isolation of soil particle-size fractions1
TRANSCRIPT
Isolation of Soil Particle-Size Fractions1
D. A. GENRICH AND J. M. BREMNER2
ABSTRACT
Studies of the properties of soil particle-size fractions havebeen restricted by the lack of a method of quantitatively iso-lating these fractions without serious modification of theirproperties. A mild method of isolating soil particle-size fractionsfor such studies is described. It involves dispersion of the soilby ultrasonic vibration in water and subsequent isolation ofthe particle-size fractions by sieving, centrifugation, and filtra-tion techniques. The method allows large-scale isolation of soilparticle-size fractions without use of chemical reagents, and itgives almost quantitative recovery of soil material in the frac-tions isolated. Data reported show that the particle-size distri-bution values obtained for 13 soils after isolation of sand, silt,and clay fractions by the method described agreed closely withthose obtained by particle-size analysis after chemical disper-sion by the peroxide-sodium polyphosphate method.
Clay fractions isolated from soils by the physical methoddescribed had higher carbon, nitrogen, sulfur, and phosphoruscontents and contained markedly higher percentages of thetotal carbon, nitrogen, sulfur, and phosphorus in the soilsfractionated than did clay fractions isolated by another physi-cal method that did not quantitatively separate clay-sizedmaterial.
Additional Index Words: ultrasonic vibration, sand, silt, clay,carbon, nitrogen, sulfur, phosphorus.
STUDIES of the properties of soil particle-size fractionshave been greatly restricted by the lack of satisfactory
methods of isolating these fractions without serious modifi-cation of their properties (7, 8, 21). In most investigationsreported, the methods used to isolate the soil-particle sizefractions studied have involved use of physical dispersiontreatments that do not effect complete separation of clay-sized particles or have utilized chemical dispersion treat-ments that can seriously affect the properties of the soilfractions isolated.
The purpose of this paper is to describe a mild physicalmethod of isolating soil particle-size fractions for studies of
their properties. This method involves dispersion of the soilby ultrasonic vibration in water and subsequent isolation ofthe particle-size fractions by sieving, centrifugation, and fil-tration techniques. It allows large-scale isolation of soilparticle-size fractions without use of chemical reagents, andit gives almost quantitative recovery of soil material in thefractions isolated. It is based on the finding that ultrasonicvibration of soil-water suspensions with a probe-type vibra-tor is an effective method for dispersion of, and yields sta-ble suspensions with, a wide range of soils (7, 9). Studies tobe reported elsewhere showed that ultrasonic vibration ofsoil-water suspensions under the conditions used for disper-sion in the method described had very little, if any, effecton various soil properties studied (e.g., pH, water-solubleC, N, S, or P content, and exchangeable Ca, Mg, K, or NH4
content).
MATERIALS AND METHODS
Soils—The soils used (Table 1) were surface (0- to 15-cm)samples selected to obtain a wide range in pH (5.1-7.7),organic-carbon content (1.3-9.0%), and texture (2-52% sand,28-62% silt, 15-42% clay). They included soils formed underforest (no. 1, 8, and 9) and prairie (no. 2-7 and 10-13) vege-tation and soils developed from glacial till (no. 1, 3, 7, and10-13), from alluvium on till (no. 2, 4, and 5), and from loess(no. 6) and lacustrine (no. 8 and 9) deposits. The virgin Supe-rior soil (no. 8) was from a forested site in Calumet County,Wisconsin, and its cultivated counterpart (no. 9) was fromadjacent farmland. The virgin Nicollet and Webster soils (no.10 and 12) were from the Kalsow Prairie, Pocahontas County,Iowa, and their cultivated counterparts (no. 11 and 13) werefrom farmland adjoining this prairie. Before use, all sampleswere air-dried and crushed to pass a 2-mm sieve.
Table 1—Analyses of soils
Num-ber
j23456789
10111213
Soil
Series*
LlndleyClydeHarpsterGlencoeOkobojlMarcusNicolletSuperior VSuperior CNicollet VNicollet CWebster VWebster C
pH
5.85.17.77.06.66.16.77.17.35.95.86.76.0
OrganicC
1.34.24.26.29.03.81.44.52.05.73.36.03.9
N
0. 130.390.350.560.850.340.140.420.200.490. 280.540.31
Total
S
0.0140.0470.0510.0640.1500.0460.0190.0580.0280.0600.0350.0870.043
P
0.0320.1000.0770.0990.0670.0680.0400.0780.0530.0580.0540.0660.056
CaCO,equivalent
005.80.20.5003.62.8000.10
V and C Indicate virgin soil (V) and cultivated counterpart (C).
