Issue 2 (178)/2019

Enhancing the solubility and reclamation efficiency of gypsum with H₂SO₄

A.I. Saqib¹, K. Ahmed^1,*, G. Qadir¹, M.Q. Nawaz¹, A.R. Naseem¹

¹Soil Salinity Research Institute (SSRI), Pindi Bhattian, Pakistan 

*E-mail: khalilahmeduaf@gmail.com 

Received: Jan. 05, 2019. Revised: Apr. 20, 2019. Accepted: June 07, 2019. Published online: Oct. 18, 2019

ABSTRACT. An effective reclamation procedure of saline sodic soils is removal of undesirable Na⁺ by addition of some Ca²⁺ source paralleled with leaching of this sodium out of root zone. Nevertheless, gypsum being a direct source of Ca²⁺ is relatively insoluble in water. Its solubility can be increased with addition of H₂SO₄. Therefore, three years (2015 to 2018) study was plan to find out the optimal and economical level of H₂SO₄, which can increase the solubi-lity and reclamation efficiency of gypsum for saline sodic soil in rice wheat crop-ping rotation. Treatments included were: T₁, Control, T₂, gypsum @ 100% of GR, T₃, gypsum @ 100% of GR+10 kg H₂SO₄ acre^-1, T₄, gypsum @ 100% of GR+50 kg H₂SO₄ acre^-1, T₅, gypsum @ 100% of GR+100 kg H₂SO₄ acre^-1. Before start of study, soil had pH_{s =} 8.85, EC_e = 4.85 (dS m^-1), SAR = 43.82 (mmol L^-1)^1/2, GR = 4.10 (t. acre^-1), BD = 1.65 (Mg m^-3), HC = 0.33 (cm hr^-1). Experiment was laid out in RCBD with three replications. Sulfuric acid and gypsum were applied (once) at the start of study in the respective treatment plots. Recommended dose of fertilizers, 150-90-60 NPK kg ha^-1 for rice (Shaheen Basmati) and 160-114-60 NPK kg ha^-1 for wheat (Faisalabad, 2008) was applied. Yield and yield determining attributes of each crop were recorded at physical maturity. After harvest of each crop, soil samples were collected and were analyzed for EC_e, pH_s, SAR, bulk density and hydraulic conduc-tivity. Pooled data analysis revealed that maximum growth and yield determining factors of rice and wheat were recorded where gypsum was applied with H₂SO₄ at the rate of 50 and 100 kg acre^-1. Soil physical and chemical properties, i.e. pH_s, EC_e, SAR bulk density and hydraulic conductivity were also substantially improved with combined application of gypsum and H₂SO₄  at the end of study. Both levels of H₂SO₄ at the rate of 50 and 100 kg acre^-1 with gypsum proved equally to be the best in enhancing the solubility and reclamation efficiency of gypsum and showed the statistically (p≤ 0.05) similar results in increasing the yield of rice and wheat crop and improving the soil physical and chemicals properties. Therefore, H₂SO₄ at the rate of 50 kg acre^-1 is recommended as most economical and optimum level, which can be used with gypsum as an effective ameliorative strategy for the salt affected soils.

Keywords: salinity; rice; wheat; ameli-oration; amendments.

INTRODUCTION

Approximately 20% of the irrigated land is salt affected with a loss of 12 billion US$ per annum (Ghassemi et al., 1995). Such proble-matic soils can be rehabilitate by calcium (Ca²⁺) source to replace the toxic sodium (Na⁺) from exchange sites, which leaches out of root zone (Oster 1982, Shainberg and Letey 1984). According to Gupta and Abrol (1990), reclamation of salt affected soils through chemical reclaiming agents is a well-established techno-logy, in which some reclaiming agents are direct source of Ca²⁺, whereas, others helps to dissolve the native CaCO₃ (Qadir and Oster, 2002).

Gypsum is most common sul-phate mineral extracted from the ground (Hand, 1997). This sulphate mineral is extensively used as reclaiming agents due to its ease of handling, economics, and accessibility (Amezketa et al., 2005). It is commonly used to neutralize the harmful effects of Na⁺ resulting in significant decrease of soil salinity indices, like EC_e and SAR (Hamza and Anderson, 2003). Gypsum application @ 100% of GR alone or in combination with organic amend-ments is a very effective strategy to ameliorate the salt affected soils and to increase the crop yield (Ahmed et al., 2015). Shahid (1993) also reported the positive effect of subsoiling and gypsum on soil physicochemical properties. Similarly, in a study conducted by Qadir et al. (2017), they reported that gypsum application at the rate of 100% and 50% of GR improved the soil properties, like pH_s, EC_e, SAR, HC, BD, and yield of wheat and rice crop under saline conditions. Nevertheless, gypsum is comparatively insoluble in water and below 40ºC it has lowest solubility (Farrah et al., 2004; Freyer and Voigt, 2003).

