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Soil Management

Efficiency of Sulfuric Acid, Mined Gypsum, and Two Gypsum By-Products in Soil Crusting Prevention and Sodic Soil Reclamation

^a Agricultural Resources Evaluation Center, Dep. of Agriculture, Gobierno de Navarra, Ctra Sadar s/n, Edificio "El Sario", 3^a planta, 31006 Pamplona, Navarra, Spain
^b Dep. of Soils and Irrigation, Research and Agronomic Technology Center, Diputación General de Aragón, Apdo. 727, 50080 Zaragoza, Spain

^* Corresponding author (espe{at}amezketa.net; esperanza.amezketa.lizarraga{at}cfnavarra.es)

Received for publication September 6, 2004.

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY AND CONCLUSIONS REFERENCES

 
Sulfuric acid and gypsum-like by-products are potentially effective amendments in preventing soil crusting and reclaiming calcareous sodic soils. However, their relative efficiencies at chemically equivalent rates are not well documented. We evaluated the efficiency of four amendments (sulfuric acid, mined-gypsum, and the by-products coal-gypsum and lacto-gypsum) in crusting prevention of two calcareous nonsodic and sodic soils and in sodic soil reclamation. Treatments for crust prevention consisted of surface-applied amendments at equivalent rates of 5 Mg pure-gypsum ha^–1. Treatments for sodic soil reclamation consisted of surface-applied acid and soil-incorporated gypsums at rates of 1 pure-gypsum requirement. The efficiency of these amendments was evaluated by comparing the final infiltration rates (FIR) of the amended vs. the nonamended soils measured in disturbed-soil columns pounded with low-salinity irrigation water. Electrical conductivity (EC) and Na in the leachates of the sodic soil were measured. In the crusting prevention experiment, FIRs (mm h^–1) of the nonsodic soil were 21 (nonamended), 33 to 35 (gypsum materials), and 53 (sulfuric acid), whereas those for the sodic soil were 0 (nonamended), 9 (lacto-gypsum), 15 to 17 (coal- and mined-gypsum), and 21 (sulfuric acid). In the sodic-soil reclamation experiment, FIRs were 0 (nonamended), 8 to 9 (gypsum-materials), and 17 (sulfuric acid) mm h^–1. All amendments were effective in crusting prevention and soil reclamation, but sulfuric acid was the most efficient due to the fastest EC and Na reductions in the leachates. The three gypsum-materials were equally effective in the reclamation process and in the nonsodic soil crusting-prevention, whereas lacto-gypsum was less efficient in the sodic-soil crusting-prevention.

Abbreviations: A, cross sectional area of the soil columns • ANOVA, analysis of variance • C, total electrolyte concentration • CEC, cation exchange capacity • CG, coal-gypsum • CV, coefficient of variation • CW, canal water • EC, electrical conductivity • ESP, exchangeable sodium percentage • FIR, final infiltration rate • FV, flocculation value • HC, hydraulic conductivity • IR, infiltration rate • LG, lacto-gypsum • MG, mined-gypsum • OM, organic matter • PGR, pure gypsum requirement • PV, pore volume • Q, volume of water collected in the leaching of soil columns • RFIR, relative final infiltration rate • SA, Sádaba (the soils' local name) • SAR, sodium adsorption ratio

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY AND CONCLUSIONS REFERENCES

 
SODIC SOILS GENERALLY exhibit poor physical conditions and are prone to clay swelling and/or dispersion, formation of impermeable surface seals (i.e., soil crusting), and decreased soil profile hydraulic conductivities (HC) that adversely affect water, solutes and air movement, soil erodibility, and plant growth (Shainberg and Letey, 1984). Soils with low levels of exchangeable sodium (ESP < 5), classified as nonsodic, may also exhibit soil crusting (Sumner, 1993), especially if they are irrigated with waters of low EC as those typical in the left bank of the Ebro River (i.e., EC < 0.4 dS m^–1) in Spain. Soil crusting per se decreases soil infiltration rates, seedling emergence, and crop production, and increases surface runoff and erosion (Sumner and Stewart, 1992). Although not properly quantified, soil crusting has been identified as a moderate to severe problem in different soils of the middle Ebro River Basin (Spain), whereas about 28% of the 536000 irrigated ha in this Basin has been degraded by salinization and/or sodification processes (Herrero and Aragüés, 1988).

