A Comparison of Column-Displacement and Centrifuge Methods for Obtaining Soil Solutions1

FRED ADAMS, CHARLES BURMESTER, N. V. HUE, AND F. L. LoNG2

ABSTRACTFour soils, ranging in texture from loamy sand to clay, were

fertilized differently and equilibrated moist for several days.Soil solutions were then separated by column-displacement,by simple centrifugation, and by immiscible displacement withCC1( via centrifugation. The ionic compositions of soil solu-tions were unaffected by the method used to obtain the solu-tions.

Additional Index Words: cations, aluminum, pH, sulfate,phosphate.

Adams, F., C. Burmester, N. V. Hue, and F. L. Long. 1980. Acomparison of column-displacement and centrifuge methods forobtaining soil solutions. Soil Sci. Soc. Am. J. 44:733-735.

BECAUSE SOIL SOLUTION is the medium in which soilchemical reactions occur and from which plants

obtain mineral nutrients, there has been a long-time in-terest in knowing its chemical composition. However,separating unaltered soil solution from solid-phase soilhas been difficult, and numerous methods have beenproposed, based on the principles of suction, pressure,displacement, and centrifugation (Adams, 1974). Thecolumn-displacement method (Parker, 1921) has beenused successfully for many years (Adams, 1974; Benianset al., 1977). A successful centrifuge method was de-veloped much later (Davies and Davies, 1963), andrecent improvements in this method suggest that itcan be used with less bother than the column-displace-ment method (Gillman, 1976; Mubarak and Olsen,1976; Yamasaki and Kishita, 1972).

The volume of soil solution needed for chemicalanalysis varies with the ionic components to be deter-mined, the concentration of these ions, and the analyt-ical methods to be used. Where soil solutions are ex-tremely dilute in several ions of interest, 25 to 30 mlmay be required (Adams, 1971; Gillman, 1976). Insuch instances, relatively large soil volumes may berequired to yield adequate solution for a completeanalysis.

The methods for separating a portion of the soilsolution from solid-phase soil are empirical, and theymay separate soil solutions of different composition.Therefore, the objective of the work reported here wasto compare the ionic composition of soil solutions ob-tained by two centrifuge methods with the column-displacement method.

MATERIALS AND METHODSFour unfertilized surface soils, differing in texture and water-

holding capacity, were bulk-sampled, air-dried, and screened(Table 1). Field capacity was determined on each soil asfollows: a weighed amount of air-dried soil was placed uni-formly in a 60 by 5 cm glass column, a measured amount of

water (50 to 75% of field capacity) was added, and the amountof soil wetted was measured after 48 hours (movement of wet-ting front had essentially stopped).

Each bulk sample was divided into subsamples that weredifferentially fertilized in order to establish different electro-lyte levels in the soil solutions. All fertilized soils received the fol-lowing rates of dry salts: 50 ppm K as K.C1, 50 ppm N as NH,NO3,50 ppm P as Ca(H2PO4)2, and 25 ppm Mg as MgSO4. A sup-plemental addition of Ca(H2PO4)2 at rates of 300, 400, and 600ppm P was added to separate samples of Lucedale (fine-loamy,siliceous, thermic Rhodic Paleudult), Decatur (clayey, kaolinitic,thermic Rhodic Paleudult), and Boswell (fine, mixed, thermicVertic Paleudalf) soils, respectively. The treated soils were thenwet to field capacity, incubated moist for 7 days, dried, crushed,rewet to field capacity, and incubated moist for at least 7 days.Soil solutions were then obtained by different methods andanalyzed for ionic composition.

Column-displacement Method—Moist soil was sieved througha 10-mm screen and poured into a 60 by 3 cm glass column,which was drawn and fitted at the bottom to a small-bore draintube (Fig. 1A); the tube was plugged with glass wool. Thecolumn was gently jarred during filling to settle the soilpartially. After filling, the soil was compacted by holding arubber-stopper assembly (Fig. ID) to protect the outlet tube andby repeatedly tapping the column (cushioned by the stopperassembly) against the desk top. The degree of soil compactionrequired for effective solution displacement had been previouslydetermined by trial and error on separate samples. Columnpacking requires a great deal of operator skill and experience.The column was placed in a suitable rack, and a collection tubeattached (Fig. 1C). The top of the soil column was firmedwith a rubber stopper attached to a glass rod (Fig. IB) andabout 100 ml of a saturated solution o£ CaSO4 containing 4%KCNS was added atop the column. The first 5 ml of leachatewas discarded as a precaution against minor contaminations;the remaining leachate was collected in 5- to 10-ml incrementsuntil CNS" appeared in the leachate (detected by spotting adrop of leachate with 5% FeCl3 in 0.1AT HC1). Each incrementwas tested immediately for pH and then transferred to poly-ethylene bottles.

