Influence of Soil Structure on Water Relations in Low Humic Latosols:II. Water Movement1

M. L. SHARMA AND G. UEHARA2

ABSTRACT

Experiments were carried out to study the effect of two as-pects of structure, macrofabric (arrangement of peds) andmicrofabric (arrangement of primary particles within the ped),on water movement. Two latosolic soils of nearly similar com-position and varying greatly in soil fabric were used. Capil-lary conductivity of the soils and their water-stable aggregateswere obtained. The results showed that the effect of macro-

fabric on water movement was more pronounced in the low-tension range (0-0,2 bar). Identical flow rates were obtainedfor the same size water stable aggregates of both soils, suggest-ing that intra-ped pores did not significantly contribute to wa-ter flow in this tension range. It was suggested, however, thatmicrofabric might influence fluid flow properties in the hightension range.

Additional Key Words for Indexing: macrofabric, micro-fabric.

WATER RETENTION and flow properties of soils depend,to a great extent, on pore-size distribution. The size

distribution of pores, which involves the effects of theshape, size, and arrangement of the primary particles whichform the compound particles and the compound particles

SHARMA AND UEHARA: SOIL STRUCTURE AND WATER RELATIONS IN LOW HUMIC LATOSOLS: WATER MOVEMENT 771

themselves, reflects the nature of soil texture and structure.With respect to water movement through saturated soils,in most agricultural soils, the structure and its stability aremore important than the textural components and theirindividual interaction with water. This is because the poresand the configurations formed between the secondaryaggregates possess much larger dimensions than thosebetween individual soil particles within the aggregates.According to Poiseuille's law the coefficient of permeabilitychanges as a fourth power of the pore radius. Severalworkers (7, 11, 13, 14) have, therefore, used hydraulicconductivity as a measure of soil structure. Swartzendruberet al. (13) tested capillary intake rate and hydraulic con-ductivity as indices of soil structure. They presented a theo-retical proof to show that hydraulic conductivity was abetter index of soil structure than capillary intake rate; thiswas also supported by their experimental results. The workof Childs and Collis-George (4), Marshall (8), Millingtonand Quirk (9), and Brooks and Corey (2) all present evi-dence on the importance of pore-size distribution in unsat-urated flow. Recently, Amemiya (1) found that capillaryconductivity was a function of aggregate size only if sizeaffected the water content-suction relationship. However,in his study the aggregate stability during capillary con-ductivity measurement was not necessarily assured, andit may be doubtful that his results show true effects ofaggregate size.

The relative importance of structure compared to tex-ture in determining the pore-size distribution—and thusthe moisture characteristics of soils—becomes greater insoils with structure of high aggregate stability. The LowHumic Latosols of Hawaii have high aggregate stability(3, 15). Although clays texturally, these soils exhibit waterretention properties that resemble those of coarse-texturedsoils (5, 12). Sharma and Uehara's (12) earlier paper dis-cussed the effect of two aspects of soil structure, macro-fabric (arrangement of peds) and microfabric (arrange-ment of primary particles within the peds), on the waterretention. The objective of this paper was to evaluate thecontribution of macro- and microfabric to water movementin the saturated and unsaturated state.

METHODS AND MATERIALS

Two soils of the Molokai and Wahiawa series belonging tothe Low Humic Latosol great soil group, representing depthranges of 50-100 cm and 50-75 cm, respectively, were sam-pled. They were nearly identical in chemical and mineralogicalcomposition, but widely different in soil structure. Evidence onthis has been presented in an earlier paper (12). Differences inthe soil structure were demonstrated by aggregate analysis andthin section study. The Wahiawa had higher aggregate stability.Thin section study showed the difference in microfabric be-tween two soils, indicating that the Wahiawa soil had higheroptical anisotropy, which represented a more preferred orien-tation of particles within the peds (3, 12), although shape andsize of primary particles were demonstrated to be very similar.

