Soil physical quality of a Brazilian Oxisol under two tillage
systems using the least limiting water range approach
Cassio Antonio Tormena
a,*, Alvaro Pires da Silva
b, Paulo Leonel Libardi
caDepartamento de Agronomia, Universidade Estadual de MaringaÂ, Av. Colombo, 5790, MaringaÂ-PR, 87090-000, Brazil bEscola Superior de Agricultura Luiz de Queiroz, Departamento de CieÃncia do Solo, Universidade de SaÄo Paulo,
Bolsista do CNPq, Piracicaba-SP, 13418-900, Brazil
cEscola Superior de Agricultura Luiz de Queiroz, Departamento de FõÂsica, Universidade de SaÄo Paulo,
Bolsista do CNPq, Piracicaba-SP, 13418-900, Brazil
Received 30 November 1998; received in revised form 2 June 1999; accepted 13 September 1999
Abstract
Plant growth is directly affected by soil water, soil aeration, and soil resistance to root penetration. The least limiting water range (LLWR) is de®ned as the range in soil water content within which limitations to plant growth associated with water potential, aeration and soil resistance to root penetration are minimal. The LLWR has not been evaluated in tropical soils. Thus, the objective of the present study was to evaluate the LLWR in a Brazilian clay Oxisol (Typic Hapludox) cropped with maize (Zea mays L. cv. Cargil 701) under no-tillage and conventional tillage. Ninety-six undisturbed soil samples were obtained from maize rows and between rows and used to determine the water retention curve, the soil resistance curve and bulk density. The results demonstrated that LLWR was higher in conventional tillage than in no-tillage and was negatively correlated with bulk density for values above 1.02 g cmÿ3. The range of LLWR variation was 0±0.1184 cm3cmÿ3in both systems, with mean values of 0.0785 cm3cmÿ3for no-tillage and 0.0964 cm3cmÿ3for conventional tillage. Soil resistance to
root penetration determined the lower limit of LLWR in 89% of the samples in no-tillage and in 46% of the samples in conventional tillage. Additional evaluations of LLWR are needed under different texture and management conditions in tropical soils.#1999 Elsevier Science B.V. All rights reserved.
Keywords:Least limiting water range; Bulk density; No-tillage; Available water; Soil resistance to root penetration
1. Introduction
In tropical soils, the loss of organic matter and the degradation of soil structure are responsible for the decline in productive potential (Cassel and Lal, 1992;
Matson et al., 1997). This process starts with mechan-ized land clearing of the areas (Alegre et al., 1986; Ghuman and Lal, 1992) and is intensi®ed with the large scale implantation of mechanized agricultural systems (Kayombo et al., 1991). Many reports are available about the structure and physical properties of tropical soils (Sanchez, 1976; Lal, 1979; Theng, 1980; Cassel and Lal, 1992; Kayombo and Lal, 1993). The responses of various crops to these modi®cations have led to
*Corresponding author.
E-mail addresses: catormen@cca.uem.br (C.A. Tormena), apisil-va@carpa.ciagri.usp.br (A.P. da Silva)
changes in crop productivity in tropical regions (Kayombo and Lal, 1994), with the magnitude of such changes depending on the soils, crops and management. The structure and physical behavior of tropical soils have been evaluated on the basis of properties and physical processes indirectly related to plant growth, such as bulk density, porosity, in®ltration, hydraulic conductivity, and aggregate stability (Kemper and Derpsch, 1981; Roth et al., 1988).
Plant growth is directly affected by soil water, soil aeration, and by soil resistance to root penetration. The least limiting water range (LLWR) is de®ned as the range in soil water content within which limitations to plant growth associated with water potential, aeration and mechanical impedance to root penetration are minimal. Once limiting values of matric potential, aeration and mechanical impedance are de®ned, the water contents are determined experimentally for each of these limiting conditions and the LLWR is com-puted. The LLWR has been proposed as an index of soil structural quality for plant growth (Da Silva et al., 1994; Topp et al., 1994). Evaluation of soils in temperate regions have demonstrated that the LLWR is affected by the soil organic matter content (Kay et al., 1997), soil structure (Da Silva et al., 1994; Da Silva and Kay, 1997; Stirzaker, 1997), and soil texture (Da Silva et al., 1994; Da Silva and Kay, 1997). Maize growth was found to be positively correlated with LLWR and negatively correlated with the frequency of occurrence of soil water content outside the LLWR limits (Da Silva and Kay, 1996). The LLWR concept has been incor-porate in a soil science text book (Brady et al., 1999). No information is available in the literature about the management±structure relations in tropical soils evaluated by joint changes in water availability, soil resistance to root penetration and soil aeration, i.e., by the LLWR. Thus, the objective of the present study was to characterize and evaluate the LLWR in a tropical clay Oxisol (Typic Hapludox) cropped with maize using no-tillage (NT) and conventional tillage (CT).
