Soil Water Ma trie Potential Effects on Aggregate Stability1

P. B. FRANCIS AND R. M. CRUSE2

ABSTRACTThe effect of soil water matric potential (i/-m) is not considered with

most techniques utilized to evaluate aggregate stability. Rather, aggre-gate stability usually is measured at 4>m of 0 Pa or at an unknown <!/„.The objective of this study was to determine $a effects, in the range of0 to —3,000 Pa, on stability of natural soil aggregates obtained froma fencerow and from plots having a range of management histories.Aggregate stability, considered inversely related to the mass of soil de-tached by impact of falling water drops, increase markedly as \l/m de-creased from 0 to -500 Pa. Similar <f>m changes at ^m < -500 Pahad smaller effects on aggregate stability. Significant differences in sta-bility of aggregates obtained from plots with different cropping and/ormanuring histories, were sometimes undetectable at <!/„ < —500 Pa butwere more consistently detected at ifrm = 0 Pa. Data from this studyindicate that aggregate stability is very sensitive to ~^m, particularly at

1 Journal Paper J-10553 of the Iowa Agriculture and Home Econ.Exp. Stn., Ames. Project 2412. Presented before Div. S-6, Soil Sci. Soc.of Am., Atlanta, 3 Dec. 1981. Funds were provided by U.S. Fish andWildlife Service through the Iowa Cooperative Wildlife Research Unit(U.S. Fish and Wildlife Service, Iowa State University, Iowa Conser-vation Commission, and Wildlife Management Institute, cooperating).Received 4 Oct. 1982. Approved 24 Jan. 1983.

2 Research Associate, Dep. of Plant and Soil Science, Univ. of Ten-nessee, Knoxville (formerly Research Assistant, Iowa State University),and Assistant Professor, Agronomy Dep., Iowa State University, Ames,respectively.

<!/„ near zero. Techniques for comparing aggregate stability betweensoils or soil treatments should ensure equal values of 4>m exist for allaggregates. Measurements made over a range of ifrm should yield bettercomparisons of aggregate stability between soils than measurements madeat one <^m.

Additional Index Words: soil strength, crop rotations, soil splash,soil detachment.

Francis, P. B., and R. M. Cruse. 1983. Soil water matric potentialeffects on aggregate stability. Soil Sci. Soc. Am. J. 47:578-581.

SOIL STRUCTURAL CONDITIONS and aggregate stabil-ity affect soil erodibility (Wischmeier and Manner-

ing, 1969; Luk, 1979), soil compressibility (Larson andGupta, 1980), and plant growth (Elson, 1940; Harris etal., 1966). Aggregate stability is generally measured ata matric potential (^m) which is either unknown or zero.Methods most frequently utilized are wet sieving (Yoder,1936) and water drop-aggregate dispersion (McCalla,1944). Although these techniques may give acceptableaggregate stability evaluation at one \j/m, recent work in-dicates that small changes in 4>m can yield significant

FRANCIS & CRUSE: SOIL WATER MATRIC POTENTIAL EFFECTS ON AGGREGATE STABILITY 579

changes in soil strength and/or soil credibility (Cruseand Larson, 1977; Al-Durrah and Bradford, 1981). Thus,changes in tym should also affect aggregate strength orstability. A testing method which compares aggregatestability between soils or soil treatments over a range of\l/m should be a more realistic measure of aggregate sta-bility differences occurring in the field because an ag-gregate in the field seldom exhibits only one \}/m duringan event such as a rainstorm.

Soil or crop management systems that include inten-sive row cropping tend to reduce aggregate stability (El-son and Lutz, 1940; Harris et al., 1966). This conclusionappears valid, however only at the \f/m that is character-istic of the aggregate stability test. The objectives of thisstudy were to evaluate tym effects on: (i) stability of nat-ural soil aggregates; and (ii) sensitivity of aggregate sta-bility tests in identifying stability differences between ag-gregates.

