Short communication
Effect of soil texture, organic carbon and water retention on
the compactability of soils from the Argentinean pampas
M. DõÂaz-Zorita
a,b,*, G.A. Grosso
aaEEA INTA Gral.Villegas, CC 153 (6230) Gral.Villegas, Argentina
bDepartment of Agronomy, University of Kentucky, N-122 Agricultural Science Center North, Lexington, KY 40546-0091, USA
Received 12 February 1999; received in revised form 31 August 1999; accepted 16 December 1999
Abstract
The Mollisols from the Pampas Region of Argentina have been described as presenting different compactability behaviors under agricultural systems. The purpose of this study was to provide information on the inherent soil factors related to the susceptibility to compaction. Total organic carbon (TOC), texture (CLAY, SILT, SAND), water retention at 0.33 kPa (WR), Proctor maximum bulk density (BDMAX) and critical water content (CWC) were determined for 26 top-soils selected from the
semiarid, subhumid and humid Pampas Region. The selected sites crossed three soil textures (loamy sand, loam and silty loam). It was observed that the compactabilty of these soils could be assessed by inherent properties routinely measured in the laboratory. Their behavior under experimental compaction revealed that TOC had a dominant effect on the susceptibility of these soils to compaction. Higher TOC levels reduced the BDMAX (r20.75, p<0.01) and the CWC (r20.82, p<0.01),
independently of textural class. The increase in BDMAXper unit of water content (susceptibility to compaction, SC) depends
on SILT and TOC levels (r20.49,p<0.01). Although the BDMAXlevels were lower than those considered critical for rooting
and diminished with increasing TOC, water and SILT contents, in addition to TOC, can determine the potential for compaction in these soils. Based on the need to prevent shallow compaction, continuous no-till systems are possible on soils from the semiarid Pampas Region but not in soils from the subhumid and humid Pampas Region with low TOC.#2000 Elsevier Science B.V. All rights reserved.
Keywords:Compaction; Soil organic carbon; Texture; Bulk density
1. Introduction
The most intensively cropped soils of the Pampas Region (Argentina) are Mollisols developed from
deep aeolian sediments under grassland with prevail-ing udic and thermic temperature regimes (Soriano et al., 1991). The intensi®cation of cultivation during the last few decades has led to soil erosion problems. Losses of organic matter and ®ne sized mineral par-ticles, formation of crusts, development of compacted layers and irregularity in crop yields have been described as the consequences of soil degradation. Rotations between annual crops and perennial pas-tures or no-till agricultural systems have been pro-posed to diminish erosion and to enhance or conserve
*Corresponding author. Present address: Department of
Agron-omy, University of Kentucky, N-122 Agricultural Science Center North, Lexington, KY 40546-0091, USA. Tel.:1-606-257-3655; fax:1-606-257-2185.
E-mail addresses: mdzori2@pop.uky.edu (M. DõÂaz-Zorita), zor-ita@inta.gov.ar (M. DõÂaz-Zorita), ggrosso@servicoopsa.com.ar (G.A. Grosso).
the fertility of these soils (Hall et al., 1992). However, a main concern under continuous no-till soils is soil compaction. Senigagliesi and Ferrari (1993) reported wheat (Triticum aestivumL.) and corn (Zea maysL.) grain yield decreases in no-till soils from the Rolling Pampas Region, probably related to shallow compac-tion. This perceived consequence has limited no-till adoption in the Pampas Region. Stengel et al. (1984) suggested that soils with low clay contents and high proportions of sand or silt were likely to be proble-matic with no-tillage systems. Results from studies developed in the semiarid Pampas Region show that no-till systems do not affect the bulk density (BD) of those soils (Kruger, 1996; Buschiazzo and Panigatti, 1996). Other reports from the humid and subhumid Pampas Region show higher BD in no-till soils as compared to plowed soils (DõÂaz-Zorita, 1996; Ferrari, 1997).
Soil compaction susceptibility or compactability is de®ned as the vulnerability of a soil to compaction. This depends on inherent factors, such as soil proper-ties, climate and management practices. The risk to compact a soil can be evaluated with different meth-odologies. The Proctor test for maximum compact-ability (American Society for Testing and Materials, 1982) is one experimental alternative and provides a standardized method for determining the soils resis-tance to compaction over a range of soil water con-tents, and with constant amount of compaction energy. This test has been widely used in different agricultural and forest soils of the world (Howard et al., 1981; Thomas et al., 1996; Smith et al., 1997).
The maximum bulk density (BDMAX) estimated in
the Proctor test has been de®ned as the highest com-pactability value for a particular soil (Stengel et al., 1984). It is strongly related to the presence of small size mineral particles, with water retention properties at low matric potential and with the organic matter content of the soils (Ekwue and Stone, 1995; Kay et al., 1997). Pecorari et al. (1993) concluded that the BD of soils from the Pampas Region could be estimated from inherent properties and that the organic matter had a greater effect on the BDMAX values than the
mechanical composition (clay and ®ne silt). Studies in the Pampas Region show that, under similar textures, the top layer of agricultural systems under no-till management practices or under pastures has lower BDMAXthan those with other tillage systems
(DõÂaz-Zorita, 1996; Kruger, 1996; Quiroga et al., 1996). The authors state that these differences are due to higher organic matter contents in soils under no-till or pas-tures than soils under conventional tillage systems.
