Journal of Agriculture Food and Development, 2016, 2 5-15

Journal of Agriculture Food and Development, 2016, 2, 5-15 5

© 2016 Revotech Press

Impact of Successive Wetting and Drying Cycles on Some Physical Properties of Gypsifereous Soils

A.W. Al-Kayssi*

Soil and Water Resources Department, College of Agriculture, University of Tikrit, Iraq Abstract: A laboratory experiment was carried out to quantify the impact of three wetting-drying cycles on physical and mechanical properties of gypsifereous soils such as bulk density, penetration resistance, drying index, shrinkage limit and pore geometry changes. The soil gypsum content treatments were 60.0 (G1), 137.6 (G2), 275.2 (G3), 314.2 (G4), 486.0 (G5), 688.3 (G6) and 860.0 (G7) g.kg-1. Soil samples were equilibrated to a matric potential -4.9, -9.8, -34 and -1500 kPa after saturation for each wetting-drying cycle. Peak cone index values showed that besides water content and bulk density, penetration resistance was positively affected by gypsum content. Drying index values at 1200 kPa matric potential were increased from 79.96% to 96.50% for the soils with gypsum content G1 to G7.

Keywords: Cone index, Wetting-drying cycles, Drying index, Shrinkage limit, Pore geometry.

1. INTRODUCTION

Gypsum is rock-forming mineral that also occurs in soils. In arid and semiarid environments, gypsum can be a major soil component and its chemical formula (CaSO4.2H2O) is quite simple. Gypsifereous soils of Iraq were studied extensively and found that gypsum crystals form mainly in the surface horizons as a result of evaporation of ground water situated at each depth that the capillary fringes reach the surface. The crys- tallization takes place essentially in voids. In some cases where leaching took place, accumulation of gypsum appears in deeper layers. In arid and semiarid areas gypsum is dissolved in the wet season and tends to be precipitated when the soil is dry and the gypsum occurs mostly in channels or tubules and packing voids [1,2]. Gypseous and gypsifereous are two adjectives used for materials containing gypsum. According to the common definitions of these words, gypsifereous was proposed to be used when the soil contains some gypsum, but it does not control most properties of the soil, while, gypseous used for soils whose main chemical and physical properties are due to the gypsum [3]. Soils with gypsum are distributed in several counties, especially in near and the Middle East, North Africa and other arid and semiarid areas where a sustainable agriculture is needed [4]. Although they have been the object of many investigations, mainly in relation to soil survey, soil genesis and irrigation, little attention has been given to the characteristics affecting plant development, and even less to the factors that condition their physical fertility [5]. Poor aggregation and structure [6,7], little available water [8], irregularities in the *Address correspondence to this author at the Soil and Water Resources Department, College of Agriculture, University of Tikrit, Iraq; E-mail: [email protected]

moisture distribution in the soil and in the uptake of water by plants [9,10] amongst other reasons, have been cited by some authors as adverse characteristics for root growth, although the conclusions are some- times contradictory due to the wide variation of gypsifereous materials and differences in experimental methods. No data exist about the mechanical impedance of gypsifereous materials, except the findings of [5]. They found that gypsum contents ranged from 0 to 900 g kg-1 were positively correlated with penetration resistance. Generally, it was found that root growth is inhibited or even completely impeded when the soil contains more than 250 g kg-1 of gypsum, in the case of Wheat [6], Cotton [9] and Pine Eucalyptus trees [11]. Moreover, the gypsifereous soils become hard when dry, and root growing within the horizon are distributed mainly along cracks or horizontally at the top of the horizon [12]. The desired soil structure following tillage of agricultural soil is often unstable and susceptible to coalescence of aggregates and reduction of inter- aggregate porosity due to wetting and drying cycles [13]. However, in the field, soils can be exposed to different wetting conditions, which can affect the role of the soil properties on seal formation [14]. Geremew [15] suggested that the successive wetting-drying cycles increased the cumulative swelling deformation; a stabilization of swell strain of each cycle is reached at the end of the third or fourth cycle.

A soil cone Penetrometer has been used widely to assess soil strength, root penetration resistance, and predict trafficability and bearing capacity for foundation [16,17].