GENRICH & BREMNER: ISOLATION OF SOIL PARTICLE-SIZE FRACTIONS 223
Analytical Procedures—Total C was determined by themethod of Tabatabai and Bremner (19), inorganic C by themethod of Anderson and Harris (1), total N by the method ofBremner (3), total S by the method of Tabatabai and Bremner(18), total P by the method of Tandon, Cescas, and Tyner(20), and pH by a glass electrode (soil/water ratio, 1:1). Or-ganic C was calculated from total C by subtracting inorganic C,and CaCO3 equivalent was calculated from inorganic C.
Method for Isolation of Soil Particle-Size Fractions—Themethod described here is for isolation of the sand (2000-50/im), silt (50-2 nm), and clay (< 2 /j.m) fractions of soils. Itcan readily be adapted for isolation of other particle-size frac-tions by appropriate modification of the sieving and centrifu-gation techniques described (11).
Place 60 g of air-dry soil in a 250-ml stainless-steel beakercontaining a 3.8-cm Teflon-coated stirring bar and add 150 mlof deionized water. Fit the beaker with a Plexiglas coolingjacket, and connect the jacket to a cold-water tap. Stir the soilsuspension magnetically, insert the tip of the probe of a Bran-son Model W-185C Sonifier (Heat Systems-Ultrasonics, Inc.,Plainview, N.Y.) fitted with a solid probe (tip diameter, 1.27cm) about 3 cm below the surface of the stirred suspension, andvibrate the suspension for 30 min with the power output of theSonifier adjusted to obtain an output meter reading of 100. Theflow of cold water through the cooling jacket of the beakershould be such that the temperature of the vibrated soil suspen-sion does not exceed 20C. Treat additional 60-g samples of soilin the same way until the desired amount of soil has been dis-persed.
To isolate the sand fraction, pour the vibrated soil suspen-sion onto a 300-mesh sieve and collect the <50 /im materialnot retained by the sieve in a 20-liter polyethylene container.Carefully wash the sand-sized (2000-50 ^m) material retainedon the sieve with deionized water (dispensed under slight pres-sure) until no further material passes through the sieve. Trans-fer the sand fraction from the sieve to a beaker, and allow it toair dry. Determine the weight and moisture content of the air-dried material.
To isolate the silt fraction from the silt- and clay-sized mate-rial collected in the 20-liter container, use an InternationalModel UV centrifuge (International Equipment Co., NeedhamHeights, Mass.) equipped with a #976 head and four 600-mIglass centrifuge bottles (Corning no. 1252, Corning GlassWorks, Corning, N.Y.). Calculate the time of centrifugationneeded to separate the silt-sized (50-2 /im) and clay-sized «2jum) particles from the following equation based on Stoke'sLaw(seeref. 11, p. 127):
(63.0xl08)n(log10«/S)[1]
where T is the time for sedimentation in minutes, n is the vis-cosity of water in poises, R is the radius of rotation of the topof the sediment in centimeters, 5 is the radius of rotation of thesurface of the suspension in centimeters, N is the speed of thecentrifuge in rpm, D is the particle diameter in microns, 5P isthe specific gravity of the soil particles, and Si is the specificgravity of water. When n is 0.00894 (25C), R is 20.6 cm, S is12.6 cm, N is 500 rpm, Sp is 2.61 g/cm3, and St is 0.997 g/cm3,the centrifugation time needed to sediment all particles largerthan 2 /im is 7.45 min.
Before centrifugation to isolate the silt fraction, mark thecentrifuge bottles to indicate levels 5 and R, adjust the concen-tration of the suspension containing the silt and clay particles toless than 5% soil particles by weight, thoroughly mix this sus-pension, and allow it to stand undisturbed for 15 min. Then fillthe centrifuge bottles with suspension to the level marked 5(step 1), centrifuge for the time calculated to sediment all par-ticles larger than 2 /im to the level marked R (step 2), removethe suspension between levels JR and S with a vacuum-operatedsiphon (step 3), and store this suspension in a 20-liter polyethy-lene container. Repeat steps 1-3 until all the suspension contain-ing the silt and clay particles has been transferred to the centri-
fuge bottles, and resuspend the sedimented material each timethe bottles are filled. After all of this suspension has been trans-ferred to the centrifuge bottles, continue to repeat steps 2 and3 by resuspending the sediment in deionized water until onlytrace amounts of clay-sized material are being separated. Trans-fer the'silt fraction from the centrifuge bottles to a beaker andallow it to air dry. Determine the weight and moisture contentof the air-dried material.
