Estimating Air Porosity and Available Water Capacity from Soil Morphology1

J. A. MCKEAGUE2

ABSTRACTField guidelines based on soil morphology and calibrated against

measured values were developed for estimating air porosity (AP,volume percent air-filled pores at 5 kPa) and available water ca-pacity (AWC, volume percent water retained between 5 and 1500kPa). The guidelines were tested by estimating and subsequentlymeasuring these properties of 24 soil horizons. The mean of theabsolute differences between estimated and measured AP and AWCwere 3.7 and 4.5% respectively. AP for the 24 horizons ranged from3 to 30% and AWC from 14 to 39%. In view of the magnitude oflocal soil variability, the discrepancies in measured values of APand AWC by different methods and the lack of standard methods,the estimates are shown to be useful. Estimation of air-water regimeproperties of soils from well-calibrated morphological guidelines isrecommended for use in soil survey and in research on effects ofmanagement on soil physical properties.

Additional Index Words: bulk density, aeration, soil water regime,soil moisture field guidelines.

McKeague, J.A. 1987. Estimating air porosity and available watercapacity from soil morphology. Soil Sci. Soc. Am. J. 51:148-152.

AERATION AND CAPACITY TO RETAIN WATER avail-able to plants have been recognized as important

soil properties for many years. Soil survey organiza-tions provide information on these and other air-waterregime properties of soils and the information is com-monly used in rating the suitabilities of soil units fora number of potential uses. Air porosity (AP) andavailable water capacity (AWC) of a soil are usuallyevaluated on the basis of either field or laboratory dataon desorption of water at different pressures (Rich-ards, 1949; Hillel, 1971). Such data may be extrapo-lated by means of models (Bouma, 1984; De long etal., 1983).

Use of different methods, limits, and models forevaluating AP and AWC may result in markedly dif-ferent estimates of these properties (Hillel, 1971;McKeague et al., 1984; Gardner, 1985). For example,using the British Soil Survey limits, 5 to 1500 kPa,rather than those of the United States and CanadianSoil Surveys, 33 to 1500 kPa (McKeague et al., 1984)results in a 200% increase in AWC and an 80% de-crease in AP for a Piperville fine sandy pedon fromthe Ottawa area (Marshall et al., 1979). Similarly, asystem for estimating AWC that does not take intoaccount layers that are inaccessible to roots, as com-pared with one that does, may give markedly differentresults (Gardner, 1985). Determinations of AP andAWC are strongly method-dependent and there are nogenerally-accepted methods.

An additional problem in the evaluation of AP andAWC is that most of the methods of measurement aretime-consuming and costly. Commonly, data for a fewsites are extrapolated to sites mapped as similar soils

1 Land Resource Res. Centre Contribution no. 86-11. Received 7Mar. 1986.2 Research Scientist, Research Branch, Agriculture Canada, Ot-tawa, Ontario K1A OC6.

even though differences in land use and managementat the sites may have contributed to major differencesin AP and AWC. A simple and rapid procedure forestimating AP and AWC on-the-spot would be usefulin soil survey, field experiments on soil-crop relation-ships, and extension work. A possible approach to thedevelopment of such a procedure is to estimate APand AWC from soil morphology. The feasibility ofsuch an approach is indicated by the degree of successin estimating vertical (McKeague et al., 1982) and hor-izontal (Wang et al., 1985b) saturated hydraulic con-ductivity from guidelines based on soil morphology£tnd calibrated against measured values. The objec-tives of this paper are to report procedures used indeveloping guidelines for estimating AP and AWC andresults of a comparison of such estimates with mea-sured values for 24 soil horizons.

MATERIALS AND METHODSSoils

The soil samples used in developing the guidelines for APand AWC were mainly from the Ottawa area (Marshall etal., 1979). The 24 horizons sampled for testing the guidelineswere from soils at new sites in Carleton and Russell coun-ties, Ontario, and the counties of Gatineau and Pontiac,Quebec (Table 1). The sites are all within 50 km of Ottawaand they include a wide range of materials, kinds of hori-zons, texture, structure and consistence.