The objective of this study was to determine the relationships, in representative soils from the Pampas Region of Argentina, among texture, total organic carbon (TOC) and soil water retention at 33 kPa (WR) with the compactability estimated, under extreme conditions (disturbed samples), using a stan-dardized procedure (Proctor test). The inclusion of soils under different management practices was to obtain a wide range of texture and TOC contents.
2. Materials and methods
This study was conducted on 26 sites from the Pampas Region of Argentina under virgin or agricul-tural soils in order to obtain a wide range of texture and organic matter contents (Fig. 1). Sites 1, 4, 6, 8, 10, 14, 16, 18, 21, 23 and 25 were under virgin conditions, grazed grasslands composed mainly of fescue (Festuca arundinaceaL.). The rest of the sites were under agricultural practices with different tillage
systems and were paired with virgin sites in each soil taxonomic group. Most of the agricultural sites were under short-term no-till or long-term conservation tillage practices. The soils were classi®ed as Entic Haplustolls (sites 1±5), Entic Hapludolls (sites 6 and 7), Typic Hapludolls (sites 8±15), Typic Argiudolls (sites 16±20) and Vertic Argiudolls (sites 21±26).
In each site, the litter was removed and composite soil samples (10 subsamples of 0.2 m0.2 m) were taken, with a ¯at shovel, in the 0±20 cm depth range. In the sites under agricultural practices, the samples were taken from interrows (0.7 m width). After air drying the samples were passed through a 2 mm sieve. Soil particle size distribution was described in terms of the percentage of clay (<0.002 mm), silt (0.002± 0.05 mm) and sand (0.05±2.00 mm). Soil samples were pre-treated with hydrogen peroxide (30 m3 100 mÿ3
) and hydrochloric acid (10 m3 100 mÿ3
) and the size fractions were determined by the pipette method (Day, 1965) after treatment with sodium hexametaphosphate. TOC was determined by wet-oxidation (Nelson and Sommers, 1982). The soil water retention at 33 kPa (WR) was estimated from the soil water content (gravimetric method) of centrifuged disturbed samples (2500 rpm for 1 h) after 24 h of water saturation (Schlichting et al., 1995).
BDMAX was determined according to the standard
ASTM method (American Society for Testing and Materials, 1982), commonly known as the Proctor test. Approximately 1 kg of 2 mm sieved soil was split into three separate portions. After compaction of the ®rst portion, successive portions were added to the cylinder and compacted in the same way. Each portion was compacted by 25 blows of a 2.5 kg drop-hammer from a height of 40 cm. The compaction test was conducted at a minimum of ®ve and occasionally six soil water contents (SWCs) to obtain a relationship between BD and SWC (gravimetric method). The BD was plotted against SWC and the BDMAXwas
esti-mated as the intersection point of the ascending and descending regression lines between soil water con-tent and BD in the proctor curves. The water concon-tent at which BDMAXwas achieved was termed the critical
water content (CWC). The slope of the ascending regression line between SWC and BD was termed the susceptibility to compaction (SC).
Multiple regression analysis (Analytical Software, 1996) was done to assess the in¯uence of TOC, CLAY,
SILT, SAND, WR contents and their interactions on BDMAX, CWC and SC. The samples were grouped
according to their texture (loamy sand, loamy, loamy silt) and the differences in soil variables between each group were compared with the mean differences ana-lysis test.
3. Results and discussion
The selected sites crossed three soil textures, loamy sand (sites 1±5), loam (sites 6±17) and silty loam (sites 18±26). This pattern is due to the distribution of the aeolian sediments from which soils of the Pampas Region developed, resulting in progressively ®ner texture from southwest to northeast. The TOC levels (4.0±32.0 g kgÿ1
) increase in the same direction as the textural classes and were inversely related with sand content (r0.76,n26,p<0.01). Similar results were reported in this area in response to particle size distribution and climate (Buschiazzo et al., 1991; Alvarez and Lavado, 1998).
The BDMAXvaried among a wide range of values
(1.10±1.72 Mg mÿ3
) with corresponding CWC levels ranging from 138 to 275 g kgÿ1
. The BDMAX was
strongly related to the CWC (Fig. 2). The ®tted parameters in the linear regression model between both compactability indices are similar to those reported by Wagner et al. (1994) for 39 soil samples. Howard et al. (1981) concluded that those soils that have ®eld capacity WR levels near or above the CWC are more likely to be compacted than those with ®eld capacity WR levels below the CWC. In this study, if
Fig. 2. CWC and BDMAXof 26 top soils from the Pampas Region,
we consider ®eld capacity as the water content at a suction of 33 kPa, the CWC was higher than the WR at ®eld capacity in the loamy sand and loamy soils, but was 26% lower in the silty loam soils (Table 1).