To isolate the clay fraction, concentrate the suspension con-taining the clay particles to about one-tenth of its original vol-ume by filtration with an auto-irrigator (15) or other ceramicfilter device, and lyophilize the concentrated suspension. De-termine the weight and moisture content of the lyophilizedmaterial.
Before analysis of the sand, silt, and clay fractions isolated,grind them to pass an 80-mesh sieve.
RESULTS AND DISCUSSION
Comments on Method
The Branson Model W-185C Sonifier is a high-intensity,probe-type ultrasonic vibrator equipped with an automatictuning device, a continuously variable power control, anda power-output meter that allows standardization of theamount of power delivered to the probe. It was used for dis-persion in the method described because previous work inour laboratory (9) showed that ultrasonic vibration of soil-water suspensions with this instrument is an effectivemethod for dispersion of, and yields stable suspensionswith, a wide range of soils. The specific conditions adoptedfor dispersion resulted from detailed studies of factors af-fecting ultrasonic dispersion of soils with the Branson in-strument (8). The tip of the probe of this instrument shouldbe polished after every 30 min of use, and the performanceof the vibrator should be checked periodically as describedby Genrich and Bremner (10).
The particle concentration of the suspension containingthe silt and clay particles is adjusted to less than 5% byweight because sedimentation in more concentrated suspen-sions does not obey Stoke's Law. It is recommended thatthis suspension be allowed to stand for 15 min before sepa-ration of the silt and clay fractions by centrifugation so thatthe coarse silt particles settle out and are not transferred tothe centrifuge bottles until the end of the separation proc-ess. An International Universal Model UV centrifuge wasadopted for centrifugation because its #976 head allowscentrifugation of 2 liters of soil suspension at one time,which reduces the time needed to process large volumes ofsuspension. It is recommended that the clay fraction be lyo-philized instead of air-dried because air-drying aggregatesthis fraction so strongly that strenuous grinding is needed toreduce it to a size satisfactory for precise chemical analysis.
Evaluation of Method
To evaluate any method proposed for isolation of soilparticle-size fractions, it is necessary to compare the par-ticle-size distribution values computed from the weights ofthe isolated fractions with corresponding values determinedby an accepted method of particle-size analysis. We usedthe Kilmer-Alexander (12) method of particle-size analysisfor this comparison because it is the most widely acceptedmethod and has been recommended by the USDA-SCS
224 SOIL SCI. SOC. AMER. PROC., VOL. 38, 1974
Soil Survey Staff (16) for particle-size analysis of a widerange of soils.
Table 2 shows that the particle-size distribution valuescomputed for 13 soils after isolation of their sand, silt, andclay fractions by the method described were similar tothose obtained by the method of Kilmer and Alexander(12). Of the many workers who have used physical meth-ods to isolate soil particle-size fractions for studies of theirproperties, only Syers, Shah, and Walker (17), Parasherand Lowe (14), and McKeague (13) have reported infor-mation that permits assessment of the adequacy of their iso-lation procedures. Their results are summarized and com-pared with those obtained in our work in Table 3. The datareported show that the average particle-size distribution val-ues obtained by our method of isolating particle-size frac-tions were virtually identical to those obtained by theKilmer-Alexander method of particle-size analysis, whereasthe physical methods used by Syers et al. (17), Parasherand Lowe (14), and McKeague (13) did not give particle-size distribution values that agreed closely with those ob-tained by accepted methods of particle-size analysis. Thededuction from Table 3 that the physical methods of isolat-ing soil particle-size fractions adopted by these workers didnot give quantitative separation of clay-sized material issupported by the findings of Syers et al. (17) and Mc-Keague (13) that silt fractions isolated by their methodscontained substantial amounts of clay. For example, Mc-
Table 2—Comparison of particle-size distribution values ob-tained for 13 soils by method described with those obtained
by the Kilmer-Alexander method of particle-sizeanalysis (12)
______ Particle-size distributionClay'
number
123456789
10111213
P5
39143510112
526
1019231917
Cl
4115399
102
526
1019231917
P5c
46564145576129605246464551
Cl& ————
44573649626028595248454649
P5
15302445323719343835313632
Cl
15282542283820353833323534
P5, method described In this paper (values reported are corrected for organicAlter); Cl, method of Kilmer and Alexander (12).