The incorporation of mined-gypsum in the soil and the addition of sulfuric acid to calcareous soils have long been used for the reclamation of sodic soils (U.S. Salinity Laboratory, 1954; Miyamoto et al., 1975; Shainberg et al., 1989). The application of these chemical amendments to the surface of both sodic and nonsodic soils is also common to prevent or reduce soil crusting and maintain or improve water infiltration rates (Agassi et al., 1982; Kazman et al., 1983; Miyamoto and Stroehlein, 1986).

More recently, sulfuric acid- and gypsum-like industrial by-products have been used for soil reclamation, soil crusting prevention, and erosion control (Mace et al., 1999; Power and Dick, 2000). The use of these by-products as soil amendments is of great interest because of their disposal and environmental problems, low costs, and the large quantities that are being produced. By-product sulfuric acid is obtained in oil refineries in the process of gasoline alkylation for production of high-octane gasoline, as well as in coal-gasification and coal-burning power plants. Gypsum-like by-products are produced in coal-burning power plants in the flue gas desulfurization process (coal-gypsum) and in the manufacture of orthophosphoric acid (phospho-gypsum), hydrofluoric acid (fluor-gypsum), lactic acid and lactates (lacto-gypsum), orthoboric acid (boron-gypsum), and organic salts (organo-gypsum). As an example of the significance of these by-products, more than 1 million Mg of coal-gypsum and 70000 Mg of lacto-gypsum are being produced yearly in industrial plants located in the Ebro River basin that could potentially satisfy the requirements for correcting the soil sodicity and crusting problems in this region.

Previous studies have demonstrated the effectiveness of some of these amendments (phospho-gypsum in particular) in soil crusting prevention and sodic soil reclamation; however, their relative efficiencies, when applied at chemically equivalent rates, are not well documented. The performance of these amendments depends on the soil water electrolyte and Ca concentrations that result from gypsum dissolution or calcite dissolution by sulfuric acid, and on the efficiency of the Na–Ca soil exchange process. This efficiency is a function of (i) the rate of Ca release, (ii) the rate of diffusion of the dissolved Ca and the exchanged Na through aggregate micropores, (iii) the mass flow processes, gravitationally induced in macropores and capillary absorption in aggregates, and (iv) the number of actively conducting macropores (Armstrong and Tanton, 1992). Also, Chan (1995) showed that the addition of gypsum to the soil surface protects surface aggregates against raindrop impact and reduces their wetting rate, therefore decreasing their susceptibility to slaking.

The objective of this work is to quantify the efficiency of four amendments (sulfuric acid, mined-gypsum, coal-gypsum, and lacto-gypsum) in (i) preventing crust formation in a sodic and a nonsodic calcareous soil, and (ii) reclaiming the sodic, calcareous soil. This quantification involves the measurement of the water infiltration rates in disturbed soil columns and the EC and Na of the leachates. The effect of three application rates of mined-gypsum, at 5, 3, and 1 Mg ha^–1, in preventing crust formation in the nonsodic soil was also investigated.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY AND CONCLUSIONS REFERENCES

 
Soils
A non saline–nonsodic soil (SA 20/1, referred as nonsodic) and a saline–sodic soil (SA 31/1, referred as sodic), located in the Bardenas I irrigation district of the middle Ebro River Basin (Spain), were sampled (0–0.2 m depth), air-dried, ground, and sieved (<2 mm). Both soils (Gypsic Haploxerept and Typic Xerofluvent, respectively) were sensitive to clay dispersion and showed severe crusting when leached with canal water (EC < 0.4 dS m^–1) and deionized water (Amezketa et al., 2004). The physical and chemical properties were given in Amezketa et al. (2004). Both soils are calcareous (CaCO₃ > 40%), high in hydrated micas and without swelling clays, low in organic matter content (<1.6%), and with a CEC around 22 cmol_c kg^–1. Soil SA 20/1 has a silty-loam texture and an ESP of 2%, whereas soil SA 31/1 has a clay-loam texture and an ESP of 25%. The two soils were analyzed in the crusting prevention experiments, whereas the sodic soil was analyzed in the reclamation experiments.

