Soil compaction is related to management practices
in the semi-arid Argentine pampas
A.R. Quiroga
a, D.E. Buschiazzo
a, N. Peinemann
b,*aEEA INTA-Anguil and Facultad de Agronomı´a, Universidad Nacional de La Pampa, 6300 Santa Rosa, La Pampa, Argentina bUniversidad Nacional del Sur, 8000 Bahı´a Blanca, Argentina
Received 19 September 1997; received in revised form 27 March 1998; accepted 20 April 1999
Abstract
The physical properties of coarse-textured soils in semi-arid regions often deteriorate with use. We hypothesize that compaction is related to the cropping systems employed in accordance with the different water balances of the soils. Surface samples of 52 Entic Haplustolls under three different regimes (24 under continuous cultivation, 18 under rotation with grass leys and 10 virgin soils) were analysed for clay, silt, organic matter and water content, bulk density, resistance to penetration, hydraulic conductivity and susceptibility to compaction. Data were statistically analysed using regression equations and soils were distinguished on the basis of organic matter content and susceptibility to compaction. In soils of similar texture we found resistance to penetration and susceptibility to compaction to be inversely related to organic matter content and therefore higher under continuous cultivation. Hydraulic conductivity was lower in cultivated soils, especially those with a ®ne texture. The results show that in sandy to loam soils an increase of about 5 g kgÿ1organic matter is required to achieve a 0.06 Mg mÿ3 decrease in bulk density at the proctor optimum. The results also indicate that the loss of organic matter occurring in the cultivated soils of the study region makes them more susceptible to compaction, which not only has adverse mechanical effects on plants but also gives rise to a considerable reduction in hydraulic conductivity.#1999 Elsevier Science B.V. All rights reserved.
Keywords: Soil compaction; Bulk density; Organic matter; Management practices; Semi-arid soils
1. Introduction
The soils of the semi-arid region of the Argentine pampas (35±38 S; 63±65 W) consist almost exclu-sively of Mollisols with small inclusions of Entisols and Aridisols (INTA et al., 1980). They were ®rst put to agricultural use about a century ago after being
cleared of their natural vegetation, consisting mainly of deciduous forest and grassland. The main species are Prosopis caldenia Burkar t and dryland bunch-grasses such asStipa L., Poa L., Bromus L., Aristida L. andPanicum L. The latter generally covers approxi-mately 70% of the soil surface. The mean annual precipitation varies between 400 and 700 mm, with the major rainfall occurring during spring and sum-mer. The mean annual temperatures are between 158C and 178C.
*Corresponding author. Fax: +549121942
E-mail address:[email protected] (N. Peinemann)
Although the soils are apparently very similar, predominantly Entic Haplustolls, they differ from one another in terms of their organic and mineral colloid content. The parent materials of these soils are mainly eolic sediments with a low clay and high silt content. Soil variability was originally caused by differences in the composition of the parent material, though the strong erosive processes particularly at the beginning of the agricultural period may have con-tributed to the present soil patterns.
Water content is the dominant factor regulating the development of the physical soil characteristics bear-ing directly on plant growth. Bulk density and pore-size distribution affect the relationship between water, air-®lled porosity and resistance to penetration in soils. Aeration decreases as water content increases, with a concomitant reduction in soil resistance (Letey, 1985); this in turn has a positive effect on plant development. Soils with poor physical characteristics as a result of degradation are likely to require special management in order to revert to favourable condi-tions for plant growth.
