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Agronomy Journal 95:526-536 (2003)
© 2003 American Society of Agronomy

TILLAGE

Soil Bulk Density and Penetration Resistance under Different Tillage and Crop Management Systems and Their Relationship with Barley Root Growth

J. Lampurlanés and C. Cantero-Martínez*

Departament de Producció Vegetal i Ciència Forestal, Universitat de Lleida–IRTA, Rovira Roure, 177, 25198 Lleida, Spain

* Corresponding author (carlos.cantero{at}pvcf.udl.es)

Received for publication March 19, 2001.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
To study the effects of fallow and tillage on soil physical properties and barley (Hordeum vulgare L.) root growth, an experiment was conducted for several years on two soils of contrasting depth: deep, a Fluventic Xerochrept of 120-cm depth, and shallow, a Lithic Xeric Torriorthent of 30-cm depth. Subsoil tillage (ST), minimum tillage (MT), and no-tillage (NT) were compared in the deep soil and MT and NT in the shallow soil. Bulk density (BD), penetration resistance, gravimetric water content, gravel content, and root length density were determined at several times during the year. In the deep soil, BD was lower in the fallow and crop-after-fallow plots (1.26 Mg m-3) than in the continuous-crop plots (1.32 Mg m-3). In this soil, NT showed the largest bulk densities (mean of 1.34 Mg m-3), followed by MT (mean of 1.27 Mg m-3) and ST (mean of 1.22 Mg m-3). Larger penetration resistance was found in NT than in ST and MT in both soils soon after tillage operations. However, root length density profiles sometimes showed greater values for NT than for the other tillage systems, revealing a good soil condition for root growth under NT. Therefore, an increase in soil strength is observed under NT in the first years after its introduction that does not greatly affect root growth in well-structured soils. Fallow reduces soil strength due to the effect of tillage and natural loosening factors like drying and wetting cycles or fauna activity. This effect extends to the following crop.

Abbreviations: BD, bulk density • CAF, crop after fallow • CC, continuous crop • GC, gravel content • GWC, gravimetric water content • MT, minimum tillage • NT, no-tillage • PR, penetration resistance • RGR, root growth rate • ST, subsoil tillage


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
INTEREST IN NT is growing in the rainfed cereal-cropping areas around the world due to the effective reduction in time and costs that this tillage system allows. This is the case in the semiarid area of the Ebro Valley of Spain. To investigate the effect of NT and other tillage systems on plant and soil properties, a series of experiments began in 1992. The results on soil water content, root length density, and yield under continuous crop (CC) and crop after fallow (CAF) have been already published in Lampurlanés et al. (2001) and Lampurlanés et al. (2002). In this paper, we investigate the effect of reducing tillage intensity on soil physical properties and the relationship between soil physical conditions and root growth.

No-tillage is not a new technique. However, the concept of not disturbing the soil still clashes strongly with most farmers, who for years have disturbed the soil to obtain a soft medium for better crop growth. For the farmer, undisturbed soil seems to be harder and more resistant to root penetration than tilled soil. In fact, high soil strength has been proved to reduce and even stop root growth (Atwell, 1993). Measurements are needed to determine whether or not reduction or suppression of tillage can increase soil strength disturbing root and plant growth.

The most common variables used to assess soil strength in tillage studies are BD and penetrometer resistance. They are interrelated, and the use of only one of these variables may lead to misleading results (Campbell and Henshall, 1991).

Bulk density is inversely related to total porosity (Carter and Ball, 1993), which provides a measure of the porous space left in the soil for air and water movement. The optimal BD for plant growth is different for each soil. In general, low BD (high porosity) leads to poor soil–root contact, and high BD (low porosity) reduces aeration and increases penetration resistance (PR), limiting root growth (Cassel, 1982).

Bulk density is related to natural soil characteristics such as texture, organic matter, soil structure (Cassel, 1982; Chen et al., 1998), and gravel content (GC) (Franzen et al., 1994). It varies over the year due to the action of several processes: freezing and thawing (Blevins et al., 1983; Unger, 1991), settling by desiccation and kinetic energy of rainfall (Cassel, 1982), and loosening by root action and animal activity. Crop operations, especially tillage, may also alter bulk soil density.

