Effect of soil texture, organic carbon and water retention on the compactability of soils from the Argentinean pampas

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1 Soil & Tillage Research 54 (2000) 121±126 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 a a EEA INTA Gral.Villegas, CC 153 (6230) Gral.Villegas, Argentina b Department of Agronomy, University of Kentucky, N-122 Agricultural Science Center North, Lexington, KY , USA Abstract Received 12 February 1999; received in revised form 31 August 1999; accepted 16 December 1999 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 (BD MAX ) 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 BD MAX (r 2ˆ0.75, p<0.01) and the CWC (r 2ˆ0.82, p<0.01), independently of textural class. The increase in BD MAX per unit of water content (susceptibility to compaction, SC) depends on SILT and TOC levels (r 2ˆ0.49, p<0.01). Although the BD MAX levels 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 * Corresponding author. Present address: Department of Agronomy, University of Kentucky, N-122 Agricultural Science Center North, Lexington, KY , USA. Tel.: ; fax: addresses: mdzori2@pop.uky.edu (M. DõÂaz-Zorita), zorita@inta.gov.ar (M. DõÂaz-Zorita), ggrosso@servicoopsa.com.ar (G.A. Grosso). deep aeolian sediments under grassland with prevailing 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 particles, 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 pastures or no-till agricultural systems have been proposed to diminish erosion and to enhance or conserve /00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved. PII: S (00)

2 122 M. DõÂaz-Zorita, G.A. Grosso / Soil & Tillage Research 54 (2000) 121±126 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 aestivum L.) and corn (Zea mays L.) grain yield decreases in no-till soils from the Rolling Pampas Region, probably related to shallow compaction. 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 problematic 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 properties, climate and management practices. The risk to compact a soil can be evaluated with different methodologies. The Proctor test for maximum compactability (American Society for Testing and Materials, 1982) is one experimental alternative and provides a standardized method for determining the soils resistance to compaction over a range of soil water contents, 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 (BD MAX ) estimated in the Proctor test has been de ned as the highest compactability 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 BD MAX 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 BD MAX than 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 pastures 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 standardized 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 agricultural 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 arundinacea L.). The rest of the sites were under agricultural practices with different tillage Fig. 1. Location of the selected sites (1±26) within the subdivisions of the Pampas Region (Aˆrolling pampa, Bˆ at pampa, Cˆinland pampa, Dˆ ooding pampa, Eˆmesopotamic pampa) showing boundaries for the subregions (*). Provinces lying partly within the area of interest are named and their boundaries shown (---).

3 M. DõÂaz-Zorita, G.A. Grosso / Soil & Tillage Research 54 (2000) 121± 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 m m 3 ) and hydrochloric acid (10 m m 3 ) and the size fractions were determined by the pipette method (Day, 1965) after treatment with sodium hexametaphosphate. TOC was determined by wetoxidation (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). BD MAX 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 drophammer 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 BD MAX was estimated as the intersection point of the ascending and descending regression lines between soil water content and BD in the proctor curves. The water content at which BD MAX was 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 BD MAX, 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 analysis 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 kg 1 ) increase in the same direction as the textural classes and were inversely related with sand content (rˆ0.76, nˆ26, 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 BD MAX varied among a wide range of values (1.10±1.72 Mg m 3 ) with corresponding CWC levels ranging from 138 to 275 g kg 1. The BD MAX 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 BD MAX of 26 top soils from the Pampas Region, Argentina.

4 124 M. DõÂaz-Zorita, G.A. Grosso / Soil & Tillage Research 54 (2000) 121±126 Table 1 CWC, water retention at a suction of 33 kpa (WR) and relative compaction (RCˆBD/BD MAX ) estimated for soils of the Pampas Region under continuous no-tillage systems in relation to their textural class a Soil texture CWC g kg 1 WR g kg 1 b BD MAX Mg m 3 c BD Mg m 3 RC Mg m 3 Author Loamy sand 149 a 101 a 1.58 a Miglierina et al., 1995 Kruger, 1996 Quiroga et al., 1996 Loam 199 b 186 b 1.51 ab DõÂaz-Zorita, 1996 Taboada et al., 1998 Silt loam 225 c 304 c 1.39 b Garay et al., 1986 Gambaudo and Marelli, 1998 a Values followed by dissimilar letters in each column are signi cantly different (aˆ0.10). b BD MAXˆmaximum bulk density, average of this study in each textural class. c BDˆpresent bulk density, average of cited studies. Table 2 Simple regression models between compactability indexes (BD MAX, CWC, SC) and soil properties (g kg 1 ) in 26 top soils from the Pampas Region of Argentina Maximum bulk density (BD MAX,Mgm 3 ) BD MAXˆ CLAY a BD MAXˆ SILT BD MAXˆ SAND BD MAXˆ TOC BD MAXˆ WR Critical water content (CWC, g kg 1 ) CWCˆ CLAY CWCˆ SILT CWCˆ SAND CWCˆ TOC CWCˆ WR Susceptibility to compaction (SC) SCˆ CLAY SCˆ SILT SCˆ SAND SCˆ TOC SCˆ WR a CLAYˆclay content, SILTˆsilt content, SANDˆsand content, TOCˆtotal organic carbon level, WRˆwater retention at a suction of 33 kpa. r 2 P 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 stepwise regression analysis only the TOC content was selected to explain BD MAX or CWC variation. The BD MAX 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 BD MAX 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 Scotland, a reduction in BD MAX of per increment of 1% TOC. Fig. 3. TOC and BD MAX of 26 top soils from the Pampas Region, Argentina.

5 M. DõÂaz-Zorita, G.A. Grosso / Soil & Tillage Research 54 (2000) 121± Fig. 4. TOC and CWC of 26 top soils from the Pampas Region, Argentina. The SC was signi catively related to TOC and SILT levels and the model tted can be written as SC ˆ 0:0553 0:0051TOC 0:00033SILT; r 2 ˆ 0:49; n ˆ 26; 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 BD MAX 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 increments 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. According to these models the BD MAX values 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 Argentina 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 textural 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 necessary 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 maintained 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. References Alvarez, R., Lavado, R.S., Climate, organic matter and clay content relationships in the Pampa and Chaco soils. Argentina Geoderma 83, 127±141. American Society for Testing and Materials (ASTM), Standard test methods for moisture relations using a 5.5 lb (2.5 kg) hammer and 12 inch (304.8 mm) drop. In: ASTM (Ed.), ASTM Standards in Building Codes, Vol. II, Philadelphia, PA, pp. 836±842. Analytical Software, Statistix for windows. User's manual, Tallahassee, FL, 333 pp. Ball, B.C., Lang, R.W., O'Sullivan, M.S., Franklin, M.S., Cultivation and nitrogen requirements for continuous winter barley on a gleysol and a cambisol. Soil Tillage Res. 13, 333±352. Buschiazzo, D.E., Panigatti, J.L., Labranzas en la regioân semiarida Argentina. Consideraciones nales. In: Buschiazzo, D.E., Panigatti, J.L., Babinec, F. (Eds.), Labranzas en la RegioÂn SemiaÂrida Argentina. INTA y SAPyA, Santa Rosa, Argentina, pp. 113±124. Buschiazzo, D.E., Quiroga, A.R., Stahr, K., Patterns of organic matter accumulation in soils of the semiarid Argentinean Pampas. Z. P anzenernaèhr. Bodenkd. 154, 437±441. Day, P.R., Particle fractionation and particle size analysis. In: Black, C.A. (Ed.), Methods of Soil Analysis. American Society of Agronomy, Madison, WI, pp. 562±566.

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