Treatments
A liquid (reagent-grade sulfuric acid with a purity of 96%) and three solid (MG, mined-gypsum; CG, coal-gypsum; and LG, lacto-gypsum) amendments were evaluated. Selected physical and chemical properties of the three gypsum sources are provided in Table 1. To eliminate the effect of particle size on dissolution rate, the three gypsums were sieved to obtain particles 30 to 100 µm in diameter. Their dissolution rates in distilled water were similar, and the ECs of the mined-gypsum, coal-gypsum, and lacto-gypsum were, respectively, 0.97, 0.98, and 1.01 dS m^–1 at 1 g gypsum L^–1 and 2.02, 2.08, and 2.11 at 3 g gypsum L^–1. The steady state saturated-gypsum EC was 2.2 dS m^–1 for the three gypsum sources, indicating the absence of other soluble salts. The heavy metal concentrations in the coal-gypsum and lacto-gypsum were variable (Table 1), but lower in all cases than the maximum concentrations permitted by the Spanish legislation in the use of sewage sludges in agriculture (Boletín Oficial del Estado, 1990).

In the sodic soil reclamation experiments, the three gypsum amendments were mixed with the soil before packing the columns while the sulfuric acid was sprayed uniformly over the soil surface after packing. The rate of application of the four amendments was 1 pure gypsum requirement (PGR), calculated after the modified Oster and Frenkel method (1980) and corrected by the purity of each amendment:

The rate of 1 PGR corresponds to soil column applications of 131 mg (sulfuric acid), 238 mg (mined-gypsum), 226 mg (coal-gypsum), and 224 mg (lacto-gypsum).

Each soil (i.e., SA 20/1 and SA 31/1) and treatment (i.e., the four amendments and the nonamended control) of the crusting prevention experiment was replicated four times, for a total of 40 (i.e., 2 soils x 5 treatments x 4 replications) soil columns. In addition, 16 columns were used in the sodic soil reclamation experiments (1 soil x 4 amendments x 4 replications) and 8 columns in the mined-gypsum rate experiment (1 soil x 2 rates x 4 replications).

Packing and Leaching of Soil Columns
The columns were prepared by packing air-dried soil samples of <2 mm into methacrylate cylinders (4.4 cm in diam. by 12.0 cm long) at bulk densities of 1.3 and 1.4 Mg m^–3 (SA 20/1 and SA 31/1 soils, respectively). The cylinders are open at the top and closed at the bottom, except for an outlet for collection of the leachates. The soils (42.6 g of SA 20/1 and 39.5 g of SA 31/1) were carefully added to the cylinders to a total thickness of 2.0 cm over a 2.0-cm layer of acid-washed quartz sand, 1 to 2 mm in diameter. They were compacted to the thickness of 2.0 cm by mechanically dropping the columns 10 times from a height of 2 cm onto a hard surface (Auerswald, 1995).

After application of amendments, the columns were leached with canal water (CW; EC 0.38 dS m^–1, sodium adsorption ratio [SAR] 0.5 [mmol L^–1]^0.5) using a constant-head device (Klute, 1965) with a constant hydraulic head of 3 cm. The leachates were collected in appropriate volume increments and analyzed for EC in both soils and for Na concentration in the sodic soil. The SAR was estimated from these values by converting EC into total electrolyte concentration C and obtaining the Ca plus Mg concentrations by subtracting Na from C. The following equations were used to convert EC into C:

and

The infiltration rate (IR) of the soil columns, reported in mm h^–1, was calculated as:

where Q is the volume of water collected during a given time period, t, and A is the cross-sectional area of the soil columns. The leaching process was continued until both a final steady state effluent EC and infiltration rate (IR) were achieved.