Developing Guidelines for AP and AWCThe first step in developing morphological guidelines was

to describe the morphology of horizons for which data wereavailable on bulk density, particle-size distribution, and waterretention by soil cores at several pressures. Properties suchas structure (pedality and porosity), texture, estimated bulkdensity, and consistence were related to measured values ofair-filled pores at 5 kPa (AP) and the difference betweenwater retained at 5 and 1500 kPA (AWC). The rationale forchoosing 5 kPa as the upper limit for available water is basedmainly on research in western Europe where an upper limitof 5 to 10 kPa is used commonly (McKeague et al., 1984).Tentative guidelines were prepared relating the propertiesassessed in the field to AP and AWC classes; the range ofeach class was 5% by volume (e.g., AWC 10-15%). Thesetentative guidelines were tested on another set of approxi-mately 20 samples for which estimates of AP and AWC weremade to the nearest percent. Discrepancies between esti-mated and measured AP and AWC values were noted andthe guidelines were adjusted accordingly. The resultingguidelines were published (McKeague et al., 1986). Exam-ples of these guidelines follow:

Very Slightly Porous, <5% AP1. Clayey, massive, few macro voids, bulk density >1.3

Mg m~3, or2. Clayey, weak to moderate coarse blocky, few channels,

bulk density >1.4 Mg m~3, or3. Loamy, bulk density >1.5 Mg m~3, or4. Sandy, cemented, intergranular spaces nearly filled, bulk

density S;1.8 Mg m~3.

148

MCKEAGUE: ESTIMATING AIR POROSITY AND AVAILABLE WATER CAPACITY 149

Table 1. Samples used in comparing estimated and measured AP and AWC.t

Soil Land use Horizon Depth, cm Texture Structure Consistence Reference

North Gower(Typic Haplaquoll)

Jockvale(Typic Haplaquept)

Piperville(Typic Haplaquept)

Bearbrook(Typic Haplaquept)

Ste Rosalie(Typic Haplaquoll)

Uplands(Aquic Dystrochrept)

Uplands(Typic Haplorthod)

Grenville(Typic Eutrochrept)

Dalhousie(Typic Haplaquept)

St. Bernard(Typic Haplaquoll)

Gatineau(Lithic Cryorthod)

Morin(Typic Cryorthod)

Achigan(Aquic Haplorthod)

Baudette(Aquic Eutrochrept)

Cereal

Pasture

Forest

Hay

Hay

Cereal

Forest

Abandoned

Cereal

Cereal

Forest

Abandoned

Forest

Hay

ApBgApBCApBgApBgApBgApBgBf

ApBApBgAp

Bf

Bf

BfBCgApBm

4-1220-28

4-1246-54

3-1123-31

4-1233-41

3-1122-30

5-1340-4810-18

3-1120-28

2-1032-40

2-10

3-11

10-18

5-1330-38

2-1023-31

LcLIf-mSfSsiLvfslsiLsicLsiCsiCfslmSfs

LLcLsiCL

sL

ms

IfSfSsiLsiL

StructurelessStructurelessStructurelessSingle grainw.f.sbkw.m.pls.f.sbkw.f.abks.f.sbkw.f.abkStructurelessSingle grainSingle grain

m.c.plStructurelessStructurelessw.f.abkm.f.sbk

Structureless

Structureless

StructurelessStructurelessm.f.sbkw.c.abk

FirmFirmFriableVery friableFriableFriableFriableFirmFriableVeryf irmFriableVery friableVery friable

FriableFriableFirmFriableVery friable

Very friable

Very friable

Very friableVery friableFriableFriable

Marshall et al. (1979)

Marshall et al. (1979)

Marshall et al. (1979)

Wicklund and Richards (1962)

Marshall et al. (1979)

Wicklund and Richards (1962)

Wicklund and Richards (1962)

Marshall et al. (1979)

Marshall et al. (1979)

Lajoie (1962)

Lajoie (1962)

Lajoie (1962)

Lajoie (1962)

Lajoie (1962)

t The soil name is that used to designate the map unit; it is an association name in the case of Marshall et al. (1975) and a series name in the other tworeports cited. Classification in Soil Taxonomy (Soil Survey Staff, 1975) is based on the data available, which are incomplete for some of the pedons. Thethickness of sample is 8 cm to conform with the height of soil cores. Textures reported were estimated by hand texturing. Structure (pedality) termsare abbreviated as follows: w,m and s weak, moderate and strong (grade), f,m and c fine, medium and coarse; sbk, abk and pi subangular blocky, angularblocky and platy.