One of the goals of tillage is to reduce BD (increasing soil porosity). This effect of tillage on BD is temporary, and after tillage, the soil rapidly settles, recovering its former BD (Hernanz and Girón, 1988; Campbell and Henshall, 1991; Franzen et al., 1994; Franzluebbers et al., 1995). In the first years of NT, BD of the soil may increase due to the repeated passes of the tractor and the lack of the loosening action of tillage.

Numerous experiments performed to compare NT with other conservation or more conventional tillage systems have given different results. In most of them, BD was greater in NT in the first 5 to 10 cm of soil (Ehlers et al., 1983; Pelegrin et al., 1988; Radcliffe et al., 1988; Hammel, 1989; Hill, 1990; Campbell and Henshall, 1991; Grant and Lafond, 1993; Rhoton et al., 1993; Franzen et al., 1994; Hubbard et al., 1994; Franzluebbers et al., 1995; Unger and Jones, 1998; Tebrügge and Düring, 1999; Wander and Bollero, 1999). In others, no differences in BD were found between tillage systems (McCalla and Army, 1961; Cassel, 1982; Blevins et al., 1983; Burch et al., 1986; Blevins and Frye, 1993; Taboada et al., 1998; Arshad et al., 1999; Logsdon et al., 1999; Ferreras et al., 2000; Logsdon and Cambardella, 2000). In a third group, BD even decreased under NT (Moran et al., 1988; Pikul and Asae, 1995; Edwards, 1996; Crovetto, 1998), especially when an increase in organic matter was observed in the first layer of the soil (Edwards, 1996; Crovetto, 1998).

Owing to the progressive increase in BD after tillage, the difference between tillage and NT becomes smaller as the time since tillage increases. In some soils, porosity under NT decreases in the first few years until the soil recovers its natural structure (Kinsella, 1995).

The most important factors affecting PR or the cone index of the soil are soil water content and BD (Cassel, 1982; Hamblin, 1985; Bradford, 1986; Klepper, 1990; Campbell and O'Sullivan, 1991; Unger and Jones, 1998). Texture, organic matter, particle surface roughness (Cassel, 1982; Campbell and O'Sullivan, 1991), and structure (Bradford, 1986; Campbell and O'Sullivan, 1991) may also produce a different PR in different soils or in different layers of the same soil.

Penetration resistance increases with depth due to the increase in shaft friction (Bradford, 1986; Campbell and O'Sullivan, 1991; Franzen et al., 1994), and values from the different soil depths are correlated with each other (Stelluti et al., 1998; Campbell and O'Sullivan, 1991). Yasin et al. (1993) found a cubic relationship between cone index and depth.

In several studies comparing tilled and nontilled soils, greater PR was found under NT, especially in the upper 10 cm (Ehlers et al., 1983; Radcliffe et al., 1988; Hammel, 1989; Hill, 1990; Pelegrin et al., 1990; Agenbag and Maree, 1991; Grant and Lafond, 1993; López et al., 1996; Wander and Bollero, 1999; Ferreras et al., 2000). Franzen et al. (1994) observed significantly smaller cone index values under NT down to 10-cm soil depth due to mulching. As for BD, differences between NT and more conventional soil-disturbing tillage methods are great soon after tillage operations but fall quickly during the growing season and may disappear at the end of the season (Pelegrin et al., 1990; Franzen et al., 1994; López et al., 1996).

The tillage system affects not only PR, but also its related variables: soil water content and BD. For this reason, some researchers have tried to separate the direct effect of tillage on cone index from its indirect effect on water content and BD in different ways to allow better comparisons. Campbell and O'Sullivan (1991) proposed measuring PR at field capacity and simultaneously measuring BD. Busscher et al. (1997) adjusted different functions to correct cone index values from water content. Others used analysis of covariance to reduce the effect of water content and BD in the cone index comparisons (Yasin et al., 1993; Franzen et al., 1994). After correction, the dependence of cone index on these variables is reduced (Busscher et al., 1997).

The root system acts as a bridge between the crop operations and plant growth response (Klepper, 1990). The most important soil physical properties affecting root growth are porosity, mechanical impedance, water content, and soil structure (Klepper, 1990; Gregory, 1994). In general, root tips are unable to penetrate pores narrower than their diameter (Taylor, 1983; Hamblin, 1985; Campbell and Henshall, 1991). They can exert a vertical pressure ranging from 0.7 to 2.5 MPa, depending on crop species (Gregory, 1994). Bulk density values that limit root growth depend on soil water content (Pabin et al., 1998) and range between 1.46 and 1.90 Mg m-3 (Campbell and Henshall, 1991).

