Soil Science Society of America Journal 65:1593-1598 (2001)
© 2001 Soil Science Society of America
DIVISION S-1 - SOIL PHYSICS
Laboratory Compaction of Soils using a Small Mold Procedure
M. Díaz-Zorita*,a,
J. H. Groveb and
E. Perfectc
a EEA INTA General Villegas, CC 153 (B6230 ZBA), General Villegas, Buenos Aires, Argentina
b Dep. of Agronomy, Univ. of Kentucky, Lexington, KY 40546-0091
c Dep. of Geological Sciences, Univ. of Tennessee, Knoxville, TN 37996-1410
* Corresponding author (zorita{at}inta.gov.ar)
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ABSTRACT
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The compactability of a soil can be determined from parameters derived from laboratory compaction curves generated using the Proctor test. However, this is a destructive, time-consuming, and labor-expensive procedure. Our objective was to evaluate a new more rapid test to determine the maximum dry bulk density (
Maxb) and soil water content at Maxb (SWCMax) from laboratory compaction curves using a mold with smaller dimensions than established for the standard procedure. Laboratory compaction curves were developed for nine soils with clay contents ranging from 232 to 385 g kg-1 using the standard Proctor test procedure and the proposed procedure, which uses a 54-mm diam. mold and a 24.4 N rammer dropped from a height of 305 mm to produce a compactive effort (CE) of 109 kJ m-3 drop-1. At a CE of 545 kJ m-3, which is similar to the 540 kJ m-3 produced by the standard test, the Maxb and SWCMax parameters derived from the proposed procedure were positively and significantly correlated with those derived from the standard Proctor test. With both procedures, Maxb decreased and SWCMax increased as the soil clay content increased. The small mold method did not affect the linear relationship between soil clay content and Maxb. However, different relationships between SWCMax and clay content were observed, depending on the compaction procedure. Use of the small mold procedure requires less dry sieved soil, saves time, and labor in evaluating soil compactability. Based only on the reduction in rammer drops, use of the small mold procedure involves 15 times less labor requirements than the standard Proctor test.
Abbreviations: CE, compactive effort • SWC, gravimetric soil water content • SWCMax, gravimetric soil water content at maximum dry bulk density • TOC, total organic C • b dry bulk density • Maxb, maximum dry bulk density
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INTRODUCTION
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COMPACTION OF AGRICULTURAL SOILS is an important problem affecting not only root growth and crop productivity (Unger and Kaspar, 1994; Boone and Veen, 1994), but also energy requirements for tillage operations (Larson et al., 1994) and the infiltration and movement of water (Horton et al., 1994), as well as erosion and pollution processes (van Ouwerkerk and Soane, 1994). Soil compactability depends on both intrinsic and transient soil properties like texture, soil organic matter, and soil moisture (Ekwue and Stone, 1995; Kay et al., 1997).
The Proctor test is widely used to characterize soil compactability (Zhang et al., 1997; Aragón et al., 2000; Barzegar et al., 2000). For this test, laboratory compaction curves are developed and then maximum dry Maxb values are interpolated (Proctor, 1933; American Society for Testing Materials, 2000). The Proctor test has also been proposed as a reference procedure for the calculation of relative bulk density when characterizing the state of soil compactness in relation to crop productivity (Carter, 1990; da Silva et al., 1994; Hakansson and Lipiec, 2000). The Proctor test is a relatively cheap procedure to characterize soil compactability when compared with approaches like the oedometric test (Burke et al., 1986). Nevertheless, the standard Proctor test requires large amounts of soil, is time-consuming, and is labor intensive because it requires measurement of the compaction response of soil to an imposed mechanical stress repeated over a range of water contents (Horn and Lebert, 1994).
The ratio between the surface area of the rammer and the surface area of the soil in the mold in the standard Proctor test is 
0.25 (American Society for Testing Materials, 2000). During compaction, the entire soil surface is only partially contacted by each direct impact of the rammer. Theoretically, the applied pressure diminishes with increasing distance from the rammer edge. Using a smaller mold with a surface area close to that of the rammer, the force applied with each blow is dissipated entirely downward with minimal horizontal spread. Consequently, when a small mold is used, a lower CE may be required to obtain the parameters similar to those found using the standard Proctor test. The smaller mold would require less soil and may involve less work.
Our objective was to evaluate the ability of a small mold procedure to generate parameters derived from laboratory compaction curves comparable with those generated using the standard Proctor test procedure.
