Effects of Maghnian Bentonite on Physical Properties of Sandy Soils Under Semi-Arid Mediterranean Climate
This research has for object to study the influence of clay addition, i.e., Maghnian bentonite, like deposit clay, in the physical properties of sandy materials from Mostaganem plateau (North-West Algeria) submitted to salinity and sodicity. The first result was to show that the clay content changes drastically the physical properties of clay-sand mixtures. Important differences were observed as a function of the sand particle size distribution. At given clay content, the saturated Hydraulic Conductivity (HCs) was lower when the sand size was small and spread. For the coarse sand the salinity was maintained, even for high clay contents, a significant hydraulic conductivity. One of the main characteristics of Maghnia clay is the presence of calcium carbonates in the natural material. In comparison to that of Mostaganem clay of other deposit, it appears less sensitive to sodicity. An important aspect is the initial state of the clay when used in addition to sands, i.e., disturbance, conditions of preparation of sand clay mixtures and presence of associated components such as carbonates. Maghnia clay appeared to be adapted to the improvement of sandy soils, not because its mineralogical characteristics, but for its natural cationic form and obviously the presence of calcite in it.
In arid and semi arid zones, sandy soils often presents in dominance, are reputed by their physical and chemical deficiencies. The recourse to clayey amendment for correct their original deficiencies was envisaged to improve particularly their available water retention (Bousnina and Mhiri, 1997; Benkhelifa, 1997; Halilat and Tessier, 2006). Essentially, the work contributions concern the physical and hydrical behavior of the sand-clay mixtures and consider very littlie the effect of the salinity and the sodicity (Halilat, 1998; Halilat and Tessier, 2000).
However the clay addition always poses problem because of its stability within a sandy material. Indeed, a partial colmatage of the porosity can lead to some important decrease of the hydraulic conductivity witch is be a problem for root penetration. This is especially right that in arid and semi-arid zones, soils are submitted to strong hydric deficits and therefore to strong variations of water potential. It can result a mobilization of clay particles during the soil brutal humectation. In addition, it is posed the problem of the local accumulation of salts in the rhizospher that for all more reason the plant practically doesn`t absorb salts and has tendency to make their accumulation at roots proximity (Bernstein, 1975). It can lead to some problems of water nutrition of culture and consequently a structural instability of the material at the humectation time (Bauder and Brock, 2001; Chaudhari and Somawanshi, 2004).
In this context, it is important to really understand the scientific bases of the properties of the clays in order to optimize the mixture properties. In the sandy soils, one of the determining parameters is the particle size distribution of the sand himself. The form of grains influences the cohesion and the deformation of the sands (Lesturgez, 2005). In the case of the Algerians soils, sands generally underwent the important aeolian transportation because of their faraway source close to the Sahara. In this context, the form is associated to the deposit mode and sands are mainly of rounded shape. This shape is bound to the many shocks that they underwent at the time of their transportation. The sand that we studied is representative of the aeolian sands of the plateau of Mostaganem (Larid, 1993).
If it is important to compare the effect of the texture (mode and size distribution) of the sands on the properties of sands clay mixtures, we must also consider the properties of the clay herself. Indeed, besides its specific role in the mixtures, this material is last used traditionally at the natural state by the local agriculturists in arid regions of Algeria (Dubost, 1992), notably in the oasis under palm for market culture (Rouvillois-Brigol, 1973; Bisson and Callot, 1990; Nasr, 2004) or for the fresh fruit tree plantations. In this last case, the burying of clay in depth is then the indispensable condition to produce citrus fruits in the gardens and the small agricultural exploitations. The clay use then as physical gate to the water of infiltration and play the role of reservoir where concentrates the root system.
Different deposits are available in the countries of Maghreb. As showing by many researchers in Algeria, Morocco and Tunisia, one of the specificities of the soils of these regions is that they are developed on clayey sedimentary materials and greatly carbonated. These materials contain quantities of clay of the order of 30 to 50% but their content in swelling minerals like smectites is weak (Ben Rhaiem et al., 1987; Laribi et al., 2005). Otherwise, considering the geology of Algeria, some important deposits of bentonite formed themselves by change of the volcanic rocks at the level of the seismic zone that crossing the whole Maghreb (Gaucher, 1974; Belaroui et al., 2004). We chose here to study specifically one clay, whose origin is descended of one of these volcanic deposits named Maghnia, that has been characterized little until then and that will be able to be compared to other clays coming from similar deposit (Sadran et al., 1955; Khalaf et al., 1997; Belaroui et al., 2004).