Mechanical impedance increases as BD increases and water content decreases (Ehlers et al., 1983). Penetration resistance measured with the penetrometer is usually 2 to 8 times greater than that actually experienced by the root tip (Bengough, 1991; Atwell, 1993; Gregory, 1994), owing to the different way in which roots and probes penetrate the soil. However, it is well correlated with the soil strength perceived by roots in soils with a relatively homogeneous matrix (Atwell, 1993).

Root growth decreases as PR increases (Taylor, 1983; Atwell, 1993; Gregory, 1994), showing a linear (Ehlers et al., 1983), inverse (Bengough, 1991; Atwell, 1993), or exponential (Hamblin, 1985) relationship. Penetrometer values greater than 2 MPa are generally reported to produce a significant root growth reduction (Atwell, 1993). However, in well-structured soils or those in which biochannels are preserved (as in nontilled soils), roots continue to extend at greater penetrometer readings because they can grow in the interaggregate spaces (Ehlers et al., 1983; Taylor, 1983; Klepper, 1990; Campbell and Henshall, 1991).

Fallow has been proved to affect water and N balances in the soil (French, 1978; McDonald and Fischer, 1991). It also seems reasonable to hypothesize that fallow can have an effect on soil strength because greater water content generally encountered under fallow than under cultivated soils may modify the restructuring process of the soil and its biological activity.

The objective of this study was to follow the evolution of BD and PR in the first few years after NT was established on fallow and continuous barley and to determine its effects on root growth.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The Experiment
The experimental fields of this study were located in El Canós, in the semiarid area of the northeast Ebro Valley, Spain (mean annual precipitation of 440 mm), on two soils of contrasting rooting depth that are representative of the soils in the area. The deep soil was a fine-loamy, mixed, mesic Fluventic Xerochrept of 120-cm depth. The shallow soil was a loamy, mixed, calcareous, mesic, shallow Lithic Xeric Torriorthent of 30-cm depth. Both soils showed high GC, mainly at the surface ({approx}15%). Some selected soil properties are shown in Table 1.


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  		Table 1. Selected properties for the layers of the deep soil (Villar, 1989) and the shallow soil.

 
The experiment was designed as a randomized complete block with four replications. The plots (10 by 6 m in area) were arranged in three contiguous strips. In the central strip, barley was cropped every year. Lateral strips were alternatively under fallow or cropped with barley each year. With this arrangement, we investigated three field situations: CC, CAF, and fallow. Three tillage systems were compared in the deep soil (ST, MT, and NT) and two in the shallow soil (MT and NT). Subsoil tillage consisted of a subsoiler tilling at 40-cm depth in August and field cultivating at 15-cm depth in October (and cultivating in spring in the fallow plots if weeds were present). Minimum tillage consisted of a field cultivator working to a depth of 15 cm before sowing (and in May in the fallow plots if weeds were present). No-tillage consisted of maintaining the soil free of weeds by total herbicide spraying {2 L ha-1 of 36% glyphosate [N-(phosphonomethyl)glycine]} in October and in spring if weeds were present.

Measurements
Rainfall and temperature were monitored daily at a weather station situated 250 m from the experimental field.

Bulk density was determined by taking two undisturbed soil cores from each plot from 0- to 7- and 7- to 14-cm depths. We took the cores by hammering into the ground stainless steel cutter edge cylinders 50 mm high and 60 mm in diameter (141.37-cm3 inner volume). The cores were stored and transported in hermetic cans to determine the gravimetric water content (GWC). The cores were dried, weighed, and washed through a 2-mm sieve to determine the GC. Bulk density of the fine soil ({oslash} < 2 mm) was calculated as (Mc - Mg)/(Vc - Vg) x 100, where Mc and Vc are the dry mass and the volume of the soil core, respectively, and Mg and Vg are the mass and the volume of the gravel, respectively. This is another form of Russos's equation (cited by Franzen et al., 1994). Gravel content was calculated as Mg/Vc to allow comparison with Franzen et al.'s (1994) results. Gravimetric water content was obtained from the fresh and dry weights of the cores. Gravimetric rather than volumetric water content was used because volumetric water content is affected by BD, which varies at the same time during penetration measurements (Campbell and O'Sullivan, 1991).

