PERSISTENT WATER-UNSATURATION OF NATURAL SOIL IN RELATION TO VARIOUS' SOIL AND PLANT FACTORS1

R. M. SMITH AND D. R. BROWNING2

MANY discussions of soil moisture mention thatcomplete replacement of air by water is diffi-

cult, and that the presence of trapped air is a com-plication in soil moisture studies; but most data givenfor wet soils are simply based upon the interactionof the two phases—solid soil and liquid water—without regard to the influence of the third phase—gaseous air.

The same might have been true in the present caseexcept for the fact that the technics -employedafforded an easy measure of the difference betweentotal porosity and total water content after labora-tory wetting. This difference, representing the'volumeof pores that failed to take up water, is convenientlydescribed as percentage unsaturation. It was deter-mined by a standard technic for a large number ofnatural soil samples incidental to estimating their poresize distributions. After considerable data had accumu-lated it became evident that pore size distribution andother characteristics were related to the volumes ofunsaturated pores, and that any accurate representa-tion of natural soil water conditions would have torecognize the pores that remain filled with air underordinary saturating conditions, as well as the poresthat take up water. For, although only laboratoryresults are given, theoretical considerations and pre-liminary tests have indicated that the relationships inthe field are similar. Field infiltration results reportedby Zwerman (i8)3 suggest that for one soil profile,the Duffield silt loam, entrapped air exerts just suchan influence as would be expected from the labora-tory data in this paper.

METHODSSoil cores for this study were collected in a 3-inch sampler

with removable sleeves, described by Baver (i) as the"Bradfield Sampler." The cores were either ij4 or 3 incheslong. Soil lumps* broken from excavated soil profiles werecollected .for studying plastic subsoils. Cores from such ma-terial are unsatisfactory on account of unavoidable compac-tion. All samples were kept in sealed cans with a few drops oftoluene added to prevent fungus growths.

In the laboratory a procedure was followed that is likeother reported technics, except in detail (4, 8, 13). It maybe outlined as follows: When soil lumps are used, they arebroken until they can be contained within 3-inch brass cylin-ders. Each lump is weighed and its volume determined by sanddisplacement.5 It is then sealed in a brass cylinder withparaffin in such a way as to leave the soil exposed at bothends of the cylinder. Duplicate soil samples are dried formoisture determination so that the volume weight of thelump can be calculated.

Porous soils are sealed by paraffin which is barely warmenough to flow. Tight, wet samples require that the paraffinbe near the smoking point. A wet blotter forms a convenientbase to prevent the paraffin from running under the sample.If the lower soil surface is broken and hence uneven, as itmust be with plastic samples to avoid puddling, the irregu-larities are filled with wet asbestus fibre to repel the paraffinand to dispose of water or air pockets during macroporositymeasurements.

Using either lumps or cores the soil is allowed to wet bycapillarity through the blotting paper base which is immersedin water. When the exposed soil surface becomes thoroughlywet, after a period of from several hours to several days,depending on the type of sample, two layers of good quality,fresh, wet blotting paper are substituted for the single layerto serve as a membrane for macroporosity measurements, assuggested by work of Learner and Shaw (8). The cylinderis clamped onto a brass base provided with a fine copperscreen spreading the water from a water column. A lowhead is supplied from below through rubber tubing causingpercolation through the sample. After 30 minutes or moreunder a head equal to the length of the sample, the water isbrought to zero tension on the soil surface, then a low tensionof about 10 cm of water is applied by adjusting the heightof the outlet of the water column. When flow stops, the tubeis lowered to about 40 cm, then to 100 cm. After completingthe measurements of the volume of water removed at thesetensions, the sample is removed for moisture determinationby oven drying.

From the indicated technic a measure of soil unsaturationwas automatically obtained by comparing the calculatedporosity with the amount of water actually taken out of the

. sample by tension and by oven drying. Some discrepancieswere anticipated, but their magnitude proved to be surprisinglylarge.

RESULTSFor approximately 200 samples the unsaturation

after laboratory wetting average.d g.i% by volume,6

'Joint contribution from the Soil Conservation Service, Office of Research U. S. Dept, of Agriculture, and the Department ofAgronomy and Genetics, West Virginia Agricultural Experiment Station, Morgantown, W. Va. Published with the approval ofthe Director as Scientific Paper No. 304.