It is important to take account, in this survey, the abiotic constraints of salinity and sodicity that characterizes the arid and semi arid regions. These constraints are bound to a soluble salt accumulation in the rhizospher due to the rare rain fall and to the elevated temperatures that characterize these regions (Halitim, 1984; Hillel, 2005). The phenomenon of secondary salinisation of the irrigated perimeters constitutes a particularly serious threat. In Algeria, 10 to 15% of the irrigated areas are reached by the secondary salinisation phenomenon (Cheverry and Robert, 1998; Bessaoud, 2006).
This study has for object to sum up the properties of sand-clay mixtures submitted to the abiotic constraints of salinity and sodicity and to compare the results gotten with others bibliographical data. It is not about in this survey, to use the bentonite of Maghnia systematically for the sandy soil improvement in arid, semi arid and coastal middles, but to optimize its use as adjusting it to best measures to use it according to the particle size of the sand and the nature of the irrigation water (concentration in soluble salts and sodium absorption ratio (SAR)).
MATERIALS AND METHODS
This study was conducted in the laboratory of soils sciences
of the Department of Agronomy at University of Mostaganem (North-West
of Algeria), during the second semester of 2005 year.
Experimental device: We create a range of materials
from 2 sands to which we adds different proportions of bentonite. With
the sand 1, we make a set of mixtures 0, 5, 10 and 15% of bentonite; with
the sand 2, another set of mixtures 0, 10, 50 and 100% of bentonite. We
irrigate these materials with a range of percolated solutions of salinity
and sodicity (SAR) variable to measuring the HCs. For the range 1, the
values of the SAR are 0, 10, 20 and 30; for the range 2, they are 0, 15,
30 and 45. The values of the salinity of the percolated solutions are
for the two ranges of 10, 100 and 1000 cmol+ L-1.
The distilled water with null salinity and sodicity is taken for the witness
solution. The measures of saturated hydraulic conductivity are repeated
three times according to the device of random complete blocks. We did
a statistical two way analysis of variance to study the relative effects
of the salinity and the sodicity on the HCs (Dagnelie, 2006).
In the first range of measure, we look for the response
of the substrata to a medium sodicity ranged as her appears in arid regions
and semi arid of Algeria (Daoud and Halitim, 1994). The choice of the
clay doses follows the works of the sandy soils improvement of sandy soils
by the bentonite in which the optimal dose of 10% is surrounded by these
of 5 and 15% (Benkhelifa, 1997). In order to permit an exhaustive survey
of the effects of the salinity, it is necessary to take more important
concentrations: 10, 100 and 1000 cmol+ L-1 (Halilat,
In the second set of measures, the range of sodicity
is more important (SAR 45), what permits to express extremely stern conditions
of aridity of the arid and semi arid regions of Algeria (Daoud and Halitim,
1994). In this last case the raised clay contents permit to determine
the behavior clean to the clay.
Characteristics and preparation: We have selected
two sandy samples in two sites located in border seaside (near the Mostaganem
town), (respectively Sablettes: sand 1 and Salamandre: sand 2) that we
have washed with water, after with HCl and in the end with the bleach
in order to assure their disinfection and to limiting their proliferation
by the micro-organisms. Sands are finally rinsed many times with distilled
water. The obtained substrates are braised at 105EC during 24 h. Its size
of particles is determinate with Laser Coulter LS230, range between 0.04
to 2000 μm divided on 116 fractions, with a solid laser of gallium
arsenide of 50 mW and λ = 750 nm (Dur et al., 2004). The particle
size distribution of sand 1 is including between 0.15 and 0.71 mm, with
a diameter φ70 equal to 0.28 mm, then of sand 2 fluctuates
between 0.16 and 0.85, with a φ70 equal to 0.40 mm. This
last sand is so relatively to coarser than the sand 1. We have estimate
the sphericity and shapely rating too for 100 grains for each sand according
to the classic method of characterization (Cailleux and Tricart, 1963).
According to the chart of estimation for this two parameters (Chretien,
1971), the sphericity index records values higher than 50% at 0.7 for
the two sands, what means that the shape has a tendency round relatively
more connoted for the second sand by report to the first one. In the other
hand, the shapely rating records, equally for the two populations of sand,
the values of cumulated percentage is less than 20% at 0.3 and higher
than 60% at 0.7.
It shows, that the particles are of general shape rounded
for the two sands, especially for the second, coarsest. This shape assigns
them an aeolian origin therefore. It is known indeed in the literature
that this type of material evolves essentially under rounded shape, due
to a long way of the sand particles under the effect of violent wind shocks
(Cailleux and Tricart, 1963; Chretien, 1986).