Centrifuge—Gillman's (1976) method, with modified soil con-tainers and solution cups, was followed. A two-tier assembly ofPlexiglas tubing and sheets were constructed to fit 600-mlcentrifuge carriers (Fig. 2). The soil containers were madefrom 11.5-cm lengths of Plexilas tubing with an 8-cm i.d. anda wall thickness of 7 mm; solution cups were 2.7 cm longand were sealed at one end with a 7-mm thick piece of ma-chine-fitted Plexiglas. A 1.5 cm thick, circular piece of Plexi-glas sheet, with a 9.4-cm diam, was drilled with 45 1-mm holes,then machine fitted to the soil container on one side and thesolution cup on the other; it was then cemented to one endof the soil container.

A sheet of Whatman no. 42 filter paper was placed in thesoil container, moist soil was packed into the container, soilswere centrifuged for 2 hours at 1,070 g (maximum speed ofcentrifuge), solution cups were removed, solution pH mea-sured, and solution stored in polyethylene bottles.

Table 1—Particle-size distribution and field capacity of soils.

1 Contribution from the Dep. of Agronomy & Soils, AuburnUniversity, Auburn, AL 36830. Received 1 Oct. 1979. Approved11 Mar. 1980.

2 Professor of Soils, Research Assistant, Research Associate,Auburn Univ., and Soil Scientist, USDA; respectively.

Particle-size distribution

Soil

Dothan loamy sand (PlinthicPaleudults)

Lucedale sandy loam (RhodicPaleudults)

Decatur silty clay loam (RhodicPaleudults)

Boswell clay (Vertic Paleudalfs)

Sand

82

61

1331

Silt

13

20

5827

Clay

5

19

2942

Fieldcapacity

6.7

13.3

19.526.8

733

734 SOIL SCI. SOC. AM. J., VOL. 44, 1980

Fig. 1—Apparatus for column displacement: (A) glass column;(B) glass rod and rubber stopper; (C) collection tube; (D)rubber-stopper assembly.

Centrifuge with CClt—The method of Mubarak and Olsen(1976) was adapted to fit available centrifugal equipment.Moist soil was packed into 50-ml centrifuge tubes; the tubeswere filled with CC14 and centrifuged for 1 hour at 22,000 g(maximum speed of centrifuge). The supernatant water wasremoved by pipette and filtered to remove organic debris.Solution pH was measured immediately, and the solution wasthen placed in polyethylene bottles.

RESULTS AND DISCUSSIONThe volume of soil solution recovered by the various

methods was greatly influenced by soil texture (Ta-ble 2). In this particular comparison, column dis-placement recovered more solution per gram of soilthan the centrifuge methods, except for the loamysand; the loamy sand could not be packed tight enoughto prevent early break-through of the CNS~ solution.The centrifuge method recovered a relatively highportion of the soil solution from the loamy sand andthe sandy loam but not from the finer-textured soils.The centrifuge-CCl4 method failed to provide any solu-tion from the loamy sand, but it did recover adequatesolutions from, the other soils. Undoubtedly, the cen-

Table 2—A comparison of the volumes of soil solutions thatwere recovered by three methods.

Centrifuge}: Centrifuge-CCU

Soil

Columndisplacement

Volume Volume VolumeDrywt. solution Drywt. solution Drywt. solutionof soil collected of soil collected of soil collected

Dothan IsLucedale siDecatur siclBoswell c

g750750780500

ml10404075

g1,0001,000

880650

ml3015106

g600640640480

ml0

151010

-Soil Cylinder

-Perforated Plate

-Collection Cup

,••—Collection Cup Plate

t One column.t One soil container.§ Sum of eight soil containers.

Fig. 2—Plexiglass apparatus for centrifuge: perorated platewas sealed to bottom of soil cylinder; collection-cup platewas sealed to bottom of collection cup.

trifuge-CC!4 method was handicapped in this compari-son because the available centrifuge equipment limitedthe size of tubes to 50 ml. Larger tubes would be ex-pected to deliver larger solution volumes.

The time required for collecting enough soil solu-tion for analysis is probably somewhat greater for thecolumn-displacement method than for the centrifugemethods. Column displacement generally required 3to 8 hours; Gillman's (1976) centrifuge method re-quired 2 hours; the centrifuge-CCl4 method required1 hour. The major disadvantage of the column-dis-placement method is the constant attention it requiresduring solution collection.