High A WC, 20 to 24.9%1. Loamy fine sands with < 10% amorphous material, or2. Fine sandy loams and loams with bulk densities of 1.4

to 1.5 Mg m~3- or3. Clay loams with bulk densities of approximately 1.4

Mg m~3, or4. Clays with bulk densities of approximately 1.2 Mg m~3

and <5% amorphous material.No specific guidelines were developed for estimating bulk

density. Estimates were based on "feel" of the weight of aclod in relation to its size, wetness and texture, and to the"feel" of samples of known bulk densities.

As indicated, the guidelines require field estimates of tex-ture, bulk density, macroporosity, pedality, consistence, andnon-crystalline material (including organic matter [OM]).When estimates were attempted to the nearest percent, acheck on their overall consistency was made. Suppose, forexample, that a loamy structureless, friable A horizon is es-timated to have the following properties: clay 20%, OM 4%,bulk density 1.3 Mg m-3, AP 15%, AWC 26%. Ifa particledensity of 2.65 Mg m~3 is assumed, total porosity should be51%

2.65 - 1.32.65

100T~

Water retained at 1500 kPa should be approximately: bulkdensity (0.4 X clay % + 1 X OM %) or 1.3 (0.4 X 20 +1 X 4) = 16%. AWC + AP should be approximately 51 -16 = 35%. This value is well below the estimate of 41% (15+ 26) based on morphology. Estimates of AP and AWCshould be rechecked to ensure that the guidelines were ap-plied correctly. If they were, the estimates of clay and OM,and the associated estimate of water retained at 1500 kPashould be rechecked. If the incompatibility of estimates can

not be resolved, the estimates based on morphologicalguidelines are used. In the example given, checking wouldshow that an appropriate estimate of AP would be 10%(McKeague et al., 1986), not the initial estimate at 15%.

Testing the GuidelinesMorphology of each horizon listed (Table 1) was described

and field estimates of bulk density, texture, amorphous ma-terial, and water retained at 1500 kPa were recorded. APand AWC were estimated to the nearest percent in the field.Triplicate cores 7.6 cm in diameter by 7.6 cm high weretaken using a core sampler, and bulk samples were taken ofthe same 7.6 cm layer. Methods of analysis (Sheldrick, 1984)were:

1. Water desorption from cores, method 84-035, tensiontanks containing either glass beads (to 10 kPa) or Aloxide powder (10-50 kPa); water retained at 0, 5, 10,and 33 kPa was determined.

2. Water desorption from <2 mm samples by pressureplate, method 84-036; water retained at 1500 kPa wasdetermined.

3. Bulk density, method 84-029, based on volume at fieldwater content (between approximately 5 and 50 kPa).

4. Particle-size distribution, method 84-026, sieving andpipette after peroxide treatment, washing, and Na me-taphosphate treatment.

5. Oxalate-extractable Fe and Al, method 84-011.6. Carbon, by LECO CHN-600 analyzer (Sheldrick, 1986).Trapped air was measured in core samples of two sandy

horizons by weighing them after "saturation" for a week inwater and again after putting the moist core samples undervacuum prior to saturating them with water.

Measured values of AP were determined in two ways:

150 SOIL SCI. SOC. AM. J., VOL. 51, 1987

1. Water lost between 0 and 5 kPa as a percentage of corevolume (AP by water loss), and

2. Total porosity calculated from bulk density and an as-sumed particle density of 2.65 Mg.m~3 minus volumepercent of water retained at 5 kPa (AP from total po-rosity).