Although signi®cant simple linear regressions were obtained between most of the inherent soil properties and the compactability indices (Table 2), in the
step-wise regression analysis only the TOC content was selected to explain BDMAX or CWC variation. The
BDMAX decreased and the CWC increased with
increasing TOC (Figs. 3 and 4). These results are in agreement with those reported by De Kimpe et al. (1982) and by Smith et al. (1997). The presence of organic matter reduces the BD in soils due to the combination of different processes such as dilution, aggregation of particles, elasticity and friction (Soane, 1990).
The ®tted parameters in the linear regression model between BDMAX and TOC are similar to those
reported by Quiroga et al. (1998) for 52 soil samples from the semiarid Pampas Region of Argentina. Ball et al. (1988) also described, on two soils from Scot-land, a reduction in BDMAXof 0.175 per increment of
1% TOC. Table 1
CWC, water retention at a suction of 33 kPa (WR) and relative compaction (RCBD/BDMAX) estimated for soils of the Pampas Region under
continuous no-tillage systems in relation to their textural classa
Soil texture CWC g kgÿ1 WR g kgÿ1 bBD
MAXMg m
ÿ3 cBD Mg mÿ3 RC Mg mÿ3 Author
Loamy sand 149 a 101 a 1.58 a 1.31 0.83 Miglierina et al., 1995
Kruger, 1996 Quiroga et al., 1996
Loam 199 b 186 b 1.51 ab 1.35 0.89 DõÂaz-Zorita, 1996
Taboada et al., 1998
Silt loam 225 c 304 c 1.39 b 1.35 0.97 Garay et al., 1986
Gambaudo and Marelli, 1998
aValues followed by dissimilar letters in each column are signi®cantly different (a0.10). bBD
MAXmaximum bulk density, average of this study in each textural class. cBDpresent bulk density, average of cited studies.
Table 2
Simple regression models between compactability indexes (BDMAX, CWC, SC) and soil properties (g kg
ÿ1) in 26 top soils
from the Pampas Region of Argentina
r2 P
Maximum bulk density (BDMAX, Mg m
ÿ3)
BDMAX1.58ÿ0.0007 CLAYa 0.01 0.230
BDMAX1.65ÿ0.0004 SILT 0.33 0.002
BDMAX1.300.0004 SAND 0.33 0.002
BDMAX1.74ÿ0.0150 TOC 0.75 0.001
BDMAX1.73ÿ0.0012 WR 0.59 0.001
Critical water content (CWC, g kgÿ1)
CWC144.70.34 CLAY 0.19 0.030
CWC131.10.16 SILT 0.55 0.001
CWC262.0ÿ0.15 SAND 0.58 0.001
CWC119.54.49 TOC 0.82 0.001
CWC119.3ÿ0.36 WR 0.69 0.001
Susceptibility to compaction (SC)
SC0.080.00017 CLAY 0.02 0.493 SC0.030.00017 SILT 0.28 0.006
SC0.17ÿ0.0015 SAND 0.25 0.009
SC0.090.0005 TOC 0.01 0.730
SC0.020.0001 WR 0.25 0.009
aCLAYclay content, SILTsilt content, SANDsand
con-tent, TOCtotal organic carbon level, WRwater retention at a suction of 33 kPa.
Fig. 3. TOC and BDMAXof 26 top soils from the Pampas Region,
The SC was signi®catively related to TOC and SILT levels and the model ®tted can be written as
SC0:0553ÿ0:0051TOC0:00033SILT;
r20:49; n26; p<0:01
The SC decreased with increasing TOC and with decreasing SILT levels. Although a strict comparison could not be carried out, previous ®eld measurements on long-term no tilled soils (Table 1) have shown that BDMAX in this study is approximately 17% higher
than the ®eld BD for loamy sand soils, 11% higher for loams and about 3% higher for silt loams. This behavior con®rms the observation that greater incre-ments in silt content of these soils, independent of the TOC, raises SC.
Bulk density has been considered the most direct quantitative measure of soil compaction. The critical levels for root development vary with the soil texture and they increase as the sand content increases (Jones, 1983). This author described equations, considering clay plus silt contents data, to estimate the optimum BD for root development and the BD level at which the rooting was 20% of the maximum growth. Accord-ing to these models the BDMAXvalues in the present
study were approximately 11% higher than optimal for rooting, and 7% below the value that would reduce root growth by 20%.
4. Conclusions
This study con®rms that the compactability of the top layer in soils from the Pampas Region of
Argen-tina can be assessed by inherent properties routinely measured in the laboratory. The behavior of these soils under experimental compaction reveals the dominant effect of TOC on mechanical properties. Higher TOC levels reduced compaction independently of soil tex-tural class. However, the development of compaction due to external factors like machinery cannot be based on water content alone (e.g., CWC). It is also neces-sary to consider the TOC and silt content of the pampas soils. According to the risk to develop shallow compaction, continuous no-till systems are possible on soils from the semiarid Pampas Region, but not in soils from the subhumid and humid Pampas Region with low TOC. In addition, for silty soils, where WR>CWC (Table 1), the TOC level should be main-tained high to counteract compactability.
Acknowledgements
We express our thanks to Dr. John H. Grove and Dr. Ed Perfect whose critically reviewed the manuscript. The ®nancial support by INTA Gral.Villegas is greatly appreciated.
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