Effect of soil texture and CaCO3 content on water infiltration in crusted soil as related to water salinity
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Irrig Sci (1985) 6:281-294 Irrigation
: clence Springer-Verlag 1985
Effect of Soil Texture and CaCO3 Content on Water Infiltration in Crusted Soil as Related to Water Salinity *
M. Ben-Hur, I. Shainberg, D. Bakker, and R. Keren
Division of Soil Physical Chemistry, ARO, Bet Dagan, Israel
Received July 29, 1984
Summary. The effect of soil texture and CaCO~ content on water infiltration rate in crusted soil was studied with the use of a rain simulator. Two types of soils with low exchangeable sodium percentage (ESP < 3.0%) were studied: (i) calcareous soils (5.1-16.3% CaCO3) with a high silt-to-clay ratio (0.82-1.47) from a region with < 400 mm winter rain; and (ii) non-calcareous soils with a low silt-to-clay ratio (0.13-0.35) from a region with > 400 mm winter rain. Soil samples with clay percentages between 3 and 60 were collected in each region. Distilled water (simulating rainfall) and saline water (simulating irrigation water) were sprinkled on the soil. The soils were exposed to "rain" until steady state infiltration and corresponding crust formation were obtained. For both types of soils and for both types of applied water, soils with ~ 20% clay were found to be the most sensitive to crust formation and have the lowest infiltration rate. With increasing percentage of clay, the soil structure was more stable and the formation of crust was diminished. In soils with lower clay content (< 20%), there was a limited amount of clay to disperse and, as a result, undeveloped crust was formed. Silt and CaCO3 had no effect on the final infiltration rate for either type of applied water, whereas with saline water, increasing the silt content increased the rate of crust formation.
The formation of surface crusts by rain has been investigated in many studies, due to their important effect on many soil phenomena, e.g., water infiltration and seedling emergence.
Studying the structure of the crust microscopically, McIntyre (1958) found the crust to consist of two parts: an upper skin seal, 0.1 mm thick, attributed to compaction by raindrop impact; and a deeper, "washed-in" region, 2 mm thick, of decreased porosity, attributed to fine-particle movement and accumulation. Con-
* Contribution from the Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel. No. 1130-E, 1984 series
282 M. Ben-Hur et al.
versely, Gal et al. (1984) found that on soils with ESP < 1.0, the crusts consisted of the compacted skin seal layer only; whereas, on soils with ESP > 1.0 exposed to rain, the crust consisted of naked sand and silt grains over a dense "washed-in" layer (Chen et al. 1980; Gal et al. 1984).
Based on the effect of rain drops' impact and the chemical properties of the soil water system, it was postulated that crust formation is due to two mechanisms (McIntyre 1958; Agassi et al. 1981; Kazman et al. 1983).
1. A breakdown of the soil aggregates caused by the impact action of the raindrops over the soil surface. The destruction of the aggregates reduced the average size of the pores of the surface layer. Also, raindrops' impact caused compaction of the uppermost layer of the soil. These factors produced the thin skin seal (Epstein and Grant 1967; Farres 1978; Morin and Benyamini 1977).
2. A chemical dispersion of the soil aggregates and the soil clays which can then migrate into the soil with the infiltrating water and clog the pores immediately beneath the surface ("washed-in" layer) (McIntyre 1958; Agassi et al. 1981; Gal et al. 1984).
The infiltration rate (IR) is affected by both the soil sodicity (Kazman et al. 1983) and by electrolyte concentration of the perculating water (Agassi et al. 1981). Increasing the soil ESP in the range between 1.0 and 5.0 resulted in a sharp decrease in the final IR when exposed to distilled water (simulating rainwater) rain. The intensity of chemical dispersion and the movement of the clay to the "washed- in" layer decreased as the electrolyte concentration of the applied water increased. By increasing the electrolyte concentration in the applied water, crust formation was diminished and the final IR of the crusted soils was maintained at higher values. Thus, it is possible that in soils containing minerals that readily release soluble electrolytes, such as calcareous soils, the formation of crust will diminish when the soils are subjected to distilled water "rain".
