A Comparison of Column-Displacement and Centrifuge Methods for Obtaining Soil Solutions1. FRED ADAMS, CHARLES BURMESTER, N. V. HUE, AND F. L. ...
Published July, 1980
A Comparison of Column-Displacement and Centrifuge Methods for Obtaining Soil Solutions1 FRED ADAMS, CHARLES BURMESTER, N. V. HUE, AND F. L. LoNG2 water (50 to 75% of field capacity) was added, and the amount of soil wetted was measured after 48 hours (movement of wetting front had essentially stopped). Each bulk sample was divided into subsamples that were differentially fertilized in order to establish different electrolyte levels in the soil solutions. All fertilized soils received the following 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 supplemental addition of Ca(H2PO4)2 at rates of 300, 400, and 600 ppm P was added to separate samples of Lucedale (fine-loamy, siliceous, thermic Rhodic Paleudult), Decatur (clayey, kaolinitic, thermic Rhodic Paleudult), and Boswell (fine, mixed, thermic Vertic Paleudalf) soils, respectively. The treated soils were then wet 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 and analyzed for ionic composition.
ABSTRACT Four 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 with CC1( via centrifugation. The ionic compositions of soil solutions were unaffected by the method used to obtain the solutions. Additional Index Words: phosphate.
cations, aluminum, pH, sulfate,
Adams, F., C. Burmester, N. V. Hue, and F. L. Long. 1980. A comparison of column-displacement and centrifuge methods for obtaining soil solutions. Soil Sci. Soc. Am. J. 44:733-735.
Column-displacement Method—Moist soil was sieved through a 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 drain tube (Fig. 1A); the tube was plugged with glass wool. The column was gently jarred during filling to settle the soil partially. After filling, the soil was compacted by holding a rubber-stopper assembly (Fig. ID) to protect the outlet tube and by repeatedly tapping the column (cushioned by the stopper assembly) against the desk top. The degree of soil compaction required for effective solution displacement had been previously determined by trial and error on separate samples. Column packing requires a great deal of operator skill and experience. The column was placed in a suitable rack, and a collection tube attached (Fig. 1C). The top of the soil column was firmed with a rubber stopper attached to a glass rod (Fig. IB) and about 100 ml of a saturated solution o£ CaSO4 containing 4% KCNS was added atop the column. The first 5 ml of leachate was discarded as a precaution against minor contaminations; the remaining leachate was collected in 5- to 10-ml increments until CNS" appeared in the leachate (detected by spotting a drop of leachate with 5% FeCl3 in 0.1AT HC1). Each increment was tested immediately for pH and then transferred to polyethylene bottles.
ECAUSE SOIL SOLUTION is the medium in which soil B chemical reactions occur and from which plants obtain mineral nutrients, there has been a long-time interest in knowing its chemical composition. However, separating unaltered soil solution from solid-phase soil has been difficult, and numerous methods have been proposed, based on the principles of suction, pressure, displacement, and centrifugation (Adams, 1974). The column-displacement method (Parker, 1921) has been used successfully for many years (Adams, 1974; Benians et al., 1977). A successful centrifuge method was developed much later (Davies and Davies, 1963), and recent improvements in this method suggest that it can be used with less bother than the column-displacement method (Gillman, 1976; Mubarak and Olsen, 1976; Yamasaki and Kishita, 1972). The volume of soil solution needed for chemical analysis varies with the ionic components to be determined, the concentration of these ions, and the analytical methods to be used. Where soil solutions are extremely dilute in several ions of interest, 25 to 30 ml may be required (Adams, 1971; Gillman, 1976). In such instances, relatively large soil volumes may be required to yield adequate solution for a complete analysis. The methods for separating a portion of the soil solution from solid-phase soil are empirical, and they may separate soil solutions of different composition. Therefore, the objective of the work reported here was to compare the ionic composition of soil solutions obtained by two centrifuge methods with the columndisplacement method.
Centrifuge—Gillman's (1976) method, with modified soil containers and solution cups, was followed. A two-tier assembly of Plexiglas tubing and sheets were constructed to fit 600-ml centrifuge carriers (Fig. 2). The soil containers were made from 11.5-cm lengths of Plexilas tubing with an 8-cm i.d. and a wall thickness of 7 mm; solution cups were 2.7 cm long and were sealed at one end with a 7-mm thick piece of machine-fitted Plexiglas. A 1.5 cm thick, circular piece of Plexiglas 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 the solution cup on the other; it was then cemented to one end of the soil container. A sheet of Whatman no. 42 filter paper was placed in the soil container, moist soil was packed into the container, soils were centrifuged for 2 hours at 1,070 g (maximum speed of centrifuge), solution cups were removed, solution pH measured, and solution stored in polyethylene bottles.
Table 1—Particle-size distribution and field capacity of soils.