"Project Supervisor and Junior Soil Surveyor, Soil Conservation Service.Special appreciation is due E. R. Leadbetter, District Conservationist, Soil Conservation Service, Moundsville, W. Va.,

formerly Soil Conservation Service, State Survey Supervisor, for assistance in collecting soil samples and identifying soil types.'Figures in parenthesis refer to "Literature Cited", p. 119.4Lump is used in this paper to indicate an irregular mass of soil with its natural structure undisturbed.6The average difference between duplicate determinations for 33 samples by this means was 1.9% of the volume of the samples.6A11 porosity and moisture values are expressed as percentage of the total soil volume.

114

SMITH AND BROWNING: WATER-UNSATURATION

with only four samples essentially saturated. Thehighest unsaturation was 22%, and the variationamong samples soon began to suggest some specialsignificance associated with unsaturation values.Longer wetting periods were allowed a number ofsamples, but there was no obvious difference in thedegree of saturation.

RELATION OF UNSATURATION TO PLANT COVER

In the early spring of 1942, 60 surface soil coreswere collected from small plots where the soil wasquite uniform and various crops were represented.The unsaturation values, summarized in Fig. i, showsome interesting relationships, although each cropwas represented by only three to five cores, and therewas considerable variability among these.

The association of three sod. plots on the low un-saturation end of the graph and of four continuous,cultivated crop plots on the high unsaturation endis clearly significant. The highest individual unsat-uration value obtained for the former group was7.3% ; the lowest value for the latter group was 8.2%.The low unsaturation for continuous corn with awinter cover crop of rye and vetch also appears sig-nificant compared to corn without winter cover. Thehighest value for rye-vetch was 8.2% ; the lowest forcontinuous corn, 8.5%. This is particularly interest-ing on account of the similarity of the soil from theseplots in other physical properties. Their volumeweights, coarse pore contents, and aggregate consti-tution were very similar, apparently indicating thatthe difference in wetting was due to some directeffect of the rye and vetch rather than to such factorsas pore size distribution.

A somewhat similar conclusion would probablyapply to the soil under wheat. Its volume weight

1.25

_ 1.20.~y

* 1.15.<L>

E .

1 1.10.

1.05.

——R. CLOVER (A YR. ROTATION)0<-BLUEGRASS SOD0<-R. CLOVER-TIMOTHY , .,(5YR. ROTATION) 00^—ALFALFA (WEEDY STAND)

t—PERMANENT TIMOTHY (WEEDY STAND)-ALFALFA (5 YR. ROTATION)

°- °- -FALLOW (SOME WEEDS)-KOREAN LESPEDEZA

—CONTINUOUS OATS—— " SUDAN—— " CORN><— " SOYBEANS

-CONTINUOUS WHEAT

2jO 6.0 8.0 10.0 12.0 14.0Pefcent Unsaturation

16.0 18.0

FIG. i.—Plant cover and volume weight of natural soil onone spring sampling date in relation to the percentageporosity remaining unsaturated after laboratory wetting.Each point is an average of three to five cores.

was exceptionally low, reflecting a high content ofcoarse and medium pores, a condition that might beexpected to lead to high unsaturation, but it showeda fairly strong tendency to become wet. The differ-ence shown under wheat and rye-vetch compared tothe other continuous crops may be related to thepresence of growing roots, for the continuous plotswith high unsaturation were bare at the time ofsampling.

The position of permanent timothy in the groupingis not clear. The fact that the stand was weedy andnot vigorous may account for its relatively high un-saturation compared to other sods. Permanentalfalfa was also thin and weedy, so the same factormay apply. Alfalfa in rotation was more vigorousand showed less unsaturation, although the differencemay not be significant. The difference in root sys-tems would probably explain the lower unsaturationunder sods than under alfalfa. • This may well be afactor in favor of alfalfa-grass mixtures. Koreanlespedeza showed rather high wetting properties,which may be due to the fact that this crop is notremoved, keeping the soil continually mulched withleguminous litter.