Table 3—Comparison of particle-size distribution valuesobtained by different workers for soils dispersed by
physical and chemical methods
Reference
Syers et al. (17)
Parasher andLowe (14)
McKeague (13)
This Investigation
Numberof soils
8
26
17
13
Method ofdispersion*
PIClP2P3C2P4C3P5Cl
Average particle-sizedistribution
Sand
..—_——
29.328.019.820.2
Silt
28.822.1.,——
48.342.548.848.8
Clay
11.115.12.93.9
25.522.429.531.431.0
P, physical method; C, chemical method. PI, mechanical shaking of aolMvatersuspension; P2, manual stirring of soil-water suspension; P3, ultrasonic vibration ofsoil-water suspension (tank-type vibrator); P4, ultrasonic vibration of soil-water sus-pension (probe-type vibrator); P5, method described In this paper; Cl, method ofKilmer and Alexander (12); C2, method of Day (6); C3, method Involving Hp2 treat-ment and Na saturation.
Keague (13) found that fine-silt and coarse-silt fractionsisolated by his method contained about 33% and 10%clay-sized material, respectively.
We have found that clay fractions isolated from soils byphysical methods that do not allow quantitative isolation ofclay-sized material can differ markedly in compositionfrom clay fractions isolated by the method described here.This is illustrated by Tables 3 and 4, which show the resultsof analyses performed on clay fractions isolated from Mar-cus and Nicollet soils by our method and by the methoddescribed by Arshad and Lowe (2), which separates onlypart of the clay-sized material in soils. It can be seen thatthe carbon, nitrogen, sulfur, and phosphorus contents of theclay fractions isolated by the Arshad-Lowe method werelower than those of the clay fractions isolated by ourmethod (Table 4) and that, in consequence of this and ofthe failure of the Arshad-Lowe method to isolate all theclay-sized material in the soils studied, the recoveries ofsoil carbon, nitrogen, sulfur, and phosphorus in the clayfractions isolated by our method were 14-30% higher thanthe corresponding recoveries in the clay fractions isolatedby the Arshad-Lowe procedure (Table 5).
To evaluate methods proposed for isolation of soil par-ticle-size fractions and research performed on fractions iso-lated by these methods, it is clearly important to know therecovery of soil material in the fractions isolated. Unfor-tunately, only a few workers have provided data that allowcalculation of recovery of soil material in soil fractionsstudied. The highest recoveries reported seem to be 90.7-99.6% (average, 96.1%) for eight soils fractionated bySyers et al. (17) and 95.9% and 98.6% for two soils frac-tionated by Chichester (4, 5). Table 6 shows that, with the13 soils used in our work, the recovery of soil material inthe sand, silt, and clay fractions isolated by the method de-scribed ranged from 96.6% to 99.8% and averaged 99.1%.
Table 4—Analyses of clay fractions isolated from soils bydifferent methods
Soilnumber
6
7
Method used to
fraction*
MaP5MSP5
Analyses of clay fractionTotal C
5.876.864.515.02
Total N———————— i
0.5680.6590.4560.512
Totals
( ————————0.07520.08180.06140. 0798
Total P
0.1250.1330.1240.141
* MS, method described by Arshad and Lowe (2) Involving disperalon of soil by manualstirring of soil-water suspension; P5, method described here Involving dispersion ofsoil by ultrasonic vibration of soil-water suspension.
Table 5—-Recovery of soil components in clay fractionsisolated from soils by different methods
Soil component
Oven-dry material
Total C
Total N
Totals
Total P
Method used toIsolate clay
fraction*
MSP5MSP5MSP5MSP5MSP5
Recovery of componentIn clay fraction
Soil 6
27.339.142.270.646.676.550.569.753.076.0
Soil?
15.720.349.971.853.077.054.283.556.971.2
MS, method described by Arsbad and Lowe (2) Involving dispersion of soil by mam***?stirring of soil-water suspension; PS, method described here Involving dispersion ofsoil by ultrasonic vibration of soil-water suspension.
SOLTANPOUR ET AL.: SOIL PHOSPHORUS AVAILABILITY 225
Table 6—Recovery of soil material in sand, silt, and clayfractions isolated from 13 soils by method described
Recovery of oven-dry soil materialSoil
number
123456789
10111213
Sandfraction
38.113.132.29.1
10.11.7
50.26.09.8
18.221.917.816.0
Siltfraction
44.954.138.443.155.758.729.157.950.745.144.642.848.5
Clayfraction
16.032.426.047.633.338.820.135.338.835.832.938.835.2
Total
99.099.696.699.899.199.299.499.299.399.199,499.499.7
With 12 of the 13 soils used, the recovery equalled or ex-ceeded 99%. Carbonate analyses of the sand, silt, and clayfractions isolated from the Harpster soil showed that thecomparatively low recovery (96.6%) with this calcareoussoil resulted partly from dissolution of CaCO3 during isola-tion of these fractions.