Different laboratories studies have been conducted to enhance the gypsum solubility with H₂SO₄ (Ling and Demopoulos, 2004; Dutrizac and Kuiper, 2006). The solubility of CaSO₄ hydrates in H₂SO₄ solutions was determined by Ling and Demopoulos (2004) and Dutrizac (2002). According to (Azimi, 2010) at temperatures of 25-45°C, gypsum solubility increases moderately with addition of H₂SO₄, while at higher temperatures, the solubility of gypsum increases monotonically with increasing concentration of acid. They stated that SO₄^2- concentration de-creases with increasing concentration of H₂SO₄, which enhances the solubility of CaSO₄ to satisfy the solubility product. Different researchers also reported that addition of acids in gypsum accelerated the transformation process and enhances the solubility of gypsum (Farrah et al., 2004; Freyer and Voigt, 2003; Dutrizac, 2002; Nyvlt, 1997).

Similarly, Farrah et al. (2007) in a study reported that maximum solu-bility of gypsum was observed when H₂SO₄ concentration is 36 g/kg solu-tion, while further increase of 72 g/kg solution recorded the smaller increase in gypsum solubility. Addition of acids, like H₂SO₄ and HCl, enhances the solubility of gypsum (Azimi and Papangelakis, 2010).

So, this study was plan to find out the optimum and economical level of H₂SO₄, which can increase the solubility and reclamation efficiency of gypsum for saline sodic soils under rice wheat cropping rotation.

 

MATERIAL AND METHODS

A field study was conducted from 2015 to 2018, at Soil Salinity Research Institute, Pindi Bhattian, Pakistan. Before start of study, soil had pH_s = 8.85, EC_e = 4.85 (dS m^-1), SAR = 43.82 (mmol L^-1)^1/2, GR = 4.10 (t.acre^-1), BD = 1.65 (Mg m^-3), HC = 0.33 (cm hr^-1). Treatments included were: T₁, Control, T₂, gypsum @ 100% of GR, T₃, gypsum @ 100% of GR+10 kg H₂SO₄ acre^-1, T₄, gypsum @ 100% of GR+50 kg H₂SO₄ acre^-1, T₅, gypsum @ 100% of GR+100 kg H₂SO₄ acre^-1.

Experimental design was RCBD having three replications. Gypsum (80% pure, 30 mesh size) and sulfuric acid (96% pure) were applied (once) at the start of study in the respective treatment plots. Land was prepared thoroughly and gypsum was broadcasted on the soil surface and H₂SO₄ was flooded according to treatment plan with irrigation of 7.5 cm depth to dissolve the gypsum. After 30 days of amendments application, during kharif season 2015 rice (Shaheen Basmati), nursery was transplanted.

Recommended dose of fertilizers (150-90-60 NPK kg ha^-1) as urea, single super phosphate and sulphate of potash was applied to rice.

All agronomic and plant protection measures were applied uniformly. Yield and yield determining attributes of rice crop were recorded at physical maturity of crop. After the harvest of rice crop, wheat crop (Faisalabad, 2008) was sown in same field. Recommended dose of fertilizer 160-114-60 NPK kg ha^-1 as urea, single super phosphate and sulphate of potash was applied. All agronomic and plant protection measures were applied uniformly. Yield and yield determining attributes were recorded at physical maturity of crop. Soil samples were collected after harvest of each crop and were analyzed for pH_s, EC_e, SAR, bulk density and hydraulic conductivity according to U.S. Salinity Laboratory Staff (1954). The collected crop data (rice and wheat) was statistically analyzed.

The treatment mean comparison was made using Least Significant Difference Test @ 5% Probability (Steel et al., 1997).