The final infiltration rates (FIR) of the nonamended controls for each soil were taken as the reference value. The relative FIR (i.e., RFIR; %) of the amended soils were obtained from the ratio "RFIR = amended FIR/control FIR," where control FIR = 100%.

Statistics
Statistical analyses were performed using the Statgraph Plus 2.1 software. One-way ANOVA was performed to compare the means of the FIR among treatments. Where the analyses showed significant differences at P < 0.05, Duncan's multiple range tests were conducted to separate treatments' FIR. Statistical significance is reported at the 0.05 and 0.01 probability levels.

	RESULTS AND DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY AND CONCLUSIONS REFERENCES

 
Eighty-six percent of the coefficients of variation (CV) of the mean FIR values obtained from the four replications in the amended soil columns were lower than 15%, and the average CV for all the treatments and soils was 8.6%, indicating that the proposed methodology is consistent and reliable.

Efficiency of Chemical Amendments in Soil Crusting Prevention
The IR of the nonamended (i.e., control) soils were lower than those of the amended soils from the start of the leaching process (Fig. 1) , indicating that clogging of pores by aggregate slaking and/or clay dispersion was almost immediate. In fact, the nonamended sodic soil SA 31/1 was basically impermeable from the beginning of the experiments. The observed initial increases in IR (Fig. 1) were attributed to the reorganization of the packed soil particles and the release of the entrapped air within the pores during the water saturation process.

View larger version (29K): [in this window] [in a new window]   Fig. 1. Efficiency of chemical amendments in soil crusting prevention: mean infiltration rate (IR) and 1 standard error (SE) bar values along the leaching process with canal water (EC = 0.38 dS m–1) of soils SA 20/1 (nonsodic) and SA 31/1 (sodic) nonamended (control) and surface-amended at equivalent rates of 5 Mg pure-gypsum ha–1 of sulfuric acid, mined-gypsum, coal-gypsum, and lacto-gypsum.

Amendment applications at equivalent rates of 5 Mg ha^–1 increased significantly (P < 0.01) the final or steady state infiltration rate (FIR) of the SA 20/1 nonsodic soil from 21 mm h^–1 in the control to 33 to 35 mm h^–1 for the three gypsum materials (i.e., the FIR for the three gypsums were not significantly different at P > 0.05) and to 53 mm h^–1 for the sulfuric acid. The corresponding FIR values for the SA 31/1 sodic soil were 0 mm h^–1 (control), 9 mm h^–1 (lacto-gypsum), 15 to 17 mm h^–1 (coal-gypsum and mined-gypsum), and 21 mm h^–1 (sulfuric acid) (Table 2). Increases in the FIR of the amended soils over those of the control soils were attributed to the dissolution of calcite by sulfuric acid and the dissolution of the gypsum materials, which increased the Ca and the total electrolyte concentrations of the soil solution, therefore, minimizing clay dispersion and soil crusting. Thus, the leachate EC of the amended soils were well above the flocculation values (FV) of these soils (0.24 and 0.22 dS m^–1 for SA 20/1 and SA 31/1, respectively; Amezketa et al., 2004) and higher than the leachate EC of the controls.

The higher efficiency of sulfuric acid in improving the infiltration rate of calcareous soils was attributed by Mace et al. (1999) to one or more of the following reasons: (i) lower SAR values, (ii) higher soluble Ca²⁺ and Mg²⁺, (iii) lower pH, and (iv) greater EC. Miyamoto and Stroehlein (1986) added other specific beneficial effects of sulfuric acid over gypsum: (v) increased dissolution of stabilizing agents such as Fe, Al, and P, (vi) development of CO₂ escape channels to the atmosphere, (vii) partial plugging of water conducting pores by very fine gypsum particles, (viii) low dissolution rate of coarse gypsum particles, and (ix) high sulfate concentrations that will limit the dissolution of gypsum due to the common ion effect.