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MATERIALS AND METHODS
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Soil samples were collected from the 0- to 10-cm depth at nine sites in Union County, KY (Table 1). After air drying (20 d) until the gravimetric (mass of water per mass of solids) SWC (Gardner, 1986) was < 25 g kg-1, the samples were manually crushed and sieved through a 4.76-mm screen. The particle-size distribution (clay, silt, and sand) and the total organic C (TOC) contents were determined on air-dried samples sieved through a 2-mm screen using the pipette method (Gee and Bauder, 1986) and dry combustion with a TOC-5000A Total Organic Carbon Analyzer (Shimadzu Corporation, Kyoto, Japan), respectively.
The mold dimensions and applied energy differences between the standard Proctor test and the proposed small mold procedure are presented in the Table 2. A minimum of five different SWC was used in developing the laboratory soil compaction curves. For the standard Proctor test, subsamples of 3 kg of dry soil were moistened by spraying with tap water to give each of the different SWC. After equilibrating at room temperature (20°C) for 24 h, samples were compacted in three layers according to the American Society for Testing Materials (2000) procedure. Only a single CE (540 kJ m-3) was performed at each moisture content.
When the small mold was used, subsamples of 130-g air-dried and sieved soil were placed in a plastic bag and sprayed with tap water to give the desired water content. Then, the samples were manually mixed, avoiding major disruption of the aggregates, and the bags were closed and left to equilibrate at room temperature (22 °C) for 24 h.
The moist soil was poured from the bag into a set of two small Al molds, which were placed over a metal disc inside a wooden mold container (Fig. 1)
. The compaction was performed by placing the rammer used for the Proctor test procedure over the small mold, with the walls of the driving cylinder aligned with the walls of the small mold and dropping the rammer from full height. Four different CEs (109, 218, 436, and 545 kJ m-3) were applied depending on the number of full height rammer drops (1, 2, 4, and 5 drops respectively). The metal disc placed at the bottom of the mold served as a push plate to facilitate removal of the mold after compaction. A wooden dowel was inserted through a hole in the bottom of the mold container and pushed upwards.
After compaction (using both procedures), the moist soil contained in the lower mold was weighed, oven dried (105 °C, 24 h), and then weighed again to determine gravimetric SWC and dry bulk density (b).
The parameters Maxb and SWCMax were estimated after fitting a quadratic model to the relationship between b and SWC (the laboratory soil compaction curve) using procedure NLIN of PC-SAS (SAS Institute Inc., 1997). All of the fits converged and had adjusted R2 values >0.85. At Maxb, the slope of the compaction curve is equal to zero (Fig. 2)
, and if,
 | [1] |
then, from the first derivative of Eq. [1], the SWC at Maxb can be estimated using the following equation,
 | [2] |
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Fig. 2. Examples of laboratory soil compaction curves for Proctor and small mold procedures determined at a compactive effort of 540 kJ m-3 (Proctor mold procedure) or 545 kJ m-3 (Small mold procedure). Data are for site C.
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Setting SWCMax equal to SWC in Eq. [1] yields the following expression for Maxb,
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Relationships between parameters from the soil compaction curves (Maxb and SWCMax) determined using the standard Proctor test and those from the proposed small mold procedure were established using regression analysis (Analytical Software, 2000). Stepwise multiple regression analysis was done to assess the influence of soil properties and compaction method, as a class variable, on Maxb and SWCMax estimated at a similar CE. Comparisons of the slope (m) and intercept (n) parameters from linear regression equations 
between texture parameters (x) and both Maxb and SWCMax (y) were performed according with Snedecor and Cochran (1989) using Statistix7 (Analytical Software, 2000).
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RESULTS AND DISCUSSION
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The Maxb values estimated using the standard Proctor test and the small mold procedures were positively related, with a high level of statistical significance and close to a 1:1 relationship, when the measurements were performed with a CE of 545 kJ m-3 (Fig. 3)
. The SWCMAX values estimated using both procedures were related positively and significantly and close to a 1:1 relationship only when the measurements were performed with CE of 436 or 545 kJ m-3 (Fig. 3). Both soil compaction parameters (Maxb and SWCMAX) estimated using the standard Proctor test and the small mold procedures were close to a 1:1 relationship when the measurements were done with a CE level similar to the one for the standard Proctor test, i.e., 540 kJ m-3 (Fig. 3). Compacting soil inside the small mold, where energy is applied over the whole soil surface with each drop, as compared with the Proctor test, where only about one quarter of the soil surface is affected by an individual rammer blow, did not have any significant effect on the parameters derived from soil compaction curves. These results imply that similar soil compaction parameters can be predicted using either the standard Proctor test or the small mold procedures at a similar CE level.