 

RESULTS

Rice crop

Pooled data of rice crop revealed that application of gypsum signi-ficantly influenced the growth and yield characteristics of rice crop, however, at the same time integrated use of gypsum and H₂SO₄ showed more positive effects than sole application of gypsum (Tables 1 and 2). Data concerning the plant height displayed that gypsum and H₂SO₄ significantly (p≤ 0.05) increased the plant height and maximum plant height of 133.67 cm was observed in T₅ (gypsum @ 100% of GR+100 kg H₂SO₄ acre^-1), which was statistically similar with T₄ (gypsum @ 100% of GR+50 kg H₂SO₄ acre^-1). While minimum plant height (122.00 cm) was documented in T₁ (control). Data regarding number of tillers and number of spikelet showed that maximum number of tillers and spikelet (231.33, 214.67) were observed with gypsum @ 100% of GR+100 kg H₂SO₄ acre^-1, followed by gypsum @ 100% of GR+50 kg H₂SO₄ acre^-1 and statistically both treatments were alike. On the other hand, minimum number of tillers and number of spikelet (217, 202) were observed where no amendments was used, i.e. in control (Table 1).

Data for 1000 grain weight revealed that maximum 1000 grain weight (32.33 g) was documented at gypsum @ 100% of GR+100 kg H₂SO₄ acre^-1, however it was statis-tically at par with gypsum @ 100% of GR+50 kg H₂SO₄ acre^-1. Minimum 1000-grain weight (24 g) was recorded by control. Data about paddy and straw yield showed inclining pattern, amongst all the treatments, combination of gypsum and H₂SO₄ performed better than individual application of gypsum; H₂SO₄ @ 100 and 50 kg acre^-1 with gypsum performed equally well in paddy and straw yield and maximum paddy (4.11 t ha^-1) and straw yield (11.45 t ha^-1) was documented in gypsum @ 100% of GR+100 kg H₂SO₄ acre^-1. On contrary, minimum paddy (2.02 t ha^-1) and straw yield (5.69 t ha^-1) was observed in control.

Wheat crop

Data regarding yield and yield attributes of wheat crop in (Tables 3 and 4) exhibited that increasing level of H₂SO₄ with gypsum significantly improved these attributes, however, H₂SO₄ remain effective only up to level of 50 kg H₂SO₄ acre^-1 and further increase did not influnced these attributes significantlly (p≤ 0.05).

Data showed that taller plants (72.66 cm) were observed in gypsum @ 100% of GR+100 kg H₂SO₄ acre^-1, followed by gypsum @ 100% of GR+50 kg H₂SO₄ acre^-1 and both treatments were statistically non significant (p≤ 0.05) from each other. Data regarding the number of tillers, grain spike^-1 and 1000-grain weight showed that maximum number of tillers (164.67), grain spike^-1 (31) and 1000-grain weight (34 g) were ensued by gypsum @ 100% of GR+100 kg H₂SO₄ acre^-1, which was statistically (p≤ 0.05), alike with gypsum @ 100% of GR+50 kg H₂SO₄ acre^-1. On the other hand, minimum number of tillers (136.33), grain spike^-1 (26) and 1000-grain weight (22 g) were recor-ded in control. Yield data revealed that maximum grain (3.51 t ha^-1) and straw yield (4.53 t ha^-1) was achieved with gypsum @ 100% of GR + 100 kg H₂SO₄ acre^-1, followed by gypsum @ 100% of GR+50 kg H₂SO₄ acre^-1 and these treatments were statistically (p≤ 0.05) non-significant from each other. Whereas, minimum grain (1.47 t. ha^-1) and straw (1.92 t. ha^-1) yield was recorded by control.

Soil properties

Soil analysis at the end of study showed that gypsum application substantially improved the soil physical and chemical properties, however, at the same time, integrated use of gypsum with H₂SO₄ was more effective in reclaiming the salt affected soil than its sole application. Data regarding the soil EC_e revealed that maximum reduction of 56.91 % over its initial value was observed with gypsum @ 100% of GR+100 kg H₂SO₄ acre^-1, followed by gypsum @ 100% of GR+50 kg H₂SO₄ acre^-1 with a reduction of 55.88% over its initial value (Table 5). Whereas, minimum reduction in EC_e (10.52%) was recorded in control.

With respect to soil pH_s, maximum reduction (7.91 %) was observed with combined application of gypsum @ 100% of GR+100 kg H₂SO₄ acre^-1 and minimum reduction (0.56%) was observed where no ammendments was used, i.e. in con-trol. Similarly, soil sodicity indicator, i.e. SAR, was also signnificantly im-proved by the amendments (Table 6).