Soil cone Penetrometer measures the soil penetration resistance, reported as cone index (penetration force/cone base area) as a function of depth [18,19].

6 Journal of Agriculture Food and Development, 2016, Vol. 2 A.W. Al-Kayssi

The system has been automated and modified to improve the data acquisition rate and evaluated to produce soil strength map [20,21]. One can observe that the use of a standardized cone Penetrometer apparatus and procedures [18,19] is and will remain to be an important tool in precision soil strength measure- ment technology. The influence of soil parameters, mainly soil moisture and bulk density, on cone index may affect the use of cone penetrometer in soil strength detection. Many studies have addressed the effect of soil moisture and bulk density on cone index in laboratory and field scale studies [22-25]. Ayers and Perumpral [22] studied soil moisture-bulk density-cone index relationships on artificial soils obtained by mixing different quantities of zircon, sand and clay. According to their report, the cone index decreased with increased soil moisture. The effect of bulk density varied with soil moisture such as that at low soil moisture, the influence of soil bulk density on cone index was high and at high soil moisture, cone index was less dependent on bulk density. Rajaram and Erbach [23] studied the effect of drying stress induced by a wetting and drying cycle on soil physical properties of a clay loam soil. It was observed that cone penetration resistance measured at 50, 100, and 150mm depths increased with increased drying stress. The study was conducted in a uniform soil density profile.

The objectives of this research were to identify the successive wetting and drying cycles on penetration resistance and peak cone index of soils containing gypsum ranged between 60.0 and 860.0 g.kg-1.

2. MATERIALS AND METHODS

2.1. Study Area

The experiment was conducted in the laboratory on soil materials collected from the surface and sub-surface gypsifereous profile from the experimental station of the University of Tikrit, Iraq. The station is located at latitude of 34° 36/ N and 43° 41/ E longitude at an altitude of 250m above mean sea level. The climate of

the study area is semi-arid and sub-tropical with an average annual rainfall of 150mm. The rainfall occurs during October to April (rainy season), which has uneven distribution. The soil of the experimental site is Typic Calcigypsids [26] with loam texture (composed 40% sand, 43% silt and 17% clay), shallow, very poor in organic matter and low plant nutrient. The water holding capacity of soil is low with high infiltration rate. Some physical characteristics of the soil at the experimental site are presented in Table 1.

2.2. Experimental Setup

Soil samples with 137.6 (G2), 275.2 (G3), 314.2 (G4), 486.0 (G5) and 688.3 (G6) g kg-1 gypsum content were prepared by mixing materials passed through a 2mm sieve from surface soil layer with low gypsum content (G1) (60.0 g kg-1) and sub-surface soil layer with high gypsum content (860.0 g kg-1) (G7) from the experimental station of the University of Tikrit, Iraq. The prepared soil samples as well as the surface and sub-surface soil materials were moistened to field capacity, mixed thoroughly and then incubated in plastic bags at room temperature (25°C±1°C) for three months. The prepared soil samples were daily mixed in the plastic-bags during the incubation period. Water retention curves were fitted to the measured water-desorption values using the Van Genuchten approximation [27]. From these curves, the equivalent pore size distribution was calculated for each soil sample (G1 – G7).

Thirty-six core samples (5.0 cm in diameter, 5.0 cm deep) were prepared for each soil (G1 – G7) passed through 2mm sieve. The cores were brought to saturation (first wetting cycle) in a sand box (matric potential -0.01 kPa). Then, three cores were equilibrated to each soil matric potentials of -4.9 kPa and – 9.8 kPa in the sand box, and to -34 kPa and -1500kPa using a pressure plate [28]. The rest 24 cores were left to air drying for 72 hours before re-wetting to the second cycle of saturation using the same above mentioned procedure. After the second wetting cycle step, three cores were equilibrated to soil matric potentials of -4.9,

Table 1: Some Physical Properties of Soil at the Experimental Site

Soil Depth Bulk Density Gypsum Particle-Size Fractions

Sand Silt Clay Soil Texture

FC* PWP** MWD (mm) Organic Matter

cm Mg m-3 g kg-1 -------- kg kg-1 -------- ---- m3 m-3 ---- mm %

0-13 1.42 060.0 410 435 155 Loam 0.231 0.073 0.231 0.53

13-72 1.37 860.0 -----† -----† ----† -----† 0.124 0.025 0.109 0.06 *, ** Field capacity at 33 kPa, and permanent wilting point at 1500 kPa soil water tension respectively. † Unable to estimate the particles size distribution because of the rapid flocculation of aqueous soil suspension with high soil gypsum content.