During the late summer, some additional coreswere collected from these same plots with the cropson the ground. The soil on the cultivated plots hadbeen well compacted by several rains since the lastcultivation. Four bluegrass sod cores averaged 4.8%unsaturated, compared to 7.5% for six from continu-ous corn, 6.8% for 5 continuous soybean cores, and6.9% for 7 cores from corn in rotation. The differ-ences between the sod and the cultivated plots areclearly significant, though much smaller than in thespring comparison, due possibly to the presence ofsome roots in the cultivated plots.

One well-protected, upland silt loam, mixed hard- .wood soil profile studied showed high unsaturationcompared to pasture or even to crop land. Ninedeterminations at different depths averaged approxi-mately 15% unsaturated. The maximum and mini-mum values were 17.9% and 12.0%, respectively.Fifteen cores from similar soil in pasture averaged7.5% unsaturated.' One other set of data involving trees is availablefrom moisture determinations under black walnutsin Ohio which showed generally higher moisturecontents in the surface soil under the trees than inthe adjacent open pasture (15). Moisture equivalentsdiffered by only about i% between the soil underthe trees and that in the open, whereas the moisturecontents averaged several per cent higher on most

n6 SOIL SCIENCE SOCIETY PROCEEDINGS 1942

sampling dates. The fact that this difference was atleast as great at high as at low moisture contentssuggests that increased wetting may be a factorunder the trees. This needs further investigationbecause the Ohio data were not designed to measureunsaturation specifically, but it appears worthy ofnote since it suggests that the nature of the organiclitter may influence soil wetting. If organic materialsdo differ as soil wetting agents, walnut litter mightbe expected to rank among the best because it be-comes thoroughly incorporated within the soil andshows no tendency to shed water like the leaves fromoaks, maples, and many other trees. Such differencesamong common organic amendments added to soilsmay be a considerable factor in the measured effectof these materials upon crops and soils.

UNSATURATION IN RELATION TO SOIL PERMEABILITY

AND ERODIBILITY

There are two general conditions probably associ-ated with extreme erodibility in the minds of mostagronomists throughout the country. One is con-tinuous cropping, particularly to soybeans and cornin the north, to cotton and tobacco in the south; theother is loose, silty soils with single grained structure.These conditions have been studied in considerabledetail under' various slopes and climates, but itseems conservative to say that no one fully under-stands why the soil losses are so great. There is apartial understanding, of course, in the recognitionof high dispersion ratios (9), low aggregation, loworganic matter, and the like, but the explanation isprobably not complete.

From the data at hand it appears that trappedair in natural soils is a factor deserving more con-sideration in the evaluation of soil erodibility andpermeability. Theoretically, unsaturated pores wouldbe expected to decrease the permeability of soil towater, and -a greater instability would be expectedwith a three-phase system containing trapped airthan with a two-phase system of soil and water.Compressibility and a tendency to explode as de-scribed in. the laboratory by Yoder (17) would beoutstanding factors in erodibility associated withhigh unsaturation. The behavior of erodible soilsand of soils that slip on steep land (7) are in accordwith these reactions.

An indication of the influence of the unsaturatedpores upon soil permeability is given by Fig. 2. Therelation between laboratory percolation rates and ameasure of noncapillary porosity involving unsatura-tion values is considerably closer than the relation

2.0.

« 1.0.tO

I 0.0.TO

ooL_OJ

"--i.o.•oOo__1

-2.0,

o >k.^ ° -^GO ^T>K ~^ff

° * ^ ,0 o «9j >|e jf?* ^K

05^ >!, ^r*

0 jL,

8 ^<0 , >k

0 0>P >kG ° >)< ^c >rc= S t a n d a r d non-capil-° °S>Kvi, ^-^ lary1 determination*

0 Q vL- 'TN^N

0 >K ° = S tandard minus0 % u n s a t u r a t i o n

16 24 32 40 48N o n - c a p i l l a r y P o r o s i t y

56

FIG. 2.—Laboratory percolation rate of natural soil samplesin relation to noncapillary porosity by two methods ofdetermination. Total porosity minus the moisture equivalent.

between percolation and noncapillary porosity esti-mated by the difference between total porosity andthe moisture equivalent. The values involving un-saturation are approximately comparable to non-capillary values obtained by wetting soils and thenmeasuring the water drained by some fixed tension(10, 8), but the determination might be more con-venient in many cases because it could be made byfield sampling at apparent saturation and again atfield capacity, or by obtaining the difference betweenthe moisture contents at maximum saturation andat the moisture equivalent in the laboratory.