 

Table 1
Effect of amendments on rice growth (average of three seasons)

Treatments	Plant height (cm)	No. of tillers m-2	No. of spikelet panicle-1
T1 Control	122.00  C	217.00  C	202.00  B
T2 Gypsum @ 100 % of  GR	127.33  B	224.00  B	210.67  A
T3 Gypsum @ 100 % of  GR + 10 kg H2SO4 acre-1	128.33   B	225.00  B	213.33  A
T4 Gypsum @ 100% of GR + 50 kg H2SO4 acre-1	131.33  AB	227.00  AB	214.67  A
T5 Gypsum @ 100% of GR + 100 kg H2SO4 acre-1	133.67  A	231.33  A	214.67  A

Different letters in the same column indicate significant differences by LSD at p ≤ 0.05

 

Table 2
Effect of amendments on rice growth (average of three seasons)

Treatments	1000 grain weight (g)	Paddy Yield (t.ha-1)	Straw Yield (t.ha-1)
T1 Control	24.00    C	2.02C	5.69C
T2 Gypsum @ 100 % of  GR	28.33   B	3.51B	9.72B
T3 Gypsum @ 100 % of  GR + 10 kg H2SO4 acre-1	29.33   B	3.63B	10.02B
T4 Gypsum @ 100% of GR + 50 kg H2SO4 acre-1	30.33  AB	3.98A	11.28A
T5 Gypsum @ 100% of GR + 100 kg H2SO4 acre-1	32.33  A	4.11A	11.45 A

Different letters in the same column indicate significant differences by LSD at p ≤ 0.05

 

Table 3
Effect of amendments on wheat growth (average of three seasons)

Treatments	Plant height (cm)	No. of tillers m-2	No. of grain spike-1
T1 Control	59.00    C	136.33    C	26.00   B
T2 Gypsum @ 100 % of  GR	64.00   B	155.33   B	29.33  A
T3 Gypsum @ 100 % of  GR + 10 kg H2SO4 acre-1	65.00   B	163.67  A	29.66  A
T4 Gypsum @ 100% of GR + 50 kg H2SO4 acre-1	70.66  A	160.00  AB	30.00  A
T5 Gypsum @ 100% of GR + 100 kg H2SO4 acre-1	72.66  A	164.67  A	31.00  A

Different letters in the same column indicate significant differences by LSD at p ≤ 0.05

 

Table 4
Effect of amendments on wheat growth (average of three seasons)

Treatments	1000 grain weight (g)	Paddy yield (t.ha-1)	Straw yield (t.ha-1)
T1 Control	22.00    C	1.47C	1.92C
T2 Gypsum @ 100 % of  GR	29.33   B	2.92B	3.85B
T3 Gypsum @ 100 % of  GR + 10 kg H2SO4 acre-1	30.00   B	3.04B	3.98B
T4 Gypsum @ 100% of GR + 50 kg H2SO4 acre-1	33.00  A	3.46A	4.57A
T5 Gypsum @ 100% of GR + 100 kg H2SO4 acre-1	34.00  A	3.51A	4.53A

Different letters in the same column indicate significant differences by LSD at p ≤ 0.05

 

Maximum reduction (79.55%) in soil SAR was recorded with gypsum @ 100% of GR+100 kg H₂SO₄ acre^-1 and minimum (7.58%) in control. Data regarding soil bulk density showed that H₂SO₄ @ 100 and 50 kg acre^-1 with gypsum @ 100% of GR performed equally in improving the soil bulk density with a maximum reduction of 5.45% over its initial value and on the other hand minimum reduction of 0.61 % was documented in control (Fig.1).

Soil hydraulic conductivity was also significantly influenced by appli-cation of amendments. Maximum in-crease (42.42%) in hydraulic conduc-tivity was noted where gypsum was applied at the rate of 100% of soil gypsum requirement +100 kg H₂SO₄ acre^-1. On contrary, minimum increase of 3.03% was observed in control (Fig. 2).

 

DISCUSSION

 

Increase in growth and yield characteristics of rice/wheat crops with gypsum + H₂SO₄ application clearly depicted the ameliorative role of these amendments, however, at the same time increasing levels of H₂SO₄ showed more positive effects on these attributes.