Preparation of Sample and Measurement ofWater Movement Through Soils

The large soil fragments collected from fields were gentlycrushed and passed through a 2-mm sieve. To collect water-

LAMP

TENSION

Fig. 1—Schematic diagram of capillary conductivity appartus.

stable aggregates, 50 g of these samples were placed in a nestof sieves and agitated for 30 min at a rate of 30 cycles/min. Ag-gregate fractions of 10-20, 20-40, 40-60, 60-140 mesh, wereair-dried and stocked for water movement studies. The 10-, 20-,40-, 60-, and 140-mesh sieve sizes correspond to 2.00, 0.84,0.42, 0.25, and 0.105 mm, respectively.

Saturated and unsaturated conductivity of the soils and theirwater-stable aggregates were measured by an apparatus whoseschematic diagram is presented in Fig. 1. The method suggestedby Nielsen and Bigger (10) was used for the steady-state capil-lary conductivity measurements in the early stages of this study.Later, a technique similar to that of Elrick and Bowman (6),which eliminates the need for tensiometers to measure potentialgradients, was utilized. The potential difference along the lengthof the soil column was measured by the tension differencesimposed at both ends of the column. The average suction ofthe system as a whole was calculated by taking the average^pftensions imposed on two ends of the sample.

It was noted in the early experiments that at low tension, thecapillary conductivity K was greatly influenced by the pore-size of the fritted glass disc. To eliminate this effect, coarse(40-60,u) and extra-coarse (170-200/i) fritted glass discs wereused at lower tensions. Beyond the air entry value of these discs,discs of finer pores (medium, 10-20/u) were used.

Since a mechanical packer was not available, the soil wascarefully placed in a cylinder positioned vertically with one endassembled. The soil was transferred into the column through along funnel, and as the column progressively filled, the funnelwas raised accordingly. The sample was filled 7.6 cm beyondthe length of the column by adding an extension to the column.The assembled lower section was connected to a water sourceand the sample was allowed to wet under a tension of 2 cm ofwater. The sample remained in that condition for approximately12 hours, after which time a tension of 200 cm of water wasimposed for about 12 hours. Thereupon, the extra soil was cutoff, and the fritted-glass-disc for this end of the column was

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SOIL SCI. SOC. AMER. PROC., VOL. 32, 1968

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Fig. 2—Capillary conductivity as a function of soil moisturetension of the Molokai soil.

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tension of the Wahiawa soil.

installed. The column was then resaturated under slight posi-tive pressure, and the saturated hydraulic conductivity wasdetermined. During saturated flow measurements, the holesin the cylinder were taped shut, but were kept open duringunsaturated flow.

By using the aforementioned technique, uniform bulk densitywas believed to be obtained as indicated by good agreementbetween K values obtained on separate samples. At each ten-sion, equilibrium was assumed to be established when outflowrates equaled inflow rates. The equation given below:

K= (Q/t) [1]

was used to calculate K at each tension; where Q is volume out-flow over the time interval t, &X is the length of soil column,A* is the head difference imposed at opposite ends of column,and A is the cross-sectional area of the column. All of thesemeasurements were made at a constant temperature 21 ± 0.5C;microbial growth was checked by using weak solutions offormaldehyde.

RESULTS AND DISCUSSION

The capillary conductivity K for the Molokai andWahiawa soils (< 2 mm, whole soil) as a function of tensionis presented in Fig. 2 and 3. It is clear from these plotsthat the saturated hydraulic conductivity of the Wahiawasoil is decidedly higher than that of the Molokai soil. Whenthe soils become unsaturated, the capillary conductivityof the Wahiawa soil decreases more sharply than does thatof Molokai.

On the basis of evidence presented earlier (12) thesesoils differ only in soil structure. Capillary conductivity dif-ferences observed under isothermal conditions can there-fore be attributed to structural differences. Two importantaspects of soil structure are pore-size distribution andaggregate stability. In soil water studies, stability is ofgreat importance, since if the aggregates cannot withstandthe effect of wetting, they may disintegrate and alter thegeometry of the system. Hence aggregate size distributionof these soils (< 2 mm) was obtained by wet sieve analysis.The results are presented in the earlier paper (12).