The higher efficiency of the sulfuric acid over the three gypsum materials is reflected in the EC, Na, and SAR values in the SA 31/1 sodic soil leachates (Fig. 2) . The leaching of salts and sodium and the concomitant reduction in the SAR values were fastest in the acid-amended soil columns, medium in the mined-gypsum and coal-gypsum amended soil columns, and lowest in the lacto-gypsum amended soil columns. Thus, a SAR value of 1 was obtained in <1 h (i.e., 2.6–3.6 pore volumes [PV]) in the leachates of the acid treatment, as compared with corresponding values of more than 2 h (i.e., 2.6–4.0 PV) in the mined-gypsum and coal-gypsum treatments, and more than 6 h (i.e., 3.2–4.2 PV) in the lacto-gypsum treatment. This increasing delay in the Na leached from the soil should be a reflection of the decreased dissolution rates of the different amendments (i.e., acid-calcite > mined-gypsum and coal-gypsum > lacto-gypsum). Although the three gypsums had similar dissolution rates when subject to stirring in deionized water, we observed that the lacto-gypsum applied on the soil columns had a soggy appearance under wetting, which could delay the water entry into the soil and its dissolution rate. Frenkel et al. (1989) and Bolan et al. (1991) indicated that the effectiveness of gypsum depends, besides its rate of dissolution, on the subsequent movement of the dissolved ions away from the site of dissolution (i.e., gypsum dissolution is transport-controlled). This lacto-gypsum soggy effect was irrelevant in the SA 20/1 nonsodic soil, probably because of its very low ESP and the electrolyte concentration being well above the flocculation value of soil clay.

View larger version (43K): [in this window] [in a new window]   Fig. 2. Efficiency of chemical amendments in soil crusting prevention: Electrical conductivity (EC), Na, and sodium adsorption ratio (SAR) values in the column leachates of the SA 31/1 sodic soil surface-amended with sulfuric acid, coal-gypsum, lacto-gypsum, and mined-gypsum at equivalent rates of 5 Mg pure-gypsum ha–1.

Efficiency of Three Rates of Mined-Gypsum in Crusting Prevention of a Nonsodic Soil
The effect of the surface-application of three rates of mined-gypsum in the IR of the SA 20/1 nonsodic soil is presented in Fig. 3 . The instantaneous IR of the 5 Mg ha^–1 amended soil were slightly higher than those for the 3 and 1 Mg ha^–1 amended soils, and the FIR were 35 (5 Mg ha^–1), 29 (3 Mg ha^–1), and 31 (1 Mg ha^–1) mm h^–1 (not significantly different at P > 0.05). The FIR of the control soil was 21 mm h^–1, significantly lower (P < 0.05) than the FIR of the amended soils. We therefore concluded that rates as low as 1 Mg ha^–1 were as effective as higher rates in minimizing crusting of this nonsodic soil.

View larger version (28K): [in this window] [in a new window]   Fig. 3. Efficiency of three rates of mined-gypsum in crusting prevention in a nonsodic soil: mean infiltration rate (IR) and 1 SE bar values along the leaching process with canal water (EC = 0.38 dS m–1) of the SA 20/1 nonsodic soil nonamended (control) and surface-amended at equivalent rates of 5, 3, and 1 Mg pure-gypsum ha–1.

View larger version (25K): [in this window] [in a new window]   Fig. 4. Efficiency of chemical amendments in sodic soil reclamation: mean infiltration rate (IR) and 1 SE bar values along the leaching process with canal water (EC = 0.38 dS m–1) of the SA 31/1 sodic soil nonamended, amended with surface-applied sulfuric acid, and with soil-incorporated mined-gypsum, coal-gypsum, and lacto-gypsum at rates of 1 pure-gypsum requirement (PGR).

