSICS RESEARCH
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
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:
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
DON KIRKHAM AND RAYMOND J. W N Z E
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
ISOTOPES IN SOIL PHYSICS RESEARCH
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+
in flux of 1011
n/cm.z/sec. for 1
1.60 and others
1.60 and others
0.6 to 2.1
Up to 0.61
u p to 1.2
Up to 1.5
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,
DON KIRKHAM Ah?) RAYMOND J. KUNZE
( 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.
ISOTOPES IN SOIL PHYSICS RESEARCH
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
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
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