One of the principal consequences of intensive soil use is compaction owing to animal or machinery traf®c. Under such circumstances bulk density can reach critical values at which plant roots are unable to penetrate the soil. The obvious susceptibility of agri-cultural soils to compaction leads in many cases to lower crop yield as a result of the effects on plant growth and water movement through the soil. By simply digging in areas of high compaction it is sometimes found that part of a plant's roots grow horizontally. Daddow and Warrington (1983) reported that heavy-textured soils have smaller pore diameters and higher resistance at low bulk densities than coarse-textured soils. These properties are affected not only by clay and silt content, but also by organic matter content. Davidson et al. (1967) found that in similar textures, maximum compaction values varied consid-erably in relation to the small changes in organic matter content produced by different soil management techniques. A study by Thomas et al. (1996) using the proctor test on a total of 36 samples from four Kentucky soils revealed that compactability is nega-tively related to the amount of organic carbon present in the soil.
Texture and organic matter content of soils in the study region are thought to be the main cause of
differences in the principal properties determining plant growth (Quiroga, 1994). Recent quantitative studies of these components show that they vary considerably: claysilt content is between 150 and 600 g kgÿ1, and organic matter levels range from 5 to 50 g kgÿ1. The claysilt: organic matter ratio ranges between 10 and 30. The reason for the variation may lie in differences in local hydrological balances and consequently in variable biological activity and resi-due accumulation, as well as to differences in the degree of organic matter protection through the formation of organo-mineral complexes (Buschiazzo et al., 1991).
Organic matter affects soil structural stability and therefore soil compaction. It in¯uences soil water retention characteristics because of its effects on soil structure and also because it can imbibe water owing to its colloidal nature. Although eolic erosion plays a major role in the deterioration of soils in the semi-arid Argentine pampas, in recent years water erosion has advanced at a faster pace (Covas and Glave, 1988), giving added importance to the study of soil compac-tion. The purpose of the present paper is therefore to study how different management practices affect soil organic matter content, which in turn in¯uences the level of compaction and the water balance of soils in the semi-arid Argentine pampas.
2. Materials and methods
Samples were collected from 52 A horizons of Haplustolls (FAO: Kastanozems) ranging in texture from sand to loam from the Argentine pampas (35± 388S; 63±658W) (Fig. 1). Occupying a total area of approximately 20,000 km2, these soils have in the past come under one of three different management sys-tems: 24 have been under long-term conventional tillage, 18 under rotation and 10 are virgin soils under grazing.
high proportion of crop residues. The more frequent tillage practices involve the use of disk and moldboard ploughs. Rotation generally comprises four years of pasture composed mainly of alfalfa (Medicago sativa
L.) and gramineous plants (Festuca arundinacea
Schreber, Agropyron elongatum Host) followed by another four or more years of grain crops. The virgin soils are not incorporated into agricultural practices and are under natural bush vegetation (Prosopis cal-deniaBurkart) which is used for extensive pasturing. After a morphological description of the soils in each site, pooled samples consisting of ®ve single samples were taken from the upper 20 cm. The fol-lowing determinations were carried out on these sam-ples: texture (pipette and sieve), total organic matter (OM) (Walkley and Black, 1934), maximum bulk density (BD), susceptibility to compaction, moisture equivalent (ME) by centrifugation at 1000 g water saturated soil samples for 40 min (Briggs and Mc Lane, 1907) and moisture limit of maximum sensi-tivity. To determine hydraulic conductivity (Klute, 1986), four cores (244 cm3) were obtained from each soil pro®le at two separate depths (0±15 and 15-30 cm). The compaction curve of the soils was deter-mined using the proctor test as adapted for agricultural soils by Faure and FieÂs (1972). The mean slope of the ascending part of the proctor curve (BD/water content) expresses susceptibility to compaction (SC) (Mettauer et al., 1983).
For determination of maximum bulk density and susceptibility to compaction, 30±50 kg samples of air-dried and sieved (<2 mm) soil were used. The proctor test was carried out according to AASHO Standard
T-99 (Stengel et al., 1984). The soil sample was com-pacted after wetting in a 947 cm3 cylinder, in three equal layers, with 25 impacts per layer using a mass of 2.5 kg from a height of 30.5 cm. This represents a compaction energy of 590 kJ mÿ3. After this treat-ment, the samples were oven-dried at 1058C and the resulting bulk densities and moisture contents plotted to obtain the compaction curve of the soils.