The average bulk density values attained by the Molokai

and Wahiawa soils in the capillary conductivity apparatuswere 1.10 g/cc and 1.03 g/cc, respectively. The data onaggregate size distribution and the bulk densities of thesesoils [Table 2 (12)] give evidence that the Wahiawasoil has a higher proportion of large pores. The Molokaisoil on the other hand contains a larger proportion ofsmall-size aggregates, and these can be presumed to havea larger proportion of smaller pores.

According to Poiseulle's law, the flow of fluid througha porous medium is proportional to the fourth power of theradius of the conducting tube. Therefore, it is not surpris-ing that the Wahiawa soil has a higher saturated hydraulicconductivity. The inter-aggregate pores of the Wahiawasoil are emptied at comparatively lower tensions, and sothe effective cross-sectional water-conducting area is alsoreduced, which in turn causes a rapid drop in the capillaryconductivity. Since the Molokai soil has a large number ofsmall size pores that are not so easily drained, this soil con-tinues to conduct water at comparatively higher tensions.

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Fig. 4—Soil moisture diffusivity as a function of volumetricwater content for the Molokai and Wahiawa soils.

SHARMA AND UEHARA: SOIL STRUCTURE AND WATER RELATIONS IN LOW HUMIC LATOSOLS: WATER MOVEMENT 773

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Fig. 5—Capillary conductivity as a function of soil moisturetension for water stable aggregates of the Molokai soil.

Soil moisture diffusivity value D(0) in cm2/sec, ob-tained by multiplying the K with the corresponding sloped*/df? of the retention curve at specified moisture contents,are presented as a function of volumetric water contentfor the Molokai and Wahiawa soils in Fig. 4. At highervolumetric water contents, the Wahiawa soil has higherdiffusivity values than the Molokai soil, but it soon dropsto lower values.

The differences in conductivity and diffusivity betweenthese two soils can be attributed to differences in soil fabricand also in the stability of the aggregates. The effect ofaggregate stability was eliminated by measuring capillaryconductivity of water-stable aggregates of sizes 20-40,40-60, and 60-140 mesh. The results are presented in Fig.5 and 6.

Comparison of saturated hydraulic conductivities of thedifferent size aggregates from the same soil suggests thatthey follow the order predicted by Poiseuille's equation:i.e., the larger aggregates have the larger fluid-conductingtubes and thus higher saturated conductivities. The capil-lary conductivity of the system remains relatively un-changed at the lower tensions until air enters the system:then it drops sharply as was indicated by the shape of thecorresponding retention curves (12). These differences showthe effect of macrofabric for constant microfabric type.

Comparison of conductivities of the same size aggregatestor both soils can show the effect of microfabric. To makesuch a comparison one must assume constancy of shapeand arrangement of peds, thus creating systems in whichthe intra-ped fabric is the only variable. As shown in Fig.5 and 6 the conductivities of two similar size aggregates arepractically the same within experimental errors.

The capillary conductivity of aggregates and whole soilattains a comparatively low value in both soils as the ten-

TENSION-CM OF WATER

Fig. 6—Capillary conductivity as a function of soil moisturetension for water stable aggregates of the Wahiawa soil.

sion reaches beyond 100 cm H2O and the curves seem tobe flattening with the increase in tension up to 180 cmH2O (Molokai—whole soil). Based on these values andshape of moisture retention curves of different size aggre-gates ( 1 2 ) , it might be safe to conclude that macrofabrichas a pronounced effect on water movement approximatelyup to a tension of 200 cm H2O, and microfabric does nothave any significant influence on K in this tension range.However, on the basis of differences in moisture retentionof these soils presented in the earlier paper (12) , it canbe suggested that intra-ped fabric may affect water move-ment in the higher tension range.

774 SOIL SCI. SOC. AMER. PROC., VOL. 32, 1968