Impact of Successive Wetting and Drying Cycles Journal of Agriculture Food and Development, 2016, Vol. 2, 7

-9.8, -34 and 1500 kPa. The rest twelve cores were left to air drying for 72 hours before re-wetting to the third saturation cycle and equilibration to the same above mentioned soil matric potential. At each matric potential, the penetration resistance was measured by means of Digital Static Cone Penetrometer HS-4210 (HUM- BOLDT MFG.CO.UK) with an angle of 60° cone. One penetration test was performed on each core with three triplicates for each matric potential. The penetration speed was 15 mm min-1, and the penetration resistance was recorded to a depth of 30 mm in each test, following the methodology of [29]. The digitally obtained cone penetrometer data for each treatment was analyzed to extract the peak cone index.

In order to compare the results of the different tests a cone index (CI) was calculated for each curve, by measuring the mean value of penetration resistance (kPa), down to a depth of 15mm. This depth was chosen because the curves had a tendency to follow a linear trend at least to 15mm, and therefore the value measured to this point allowed a better comparison between the different tests [30]. Soil particle size analysis, bulk density was determined for each soil sample Soil shrinkage index was determined from the volume decrease per unit weight of soil [31]. Porosity, f, was calculated using the relationship f=[1-ρb / ρs].100 (ρb= bulk density, Mg.m-3, and ρs=density of solids, Mg.m-3).

2.3. Statistical Analysis

ANOVA and paired t-test (SYSTAT8, Systat Inc., US) were carried out at p < 0.01 to determine the treatments significant differences. A Hewlett-Packard Regression Analysis Package was used to analyze the data set. Treatment means were compared with a least significant differences procedure. Statistical compari- sons were considered significant at p < 0.01.

3. RESULTS AND DISCUSSION

3.1. Soil Bulk Density of Gypsifereous Soils as a Function of Matric Potential

Figure 1 depicts the relationship between bulk density values as a function of soil gypsum content during the three wetting-drying cycles. The examined gypsifereous soil samples demonstrated distinctly different bulk density values during the course of the three sub- sequent wetting-drying cycles. It can clearly be seen from the figure that bulk density values at 1200 kPa matric potential were significantly increased from its initial value (1.2 Mg.m-3) to the maximum values (1.53 to 1.43 Mg.m-3) after the second wetting-drying cycle for the soil samples with gypsum content G1 to G5. While, the soil samples with a gypsum content of G6 and G7 exhibits maximum bulk density vales (1.38 and 1.37 Mg.m-3) after the first wetting-drying cycle.

When soil cores were filled with gypsifereous soil samples, the first wetting process promotes the slump- ing of beds, which is identical to a complete settlement resulting from wetting and aggregate slaking. The aggregate disintegration caused by the slaking process [32] is important mechanism of seal formation. This mechanism of aggregate breakdown appears when the aggregate is not strong enough to withstand the stress- es produced by differential swelling and the mechanical action of moving water [33,34]. The adhesion forces between soil particles inside an aggregate decrease during wetting [13], which in turn, should make the aggregates easier to breakdown. Additionally, the decrease in clay content of the soil (Table 1) decreases aggregate strength, but at the same time increases the slaking forces [14]. Hence, in gypsifereous soils (G5 and G6) with low clay content and aggregate stability (Table 1), one cycle of wetting was sufficient to break-

Figure 1: Bulk density versus soil gypsum content for three wetting drying cycles.


down the aggregate and to develop a seal. In contrast, in high aggregate stability, slaking plays an important role in aggregate breakdown and seal formation. Thus, the first drying process was the most important one with respect to the increase of the bulk density. More- over, one cycle of wetting and drying was enough to obtain a bulk density value very close to the final value after three cycles.