Under dry ®eld conditions, resistance to penetration (O'Sullivan et al., 1987) was determined every 30 cm over a distance of 3 m, taking measurements at inter-vals of 5 cm down to a depth of 50 cm (cone diameter: 12.8 mm; angle 308). Isoresistance lines of 0.4, 0.8, 1.2 and 2.0 MPa were used to represent the pattern of soil resistance. Simultaneously, the water content of 10 cm layers, down to a depth of 40 cm in four of the measured pro®les was determined for each site and averaged in order to relate the ®gures to the penetra-tion resistance values.
3. Results and discussion
Within the population of the studied samples (Fig. 1), the following frequency of textural classes was encountered: sand (3), loamy sand (10), sandy loam (24), and loam (15). Clay content varied between 50 and 250 g kgÿ1, silt between 60 and 490 g kgÿ1 and the sum of both ranged between 120 and 650 g kgÿ1.
The pro®le pairs of similar texture (loam sand (A and B) and loam (C and D)) in Fig. 2 and the different organic matter contents as a result of differing soil use Fig. 1. Textural distribution of the studied Kastanozem soils from
the semi-arid Argentine pampas.
Table 1
Claysilt content, organic matter content and hydraulic conduc-tivity of two soil pairs (A and B) and (C and D) under different management treatments
aA and B: coarse textured soils; C and D: loam soils. bDifferent letters indicate significant differences (p<0.05)
(Table 1) show that higher resistance to penetration and lower subsoil hydraulic conductivity values cor-respond to lower organic matter content.
Comparing the coarse-textured soil pair (A and B), it was found that the cultivated soil (A) showed greater variation in resistance to penetration values despite its higher moisture content of 0.137 kg kgÿ1, compared with 0.114 kg kgÿ1for the rotation soil (B) (Fig. 2). At depths of 20±35 cm the mean resistance to pene-tration of soil A was 1.21 MPa, compared with 0.52 MPa under rotation soil. The former also exhib-ited a very low impedance peak and a more uniform moisture distribution. In cultivated soil under conven-tional tillage, the maximum moisture content was found in the zone of highest impedance, presumably because of the higher proportion of small-size pores, in agreement with the results of Taylor and Brar (1991) who found greater water content in compacted plots.
The heavier loam textured pro®les (C and D) dif-fered considerably in their moisture content (Fig. 2). Cultivated soil (C) had a mean water content of 0.195 kg kgÿ1and virgin soil (D) only 0.106 kg kgÿ1. However, cultivated soils had maximum resistance between 15 and 30 cm depth, whereas in virgin soils the maximum resistance was at the surface and
dimin-ished with depth. The spatial variability of resistance data was higher in the cultivated than in the virgin soil. In concordance with the resistance to penetration, subsoil hydraulic conductivity (depth: 15±30 cm) was signi®cantly lower in cultivated soils (Table 1).
In order to evaluate the mutual dependence between resistance and moisture, periodic determinations of the two parameters were carried out in loamy sands and loam soils covering a range of moisture content between 0.07 and 0.22 kg kgÿ1. The linear regressions obtained indicate that the relationship was only sig-ni®cant when comparing soils of the same depth (R20.46±0.83). Considerable variation between depths in one and the same pro®le and between different pro®les was found, making it dif®cult to compare results (Table 2). The different ploughing depths of alternative tillage systems may give rise to different resistance±moisture behaviour patterns in the various layers.
The proctor test data show that for any compaction energy level it is necessary to de®ne the moisture content of the samples corresponding to the liquid, plastic and solid limits. Although these limits are dependent upon the clay content and its mineralogical characteristics, the behaviour of agricultural soils cannot be explained on the basis of the clay fraction alone (Faure, 1978) since organic matter content also strongly affects the relationship.