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Fig. 3. Effect of four compactive efforts (CE, kJ m-3) on the relationships between Maxb or SWCMax values estimated using the standard Proctor test procedure and the small mold technique.
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Soil texture (clay and sand contents) was significantly related to the compactability parameters (Maxb and SWCMax) estimated using the small mold procedure with a CE of 545 kJ m-3. Wagner et al. (1994), Ball et al. (2000), and Díaz-Zorita and Grosso (2000) described similar trends between sand and clay contents with Maxb and SWCMax in soils from the USA, United Kingdom, and Argentina respectively. These researchers and others, for example De Kimpe et al. (1982) and Thomas et al. (1996), also described strong relationships between compactability parameters and TOC. In our study, although TOC correlated negatively with Maxb and positively with SWCMax, TOC was not selected in the stepwise multiple regression analysis. The effect of TOC on Maxb and SWCMax did not seem to be important, probably because it is a covariate with clay and sand contents. Independent of the laboratory compaction procedure, increasing the sand content or decreasing the amount of clay caused the Maxb to increase,
 | [4] |
where SAND and CLAY represent the soil sand and clay contents in units of g kg-1. In Fig. 4
, we present the relationships between soil clay content and Maxb derived from either the Proctor test or the small mold procedure and the predicted values from Eq. [4] for the maximum and minimum sand contents determined in this study (Table 1). There were no significant differences between the two procedures in terms of the values of the regression parameters (slope, P = 0.74 and intercept, P = 0.64).
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Fig. 4. Effect of clay content on the maximum bulk density (Maxb) parameter determined using the standard Proctor test or the small mold procedure at a compactive effort of 545 kJ m-3 and predicted values from Eq. [4] for two sand contents.
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The SWCMax values were positively related to the clay content of the soils and the laboratory compaction procedure,
 | [5] |
where METHOD is equal to zero or one when the Proctor test or the small mold procedure was used, respectively. The SWCMax increased as the clay content increased (Fig. 5)
. However, the relationships between SWCMax and clay content exhibited significant differences in their linear regression parameters (slope, P = 0.12 and intercept, P = 0.04), depending upon the compaction procedure used to generate. The SWCMax values derived from the Proctor test were less responsive to soil clay content than those derived from the small mold procedure. The difference between observed and predicted SWCMax values (residuals) was not homogenous among samples. The greater variability in residuals values was observed for samples with the highest clay content, the Nolin soils (fine-silty, mixed, active, mesic Dystric Fluventic Eutrudepts) (Table 1). When samples G, H, and I were not included in the regression analysis between SWCMax and clay content, there were no significant differences between the standard Proctor and the small mold procedures in terms of the regression parameters (slope, P = 0.13 and intercept = 0.17). The resultant relationship between SWCMax and clay content was,
 | [6] |
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Fig. 5. Effect of clay content on water content at maximum bulk density (SWCMax) parameter determined using the standard Proctor test or the small mold procedure at a compactive effort of 545 kJ m-3 and predicted values from Eq. [5] for the two laboratory compaction procedures.
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Our results indicate that the small mold procedure, at a CE similar to that of the standard Proctor test, can be employed to estimate soil compactability. When performing the small mold procedure, 15 times less drops of the rammer (less labor) and almost 10 times less dry sieved soil sample are required.
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CONCLUSIONS
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The Maxb of soils with clay contents ranging from 237 to 387 g kg-1 and TOC's ranging from 11.1 to 26.3 g kg-1 was estimated accurately from laboratory soil compaction curves performed with a 68.7 cm3 mold. The SWCMax of soils with clay contents smaller than 370 g kg-1 could also be estimated accurately using the proposed small mold procedure. This procedure permits a great reduction in the amount of labor and soil required. The use of less soil may permit a more precise evaluation of soil compactability using more soil moisture points on the compaction curve. The small mold procedure should allow researchers to investigate the relationship between SWC and b in small plots with less soil disturbance and at a significant reduction in labor. Further research is required to determine the suitability of this procedure over a wider range of soil properties.
Received for publication January 31, 2001.
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ABSTRACT
INTRODUCTION