In order to provide a relatively wide range in solu-tion composition as an additional test of the validityof the methods, fertilizer nutrients were added to eachsoil and allowed to equilibrate. A comparison of thecomposition of soil solutions obtained by differentmethods under a range of electrolyte contents was themajor concern of this experiment. Within experi-mental error, the three methods generally recoveredsoil solutions of identical composition (Table 3). Someof the very low concentrations of Al and P appear todiffer considerably, but those values were of very lowprecision because they were pressing the detectionlimits of the analytical methods. Calcium, magnesium,potassium, and sulfate compositions were strikinglysimilar for all three methods. There were minor varia-tions in pH, which may well have been associated withdissolved CO2 contents (solution pH rose upon stand-ing). The only serious discrepancy among all the datawas pH and Al for the Dothan loamy sand (fine-loamy,siliceous, thermic Plinthic Paleudult). Unfortunately,there was inadequate soil sample for a second determi-nation, but the differences are believed to have beenan artifact because of the close agreement of all otherpH and Al measurements.

CONCLUSION

The soil-solution composition was not affected bythe method used to separate the solution from the solid

ADAMS ET AL.: COMPARISON OF METHODS FOR OBTAINING SOIL SOLUTIONS 735

Table 3—A comparison of soil-solution composition obtained by different methods from four soils at different fertilizer levels.No fertilizer Fertilizer 1 Fertilizer 2

Chemical component

pHCa,mAfMg,mMK,mMNH,,mMAluAfSO., mAfVOt.fM

PHCa, mAfMg,mMK,mAfNH,,mAfAlpMSO,, mAfPO,,^M

Colt

5.050.880.330.370.141.50.281.3

5.121.700.790.460.011.50.261.3

Centt

4.630.900.330.370.151.10.280.6

5.021.580.750.450.01

<0.40.251.0

cat

--------

5.041.680.750.430.01

<0.40.251.0

Cot.

4.7611.410.27.08.9

10.78.4

12.6

4.6824.58.81.85-

3.01.38

72

Cent.Dothan loamy sand

4.4311.610.57.18.4

17.48.5

14.5Lucedale sandy loam

4.7224.89.01.89-

1.51.40

69

CC1.

--------

4.6722.28.01.88-1.31.32

71

Col.

--~----.-

4.609.85.91.911.16

590.892.9

Cent.

--------

4.6410.16.22.021.19

670.832.3

CC1.

--------

4.7810.16.01.841.22

710.831.9

Decatur silty clay loampHCa, mAfMg, mAfK,mAfNH,,mAfAllMSO,, mAfPO,,^Af

pHCa,mAfMg,mAfK,mAfNH,,mAfAl,^AfSO,, mAfPO.,^Af

5.341.650.500.220.06

<0.40.271.0

4.480.400.210.130.038.10.260.6

5.431.430.460.200.05

<0.40.260.6

4.560.450.210.100.008.50.261.0

5.491.550.460.220.07

<0.40.290.6

4.660.550.290.120.008.50.261.0

5.1020.5

4.670.94-

0.41.39

59

3.854.552.380.430.46

1220.53

15.8

5.1618.84.751.09-

1.51.30

61Boswell clay

3.904.552.420.450.49

1210.53

13.2

5.1020.94.831.11-

0.41.43

58

3.794.482.290.450.42

1240.53

14.8

4.808.22.960.960.363.30.831.3

3.951.701.920.370.31

870.512.9

4.737.82.960.930.313.30.791.3

4.062.051.920.340.31

830.542.3

4.788.93.250.980.334.10.831.0

4.161.952.000.400.32

700.513.2

T Col. = column displacement, Cent. = large centrifuge cups, and CC1, = centrifuge-CCl, method.

phase. Column displacement requires a degree of skillthat the novice must acquire for its successful applica-tion; it is not well-suited for sands and loamy sandsbut is well-suited for soils that contain enough clayto support aggregation. Gillman's (1976) centrifugemethod requires no special skills and is easy to use;it works on all soils, but it is particularly good oncoarse-textured soils. It has the additional advantageof being nondestructive on the sample, and the soilcan be used subsequently for other studies. The cen-trifuge-CCl4 method was not evaluated in its best lightbecause of equipment limitations. There is reason tobelieve that larger cups and a greater centrifugal forcewould have produced more solution per gram of soil.Methods can be modified to provide the requiredvolume of soil solution. The significant conclusionfrom these experiments is that ionic compositions ofsoil solutions obtained by the three methods were es-sentially the same.