32-

28-

24-

20-

16-

12-

8-

4-

Difference (%)0-3 154-6 57-10 2

11-13 2• mean absolute difference 3.7%- standard deviation of difference 5.1%

sandX loamy sand or sandy loamQ loam or silt loamO clay loam or clay

12 16 20 24 28 32

MEASURED AIR POROSITY

Fig. 1. Comparison of measured and estimated air porosites (AP)of 24 soil horizons with field texture ranging from sand to clay.Measured AP is based on total porosity calculated from bulk den-sity and an assumed particle density of 2.65 Mg.m~3 minus water(vol.) retained at 5 kPa (triplicate cores). Estimated AP is basedon morphological guidelines. "Difference" refers to the differencebetween measured and estimated AP's. The diagonal line has aslope of 1.

1urrui

Iui

<QUII

44-1

40

36-

32

28

24.#

20

16-

12-

8-

4-

Difference (%)0-3 104-6 87-9 5

10-12 1• mean absolute difference 4.5%- standard deviation of difference 5.5%

• sandX loamy sand or sandy loiD loam or silt loamO clay loam or clay

0 4 8 12 16 20 24 28 32 36 40 44

MEASURED AVAILABLE WATER CAPACITY (5-1500 kPa)

Fig. 2. Comparison of measured and estimated available water ca-pacities (AWC) of 24 soil horizons. Measured AWC is the volumepercent of water lost between 5 and 1500 kPa (triplicate cores).Estimated AWC is based on morphological guidelines. "Differ-ence" refers to the difference between measured and estimatedAWC's. The diagonal line has a slope of 1.

RESULTSAir Porosity

During the period of development of guidelines forAP, it became evident that for most samples "AP fromtotal porosity" exceeded "AP from water loss" as de-nned in the preceding paragraph. In the case of sandysoils, the difference was commonly from 3 to 7%. Forfine sands, rapid water loss during removal of the corefrom the saturation tank for weighing was found to bea minor factor. Trapped air in medium and fine sandsamples accounted for 7 and 11%, respectively, of thecore volume (means of triplicate). AP from total po-rosity was used in developing the guidelines that weretested on the 24 samples (Table 1).

Both measured arid estimated AP values rangedfrom 3 to 30% (Fig. 1). AP was underestimated forapproximately 2/3 of the 24 samples but estimateswere within 3% of the measured value for 15 of the24. The mean of the absolute difference between es-timated and measured AP was 3.7%. Two very lowestimates (Piperville AP and Upland Bf, Table 1) con-tributed substantially to this difference. Standard de-viations of measured AP (triplicate cores) ranged from0.1% to 5%, and exceeded 2% for 6 of the 24 samples.The sands have high to medium AP and the clay loamsor clays, low to medium values (Fig. 1).

Available Water CapacityMeasured AWC ranged from 14 to 39% and esti-

mates from 11 to 44% (Fig. 2). AWC was overesti-mated for 13 samples and underestimated for 9. Es-timates and measured values differed by 3% or lessfor 10 samples and by 4 to 6% for 8 of the 24 samples.The mean of the absolute difference between esti-mated and measured AWC was 4.5%. Standard de-viations of measured AWC (triplicate cores) rangedfrom 0.1 to 7%, and exceeded 2% for 6 of the samples.The loamy sands to sandy loams have relatively highand the clay loam and clay samples relatively low AWCvalues (Fig. 2).

Bulk DensityMeasured bulk densities ranged from 0.94 to 1.65

Mg.m~3 and estimated from 0.8 to 1.6 Mg m~3 (Fig.3). The mean absolute difference between estimatedand measured bulk densities was 0.10 Mg m~3 andestimates differed from the measured values by <0.10Mg m~3 for 14 of the 24 samples. Standard deviationsof measured bulk densities (triplicate cores) rangedfrom 0.01 to 0.09 Mg m~3. No obvious relationshipis evident between texture and bulk density (Fig. 3).