Application of gypsum accelera-ted the leaching of salts out of root zone and promotes the flocculation of dispersed soil (Qadir et al., 2001; Zaka et al., 2005). Removal of exchangeable Na⁺ by Ca²⁺ provided by gypsum improve the soil physical and chemical properties (Muhammad and Khattak, 2011).

Improvement in the soil physical and chemical properties may be the possible reasons for increased yield and yield contributing factors of rice and wheat crops in the treatments receiving the combined application of gypsum and H₂SO₄. Our results are in consistent with previous findings that gypsum application with organic and inorganic amendments improved the crop yield and salinity sodicity indices of salt affected soils (Qadir et al., 2001; Ghafoor et al., 2008; Qazi et al., 2009; Nan et al., 2016; Murtaza et al., 2017).

 

Table 5
Effect of amendments on soil chemical properties at the end of study

Treatments	ECe	% decrease over initial value	pHs	% decrease over initial value
T1 Control	4.34	10.52	8.80	0.56
T2 Gypsum @ 100 % of  GR	3.65	24.74	8.28	6.44
T3 Gypsum @ 100 % of  GR + 10 kg H2SO4 acre-1	2.62	45.98	8.26	6.67
T4 Gypsum @ 100% of GR + 50 kg H2SO4 acre-1	2.14	55.88	8.15	7.91
T5 Gypsum @ 100% of GR + 100 kg H2SO4 acre-1	2.09	56.91	8.15	7.91

Different letters in the same column indicate significant differences by LSD at p ≤ 0.05

 

Table 6
Effect of amendments on soil chemical properties at the end of study

Treatments	SAR	% decrease over initial value
T1 Control	40.50	7.58
T2 Gypsum @ 100 % of  GR	11.92	72.80
T3 Gypsum @ 100 % of  GR + 10 kg H2SO4 acre-1	11.96	72.71
T4 Gypsum @ 100% of GR + 50 kg H2SO4 acre-1	9.08	79.28
T5 Gypsum @ 100% of GR + 100 kg H2SO4 acre-1	8.96	79.55

Different letters in the same column indicate significant differences by LSD at p ≤ 0.05

 

Positive effect on crop growth and yield due to gypsum application with increasing levels of H₂SO₄ may also be ascribed, as the addition of H₂SO₄ results in a significant increase in the calcium sulphate solubility, compared to that in water (Azimi et al., 2007), because gypsum is compara-tively insoluble in water and below 40ºC it has lowest solubility (Freyer and Voigt, 2003; Farrah et al., 2004; Azimi et al., 2007). While the addition of H₂SO₄ increases the solubility of gypsum up to 10 times in water over the temperature range of 25–250ºC (Azimi, 2010). This increased solubili-ty of gypsum also increases its recla-mation efficiency and consequently a notable improvement in physical and chemical properties of salt affected soil was recorded where gypsum was applied with H₂SO₄ at rate of 50 and 100 kg acre^-1. So, this positive effect of gypsum in combination of H₂SO₄ was responsible for the improved yield of wheat and rice crops in these treatments. In addition, crop was also benefited through improved nutri-tional status of soil, as gypsum application also increases the solubi-lity of essential plant nutrients like N, K, Ca, Mg and S (Elrashidi et al., 2010).

 

 

 

Combined application of gypsum and H₂SO₄ significantly improved the soil physical and chemical properties, as compared to control and gypsum alone. Results of the current study demonstrated that integrated use of gypsum and H₂SO₄ significantly lowered the soil pH_s. Application of gypsum with H₂SO₄ at the rate of 50 and 100 kg acre^-1 reduces the soil pH_s 7.91%, as compared to initial soil pH_s value. This lowered value of pH_s due to gypsum application was the result of replacement of Na⁺ by Ca²⁺ and formation of soluble salts of SO₄^2- (Abdel-Fattah, 2012). Sharp decline in pH_s of soil due to H₂SO₄ + gypsum may be ascribed as addition of acid create the conditions, which increases the solubility of gypsum owing to enhanced activity of coefficient of Ca²⁺ and SO₄^2-, as a result of increased ionic strength of solution (Calmanovici et al., 1993; Abdel-Fattah, 2012), furthermore, addition of acid has direct effect on lowering the soil pH_s.