MATERIALS AND METHODSSoil cropping histories utilized in this study were: (i) fence-

row (FR); (ii) continuous corn (CC); (iii) corn-corn-oats-meadow with manure at 4,480 kg/ha per year average (C-C-O-M + m); (iv) corn-corn-oats-meadow without fertilization(C-C-O-M); (v) corn-corn-oats-meadow-meadow with man-ure at 4,480 kg/ha per year average (C-C-O-M-M + m); and(vi) corn-corn-oats-meadow-meadow without fertilization (C-C-O-M-M). The continuous corn treatment was established in1915 and existed in 1980 at time of soil material collection.The FR has been in grass since at least 1915. The corn, oats,and meadow rotations were established in 1941 and were ter-minated in 1973. Since that time these plots have been in con-tinuous alfalfa meadow. Soil materials were collected in thespring of 1980 from the upper 9 cm of soil from each treatment.The continuous alfalfa meadow since 1973 possibly minimizedaggregate stability differences between cropping treatmentsnumbered 3-6. These plots therefore seemed to be a useful toolfor testing sensitivity of the proposed technique, i.e., for testingthe ability to detect the aggregate stability differences betweensoils over a range of $„. The FR, C-C-O-M + m, C-C-O-M,C-C-O-M-M + m, and C-C-O-M-M treatments were on Clar-ion loam (fine-loamy, mixed, mesic Typic Hapludolls). The CCtreatment was on Nicollet loam (fine-loamy, mixed, mesic AquicHapludolls).

The soil materials when collected were approximately mid-way between field capacity and air-dry water contents. The CC

Drop former-

Wind shield -

Aggregate, catchment cup-

Water reservoir- St°Pcock-

Capillary tube

||— Drain

Tension plate

Fig. 1—Schematic diagram of tension table and raindrop tower used inaggregate stability tests.

treatment had been moldboard plowed in the fall of 1979. Thecorn, oats, and meadow rotations were in meadow at time ofsampling.

The soil material was air-dried and sieved through a 1.9-cmsieve to remove large plant residue pieces. Soil organic matter(Table 1) was then determined by the Walkley-Black methodas outlined by Allison (1965). Particle-size analysis (Table 1)was measured by the pipet method (Walter et al., 1978).

Tension Plate and Hanging Water ColumnThe tension plate (Fig. 1) on which the aggregates were placed

for the raindrop-impact, soil-detachment tests was constructedfrom fused glass beads. The end of the hanging water column,constructed with a 1.5-mm i.d. capillary tube, was supportedon a horizontal platform that can be raised or lowered to agiven position with respect to the tension plate for adjusting the\i/m of the aggregate resting on the plate. The capillary tubewas connected to the tension plate by 0.5-cm i.d. tygon tubing.

Water-Drop DeviceThe water-drop former, producing drops 3.37 mm in diam-

eter, was of the same type used by Mutchler and Moldenhauer(1963). The drop former was constructed with four telescopingtubes (19, 14, 12, and 9 gauge), each 2.5 cm in length andoverlapping the next smaller by 1.9 cm. The drop former wasattached to a burette so that time intervals between drops couldbe regulated (Fig. 1). The fall height was 1 m, producing about49% of terminal velocity for this drop size, based on data ofLaws (1941). To reduce drifting of the waterdrops from airturbulence, an acrylic tube 95 cm in length with a 3.1 cm i.d.was used as a windshield (Fig. 1). Deviations of waterdrop falldue to static electricity within the acrylic windshield were re-duced by spraying the inside of the windshield with a com-mercially available antistatic spray commonly used on fabrics.

Aggregate Stability MeasurementTen air-dry soil aggregates, each weighing between 0.25 and

0.30 g, were randomly selected from each management treat-ment for aggregate stability testing. A soil slurry was made bycombining a 1:1 weight ratio of water and soil from the soiltreatment to be tested. The soil slurry, which promoted goodcontact between the tension plate and the aggregate, was placedon the tension plate and allowed to drain of free water precedingaggregate placement. The aggregate was then placed on thetension plate and allowed to reach equilibrium with the watertension applied by the hanging water column. Equilibrium wasassumed to exist when water uptake by the aggregate ceased.This was determined by monitoring the meniscus movementwithin the horizontal glass tubing at the end of the hangingwater column (Fig. 1).

Splash catchment cups were constructed by molding alumi-num foil over the bottom of a 50-mL beaker. A circular opening1 cm in diameter was cut in the bottom of the cup so that thealuminum foil cup could be set down over the aggregate andthe soil splash contained. Each aggregate was subjected to aset of 10 simulated raindrops. After each drop, the aggregatewas allowed to come to equilibrium with the water tension in-

Table 1—Properties of soils from which aggregates wereobtained for aggregate stability tests.

Soil O.M. PH Silt Clay

FRC-C-O-M-M + mC-C-O-M-MC-C-O-M + mC-C-O-MCC

6.95.75.46.45.74.5

7.47.17.07.16.96.7

55.640.848.147.843.737.2

24.120.622.222.019.318.8

580 SOIL SCI. SOC. AM. J., VOL. 47, 1983

Table 2—Crop rotation and i/-m effects on soil splash fromaggregates. Each value is an average

of 10 observations.