2. Material and methods
2.1. Experimental site and tillage
Undisturbed soil samples were collected in August 1996 from a commercial farm located in the
north-eastern region of the State of SaÄo Paulo, Brazil (2081901300 latitude South and 48
81800300 longitude West). The climate of the region is of the tropical type, with mean annual temperatures and precipitation
of 22.78C and 1420 mm, respectively. The soil is
classi®ed as Rhodic Ferralsol (Typic Hapludox) with
particle-size distribution consisting of 800 g kgÿ1
clay, 150 g kgÿ1 silt and 50 g kgÿ1 sand. The clay
fraction is dominated by kaolinite and various sesqui-oxides of iron and aluminum (Costa, 1996).
The study was performed using two contiguous plots cultivated by the NT and CT systems. In the NT area, the system had been set up 4 years before, and in the CT area the system had been used for 10 years. Conventional tillage was carried out with a disk plough followed by cultivation in April 1996. Both areas were irrigated with a central sprinkler. By the time of sampling, water had been applied in the area 20 times with 16 mm water head each time. The irrigation control was based on a class-A evaporation pan. In both areas, crop rotation consisted of soybean (Glycine max, L. Merril), maize and beans (Phaseolus vulgaris, L.). At the time of sampling (silking stage), both areas were cropped with maize at row spacing of
0.90 m. Basic fertilization was 330 kg haÿ1 04±20±
20Zn and additional fertilizations were performed
20 days after plant emergence (APE) with 145 kg haÿ1
20±00±20 and at 35 and 50 APE with 40 kg haÿ1urea.
2.2. Soil sampling and analysis
Sampling was performed in August 1996. Undis-turbed cores (5 cm diameter, 5 cm length) were taken from the center of the layer at 0±0.10 m depth. The sampling points were located in a transect of 43.2 m transverse to the culture rows for both tillage systems. Samples were taken at 0.45 m intervals, resulting in 96 samples per tillage system sequentially located along the row and between rows.
The soil water retention curve was determined by the procedure of Da Silva et al. (1994). The samples were divided into 12 groups of 16, with four samples per position and potential for each tillage system. The following potentials were applied using a tension table
adapted from Topp and Zebtchuck (1979): ÿ0.001,
ÿ0.003,ÿ0.005,ÿ0.006, andÿ0.008 MPa. Pressure
plates (Klute, 1986) were used to equilibrate samples
ÿ0.5 and ÿ1.5 MPa. After equilibrium, the samples were utilized to determine soil resistance to
penetra-tion (SR) and then dried in an oven at 105±1108C for
the determination of soil water content and bulk density (Ds).
The SR was measured using an electronic penet-rometer with a cone of 4 mm diameter and semi-angle
of 308. The rate of penetration was set up to
1.0 cm minÿ1. The measurements obtained from 1
to 4 cm of depth were averaged for each core. The soil water retention curve was ®tted to the equation proposed by Van Genuchten (1980).
r sÿr= 1 n1ÿ1=n
h i
; (1)
whereis the volumetric water content (cm3cmÿ3),
the matric potential (cm), r is the residual water
content (cm3cmÿ3), and(cmÿ1) andnare constants.
The Ds, position and tillage effects on the model
parameters were evaluated following the procedure described by Da Silva and Kay (1997) using SAS Institute (1991).
The SR data were regressed againstDs(g cmÿ3) and
soil water content () using the model proposed by
Busscher (1990).
SRabDcs; (2)
where a, b and c are constants and SR is the soil
resistance (MPa). The influence of tillage and sam-pling position were assessed according to Da Silva et al. (1994).
The LLWR was determined for each core by the method of Da Silva et al. (1994). The soil water content () at the critical limits of the matric potential, soil resistance and air-®lled porosity were obtained
considering ®eld capacity (fc) to be the soil water
content at ÿ0.01 MPa (Haise et al., 1955). For
the permanent wilting point (wp) we considered soil
water content at ÿ1.5 MPa (Savage et al., 1996),
for SR (sr) we used the 2.0 MPa value (Taylor et al.,
1966), and for air-®lled porosity (afp) we used the
value of 10% (Grable and Siemer, 1968). Both fc
and wp were obtained using Eq. (1). The sr was
obtained by Eq. (2), while afp was obtained as
[(1ÿDs/Dp)ÿ0.1], where Ds is the measured bulk
density andDpis the particle density (assumed to be
2.65 g cmÿ3). At eachDs, the LLWR is the difference
between the upper limit and the lower limit.
The upper limit is the drier of either fc or afp
whereas the lower limit is the wetter of eitherwp
or sr.