To measure PR, we used a hand-held penetrograph (Stiboka penetrograph, Eijkelkamp Agrisearch Equipment, Giesbeek, the Netherlands) that draws a graph of the resistance to penetration vs. depth to a depth of 80 cm. The conical point was 1 cm2 in area, and the point angle was 60°. The measurement range was 0 to 5 MPa. At each measurement time, we obtained two graphs per plot with a total of eight replications per treatment.

The dates on which tillage operations and BD and penetrometer resistance determinations were made are shown in Table 2.


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  		Table 2. Dates of tillage operations, penetration resistance (PR) or bulk density (BD) measurements, and days and rainfall from the last tillage operation in the deep soil and the shallow soil.

 
Gravimetric water content and root length density profiles were obtained by taking soil cores between rows with Edelman or Riverside 5-cm-diam. augers (Eijkelkamp Agrisearch Equipment, Giesbeek, the Netherlands) at important developmental stages of the barley tillering, stem elongation, anthesis, maturity, and harvest. Additional samples for water content were taken at sowing and during winter. In each plot of the deep soil, soil cores were taken from 0- to 25-, 25- to 50-, 50- to 75-, and 75- to 100-cm depth. In the shallow soil, the cores sampled the profile from 0- to 10- and 10- to 30-cm depth.

Roots in each core were washed out by elutriation (Pearcy et al., 1989) and stained following the procedure of Ward et al. (1978). Root length was determined by the line intersection method (Newman, 1966) and root length density calculated as the quotient between root length and volume of the sample. Root growth rate (RGR) between two sample dates was calculated as the difference in root length density divided by the number of days.

Gravimetric water content was obtained by drying the samples (Campbell and Mulla, 1990).

Statistics
Statistical analyses were accomplished using SAS software. The data were analyzed as repeated measures over time and space (Steel and Torrie, 1980; Gómez and Gómez, 1984). Due to unequal cell size, this analysis was done as a split-split plot (Littell et al., 1991), with tillage as a main plot and sampling date and depth as successive subplots. Separate analyses of variance were computed for every strip: CC, CAF, and fallow.

Gravimetric water content, GC, or both were used as covariables in the analysis of BD data when significant linear relationship was found between them and BD. There was a significant linear relationship between PR and GWC and BD in all strips. For this reason, we use both GWC and BD as covariables for the PR data analysis. Least-square means (corrected by the covariables) were used to compare treatments, and differences for main effects and interactions were tested with the PDIFF option of the LSMEANS statement (Littell et al., 1991).

To quantify the relationship between soil strength and root growth, a multiple regression analysis was performed on each soil, with the RGR as the response variable and GWC, BD, and PR as the predictors.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Rainfall
Daily rainfall and tillage operations are shown in Fig. 1 . The driest year was 1994–1995, with little winter and spring rainfall. The wettest years were 1995–1996 and 1996–1997, with high winter rainfall. In 1995–1996, spring rainfall was also high.



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  		Fig. 1. Daily rainfall and tillage operations. T1, subsoiling and cultivation in subsoil tillage (ST) plots; T2, cultivation in ST and minimum tillage (MT) plots; T3, cultivation in ST and MT fallow plots.

 
In 1994–1995, rainfall between the first and second tillage operations (142 mm) was higher than in 1995–1996 (74 mm) or in 1996–1997 (52 mm). On the other hand, rainfall between the second and third tillage operations was higher in 1995–1996 (294 mm) and 1996–1997 (275 mm) than in 1994–1995 (69 mm). Precipitation between the third and first tillage was also higher in 1995–1996 (293 mm) than in 1994–1995 (122 mm).

Bulk Density
There was a positive linear relationship between GWC and BD for CC and fallow strips of the deep soil and for the fallow strip of the shallow soil (Table 3). The linear relationship between GC and BD was negative for the fallow strip of the deep soil and for all of the shallow-soil strips.


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  		Table 3. Regression coefficients of bulk density (BD, t m-3) vs. gravimetric water content (GWC, %) and gravel content (GC, t m-3) by strip for the deep and shallow soil.