^m(Pa x 10-')

Rotationf

-5 -10 -15 -20 -25 -30

Soil splash

FRCCC-C-O-MC-C-O-M + mC-C-O-M-MC-C-O-M-M + m

168373471302312394

67219100988076

—— 1251901038513856

X lO-i341159110567110

S596944678251

489060626451

377968594847

tFR = fencerow, CC = continuous corn, C = corn, O = oats, M =meadow, and m = manure application.

duced by the hanging water column before the next waterdropimpact. Upon completion of 10 waterdrop impacts on each ag-gregate, the amount of soil detached was determined by thedifference in pretest and post-test oven-dry weights of the alu-minum cups. If any of the 10 waterdrops failed to land com-pletely within the perimeter of the aggregate, the test was dis-carded and rerun with a new aggregate. Aggregate stabilitywas considered to be inversely related to mass of soil detachedby waterdrop impact. Tests were made at \(/m of 0, —500,-1,000, -1,500, -2,000, -2,500, and -3,000 Pa. Ten ag-gregates were tested from each management treatment at each*m.

An analysis of variance utilizing selected independent con-trasts was utilized to test for significant differences in splashamounts at each $m.

RESULTS AND DISCUSSIONMatric potential had a statistically significant effect,

at the 0.01 level, on soil splash amounts (not shown).Mass of detached soil from aggregates of each soil man-agement and \f/m treatment is given in Table 2. A slightreduction in \f/m from 0 to —500 Pa markedly reducedsoil splash while \l/m reductions beyond —500 Pa hadsmaller impacts on soil detachment. In other words, smallchanges in \l/m near 0 Pa had a relatively large effect onaggregate stability measurements compared to compa-rable \l/m changes at lower \f/m values.

Independent contrast results (a single degree of free-dom F-test), which compares stability of aggregates fromeach plot over a range of \f/m, are given in Table 3. Con-clusions drawn concerning significant differences be-tween aggregate stabilities varied with ̂ m applied at timeof water-drop impact. In general, lower \l/m treatmentsresulted in fewer significant differences between sourcesof aggregates than did higher \f/m treatments, i.e., for thefive contrasts in Table 3 only 1 significant difference wasobserved at $„ < -2,000 Pa while at $m > -1,000 Pa

seven significant differences were observed. As ^m in-creases, aggregate stability tends to decrease (Table 1),and statistically significant differences in aggregate sta-bility between treatments are more readily detected (Ta-ble 3). While cropping history may be affecting aggre-gate stability, it should be noted that textural differences(Table 1) between soils may also contribute to these dif-ferences.

Techniques, such as wet sieving (Yoder, 1936) whichcompare aggregate stability at a \{/m of zero are likelyobserving the maximum aggregate stability differenceswhich will occur between soils for common values of \f/m.Since \j/m in field soil is seldom zero, the validity of re-lating aggregate stability differences observed at \j/m =0 Pa to soil conditions .in the field is questionable. Rather,techniques which compare aggregate stabilities over arange of \frm seem more desirable. Furthermore, any ag-gregate stability comparison test which does not assureuniform \l/m treatments for all soils will likely result ininvalid comparisons.

Structural alteration differences found between differ-ent field soils or soil management treatments will be ata maximum when rainfall is sufficiently intense and/ordrainage is sufficiently slow to maintain \j/m = 0 Pa withinsurface aggregates. Management practices which en-hance internal drainage and promote even small \l/m re-ductions in the plow layer during heavy rainfall will pro-mote structural stability. Conversely, practices whichresult in slow internal drainage may have significant neg-ative effects on structural stability through their impactof increasing \[/m.

l.

Table 3—F values for independent contrasts made over a range of matric potentials (values rounded to two decimal points).

F value

Matric potential (Pa x 10~"l

Contrast -5 -10 -15 -20 -25 -30

CC and FR vs. All rotationsC-C-O-M and C-C-O-M + mvs.

C-C-O-M-M and C-C-O-M-M + mCC vs. FRC-C-O-M vs. C-C-O-M + mC-C-O-M-M vs. C-C-O-M-M + m

4.60*

0.397.19**4.98*1.16

9.57**

1.1228.80**

0.000.02

0.31

0.0221.91**

0.275.50*

1.28

0.248.76**0.252.38

0.06

0.620.211.142.12

0.51

0.063.580.000.32

0.06

1.434.88*0.260.00

*,** Statistically significant at the 0.05 and 0.01 level, respectively.

SHARPLEY & SMITH: DISTRIBUTION OF P FORMS IN VIRGIN & CULTIVATED SOILS 581