Data also illustrated that application of gypsum also reduces the soil EC_e, however sharp decline in soil EC_e was observed where gypsum was applied with H₂SO₄ and maximum reduction of 56.91% over initial value was recorded where gypsum was applied with H₂SO₄ at rate of 100 kg acre^-1. The possible reason for this lowered value of EC_e could be the leaching of soluble salts out of root zone (Ghafoor et al., 2008). Similar findings are reported by Van-Camp et al., 2004; Blum et al., 2004; Chitravadivu et al., 2009, which reinforced the findings of current study. Similarly, same trend was observed in case of SAR, maximum reduction in SAR was noted where gypsum was applied with H₂SO₄ at rate of 50 and 100 kg acre^-1. Increased exchangeable Ca²⁺ due to enhanced dissolution of gypsum with H₂SO₄ replaces the Na⁺ and consequently a sharp decreased in soil SAR was observed. Our results are in agreement with findings of Muhammad and Khattak (2011), who observed a significant reduction in soil SAR with gypsum application.

Soil physical properties, i.e. bulk density and hydraulic conductivity, were also positively influenced with application of gypsum and H₂SO₄. Bulk density was reduced by 5.45% and hydraulic conductivity was in-creased by 42.42%, as compared to its initial value at the start of study with combined application of gypsum and H₂SO₄ at rate of 100 kg acre^-1.

Application of Ca²⁺ as gypsum (Ca SO₄.2H₂O) replaced the Na⁺ from clay complex that was pushed into soil solution and subsequently leached down into lower profile. The effect of soil EC_e, pH_s and SAR is directly translated into increase or decrease of soil hydraulic conductivity and bulk density. The higher value of BD indicates harder and less porous soil. Generally, BD of saline sodic soil with dominancy of Na⁺ are higher than equivalent normal soil. As stated earlier the decrease in soil pH_s, EC_e and SAR with application of gypsum and H₂SO₄ decreased soil dispersion, increase soil porosity and resultant net reduction in BD was recorded (Fig.1).

An improvement in soil porosity through gypsum application was also obtained by Shainberg et al. (1989). Similarly decreased BD values with the use of calcium sources were previously reported (Peters and Kelling, 2002). Hydraulic conductivity plays its role in water relation of soil and plants in general, but is of prime importance in soil reclamation process. Because if water cannot permeable in to lower profile it will not take salt with it. The initial soil data presented very low value of HC. It was as low as 0.33 cm hr^-1.

However, it increased manifold during three years with a maximum of 0.47 cm hr^-1 (42.42 % increase), when gypsum and H₂SO₄ at rate of 100 kg acre^-1 were applied in combination (Fig. 2). It might be due to reduction in SAR, which resultantly decrease soil dispersion and encouraged coagulation of soil particles. The increase in pore spaces caused by aggregation increased the HC (Kauraw and Verma, 1982). Various research workers have reported an appreciable increase in HC due to application of inorganic or organic amendments, cultural practices and growing of crops (Qadir and Schubert, 2002; Carter et al., 2004; Evanylo et al., 2008).

 

CONCLUSION

An effective reclamation proce-dure of saline sodic soils is removal of undesirable Na⁺ by addition of some Ca²⁺ source paralleled with leaching of this sodium out of root zone. Nevertheless, gypsum being a direct source of Ca²⁺ is relatively insoluble in water. Its solubility can be increased with addition of H₂SO₄. Combination of gypsum with H₂SO₄ at the rate of 50 and 100 kg acre^-1 proved equally to be the best in enhancing the solubility and reclama-tion efficiency of gypsum; however, H₂SO₄ at the rate of 50 kg acre^-1, is the most economical and optimum level for reducing soil pH, salinity, SAR and improving bulk density and hydraulic conductivity. So ultimately, the paddy and grain yield of rice and wheat increased with the improve-ment of soil health. The message derived for the farmers from this study is that they can effectively rehabilitate their salt affected lands to obtain original potential if they apply gypsum at the full rate (100% GR) by combining it with H₂SO₄ at the rate of 50 kg acre^-1.

 

REFERENCES

Abdel-Fattah, M.K. (2012). Role of gypsum and compost in reclaiming saline-sodic soils. J.Agric.Vet.Sci., 1: 30-38.