 
Deep Soil
Measured BDs ranged between 0.69 and 1.66 Mg m-3. Mean BD (Table 4) was similar for CAF and fallow strips (1.26 and 1.27 Mg m-3) and greater for CC strip (1.32 Mg m-3). The smallest BDs were found during the fall and winter months (about 1.12 Mg m-3) and the greatest in spring and summer (1.36 Mg m-3). Bulk density was significantly smaller (P < 0.0001) in the 0- to 7-cm soil layer (1.22 Mg m-3) than in the 7- to 14-cm layer (1.34 Mg m-3).


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  		Table 4. Probabilities of the bulk density (BD, Mg m-3) analysis of covariance and LSMEANS separation for the different field conditions in the deep soil.

 
The effect of tillage on BD was significant in the three strips. Bulk density was greater, in general, for NT (mean of 1.34 Mg m-3), medium for MT (1.27 Mg m-3), and lower for ST (1.22 Mg m-3), according to tillage intensity.

Bulk density trends in Fig. 2 show that over the three strips, BD was greater under NT in the 0- to 7-cm layer and lower under ST in the 7- to 14-cm layer though differences were more significant in fallow, the only strip with significant tillage system x soil depth interaction (P < 0.0001, Table 4). The greatest difference between tillage systems was found in fallow in June 1996 from 0- to 7-cm depth, 30 d and 109 mm of rain after the last tillage operation.



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  		Fig. 2. Bulk density (BD) trends for three tillage systems in the deep soil: subsoil tillage (ST), minimum tillage (MT), and no-tillage (NT).

 
Shallow Soil
Bulk density ranged from 0.67 to 1.46 Mg m-3. Mean BD for both tillage systems was 1.10 Mg m-3. The results of the analysis of covariance for BD (Table 5) show that neither tillage system nor tillage system x soil depth interaction was significant in any strip. Clearly, soil depth had a significant effect (P < 0.0001), with a mean BD of 1.04 Mg m-3 from 0- to 7-cm depth and 1.17 Mg m-3 from 7 to 14 cm. For CC and fallow strips, BD increased with time (from 1.01 Mg m-3 in March 1995 to 1.20 Mg m-3 in March 1997 for CC and from 1.11 to 1.25 Mg m-3 for fallow on the same dates), whereas for CAF, it decreased (from 1.21 to 1.14 Mg m-3).


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  		Table 5. Probabilities of the bulk density (BD, Mg m-3) analysis of covariance and LSMEANS separation for the different field conditions in the shallow soil.

 
Penetration Resistance
For both soils and all strips, there was a negative linear relationship between GWC and PR and a positive one between BD and PR (Table 6).


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  		Table 6. Regression coefficients of penetration resistance (MPa) vs. gravimetric water content (GWC, %) and bulk density (BD, t m-3) by strips for the deep soil and shallow soil.

 
Deep Soil
Though tillage system was not significant for this variable, tillage system x soil depth interaction was very significant in the three strips (P < 0.0001; Table 7). Tillage system x measurement date x soil depth interaction was also significant in the CAF (P < 0.02) and fallow (P < 0.007) strips. Differences between tillage systems were more evident in the first 20-cm depth, principally on the following dates (Fig. 3) : May 1995 in the fallow strip, 34 d after cultivator tillage (23 mm of accumulated rainfall); September 1995, 30 d after tillage (72 mm); and March 1997, 176 d after last tillage (274 mm). On these dates, PR in NT was 0.5 to 1 MPa greater than that in MT and ST in the first 10-cm depth, especially in the fallow strip. In addition, ST showed a PR that was about 1 MPa lower than that in MT and NT from 10- to 25-cm depth in September 1995, especially in the CAF and fallow strips.


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  		Table 7. Probabilities of the penetration resistance analysis of covariance for the different field conditions in the deep soil.

 


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  		Fig. 3. Penetration resistance (PR) profiles in the three strips at different times for the three tillage systems in the deep soil: subsoil tillage (ST), minimum tillage (MT), and no-tillage (NT).

 
The greatest differences between CC and CAF strips were found in the first 20 cm of soil. In this layer, mean PR was greater for CC than for CAF (0.43 MPa in ST, 0.27 in MT, and 0.36 in NT).