An illustration of the altered picture of a soilprofile that is' obtained by including the measuredvolume of unsaturated pores in a soil profile graphis shown in Fig.- 3. If all the pores that resist wettingwere designated as noncapillary, there would not bethe distinct constriction that is shown by the graph.The constricted condition is more in harmony withfield observations of the tightness of this subsoil andand with measurements of laboratory percolationrates. This type of graph may be readily obtained,and it appears to be an improvement over the stand-ard porosity graph as used by Baver (i), Fauser(5), and others. It is apparent from graphs likethis that the warmth and supposed rapid drainageof some soils are predominantly unsaturation phe-nomena, 'and that the drought resistance of othersoils is probably related to their more complete wet-ting. The supply of air for plant roots would, there-

SMITH AND BROWNING: WATER-UNSATURATION 117

Depthinches

8.

16.

24.

32

40

S o l i d s o i lp a r t i c l e s

Capi l larypores

20 40 60Per-cent o f t o t a l v o l u m e

80

FIG. 3.—Porosity graph of a silt loam soil profile' with imper-fect internal drainage, illustrating the importance of poresthat resist wetting.

fore, be dependent upon the volume of trapped airas well as upon the capacity for drainage.

Regarding the direct relation of soil erodibility tounsaturation, Fig. 4 shows a tendency toward higherdispersion ratios by Middleton's method (9) forincreasing unsaturation values. This relation wouldbe closer if some restrictions as to plant cover andsoil profile horizons were imposed. The high unsat-uration values shown corresponding to relativelylow dispersion ratios were from the soil profilesamples collected under mixed hardwood timber,

80.

64.

48.

- 32.

16

8 12Per-cent Unsa tu ra t ion

16

FIG. 4.—The relation between dispersion ratios arid the per-centage of soil pores remaining unsaturated'after labora-tory wetting. Samples are from various depths in severalsoil profiles.

already referred to as showing high unsaturationcompared to similar soil under grass.

SOIL WETTING IN RELATION TO FIELD CAPACITY AND

EFFECTIVE PORE SIZE

In considering the mechanism of soil wettingunder field or laboratory conditions, it appears likelythat the maximum trapping of air would ordinarilyoccur in the intermediate range of pore sizes. Amongthe finest pores, strong surface sorption, strongcapillary tension, and swelling would account forthe air, either by taking it into solution or by com-pressing it and forcing it out. These processes ap-parently account for a rather complete filling of thesoil pores up to the tension of the moisture equivalentin silt and clay soils, as evidenced by the relativeconsistency of the moisture equivalent regardless ofpre-treatments (16) and the persistent relation tofield capacity (2, n). This is further confirmed bythe fact that evacuation previous to wetting hasfailed to cause any considerable change of themoisture equivalent in the present studies.

The consistent filling of the finest pores contrastswith the behavior of the coarse pores, whose meas-ured volumes are difficult to duplicate (13). Thesepores fill in part by tension, but much air evidentlybecomes surrounded by water films and trapped.When a hydraulic head is applied and the waterflows through the sample, there is sufficient velocityand momentum through ,the coarsest pores to effect arather complete removal of air bubbles, but withprogressively finer pores this mechanism becomesless effective so that there is a concentration of airas bubbles in the intermediate range, giving rise tovarious peculiarities associated with the intermediatepores. This partial air gap between the coarse andthe fine pores is suggested as the primary basis forthe'concepts of capillary and noncapillary porosity.

It is undoubtedly true that some coarse pores 're-main filled with air in some samples, but many ofthese pores that appear large may be intermediatein terms of tension, which is the basis for mostpresent measurements of pore sizes. Large poresfrom worm or insect activities undoubtedly accountfor part of the unsaturation in certain cases.

Compact clay soils or clay soils composed of com-pact aggregates with coarse pores between wet prac-tically to the limit of their calculated porosity. Loosesilty soils with few coarse pores have the highestunsaturation values.