MATERIALS AND METHODS
Particle-size distribution
Four unfertilized surface soils, differing in texture and waterholding capacity, were bulk-sampled, air-dried, and screened (Table 1). Field capacity was determined on each soil as follows: a weighed amount of air-dried soil was placed uniformly in a 60 by 5 cm glass column, a measured amount of
Soil Dothan loamy sand (Plinthic Paleudults) Lucedale sandy loam (Rhodic Paleudults) Decatur silty clay loam (Rhodic Paleudults) Boswell clay (Vertic Paleudalfs)
1
Contribution from the Dep. of Agronomy & Soils, Auburn University, Auburn, AL 36830. Received 1 Oct. 1979. Approved
11 Mar. 1980.
2 Professor of Soils, Research Assistant, Research Associate, Auburn Univ., and Soil Scientist, USDA; respectively.
733
Sand
Silt
Clay
Field capacity
82
13
5
6.7
61
20
19
13.3
13
58 27
29 42
19.5 26.8
31
SOIL SCI. SOC. AM. J., VOL. 44, 1980
734
-Soil Cylinder
-Perforated Plate
-Collection Cup
,••—Collection Cup Plate Fig. 2—Plexiglass apparatus for centrifuge: perorated plate was sealed to bottom of soil cylinder; collection-cup plate was sealed to bottom of collection cup.
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 tubes were filled with CC14 and centrifuged for 1 hour at 22,000 g (maximum speed of centrifuge). The supernatant water was removed by pipette and filtered to remove organic debris. Solution pH was measured immediately, and the solution was then placed in polyethylene bottles.
RESULTS AND DISCUSSION The volume of soil solution recovered by the various methods was greatly influenced by soil texture (Table 2). In this particular comparison, column displacement recovered more solution per gram of soil than the centrifuge methods, except for the loamy sand; the loamy sand could not be packed tight enough to prevent early break-through of the CNS~ solution. The centrifuge method recovered a relatively high portion of the soil solution from the loamy sand and the sandy loam but not from the finer-textured soils. The centrifuge-CCl4 method failed to provide any solution from the loamy sand, but it did recover adequate solutions from, the other soils. Undoubtedly, the cenTable 2—A comparison of the volumes of soil solutions that were recovered by three methods. Column displacement Soil Dothan Is Lucedale si Decatur sicl Boswell c
Centrifuge}:
Centrifuge-CCU
Volume Volume Volume Drywt. solution Drywt. solution Drywt. solution of soil collected of soil collected of soil collected g
ml
750 750 780 500
10 40 40 75
t One column. t One soil container. § Sum of eight soil containers.
g 1,000 1,000 880 650
ml 30 15 10 6
g 600 640 640 480
ml 0 15 10 10
trifuge-CC!4 method was handicapped in this comparison because the available centrifuge equipment limited the size of tubes to 50 ml. Larger tubes would be expected to deliver larger solution volumes. The time required for collecting enough soil solution for analysis is probably somewhat greater for the column-displacement method than for the centrifuge methods. Column displacement generally required 3 to 8 hours; Gillman's (1976) centrifuge method required 2 hours; the centrifuge-CCl4 method required 1 hour. The major disadvantage of the column-displacement method is the constant attention it requires during solution collection. In order to provide a relatively wide range in solution composition as an additional test of the validity of the methods, fertilizer nutrients were added to each soil and allowed to equilibrate. A comparison of the composition of soil solutions obtained by different methods under a range of electrolyte contents was the major concern of this experiment. Within experimental error, the three methods generally recovered soil solutions of identical composition (Table 3). Some of the very low concentrations of Al and P appear to differ considerably, but those values were of very low precision because they were pressing the detection limits of the analytical methods. Calcium, magnesium, potassium, and sulfate compositions were strikingly similar for all three methods. There were minor variations in pH, which may well have been associated with dissolved CO2 contents (solution pH rose upon standing). The only serious discrepancy among all the data was pH and Al for the Dothan loamy sand (fine-loamy, siliceous, thermic Plinthic Paleudult). Unfortunately, there was inadequate soil sample for a second determination, but the differences are believed to have been an artifact because of the close agreement of all other pH and Al measurements. CONCLUSION The soil-solution composition was not affected by the method used to separate the solution from the solid
735
ADAMS ET AL.: COMPARISON OF METHODS FOR OBTAINING SOIL SOLUTIONS Table 3—A comparison of soil-solution composition obtained by different methods from four soils at different fertilizer levels. No fertilizer
Fertilizer 1
cat
Cot.
-
4.76 11.4
0.14 1.5 0.28 1.3
4.63 0.90 0.33 0.37 0.15 1.1 0.28 0.6
PH Ca, mAf Mg,mM K,mAf NH,,mAf AlpM SO,, mAf PO,,^M
5.12 1.70 0.79 0.46 0.01 1.5 0.26 1.3
5.02 1.58 0.75 0.45 0.01