Shallow Soil
Penetration resistance was affected by tillage systems, as a main factor, in the fallow strip only (P < 0.012) (Table 8). However, tillage system x soil depth interaction was significant in the three strips (P < 0.009 for CC and P < 0.0001 for CAF and fallow strips). Figure 4 shows PR profiles for February 1995, February 1996, and March 1997 at 112, 119, and 140 d after the last tillage operation, respectively. In CC, PR increased with year from about 3 MPa in 1994 to nearly 4 MPa in 1996. The differences between MT and NT also increased from 0 to 1 MPa in the 0- to 10-cm layer for the same period of time. On the CAF strip, PR increased with year, and the differences between MT and NT also increased from 0.5 MPa in February 1996 to 1 MPa in March 1997. In the fallow strip, both mean and differences in PR between tillage systems decreased with year. The largest differences in PR between MT and NT were found in the first 10 to 15 cm of soil in the fallow strip in May 1995 (34 d after tillage) and June 1996 (30 d after tillage) (Fig. 5) .


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  		Table 8. Probabilities of the penetration resistance analysis of covariance for the different field conditions in the shallow soil.

 


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  		Fig. 4. Penetration resistance (PR) changes in time for the three strips and the two tillage systems in the shallow soil: minimum tillage (MT) and no-tillage (NT).

 


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  		Fig. 5. Two penetration resistance (PR) profiles in the fallow strip showing the largest PR differences between the tillage systems in the shallow soil: minimum tillage (MT) and no-tillage (NT).

 
In the first 20 cm of soil, PR was greater in the CC than in the CAF strip, with a difference of 0.55 MPa for MT and 0.32 MPa for NT.

Soil Strength and Root Growth Rate
The relationship between RGR and GWC was positive, the relationship between RGR and BD was not significant, and the relationship between RGR and PR was negative for the deep soil and positive for the shallow soil (Table 9).


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  		Table 9. Regression coefficients of the growth rate of the root length density (cm cm-3 d-1) vs. gravimetric water content (GWC, %), bulk density (BD, t m-3), and penetration resistance (PR, MPa) for the deep soil and the shallow soil.

 
The RGR was significantly higher for the CAF strip than for the CC strip in the deep soil (P < 0.013) but not in the shallow soil (Table 10). This reveals the beneficial effect of fallow in the following crop. Differences between tillage systems were not significant.


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  		Table 10. Growth rate of the root length density (cm cm-3 d-1) in the preanthesis stages for three tillage systems [subsoil tillage (ST), minimum tillage (MT), and no-tillage (NT)] and two field situations [continuous crop (CC) and crop after fallow (CAF)] in the deep and shallow soil. Three-year data.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
Fallow Effects on Soil Strength
During fallow, tillage was performed to control weeds in ST and MT plots. This tillage also had a loosening action that reduced BD and PR in these plots. This effect extended to the following crop, as is shown by the lower BD and PR found on the CAF strip than on the CC strip. In NT, lower soil strength of plots was also found on the CAF strip than on the CC strip. In these plots, only natural soil-loosening factors, such as drying and wetting cycles or fauna activity, could reduce soil strength because weeds were removed chemically. Strength reduction due to fallow in NT plots was greater than in MT plots of the deep soil and lower than in MT plots of the shallow soil. This indicates that in some situations, natural factors induced by fallow may be as effective as tillage in reducing soil strength.

Tillage System Effects on Soil Strength
Bulk density in NT was greater than in tilled plots in the first 7 cm of the deep soil, as has also been reported by a number of authors (Pelegrin et al., 1988; Radcliffe et al., 1988; Hammel, 1989; Hill, 1990; Grant and Lafond, 1993; Rhoton et al., 1993; Franzen et al., 1994; Hubbard et al., 1994; Franzluebbers et al., 1995; Unger and Jones, 1998; Tebrügge and Düring, 1999; Wander and Bollero, 1999), and increased from 1.29 Mg m-3 in March 1995 to 1.44 Mg m-3 in March 1997. This effect was especially clear on fallow plots (Fig. 2).

There could be two reasons for these results. First, we measured BD in the first 5 yr after the change from conventional tillage to NT. According to Kinsella (1995), the soil was in the transition period in which it builds humus, regains its structural stability, and restores the pore space. During this period, there is first an increase in BD up to a maximum and then a decrease due to the restructuring process until an equilibrium level is reached when the structure is fully restored. The second reason was the low quantity of residues left on the soil (straw was packed and removed after harvest, as it is a normal practice in the area), which delayed the increase in organic matter and the restructuring process of the soil.

Subsoiling was effective in reducing BD in depth (7–14 cm). Larger differences would probably have been found if BD had been measured at greater depths.