The proctor compaction curve of an A horizon from a soil under zero tillage for eight years (Fig. 3) is similar to the theoretical model. In this case, density is an increasing function of moisture content up to the maximum compaction limit (maximum bulk density),
where air porosity is almost completely eliminated. The susceptibility of soil to form impedances can be expressed by a series of parameters normally derived from these curves. Thus, from the curve presented in Fig. 3, the information presented in Table 3 was obtained.
Mettauer et al. (1983) consider the slope of the curve (SCBD/WC) to be the best indicator of susceptibility to compaction, whereas according to GarcõÂa Pita (1985) the difference between maximum bulk density and the initial value of the ascending part of the curve is a better indicator. Stengel et al. (1984) propose BDmax (maximum bulk density) among other parameters, and this value would appear to best represent the least favourable conditions of soil porosity. Perez-Moreira and Diaz-Fierros (1989) indi-cate that this index best discriminates the studied horizons.
The effect of organic matter on BDmax and sus-ceptibility to compaction (SC) is con®rmed in the Table 2
Relationship between resistance to penetration (MPa) and water content (WC) (kg kgÿ1) of loamy sand (Soil 1) and loam soils (Soil 2)
Depth (cm) Regression equationa R2 Std. error Claysilt (g kgÿ1) OMb(g kgÿ1)
Soil 1
0±10 RP0.53ÿ2.19 WC 0.77 0.51 325 12.2
10±20 RP0.39ÿ1.30 WC 0.62 0.44 328 11.6
20±30 RP0.32ÿ1.13 WC 0.75 0.35 347 9.9
30±50 RP0.22ÿ0.91 WC 0.83 0.68 309 7.5
Soil 2
0±10 RP0.62ÿ2.25 WC 0.70 0.64 479 21.7
10±20 RP0.69ÿ2.43 WC 0.46 0.82 471 19.9
20±30 RP0.44ÿ1.68 WC 0.64 0.35 496 17.6
30±50 RP0.28ÿ1.12 WC 0.68 0.15 475 10.6
aRPresistance to penetration; WCwater content. bOMorganic matter.
Fig. 3. Proctor compaction diagram showing maximum bulk density vs. soil water content of a surface soil sample under no till/sod seeding (clay: 160 g kgÿ1, silt: 343 g kgÿ1, organic matter:
31 g kgÿ1). State change limits Wc: plasticity limit or compaction
sensibility, Wm: fluidity limit or water content at maximal tension, BDmax: maximal bulk density.
Table 3
Values of different indexes proposed to characterize the proctor curve of Fig. 3
Wc (kg kgÿ1)
Wm (kg kgÿ1)
BDmax (Mg m3)
BD SC
0.066 0.195 1.42 0.113 0.87
Wc: hydric threshold of compaction sensibility; Wm: hydric content at maximum tenacity; BDmax: maximum bulk density; BD: difference between maximum and minimum bulk density; SCBD/WC: ascendant slope of the Proctor curve, where
following regression equations for all the studied soils:
BDmax1:775ÿ0:0125 OM
R20:67;n52;std:error0:07; (1)
SC2:13ÿ0:04 OM
R20:39;n52;std:error0:36: (2)
Distinguishing on the basis of management systems gives:
for cultivated soils : BDmax1:751ÿ0:0126 OM
R20:52;n24; std:error0:06; (3)
for soils under rotation : BDmax1:914ÿ0:0170 OM
R20:49;n18; std:error0:06; (4)
for virgin soils : BDmax1:554ÿ0:0066 OM
R20:60;n10; std:error0:11; (5)
where it can be seen that the intercept values was lower in the virgin soils. This is expressed in Table 4, where the statistical signi®cance of BDmax in virgin soils as against cultivated and rotation soils is shown. In soils of similar texture, organic matter content depends on the management system (lower in culti-vated than in virgin soils). We found that decreases in organic matter content led to increases in the bulk densities mainly of cultivated and rotation soils rather than of virgin soils. This con®rms the results of Davidson et al. (1967), who observed a close relation-ship between maximum density and organic matter. Stengel et al. (1984) found a regression equation that shows the necessary increases in organic matter to achieve a 0.1 decrease in bulk density at the proctor optimum. Our results show that an increase of
approximately 5 g kgÿ1 in organic matter content produced a decrease of 0.06 in maximum bulk density. The relationship between susceptibility to compac-tion (SC) and organic matter content (OM) serves to clearly distinguish soils in terms of their treatment under different agricultural usage (Fig. 4). Grouping soils according to management type did not increase the signi®cance of the relationship between properties, owing to the narrow range of organic matter found in the samples from each management group.