The bentonite comes of a deposit located in the right
bank of the Tafna on the South-East of Maghnia city (North-West of Algeria)
with some characteristics shown in Table 1.
After drying in the dryroom at 105°C during 24 h,
the mixtures are done by hand then for the two sets at dry state and decanted
in the columns of the device of measure of HCs (Kheyrabi and Monnier,
1968; Fies, 1971) that consist to introducing the sand-clay mixture by
successive thin layers in order to assure the homogeneity of the whole.
The saline solution is composed by two salts, the sodium chloride (NaCl)
and calcium le chloride (CaCl2). Table 2 shows
relative contents of the two salts for obtaining the values ranges of
SAR and saline concentrations used for the two sets of measures. This
values are obtained using the following equations: SAR = [Na+]/([Ca++]+[Mg++]/2)0.5
and [Na+]+[Ca++] = 20[C], with [X]: concentration
of element X in meq L-1 in the solution and [C]: concentration
of saline solution in cmol+ L-1.
Physical and chemical
characteristics of bentonite (Bendella, 1994)
|*spe: saturated paste extract
of NaCl and CaCl2 solution (g L?1)
The measure devise of HCs (Aubert, 1978), is composed
by four gradual columns connected to a horizontal tube linked by a faucet
to a other balloon of Woolff itself alimented from another small balloon
of Woolff with another faucet. The superior Woolff balloon aliments the
one situated to a lower level whose role is to assure a constant level
on the samples columns. This last nourishes the columns by the horizontal
tube. After establishing balance between the solution and the sample,
the introverted solution volumes are measured at the end of 1 h of percolation.
The saturated hydraulic conductivity HCs (cm h-1)
is obtained with the following relation (Aubert, 1978):
||Height (cm) of the sample column,
||Height (cm) of the water load,
||Volume (mL) collected water during 1 h,
||Interior section (cm2) of the column.
Before undertaking the measure of HCs, we did percolating
the saline solution through the sample until the Electrical Conductivity
(EC) of collected leaching solution equalizes the EC of saline solution.
The balance times changed according to the bentonite dose and relative
contents of salinity and sodicity in the middle. The EC is measured with
a conduct meter in saturated paste extract (Allison et al., 1954;Rhoades
et al., 1989).
RESULTS AND DISCUSSION
First set: sand 1: The EC of saturated paste extract for substrate
increases under growth effect of saline concentration and SAR (Fig.
1). This parameter increases
Effects of salinity
(cmol+ L-1) on the EC according to the dose
of bentonite and the sodicity (SAR, 0 to 30) for the mixture clay-sand
1, (a) 10% of bentonite, (b) 5% of bentonite, (c) 10% of bentonite
and (d) 15% of bentonite
slightly with the growth of the clay dose and becomes
important for the concentration of 1000 in relation to 10 and 100 cmol+
The HCs decreases under growth of the bentonite dose and SAR (Fig.
2). The saline concentration effect for the dose 10% of bentonite
becomes particularly apparent in relation to the witness. The differences
in the values of the HCs between the SAR 0 and 30 are especially raised
than the saline concentration is weaker. The HCs constitutes one of the
most important parameters to debate effects of the salinity and sodicity
on soils (McIntyre and Loveday, 1979; Chaudhari, 2001; Dikinya et al.,
Second set: sand 2:
The results of HCs to mixtures (Fig. 3), show that it
is in the first place function of the clay content. This parameter passes
200 to 0.1 cm h-1 between the sand to the pure clay what is
coherent with the data of the literature. This parameter (HCs) varies
also according to the saline concentration. Globally, the HCs
HCs (cm h-1)
of mixture bentonite-sand 1, according to the salinity (cmol+
L-1) and sodicity (SAR, 0 and 30)
increases with the saline concentration in the domain
of the weak SAR, that is to say of 0 to 15. It agrees to note the existence
of a significant variation of HCs also, even
Effects of salinity
(cmol+ L-1) on the HCs (cm h-1)
of mixture bentonite-sand 2, according to the dose of clay and
sodicity (SAR, 0 a 45), (a) Sand, (b) Sand+10% of bentonite, (c)
Sand+50% of bentonite and (d) 100% of bentonite
for the pure sand, between 10 and 100 cmol+
L-1 on the one hand and for 1000 cmol+ L-1
on the other hand.
The influence of the SAR permits to decrease strongly
the HCs to the weak saline concentrations and for the high contents of
clay. In this case, the HCs can reach extremely weak values (0.1 cm h-1).