3. Results and discussion
The soil physical properties determined in the
samples are presented in Table 1. Estimates of fc
and wp were made using Eq. (1). Only Ds was
incorporated in the model vian, i.e.,
0:1342 sÿ0:1342= 11:3355 n1ÿ1=n
The soil resistance curve was in¯uenced by the tillage system but not by sample position. The coef®-cients of the models demonstrated that SR was
posi-tively correlated with Dsand negatively with. The
increase in SR with decreasing is a well-known
process and is due to an increase in effective stress (Snyder and Miller, 1985), which is magni®ed by the increasedDs.
The model used to estimate SR in both tillage systems were
NT: SR0:0223ÿ2:6908D8s:2080; (4)
CT: SR0:0194ÿ2:6908D8s:2080; (5)
R2 0:88:
Table 1
Soil physical parameters measured in NT and CT in an Oxisol (Typic Hapludox) cropped with maize, at a depth of 0±0.10 ma
Variable Mean Standard deviation Minimum Maximum
NT
SR 1.426 0.936 0.306 5.082
Ds 1.153 0.065 0.950 1.320 0.356 0.059 0.239 0.459
CT
SR 1.116 0.745 0.312 3.603
Ds 1.129 0.075 0.930 1.330 0.346 0.058 0.213 0.457
aSR: soil penetrometer resistance (MPa), D
s: bulk density
Several studies have demonstrated a higher SR in NT compared to CT (Cornish and Lymbery, 1987; Hill, 1990; McCoy and Cardina, 1997; Opoku et al., 1997) and the differences were as explained by the
variation in Ds and . The results obtained
demon-strated that, under the same soil moisture andDs, SR
was higher in NT, in agreement with data reported by Cornish (1993). In CT, mobilization of the soil results
in the break of bonds between particles and/or aggre-gates, reducing SR (Dexter et al., 1988). The greater SR in NT may be related to the occurrence of the process of ``age hardening'' of the aggregates by which the aggregates reacquire and maintain resis-tance a long time after the initial mobilization of the soil (Utomo and Dexter, 1981; Kemper and Rosenau, 1984). According to Grant et al. (1985) and Semmel et al. (1990), the persistence of the effects of drying and wetting cycles as well as traf®c results in larger and denser aggregates, leading to higher SR in the NT system (Cornish, 1993).
The LLWR limits, i.e., fc, wp, sr and afp are
presented in Fig. 1a and b for both tillage systems.Ds
increased fc up to Ds of 1.27 g cmÿ3 in NT and
1.26 g cmÿ3 in CT. According to Hill (1990), the
increase in water retention with Ds under elevated
potentials occurs due to the reduction in macroporos-ity. In contrast,wpwas positively affected throughout
the Ds range in both systems. The magnitude of the
effects of Ds on water retention was lower under
higher than under low , resembling the behavior
of sandy soils described by Hill and Sumner (1967). This is related to the fact that clayey Oxisols have stable and well developed microstructure. According to Van den Berg et al. (1997), in tropical soils with strongly microaggregated structures, the greater water retention at lower potentials with increasingDsis due
to a larger amount of particles available for water absorption allied to an increase in soil microporosity. Other investigators have demonstrated a negative
effect ofDson water retention under elevated
poten-tials and a positive effect at low potenpoten-tials (Sme-demma, 1993; Gupta and Larson, 1979). These investigators argue that, in the presence of elevated , soil water retention is in¯uenced by total porosity,
whereas at low , soil water retention is controlled by
the volume of micropores, which in turn depend onDs
(Carter, 1988). The available water content
(AWCFCÿWP) varied positively up to a Ds of
1.02 g cmÿ3 in both systems and, starting from this
value, AWC was reduced by the positive effect ofDs
on wp and its negative effect on fc. The greater
reduction in AWC under NT conditions is due to
higherDscompared to CT.
An increase insrand a decrease inafp occurred
with increasingDsin both tillage systems (Fig. 1a and
b).afpwas progressively reduced with increasingDs,
as also reported by Archer and Smith (1972) and Da Silva et al. (1994). The observationsafp>fcsuggests
that, even in the presence of greater Ds, the stable
microstructure preserves the porous space necessary for gas exchange in soil. These results contrast with those obtained for clay soils by Topp et al. (1994), who reported that air-®lled porosity frequently reached values considered to be limiting for an appropriate
aeration of the plant root system. For the Ds values
determined,afpdid not replacefcat the upper limit of
water availability. For higherDs's,afpmay represent a
limitation, especially under conditions of high oxygen demand in soil (Hadas, 1997). Hamblin (1985) sug-gested that a limitation caused by aeration may fre-quently occur in clay soils since with increasingDsthe
roots occupy pores of smaller size with decreasing drainage. Furthermore, soil compression during root growth contributed to a reduction of the proportion of root surface exposed to free oxygen ¯ow in soil. The low bulk densities values associated with high poros-ities may be associated with the microstructure present in the tropical Oxisol (Sanchez, 1976; Igwe et al., 1995). TheDshad a strong effect onsrin both tillage
systems. This was more pronounced in NT wheresr
was the lower limit in 89% of the samples and replaced
wpatDsvalues1.06 g cmÿ3. In contrast, in CT,sr
was the lower limit in 46% of the Ds value and
replaced wp for Ds1.13 g cmÿ3. Similar results
were obtained by Topp et al. (1994) and Da Silva et al. (1994) in Canadian soils.