The relationship between maximum density and moisture equivalent (ME) was expressed by the gen-eral relation
water content at BDmax87:610:057 ME
R20:68;n52;std:error2:37: (6)
A difference was be discerned among soils under different management systems
cultivated soils :
water content at BDmax88:310:051 ME
R20:78;n24;std:error1:77; (7)
under rotation :
water content at BDmax70:210:070 ME
R20:69;n18;std:error1:80; (8)
virgin soils :
water content at BDmax121:400:048 ME
R20:49;n10;std:error3:66: (9) Table 4
Susceptibility to compaction (SC) and maximum bulk density (BDmax) of surface soil samples under different management regimesa
Management n BDmax
(Mg mÿ3)
SC
Agriculture 24 1.57ab 1.61a
Rotation 18 1.53a 1.17b
Virgin 10 1.31b 0.68c
aCalculated from Eqs. (3) and (4) in text.
bDifferent letters indicate significant differences (p<0.05).
Although organic matter content depends on soil texture, this parameter is strongly in¯uenced by soil management. In horizons with a similar texture but under different management systems it was found that those with higher organic matter levels presented lower susceptibility to compaction (BDmax values), as shown in Table 4.
Fig. 5 shows the proctor curves of three textural groups of soils under different management. It can be observed that independent of their texture, virgin soils with higher organic matter content were susceptible to compaction. Their bulk density was lower and they reached maximum density at higher moisture levels when compared with cultivated or rotation soils, owing to the higher organic matter content in top soil horizons where no tillage had been practised. Thus, the tendency was towards less compaction (Thomas et al., 1996; Ball et al., 1996). Soils under rotation showed a slightly different behaviour from continu-ously cropped soils and fall somewhere between agricultural and virgin soils in this respect. This implies that within similar textural groups, different moisture limits were caused by differences in the organic matter level, as indicated by Guerif and Faure (1979).
4. Conclusions
It can be concluded that decreases in organic matter content as a consequence of more intensive soil use under different management systems rendered soil more susceptible to compaction, resulting in massive soil structure in the semi-arid pampa region.
The relationship between susceptibility to compac-tion and soil organic matter content was useful in distinguishing between different soil treatments and grouping soils according to their use. Higher compac-tion values and lower hydraulic conductivity corre-sponded to lower organic matter content. Virgin soils with higher organic matter content were less suscep-tible to compaction; their bulk density was lower; and they reached maximum density values at higher moist-ure levels than soils under cultivation or rotation. Under conventional tillage, the maximum moisture content was found in the zone of highest impedance and the spatial variability of resistance was higher than in soils under more conservative management systems.
The relationship between certain physical proper-ties and organic matter content in soil raises the question of whether or not tillage actually helps in combating soil compaction. On the basis of the results of the present paper we consider tillage to be in many cases inappropriate; the parallel loss of organic matter can have the effect of worsening the problem since re-compaction is likely to result in even higher resistance values.
Acknowledgements
The authors wish to thank to the Consejo Nacional de Investigaciones Cientõ®cas y TeÂcnicas (CONICET) which provided founds and personnel for this research.
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