The tendency of soils to form a crust depends also on the stability of their structure. Stability of the soil's aggregates increases with increase in clay content. Thus, the stability of the aggregates against the impact action of the rain drops should also increase with an increase in clay content. Kemper and Koch (1966) found a good correlation between clay content (in the range between 5 and 90%) and wet sieve aggregate stability. Increasing the clay content resulted in a hyper- bolic increase of the wet s ieve aggregate stability. This was attributed to clay particles acting as cementing material holding the particles together in the aggregates. Thus, as a result of this mechanism, formation of crust should be slowed down by increase in clay content. Conversely, Moldenhauer and Kemper (1969), studying the effect of clay content and soil aggregate stability on the infiltration rates of four types of soils that were exposed to rain, found that the final infiltration rates of the crusted soils decreased as the percentage of clay in the soil increased.
Organic matter and silt content in the soil also have an effect on the aggregate stability (Cary and Evans 1974; Wischmeier and Mannering 1969). They found that soils with low organic matter and high silt content usually have low aggregate stabilities. Similar results were obtained by Moldenhauer and Long (1964), who investigated the effect of the soil texture on the energy required to initiate runoff (an indication of crust rate formation); there was good agreement between the silt content and the rate of crust formation.
Effect of Soil Texture and CaCO3 Content on Water Infiltration in Crusted Soil 283
The objective of the current study was to determine the effect of soil texture and CaCO3 content on crust formation and its properties by the use of a rain simulator.
Materials and Methods
Two types of soils with various clay contents were used in this study: 1. Calcareous soils from the northern Negev and the Pleshet Plain in Israel.
The average annual precipitation in these regions is 200-400 mm. These soils contain 5-16 % lime and have a high silt/clay ratio (0.82-1.47) (Table 1).
2. Non-calcareous soils that were sampled from the Coastal Plain and from the Golan Heights in Israel. The average annual precipitation in these regions is 600 and 1,000 mm, respectively. The silt/clay content in these soils is low (0.13-0.35). Some chemical and physical properties of these soils are presented in Table 1. The low silt content of the non-calcareous soils suggests that these soils might be more stable to crust formation.
In order to maintain the same ESP ( ~ 2.0), in all the soil samples, 1 m 2 plots of soil were leached in the field, with 120 liters of 0.2 M chloride solution with a Sodium Adsorption Ratio (SAR) of 2.0. Leaching was performed by three applica- tions of 401 portions of the 0.2 M solutions, with a one-week interval between applications. Finally, the plots were leached with 40 1 of 0.01 M solution of the same SAIl, air-dried (in the field), and soil samples from the 0.10 m layer were taken to the laboratory for further studies. This technique of leaching the soils in the field was used to reduce soil structure breakdown, which usually takes place when soil samples are leached in the laboratory with the desired solutions. The soil samples were screened to 0-4 m m aggregate size and the soil texture, CaCO3 content and final ESP were determined by standard methods (US Salinity Lab. Staff 1954). Soil samples were placed in 30x50 cm perforated trays, 2.0 cm deep, over a layer of coarse sand (four replicates). The trays were placed in a rainfall simulator (Morin et al. 1967), at a slope of 5%, and saturated with tap water (0.85 dSm -1) from underneath. Thereafter, the soils were subjected to rainfall of distilled water (DW) with electrical conductivity (EC) of =0.01 dSm -1 (simulating rainwater) and of saline water (SW) with SAR 2 and EC--- 5.0 dSm -~ (simulating saline irrigation water). Typical mechanical parameters of the applied rain were: rainfall intensity of 31.6 mmh -1, rainwater median diameter, 1.9 mm; median drop velocity, 6.02 ms-l; and total kinetic energy, 570 J m -2 h-L The volumes of runoff and of water infiltration were recorded. The soil loss was measured by drying the runoff water and weighing the eroded material.