This apparent concentration of air in the inter-mediate pores probably accounts for the close rela-

n8 SOIL SCIENCE SOCIETY PROCEEDINGS 1942

tion that has been shown between the moistureequivalent and the field capacity of silt and clay soils(2, 11). The tensions of the two may not be thesame, but the dominance of air in the pores of thissize range leads to similar moisture contents. In afew cases the water content of natural soil cores afterdraining under a loo-cm tension has been foundactually lower than moisture equivalent, whereas inmost cases it is only a few per cent higher. Moistureequivalents were determined by the Gooch cruciblemethod and corrected to standard (3). On this basisfield capacity should be considered as much a wettingphenomenon as one of drainage, and it is approxi-mated almost equally well by a rather wide rangeof tensions.

Regarding various soil moisture tension curvesin the literature where wetting has not involvedevacuation (i, 12, 10), the presence of sharp flexpoints in the pF range from 1.5 to 2.0 is probablya reflection of the concentration of air beginning inthis range and continuing to the tension of about themoisture equivalent. If the moisture equivalent rep-resents a tension of about pF 2.7 as indicated byRussell and Richards (14), this would seem to definethe effective pore size of the persistent air gaps. Sucha concentration of air would clarify the significanceattached to the flex point by Baver (i) in his studyof laboratory percolation rates, and to pF 1.6 byNelson and Baver (10).

APPLICATION OF RESULTS

The suggested relations of persistent unsaturationof natural soil to various soil and plant factors ap-pears to offer an opportunity for altering certainthings that have been considered more or less fixedand .definite. If some control can be exerted over thefactor of soil wetting,-the influence upon crop pro-duction, runoff, and erosion might be very great.Under some conditions an increase in water-holdingcapacity of a few per cent would mean the differencebetween the consistent success or failure of a par-ticular crop, whereas the elimination of that muchair from the soil-water system might effect a con-siderable measure of erosion and runoff control. -Thedata given show differences among crops and soilsthat amount to enough to exert such influences, whichprobably account for many conservation and produc-tion phenomena that have been attributed to othercauses. It appears not unreasonable that an under-standing of the present role of unsaturation willpermit an improvement in the systematic arrange-ment of crops and the use of soils. Further, the sug-

gested relation of unsaturation to pore size and the'possibility of differences among organic materials assoil wetting agents affords an opportunity for devis-ing ways and means of consciously altering the wet-ting properties of soils. Various mechanical manipu-lations and organic matter treatments are probablyaccomplishing such alterations in many cases already;but here again, attention to the wetting factor anda conscious effort to control it would be expected toprovide more profitable,results.

These practical benefits to be expected from un-saturation studies are additional to the theoreticaland experimental interest. attached. Experimentally,as shown, recognition of the extent of unsaturationshould provide a useful measure of noncapillaryporosity. This and other adapted measurementsshould provide an improved basis for estimating andpredicting the behavior of water in various hydrologicand engineering projects and practices.

SUMMARY

The unsaturation of about 200 varied soil samplesaveraged 9% of the total soil volume after laboratorywetting, measured by the difference between calcu-lated total porosity and actual water removal bywater column tension and by oven drying.

Variations in unsaturation with different cropswere clearly significant on one spring sampling date.Highest unsaturation was for continuous soybeansand other continuous cultivated crops; the lowest forgood sods and for rye-vetch winter cover. Similar,though smaller, differences were obtained withsamples collected in late summer. Other relationsto plants and types of organic material are suggested.

Erodibility seems definitely related to persistentunsaturation, as indicated by the connection betweenunsaturation and crops, percolation rates, types ofmaterials, dispersion ratios, and theoretical consid-erations.

A discussion of soil wetting suggests that thereis normally a concentration of air as bubbles in theintermediate pore sizes which provides a theoreticalbasis for the distinction between capillary and non-capillary porosity and explains how a rather widerange of tensions, including the moisture equivalent,can serve almost equally well as indices of fieldcapacity. Many peculiarities of silty soils are con-sidered associated with this concentration of air.

The possibility of some control over soil wettingappears to offer an opportunity for improved pro-duction, conservation, and experimentation.

SMITH AND BROWNING: WATER-UNSATURATION 119