This survey raises several questions notably the importance
of the texture of the sands in the addition of clay to one soil, the role
of the clay according to its nature and the one of the constituents associated
and finally the respective roles of the salinity and the sodicity in the
Effect of the sands textures on the behavior of the
In this work, the mixtures of sand-clay are studied while considering
two sands with different particles size distributions. The results show
that the HCs (Fig. 2, 3) is superior,
at identical clay content, for the sand 2, coarsest (D70 =
0,40 mm) in relation to the sable1, fine (D70 = 0.28 mm). Otherwise,
we observed previously that the content of clay is the main parameter
controlling the HCs (Fig. 3). However it is interesting
of salinity and sodicity on the global variations of the HCs for
the two ranges of clay content
analyze for the two sands, the effects of the variables
of sodicity and salinity, very important in the arid region soils in relation
to the global variations of the HCs.
The results by the statistical two-way analysis of variance (Fig.
4) indicate that the influence part of sodicity in the global variations
of the hydraulic conductivity is not the same in the two mixtures clay-sand.
For the more fine sand (sand 1), we observe an increase of sodicity effect
in the hydraulic conductivity for 5% of bentonite, then a decrease for
10% and at last a fall for 15% of clay. On the other hand, the specific
effect part of salinity in the hydraulic conductivity increases continuously
next to the augmentation of clay content. To regarding, what we know about
the salts and exchangeable sodium role, it means that the salinity controls,
via the swelling limitation of clay in saline middle, one good part of
intergrains porosity, same at high clay content (Radi, 1991).
In the case of coarse sand (sand 2), the sodicity effect
increases progressively with the clay content at the contrary of the decreasing
In this way for 100% of bentonite, the sodicity explains
nearly 97% of clay behavior, however the salinity effect is only near
0.35%. The interaction effect goes to the same salinity variations but
in more high proportions. It means that the sodicity is the principal
factor of porosity colmatage since it allows to the clay to express itself
swelling in the porous space delimited by the sand particles. That`s confirmed
by other works, where the saline solution can make multiplication with
a factor of 3, the clay swelling at a water potential near the saturation
state (Tessier, 1984).
To analyze the evolution of the HCs it is not only important
to refer to the modal size of the grains, but also to the particle size
distribution that really controls the porous space. To this effect, Molle
(2003) show that more the grains are of small size and spread size distribution,
more the HCs is weak. The display of the size distribution of a sand is
expressed by the coefficient of uniformity (CU) equivalent to the report
of two diameters of the particles of a sandy material while taking the
sizes of grains corresponding respectively to 60 and 10% of the particles
of the material: CU = d60/d10, (d60 is
a value of the diameter of which 60% of the material = d60
mm and d10 is a value of the diameter of which 10% of the material
= d10 mm). If the CU<2, the size distribution is uniform
and so CU>2 it is dispersed.
From the measures with the laser granulometer the CU
coefficient can be defined: for the sand 1 this coefficient is of 1.47,
whereas it is of 1.32 for the sand 2. It means that the two sands have
a uniform size distribution, although it is for the sable 1, the fine,
relatively more important than for the sand 2, the coarse. It is why,
the values of the HCs gotten for the first sand are even weaker than those
waited considering its modal size. It confirms a hydraulic behavior difference
between the two sets of measure essentially linked to the textural ranges
of the two sands. In all survey on the improvement of sandy soils properties
is thus, therefore important to know its modal size, but also to have
some information on the uniformity of the size distribution of sands.
Otherwise it is interesting to compare our results with
those of Halilat and Tessier (2000, 2006). With a CU = 1.5, the sand used
by this author presents a spread size distribution that comes closer of
the sand 1 of our survey. Let`s note that in this case, the sand is slightly
different to a textural point of view but also on a morphological level
since its facies are at a time oval and angular, whereas it is rounded
in our case. To this topic, Chretien (1986), working on the porosity of
mixtures of clay and populations of sand with different size distributions
and shapes, concludes that the variations of the porosity are not owed
to the nature of the clay, nor to the mode of samples preparation, but
essentially to the differences between the textural ranges of the skeleton.