Results and Discussion
The IR of the two types of soils as a function of the cumulative rainfall when subjected to DW rain are presented in Fig. l. It is evident that the IR of all the soils dropped sharply as the depth of cumulative rain increased until final or steady state IR values were obtained. The decrease in the IR is due to the formation of crust on
284 M. Ben-Hut et al.
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Effect of Soil Texture and CaCO3 Content on Water Infiltration in Crusted Soil 285
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CUMULATIVE RAINFALL (ram)
Fig. I. The infdtradon rates of two types of soils as a function of the cumulative rainfall when subjected to distilled water (DW) rain
the soil surface (Morin and Benyamini 1977). The final IR values and the rates of decrease of the IR's with the cumulative rain were different for the various soils. In the non-calcareous sandy soil (2.3% clay) there was no measurable drop in IR and therefore the rate of rain application was determining the measured IR. However, the final IR recorded - 30.6 mm h -1 - had a high value compared with other soils which suggest that only a weak crust was formed due to the low percentage of clay in this soil. There was not enough clay in the soil to clog the soil pores or to form a crust.
With increase in clay concentration in the range up to 19.2% clay, the rate o f the drop in IR increased and the final IR decreased. As the clay percentage increased above 19.2%, the rate at which the IR dropped was lower and the final IR was maintained at higher values compared with the other soils. It is evident that the clay fraction of the soil has two opposing effects: (a) The clay is the substrate for crust formation; thus, in soils with a low clay content, the rate o f crust formation
286 M. Ben-Hur et al.
increased and the steady state hydraulic conductivity of the crust decreased as the clay content increased. (b) The clay acts as a cementing material, stabilizing the soil aggregates against the beating action of the raindrops; thus, increasing the clay content of the soil aggregate prevents their disintegration and crust formation.
Similar results were obtained with the calcareous soils exposed to DW rain (Fig. 1). The rate of the drop in IR with the depth of rain increased, and the final IR decreased, with an increase in clay percentage up to the loamy soil with 22.4% clay. As the clay percentage increased to 39.9%, the final IR was maintained at a greater depth of rain and its value increased.
The IR's of the calcareous and non-calcareous soils, exposed to SW rain, are presented in Fig. 2. It is evident that when saline water was used instead of distilled water the final IR is maintained at higher values. As indicated in the Introduction, with SW the chemical dispersion of the soil clays is not as great. Thus, it is primarily the mechanical impact of the rain drops which forms the crust. Consequently, the rate of crust formation is slower, the structure of the crust consists of only a thin skin layer on the surface of the soil, and the hydraulic properties of this layer are such that the final IR is higher than the IR of the crust formed on soils "rained on" with DW.
The same trend observed in DW, namely that soils with 20% clay are the most susceptible to crust formation, is also obtained with SW.
The final IR of the soils may serve as a good index to characterize the structure and the strength of the soil's crust. Thus, in order to study the effect of soil texture and CaCO~ content on crust properties, the final IR's of the various soils, as shown in Figs. 1 and 2 as a function of the clay percentage are presented in Fig. 3 and in Table 2 with their standard deviation (SD). The final IR's of the same soil with ESP > 4.6% (Kazman et al. 1983) are also presented in Fig. 3. It should be remembered that in the present study the ESP of the soils was _-< 2.5 (Table 1).
The following characteristics are noted: 1. At any given clay content in each soil type, the final IR's obtained with DW
rain were lower than the final IR's obtained with SW rain. The final IR's of the soils exposed to SW were 4.0-6.0 m m / h higher than the final IR's obtained with DW. Similar results were obtained and discussed by Agassi et al. (1981).
2. The final IR's of the calcareous and non-calcareous soils at any given clay content were similar for both the DW and SW. The high concentrations of CaCO3 in the calcareous soils (5-16%) did not have any effect on the final IR. Similar results were obtained by Kazman et al. (1983) with sandy loam and silty loam soils.
However , in studies on the effect of CaCO3 on the hydraulic conductivity (HC) of sodic soils, it was found that the presence of CaCO3 maintained the structure of the soils and prevented the HC decline of sodic soils (Felhendler et al. 1974; Shainberg et al. 1981; Shainberg and Gal 1982). It was proposed (Shainberg et al. 1981) that when calcareous soils were leached with DW, the CaCO3 released electrolytes at a rate sufficient to maintain the concentration of the soil solution above the fl...