Ahmed, K., Qadir, G., Jami, A.R., Nawaz, M.Q., Rehim, A., Jabran, K. & Hussain, M. (2015). Gypsum and farm manure application with chiseling improve soil properties and performance of fodder beet under saline-sodic conditions. Int.J.Agric. Biol., 17(6): 1225-1230, DOI: 10.17957/IJAB/14.0036

Amezketa, E., Aragüés, R. & Gazol, R. (2005). Efficiency of sulfuric acid, mined gypsum and two gypsum by-products in soil crusting prevention and sodic soil reclamation. Agron.J., 97: 983-989, DOI: 10.2134/agronj 2004.0236

Azimi, G. & Papangelakis, V.G. (2010). Thermodynamic modeling and experimental measurements of calcium sulphate in complex aqueous solutions. Fluid Ph. Equilibria, 290(1-2): 88-94, DOI: 10.1016/j.fluid.2009.09.023

Azimi, G. (2010). Evaluating the potential of scaling due to calcium compounds in hydrometallurgical processes. Phd. Thesis. Graduate Department of Chemical Engi-neering and Applied Chemistry University of Toronto.

Azimi, G., Papangelakis, V.G. & Dutrizac, J.E. (2007). Modelling of calcium sulphate hydrates solubility in concentrated multi-component sulphate solutions. Fluid Ph. Equilibria, 260(2): 300-315, DOI: 10.1016/j.fluid.2007.07.069

Blum, W.E.H., Büsing, J. & Montanarella, L. (2004). Research needs in support of the European thematic strategy for soil protection. Trends Analyt.Chem., 23(10-11): 680-685, DOI: 10.1016/j.trac.2004. 07.007

Calmanovici, C.E., Gabas, N. & Laguerie, C. (1993). Solubility measurements for calcium sulfate dihydrate in acid solutions at 20, 50, and 70^oC. J.Chem.Eng. 38: 534-536, DOI: 10.1021/je00012a013

Carter, M.R., Sanderson, J.B. & MacLeod, J.A.  (2004). Influence of compost on the physical properties and organic matter fractions of a fine sandy loam throughout the cycle of a potato rotation. Can.J.Soil Sci.,  84: 211-218

Chitravadivu, C., Balakrishnan, V., Manikandan, J., Elavazhagan, T. & Jayakumar, S. (2009). Application of food waste compost on soil microbial population in groundnut cultivated soil, India. Middle East J.Sci.Res., 4(2): 90-93.

Dutrizac, J.E. & Kuiper, A. (2006). The solubility of calcium sulphate in simulated nickel sulphate-chloride processing solutions. Hydrometallur-gy, 82(1): 13-31, DOI: 10.1016/j. hydromet.2005.12.013

Dutrizac, J.E. (2002). Calcium sulphate solubilities in simulated zinc processing solutions. Hydrometallur-gy, 65(2-3): 109-135, DOI: 10.1016/ S0304-386X(02)00082-8

Evanylo, G., Sherony, C., Spargo, J.T., Starner, D., Brosius, M. & Haering, K. (2008). Soil and water environmental effects of fertilizer-, manure-, and compost-based fertility practices in an organic vegetable cropping system. Agric.Ecosys. Environ., 127(1-2): 50-58, DOI: 10. 1016/j.agee.2008.02.014

Farrah, H.E., Lawrance, G.A. & Wanless, E.J. (2004). Gypsum-anhydrite transformation in hot acidic manganese sulphate solution. A comparative kinetic study employing several analytical methods. Hydrometallurgy, 75: 1-98. DOI: 10.1016/j.hydromet.2004.07.002

Farrah, H.E., Lawrance, G.A. & Wanless, E.J. (2007). Solubility of calcium sulphate salts in acidic manganese sulphate solutions from 30 to 105°C. Hydrometallurgy, 86: 13-21. DOI: 10.1016/j.hydromet. 2006.10.003

Freyer, D., Voigt & W. (2003). Invited review. Crystallization and phase stability of CaSO₄ and CaSO₄ -based salts. Monatshefte fur Chemie, 134: 693-719, DOI: 10. 1007/s00706-003-0590-3

Ghafoor, A., Murtaza, G., Ahmad, B. & Boers, T.M. (2008). Evaluation of amelioration treatments and eco-nomic aspects of using saline-sodic water for rice and wheat production on salt‐affected soils under arid land conditions. Irrig.Drain., 57: 424-434, DOI: 10.1002/ird.377

Ghassemi, F., Jakeman, A.J. & Nix, H.A. (1995). Salinisation of land and water resources: human causes, extent, management and case studies. CABI Publishing, Wallingford, Oxon, UK, p. 517.