Other EstimatesSeveral properties were estimated without devel-

opment of formal guidelines calibrated against mea-sured values. Texture was estimated in the same classas that measured for 14 of the 24 samples. Water re-tained at 1500 kPa was estimated within 4% of themeasured value for 3A of the samples. Amorphous ma-terial (OM plus twice oxalate-extractable Fe + Al) wasestimated, on the basis of color and kind of horizon,

MCKEAGUE: ESTIMATING AIR POROSITY AND AVAILABLE WATER CAPACITY 151

within 2% of the measured value for 2/3 of the sam-ples.

DISCUSSIONThe results indicate that AP and AWC, as denned,

can be estimated from morphology within 3 to 4% and4 to 5%, respectively, of the measured values in ap-proximately 2/3 of cases. The estimates can be maderapidly on the spot; approximately 5 min are adequateper horizon. The question of whether estimates of thisdegree of reliability are worthwhile requires consid-eration. The possible alternative of measuring AP andAWC of several horizons of soils in all fields wheresuch data are needed is not even remotely feasible ina vast country with relatively few soil scientists. Mea-sured data should be used mainly to calibrate otherapproaches to estimating air-water regime propertiessuch as AP and AWC. Another approach is that ofextrapolating measured data by use of models and soilmaps (Bouma, 1984). Consideration of data (Table 2)for a few of the 24 horizons studied, and of results ofBeke and MacCormick (1985), however, suggest thatlarge errors may result from use of simple models toextrapolate data to "similar" soils.

If a model based on particle-size distribution andorganic C were used to estimate AP and AWC, theestimates for the Bearbrook and Dalhousie Ap hori-zons would be nearly identical, as would those for theAchigan and Uplands samples (Table 2). In fact, APand AWC as defined in this work are widely differentfor both pairs of samples. For the first pair, the dif-ference in bulk density associated with different landuse accounts for much of the difference in AP andAWC. The Dalhousie soil had been compacted duringlong term corn production (Wang et al, 1985a). Themarked difference in AP and AWC of the sandy soilsis almost certainly associated with the fact that 71%of the Uplands sample is medium sand and 75% ofthe Achigan sample is fine sand.

The major effect of the criteria used to define APand AWC are demonstrated clearly by data for thetwo pairs of samples (Table 2). Setting the upper limitof AWC at 33 kPa results in a substantial increase inAP and a decrease in AWC for all four samples. Theeffect is most striking for the Achigan sample. If Gard-ner's (1985) statement "differences in rooting depthand rooting activity at depth have a far larger effecton variation in profile PAWC (plant available watercapacity) between soils than do differences in availablewater storage per unit depth of soil" is taken seriously,the AWC for the Dalhousie Ap might be more cor-

1.7-

1.6-

1.5-

I 1.<HzUJ0 1.3 Hj W

S |1.2-D EUJ

< 1'1 "

§ U°

0.9

0^

Difference (Mg.rn'3)<0.1 140.1-0.19 60^-0.26 4- mean absolute difference 0.095 Mg.rrT3

• standard deviation of difference 0.12 Mg.rrT3

XD D X O

sandX loamy sand or sandy loamD loam or silt loamO clay loam or clay

0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7

Mg-itT3

MEASURED BULK DENSITY

Fig. 3. Comparison of measured and estimated bulk densities of 24soil horizons. Measured bulk density is based on data for tripli-cate cores trimmed at field soil water content. Bulk density wasestimated in the field on the basis of "feel" and comparisons withsamples of known bulk densities. No formal guidelines were used."Difference" refers to the difference between estimated and mea-sured values. The diagonal line has slope of 1.

rectly designated as 0 than as 15%. The horizon wascompact and firm and very few roots penetrated themassive material. The lower limit of AWC (1500 kPa)is probably not appropriate for the lower part of theroot zone and adjustments have been made by severalorganizations (McKeague et al., 1984; Gardner, 1985).

The choice of 5 kPa as the upper limit of AWC maybe questioned but a limit of 5 to 10 kPa is almostcertainly more appropriate than the limit of 33 kPaused generally in soil survey in North America. Rich-ard's (1949) statement is relevant, "Results from anumber of years experience in the West with tensiom-eters in a variety of soils in the field indicate that un-der conditions ordinarily designated as field capacity,tensiometer readings usually lie in the range from 30to 150 cm of water." He noted that a slightly lowerrange, 25 to 125 cm of water, had been reported forthe midwestern USA. These upper limits are compat-ible with those used in western Europe (McKeague etal., 1984).