In the shallow soil, with greater GC than the deep soil, no differences were found in BD among tillage systems due to the structural effect of gravel (Franzen et al., 1994), which protected the soil against compaction in both MT and NT plots.

When significant differences in PR were found between tillage systems, plots under NT showed greater PR in the first 10 to 20 cm of soil than tilled plots, as has also been observed by a number of authors (Radcliffe et al., 1988; Hammel, 1989; Hill, 1990; Pelegrin et al., 1990; Agenbag and Maree, 1991; Grant and Lafond, 1993; Franzen et al., 1994; López et al., 1996; Wander and Bollero, 1999; Ferreras et al., 2000). In the deep soil, differences found in the first 20-cm depth were according to tillage intensity: lower PR for ST, medium for MT, and higher for NT (Fig. 3). In March 1997, far from tillage operations, great differences were found between NT and ST or MT on CC in the deep soil (Fig. 3) and between NT and MT on CC and CAF in the shallow soil (Fig. 4). This seems to indicate, as in the case of BD, that the soil under NT is in the transition period when the soil strength increases.

Soil Strength and Root Growth
The maximum BDs recorded in the deep soil were between the critical (1.67 Mg m-3) and the nonlimiting (1.46 Mg m-3) BD values for root growth stated by Pierce et al. (1983; cited by Godwin, 1990). In the shallow soil, BD was below the nonlimiting value.

Fifty-nine percent of the PR recorded in the deep soil and the shallow soil ranged from 1.3 to 3.7 MPa. These PR values are reported to produce a 50 to 100% reduction in elongation rate for barley (Hadas, 1997). On the other hand, root length density profiles (Lampurlanés et al., 2001, 2002) do not indicate poor conditions for root growth in these soils because the greatest root length densities were found in NT, the tillage treatment that also showed the greatest soil strength.

Several factors could contribute to these high PR readings. First, the soil has high GC. The gravel interfered with the penetrometer measurements, increasing the values (Hamblin, 1985) and the variance of the PR readings (Campbell and O'Sullivan, 1991). Second, these soils have moderately high clay content (Table 1) that increases with depth and leads to the formation of strong columnar aggregates (Villar, 1989), which increase the PR readings (Atwell, 1993). Finally, the organic matter was relatively high for these semiarid soils (2–3% in the top layer of the soil, Table 1), which is also reported to increase PR (Campbell and O'Sullivan, 1991).

The stress required to drive a probe into compacted soil is four to eight times that required for the roots to penetrate the soil (Bengough, 1991; Atwell, 1993). This is because roots grow along the boundaries between the peds, thereby avoiding the resistance to penetration of the bulk soil (Campbell and Henshall, 1991; Atwell, 1993), as was observed by Villar (1989) in the deep soil.

The significant relationship between RGR and GWC (Table 9) was positive because most data were from the preanthesis period in which roots are still extending. The fact that the relationship with BD was not significant corroborates the idea that BDs in these soils were below the critical range for root growth. The positive regression coefficient found between RGR and PR in the shallow soil may be due to the high GC in this soil, especially in the first layer where root length density was also greater.

The reduction in soil strength in the CAF strip due to fallow resulted in higher growth rates of the root length density in the CAF strip than in the CC strip (Table 10). This effect was observed in both the deep and shallow soil, demonstrating the favorable effect that fallow can have on root growth in the first stages of the crop.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
Fallow has been effective in reducing soil strength for the following crop. This effect is as important in tilled as in nontilled fallows, indicating that natural loosening factors, like drying and wetting cycles or fauna activity, may be as effective as tillage in reducing soil strength.

After the introduction of NT, there is an increase in soil strength compared with tilled soils. In our well-structured soils, this increase in strength does not limit root growth because roots can grow between the aggregates. In gravelly soils, the increase in strength is smaller due to the structural supporting effect of gravel.

Differences in root growth due to the cropping (fallow or CC) or tillage system were small because the soil strength did not reach limiting levels for root growth.


   ACKNOWLEDGMENTS
 
This work was funded by the Comisión de Investigacion Científica y Técnica (CICYT), projects AGR91-312 and AGF94-198. We thank the Ministerio de Educación y Cultura (MEC), which funded the doctorate studies of J. Lampurlanés. We also thank J. Boixadera and C. Herrero from the Secció de Noves Tecnologies (DARP), who lent us the penetrograph.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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