Shrinkage limit values of each wetting-drying cycle at 1200kPa matric potential were plotted against soil gypsum content and presented in Figure 2.

A good exponential regression equation is deduced for each wetting-drying cycle. There is an indication from the data in this Figure that there are significant considerable reductions in shrinkage limit values with increasing soil gypsum content. Moreover, the derived linear relationship between shrinkage limit values and bulk density by pooling all the data obtained for the three wetting-drying cycles (Figure 3) explain the reduction of shrinkage limit as a function of bulk density.

Figure 3 reveals that shrinkage limit values were considerably decreased with increasing bulk density for all gypsifereous soil samples. The reduction range of shrinkage limit values of the soil with gypsum contents G1 was much higher than the reduction values range with the soil gypsum content G2 – G7. This could be ascribed to the reduction of soil clay content and organic matter with increasing gypsum content (Table 1) [35].

To better understand the changes induced in the soil bulk density by the successive wetting-drying cycles, the equivalent pore size distribution was calculated for the soil samples with different gypsum content from the water retention curves fitted to the measured water-desorption values using the Van Genuchten approxi-

mation [27]. Figure 4 show the relationships between the percent of pore size classes for each wetting-drying cycle and soil gypsum content.

A look at the Figure quickly reveals that the lower percent of pore size (100 – 300 µm) was conjugated with the lower soil gypsum content (G1). These values were increased gradually with increasing soil gypsum content. Thus, the soil with higher soil gypsum content (G7) gives higher percent of the pore size class 100 – 300 µm. Contrary, the lowest percent of pore size class < 0.2 µm has been shown with soil gypsum content of G7 and increased with decreasing soil gypsum content. Generally, percent of pore size classes 100 – 300 µm and 60 – 100 µm were increased with increasing soil gypsum contents. While, the percent of pore size classes 30-60, 10-30, 0.2-10 and < 0.2 µm were decreased with increasing soil gypsum content (Figure 4). This supports the view that there was no cementing and that the soil with high gypsum content behaves like a typical granular soil [5]. Thus, with increasing soil gypsum content, its contribution to the total variation of pore size distribution will considerably decreased. The pore-size-network attributes of the soil samples were also found to vary as a function of the number of wetting-drying cycles. As shown in Figure 4, cyclic wetting and drying results in a much higher increasing in pore size 10-30, 0.2-10 and < 0.2 µm compared with a reduction in pore size classes of 100-300, 60-100 and 30-60 µm. Generally, the first wetting-drying cycle was enough to obtain a pore size distribution value very close to the final values after the third wetting-drying cycle. A look at Figure 4 shows that the impact of wetting-drying cycles was not effective on the soils with gypsum contents of G5, G6 and G7, and the pore size distribution was kept constant during the wetting-drying cycles.

Figure 2: Shrinkage limit versus soil gypsum content for three wetting-drying cycles.


Figure 3: Soil shrinkage limit versus bulk density of the investigated gypsifereous soils 60.0 (G1), 137.6 (G2), 275.2 (G3), 314.2 (G4), 486.0 (G5), 688.3 (G6) and 860.0 g.kg-1 (G7) for the three wetting-drying cycles.


Figure 4: Pore size distribution of soils with different gypsum content for the three wetting-drying cycles.

Finally, we have to draw our attention also to the fact that the gypsifereous soils were poor structured. Nevertheless, in their pedogenetical processes, there are no evidences of C accumulation or translocation. Thus, the low C amount as well as clay content (Table 1), results in an extremely aggregate water dispersion [36].

3.2. Cone Index (CI ) of Gypsifereous Soils as a Function of Matric Potential

The mean values of CI for each gypsifereous soil sample (G1-G7) have been plotted against the corres- ponding matric potentials (Figure 5). The general tendency observed in these curves is that CI increase fallowing a logarithmic trend. Peak cone indices at mat-

ric potential of 1200 kPa were 2.06, 2.75, 3.05, 3.73, 3.63, 3.70 and 3.73 for the soils with gypsum contents G1, G2, G3, G4, G5, G6 and G7 respectively. On the other hand, CI values were linearly correlated with shrinkage limit values when pooling all the data obtained during the three wetting-drying cycles (Figure 6). The good linear relationships explain the considerable increasing cone index with decreasing shrinkage limit. Whatever the explanation, this behavior could be explained by the differences in the contribution of water at different matric potentials to soil strength. Water content has been reported as an important factor controlling penetration resistance of gypsifereous horizons [5,29]. But there is no available information about the effect of moisture on the penetration resistance of gypsifereous soils and the change in its strength upon drying.