The operational definition used in the determina-tion of AP at a stated pressure has a major influenceon the value obtained for that parameter. As shown,trapped air may account for 10% or more of the vol-

Table 2. Comparison of AP and AWC values for three different upper limits of AWC for two pairs of horizons of similar textures, t

Soil

BearbrookDalhousieAchiganUplands

Horizon

ApApBCgBg

Sand

46439895

Silt

3132

23

Clay7o ——————

232412

C

2.41.60.60.4

Bulkdensity'Mgm-'

1.281.661.401.38

5

143

1225

AP at kPa10

174

2233

AWC at kPa33

216

3135

5-1500

23153220

10-1500

20142212

33-1500

16121310

t See Table 1 for other information on the samples. The AWC limits are those used by the soil survey organizations of England (5-1500), the Nether-lands (10-1500) and the United States and Canada (33-1500).

152 SOIL SCI. SOC. AM. J., VOL. 51, 1987

umes of some samples. The operational definition ofAP used in this work resulted in the inclusion of trap-ped air as a part of air porosity. Such air may be rel-atively stagnant and of little significance in supplyingoxygen to plant roots. Both methods of measuring andoperational definitions of AP require further work.

Though field estimates based on morphology andsupported by measured data seem to be the best prac-tical approach to characterizing and interpreting theair-water regime properties of soils, the approach sug-gested is only one of the possibilities. In Britain, forexample, AP and AWC are estimated on the basis ofpacking density (bulk density + 0.009% clay) and tex-ture (Hall et al, 1977). Limited testing of that ap-proach in the Ottawa area gave disappointing resultsbut it yielded reasonable estimates for horizons of apedon in England (Bullock and McKeague, 1984). Themodelling approach (Bouma, 1984) could be used toextrapolate data estimated by the morphological ap-proach. Relying on "hard" data for a few sites is notadequate because of both inherent soil variability andthe major effects of land use and management on soilphysical properties in the root zone (Topp et al., 1980;Wang et al., 1985a; Kooistra et al., 1985).

Use of morphological guidelines to estimate the air-water regime properties of soils has great potentialusefulness in soil survey. Such estimates require onlya minor additional expenditure of time at the pit.Making the estimates gives purpose to the task of de-scribing soil morphology. The practice of estimatingwould probably result in more reliable and uniformsoil descriptions, improved guidelines for estimating,and more and better measurements of air-water prop-erties of soils. Estimators are keen to check the reli-ability of their estimates and to resolve problems ofdifferent estimates by two or more pedologists. Cur-rently, descriptions of soil morphology are used verylittle in making interpretations and the incompatibil-ity of descriptions has little practical consequence(McKeague et al., 1986).

Limited testing in field workshops in the AtlanticProvinces in 1985 indicated that pedologists and othersworking with soils learned in < 1 d to apply consist-ently the guidelines for estimating Ksat. It is probablethat application of the guidelines for AP and AWCcan be learned as readily.

CONCLUSIONS1. Estimates of AP and AWC based on morpho-

logical guidelines were close enough to measuredvalues to be useful. The differences between es-timated and measured values was commonly lessthan that between measured values involving dif-ferent, but commonly used, limits for AP andAWC.

2. Field estimates of AP and AWC based on guide-lines and checked regularly against measuredvalues could be used to advantage in soil surveyand in the evaluation of soil physical conditionas affected by management.

3. The morphological approach to estimating air-water regime properties of soils requires further

testing on a wide range of soils and more cali-bration of guidelines against measured values.

ACKNOWLEDGMENTContribution of co-workers are gratefully acknowledged:

C. Wang for participation in field work including workshops;G.C. Topp and D. Kitching for providing some of the coredata used in developing guidelines; the Service Laboratorystaff, B.H. Sheldrick, E. Vendette, and D. Anderson for pro-viding the laboratory data, and the Graphics Units for pre-paring the figures.