View larger version (39K): [in this window] [in a new window]   Fig. 5. Efficiency of chemical amendments in sodic soil reclamation: Electrical conductivity (EC), Na, and sodium adsorption ratio (SAR) values in the column leachates of the SA 31/1 sodic soil amended with surface-applied sulfuric acid and with soil-incorporated mined-gypsum, coal-gypsum, and lacto-gypsum at rates of 1 pure-gypsum requirement (PGR).

Since crusting is a soil surface process (i.e., structural crusts due to clay dispersion are very thin, usually a few millimeters), it is best prevented when the applied water dissolves the surface-applied amendments and increases the electrolyte (i.e., EC) and Ca concentrations before reacting with the soil particles (Sumner and Stewart, 1992). The rate of dissolution of the amendment (i.e., the structural characteristics of the amendment and, in particular, its particle size) is therefore critical in promoting EC and Ca of the infiltrating waters above the FV of the soil clays.

On the other hand, the efficiency of sodic soil reclamation depends on the water transmitting properties of the soil profile, which are promoted by the enhanced solubility of the Ca-amendments when mixed with the soil. This enhanced solubility may be more important than the rate of dissolution because it increases even more the EC and Ca levels of the soil water, therefore promoting the Na–Ca exchange process (Frenkel et al., 1989).

Our results indicate that, despite the similar dissolution rates of the three gypsums in deionized water, the soggy texture of the surface-applied lacto-gypsum limited its beneficial effect in soil crusting prevention as compared with the surface-applied mined-gypsum and coal-gypsum (Table 2). Apparently, when lacto-gypsum was mixed with the soil, the soggy effect was irrelevant and performed as the other two gypsums (Table 3).

When the three gypsums where mixed with the soil, their residual amounts on the soil surface seemed to be insufficient to prevent clay dispersion and/or aggregate slaking, producing FIR values (7.8–8.9 mm h^–1; Table 3) 50% lower (P < 0.05) than the 15 to 17 mm h^–1 values obtained in the coal-gypsum and mined-gypsum surface-amended soil columns (Table 2). Amezketa et al. (2004) concluded that this soil was very sensitive to clay dispersion and aggregate slaking when subject to the canal irrigation water. These results suggest that soil crusting rather than the water transmitting properties was the limiting factor in the reclamation process of this soil. However, the lower amounts of amendments used in the reclamation experiments (i.e., average of 229 mg of gypsum) than in the crusting-prevention experiments (i.e., avg. of 787 mg of gypsum) could also partially explain these results.

	SUMMARY AND CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY AND CONCLUSIONS REFERENCES

 
All the amendments were effective in crusting-prevention and sodic soil reclamation. Sulfuric acid was the most efficient amendment in both experiments as indicated by the fastest reductions of EC, Na, and SAR in their leachates. The three gypsum materials were equally effective in preventing soil crusting in the nonsodic soil, while lacto-gypsum was less efficient in the sodic soil. In contrast, the three gypsum materials were equally effective in the reclamation process of the sodic soil. These findings indicate that all amendments will be beneficial in correcting the crusting and sodicity problems of the study soils.

Taking into account that (i) by-product sulfuric acid is not fully available in the region and poses management risks, (ii) coal-gypsum is produced at rates of 1 million Mg yr^–1 in this region, (iii) the rate of lacto-gypsum production is much lower and its efficiency is lower than that of coal-gypsum for crusting prevention of the sodic soil, and (iv) the higher cost of the mined-gypsum because of its crushing process before application, we concluded that coal-gypsum is the amendment of greater technical and economical interest for soil conservation and reclamation in the study area. Furthermore, the use of the low-cost and largely available coal-gypsum as a soil amendment will alleviate the present environmental problems derived from its hazardous disposal.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY AND CONCLUSIONS REFERENCES

 
This research was financially supported by the Spanish National Institute for Agricultural and Food Research and Technology (INIA).

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY AND CONCLUSIONS REFERENCES

 

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