Figure 5: Cone index versus soil matric potential of the investigated gypsifereous soils 60.0 (G1), 137.6 (G2), 275.2 (G3), 314.2 (G4), 486.0 (G5), 688.3 (G6) and 860.0 g.kg-1 (G7) for the three wetting-drying cycles.


Figure 6: Cone index versus soil shrinkage limit of the investigated gypsifereous soils 60.0 (G1), 137.6 (G2), 275.2 (G3), 314.2 (G4), 486.0 (G5), 688.3 (G6) and 860.0 g.kg-1 (G7) for the three wetting-drying cycles.


Figure 7: Relationships between drying index and number of hours passed after saturation for soils with gypsum content 60.0 (G1), 137.6 (G2), 275.2 (G3), 314.2 (G4), 486.0 (G5), 688.3 (G6) and 860.0 g.kg-1 (G7) during three wetting-drying cycles.

3.3. Soil Drying Index

The magnitude of soil dryness was quantified using soil drying index (Equation 1), computed by comparing the soil moisture content at -4.9, -9.8, -34 and -1500 kPa matric potentials after each wetting-drying cycle with the soil moisture content at -0.01 kPa matric potential for each gypsifereous soil sample [37]:

Soil Drying Index (%) = !"#$!!"#$%&'(!!"#!! !− !!"#$!!"#$%&'(!!"#!!

!"#$!!"#$%&'(!!"#!! !*100 (1)

Where:

i = hour index 1, 2, 3 ... etc. that shows the sampling hours.

The soil drying index values were affected both by the number of hours passed and the soil gypsum content for each wetting-drying cycles. As more hours passed, the drying index values increased linearly for all soil samples with different gypsum content and wetting-drying cycles (Figure 7).

Figure 7 reveals that maximum drying index values were confined between 90.54% and 92.39% after 24 to 25 hours of drying cycle for the soil samples with gypsum content G5, G6, and G7, but this time was extended from 34 to 42 hours for the maximum drying indices 88.05%, 85.40%, 83.93% and 81.85% of the soil samples with gypsum content of G4, G3, G2 and G1

respectively. This trend was shown for all the wetting-drying cycles. Soil drying index was positively correlated with the peak cone index values for the three wetting-drying cycles (Figure 8). It can clearly be seen a pronounced impact (about 74%) of the drying index on peak cone index for the soils with gypsum content G1 to G4, while such impact did not exceeds 2.7% for the soils with gypsum content G5 to G7. This could be ascribed to the limited clay, organic matter (Table 1) and weak soil structure [38].

4. CONCLUSIONS

In the light of the obtained results, the strength of the studied gypsifereous soils increased upon the three successive wetting-drying cycles due to the changes in bulk density as a result to the porosity reduction especially relevant in the largest pores size classes. One wetting-drying cycle was enough to obtain a bulk density and cone index values very close to the final values after the third cycle for the soils with gypsum content G6 and G7. While, the soils with gypsum contents G1-G5 exhibit a maximum bulk density and cone index values after the second wetting-drying cycle. Drying index values were linearly increased as more hours passed after saturation for all the studied gypsifereous soil samples. A pronounced impact (about 74%) of the drying index on peak cone index was shown for the soils with gypsum content G1 to G4, while such impact did not exceeds 2.7% for the soils with gypsum content G5 to G7.


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Received on 26-12-2015 Accepted on 07-01-2016 Published on 18-01-2016 © 2016 A.W. Al-Kayssi; Licensee Revotech Press. This is an open access article licensed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/), which permits unrestricted, non-commercial use, distribution and reproduction in any medium, provided the work is properly cited.

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Journal of Agriculture Food and Development, 2016, 2 5-15