Specificity of the bentonite of maghnia: It is to notice that,
in our survey, the clay doses implicated to reducing the HCs strongly
are a lot weaker than in the clayey soils. So, Chretien (1986) show that
it is necessary to reach a clay content of 30% to assure a replenishment
of the inter grains porosity of the sands. It becomes therefore difficult
to compare clay of deposit directly like the bentonite of Maghnia and
clay of soil, how much it is a smectite. For the bentonite of Maghnia,
the swelling properties are so strong that a dose of 12% is sufficient
to modify the properties completely. It is in agreement with the results
gotten by Halilat and Tessier (2000). Otherwise, Tessier (1984) showed
that the behavior of the clays of deposit as the montmorillonite and those
of soils as Bethonvilliers are of very different scale. The comparison
cannot make itself directly enters clays of soils of sedimentary origin
and the clays of deposit descended of the volcanic origin alteration
Characteristics of the deposit smectite compared to those
of soils can probably be joined to the value of the superficial electric
charge and to its localization (Ben Rhaiem et al., 1987). All works
of the literature demonstrate that the deposit clays descended of the
volcanic origin alteration are very sensitive to the sodicity and that
it drives to a dispersion of the clays. One of the possible ways to use
of this clay type, in order to maintain sufficient HCs to the limitation
of the colmatage risks linked to the effects of salinity and sodicity
of substrate, is to optimize the choice of the dose to bring. This optimization
must be reflexive according to the size and the distribution uniformity
of the sand as well as the salinity of the water irrigation and its degree
of sodic aggressiveness (Mamedov et al., 2001). The quality of
the waters irrigation is not of the best in the arid and semi-arid zones
of Algeria, for example in the region of Ouargla (South-East of Algeria),
where the SAR fluctuates between 2,3 to 32 and drags a salinisation by
irrigation of 3 to 5 times the one of water irrigation in the superficial
horizon (Daoud and Halitim, 1994). In this case, the clay dose to adopt
must be as the sodicity doesn`t drag a significant decrease of HCs. It
can explain itself like a limited swelling of the clay in the matrix of
mixture so that the continuity of the pores between the grains of sands
is not colmated by the clayey colloids. So to strong saline concentration
(1000 cmol+ L-1), the force of cohesion between
the grains of sands are increased strongly in presence of salts, so that
the downfall of the grains assembly during the tests of HCs is limited
more that to the weakest concentrations in salts. This property is verified
for the values of the active SAR until 30.
The clays have the swelling properties that depend at
a time on their purity, their mineralogical type, but also the presence
of associated substances who can orient their properties, for example
the presence of salts or carbonates (Caillere et al., 1982; Keren
and Ber-Hur, 2003; Sally and David, 2004). Beside the fact that the mode
of preparation of clay amendment is important, the fact to modifying a
sample in water increases considerably its hydratation properties. It
is the reason for which in our survey the clay has been added to the dry
state, therefore no remanied in water, in order to limit the colmatage
of the porosity. Besides it is a protocol as used traditionally in Algeria
to improve the sandy soil properties.
In this study, we tried to show the interest of the bentonite
of Maghnia as mineral amendment for essentially sandy soils submitted
to abiotic constraints of salinity and sodicity. It constitutes an interesting
setting for the field survey of amendments to basis of clay.
This clay, apart from the calcite that is intimately
associated, is essentially saturated by calcium that limits its swelling.
It is necessary to reach values of the water SAR of irrigation of 45 so
that the HC in saturated state of the mixtures approaches the threshold
of impermeability. It means that the proportion of sodium in the soil
solution must be raised so that there is a significant exchange with sodium.
Our results show that the salinity has an effect contrary to the one of
the sodicity on the HCs, since it has the tendency to reduce the swelling
properties of the clay and therefore to limit the volume of the clay within
the sand matrix. It results that the porosity of assembly of the grains
of sands is not completely full by the clay assuring a certain HCs. It
is to note that the salinity threshold of 1000 cmol+ L-1
and sodicity of 45 used in this survey, represent some extremely stern
conditions of aridity. These thresholds demonstrate that the use of this
bentonite is much appropriated for the physical and hydric sandy soil
improvement and so attenuation of deterioration effects of the structure
by abiotic constraints of salinity and sodicity in arid and semi-arid
area. For this clay, the effects of the sodicity and the salinity are
a lot more tamponed to the one of Mostaganem. Otherwise, although in the
case of fine sand and under an important saline-sodic condition of middle,
the conduct of the irrigation should be do with a particular attention
especially. The bentonite use under brut form extensively reserved to
local practices, notably in the setting of market or domestic productions,
can also apply to the plantation of trees by burying to the basis of the
root system. This practice can nearly constitute an alternative for arboriculture
practically absent in arid and semi-arid zones and same littoral of Algeria.
This survey, do not recommend to using systematically
the bentonite of Maghnia for the sandy soil improvement in arid, semi
arid and littoral regions, but it is in purpose to optimize its use by
adjusting to the best the dose brought in according to the size distribution
of sand and nature of the water irrigation too (concentration in soluble
salts and SAR).
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