The LLWR was positively correlated where
Ds< 1.02 g cmÿ3, and negatively correlated with
Ds> 1.02 g cmÿ3in both tillage systems (Fig. 2). This
behavior was similar to that reported by Topp et al. (1994), Da Silva et al. (1994) and Stirzaker (1997). For
sameDs, LLWR NT < LLWR CT. The LLWR ranged
from 0 to 0.1184 cm3cmÿ3 in both tillage systems,
with mean values of 0.0785 cm3cmÿ3 for NT and
0.0964 cm3cmÿ3 for CT, which were statistically
different (p< 0.05). At the row position, LLWR
CT0.1078 cm3cmÿ3 and LLWR NT0.0869
cm3cmÿ3 whereas in the interrow position, LLWR
CT0.0857 cm3cmÿ3 and LLWR NT0.0701
cm3cmÿ3. These values were statistically different
(p< 0.05). At the averageDsvalues there were
mini-mal physical limitations to plant growth in both CT
and NT. However, the temporal variability of Ds
associated with severe physical limitation to crop growth (Carter, 1990; Carter et al., 1999).
Bothsrandafpwere more strongly affected byDs
thanfcorwp. The effect ofDswas more marked on
sr, suggesting that in this soil LLWR is more sensitive
to the effects of structure on SR than on available water. Da Silva et al. (1994) reported that the
sensi-tivity of LLWR toDsis dependent on the limits of SR.
In the soil studied, SR was the most limiting factor. The limit values of SR selected to analyze the sensi-tivity of LLWR were 1.0, 2.0, 3.0 and 4.0 MPa. The sensitivity of LLWR variation differed between the tillage systems (Fig. 3a and b), being higher in NT.
The effect of high SR on root growth may be minimized by the presence of macropores formed by the mesofauna and by the crop roots. Macropores favor root growth, although the ef®ciency of these roots in absorbing water and nutrients has been ques-tioned by Passioura (1991) and Smucker and Aiken (1992). However, several studies have demonstrated that NT increases the frequency and number of macro-pores compared to CT and that these macromacro-pores are preserved due to lower soil mobilization. The utiliza-tion of these biopores as alternative routes permits root growth under conditions of higher SR, as observed by Ehlers et al. (1983), Cornish (1993) and Martino and
Shaykewich (1994) under ®eld conditions, and by Stirzaker et al. (1996) in a study on potted plants. Ehlers et al. (1983) observed that the limit SR values
for oat (Avena sativa L.) root growth were 3.6 and
4.9 MPa, respectively, for CT and NT, and these results were attributed to the presence of biopores that are not detected by penetrometers.
Considering the occurrence of these conditions in the present study and assuming the critical SR estab-lished by Ehlers et al. (1983), the LLWR was recal-culated. LLWR was similar (p> 0.05) for both tillage
systems (Fig. 4). However, at higher Ds, LLWR
NT > LLWR CT.
Excessive tillage and the absence of a soil cover may expose these soils to high drying rates and an abrupt increase in SR, as suggested by Weaich et al. (1992) and Townend et al. (1996). In NT system the presence of residues contributes to a greater water content in soil, thus maintaining the physical proper-ties within an optimum range for crop productivity (Kladivko, 1994).
Evaluations of the physical quality of tropical soils in the presence of a wide variation of mineralogy, texture and management conditions should be per-formed by employing the LLWR. The use of pedo-transfer functions may be an alternative to facilitate
the LLWR estimation from routinely measured soil properties (Da Silva and Kay, 1997; Kay et al., 1997).
4. Conclusions
The use of the LLWR concept allowed the identi-®cation of physical factors that control the physical quality of the soil studied in terms of plant growth. The SR was the physical parameter that limited the LLWR in both tillage systems. Air-®lled porosity did not represent a limitation of the LLWR for either tillage system studied. Detailed studies are needed to estab-lish the limits of SR of plant growth, with priority in tropical soils, in order to establish the lower LLWR limits for the determination of the physical quality of these soils.
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