Gupta, R.K. & Abrol, I.P. (1990). Salt-affected soils: their reclamation and management for crop production. Adv. Soil Sci., 11: 223-288.

Hamza, M.A. & Anderson, W. (2003). Responses of soil properties and grain yields to deep ripping and gypsum application in a compacted loamy sand soil contrasted with a sandy clay loam soil in Western Australia. Aust.J.Agric.Res., 54(3): 273-282, DOI: 10.1071/AR02102

Hand R.J. (1997). Calcium sulphate hydrates: a review. Br.Ceram.Trans., 96 (3): 116-120.

Kauraw, D.L. & Verma, G.P. (1982). Improvement of soil structure of degraded soil with organic amendments. J. Indian Soc.Soil Sci., 30: 528-530.

Ling, Y. & Demopoulos, G.P. (2004). Solubility of calcium sulphate hydrates in (0 to 3.5) mol.kg⁻¹ sulphuric acid solutions at 100ºC. J.Chem.Eng. Data, 49(5): 1263-1268, DOI: 10.1021/je034238p

Elrashidi, M.A., West, L.T., Seybold, C.A., Benham, E.C., Schoeneberger, P.J. & Ferguson, R. (2012). Effects of gypsum addition on solubility of nutrients in soil amended with peat. Soil Sci., 175 (4):162-172, DOl: 10.1097/SS. 0b013e3l81ddSId0

Muhammad, D. & Khattak, R.A. (2011). Wheat yield and chemical composition as influenced by integrated use of gypsum, pressmud and FYM in saline-sodic soil. J.Chem.Soc.Pak., 33(1): 82-86.

Murtaza, B., Murtaza, G., Sabir, M., Owens, G., Abbas, G., Imran, M. & Shah, GM. (2017). Amelioration of saline-sodic soil with gypsum can increase yield and nitrogen use efficiency in rice-wheat cropping system. Arch.Agron.Soil Sci., 63(9): 1267-1280, DOI: 10.1080/03650340. 2016.1276285

Nan, J., Chen, X., Wang, X., Lashari, M.S., Wang, Y., Guo, Z. & Du, Z. (2016). Effects of applying flue gas desulfurization gypsum and humic acid on soil physicochemical properties and rapeseed yield of a saline-sodic cropland in the eastern coastal area of China. J. Soils Sediments, 16: 38-50, DOI: 10.1007/s11368-015-1186-3

Nývlt, J. (1997). On the kinetics of solid-liquid-solid. Phase transformation. Crys.Res.Technol., 32 (5): 695-699, DOI: 10.1002/crat.2170320513

Oster, J.D. (1982). Gypsum usage in irrigated agriculture: a review. Fertil.Res., 3(1): 73-89, DOI: 10. 1007/BF01063410

Peters, J. & Kelling, K. (2002). Should calcium be applied to Wisconsin Soils? Focus on Forage, 4(3): 1-3.

Qadir, G., Ahmad, K., Qureshi, M.A., Saqib, A.I., Zaka, M.A., Sarfraz, M., Warraich, I.A. & Ullah, S. (2017). Integrated use of inorganic and organic amendments for reclamation of salt affected soil. Int.J.Biosci., 11(2): 1-10, DOI: 10.12692/ijb/ 11.2.1-10

Qadir, M. & Oster, J.D. (2002). Vegetative bioremediation of calca-reous sodic soils: history, mecha-nisms, and evaluation. Irrigation Sci., 21: 91-101, DOI: 10.1007/s00271-001-0055-6

Qadir, M. & Schubert, S. (2002). Degradation processes and nutrient constraints in sodic soils. Land Deg.Devel. 13(4): 275-294, DOI: 10.1002/ldr.504

Qadir, M.A., Ghafoor, A. & Murtaza, G. (2001). Use of saline-sodic waters through phytoremediation of calcareous saline-sodic soils. Agric. Water Manag., 50(3): 197-210, DOI: 10.1016/S0378-3774(01)00101-9

Qazi, M.A., Akram, M., Ahmad, N., Artiola, J.F. & Tuller, M. (2009). Economical and environmental implications of solid waste compost applications to agricultural fields in Punjab, Pakistan. Waste Manag., 29(9): 2437-2445, DOI: 10.1016/ j.wasman.2009.05.006

Shahid, S.A. (1993). Effect of saline-sodic waters on the hydraulic conductivity and soil structure of the simulated experimental soil conditions. In: Environmental Assessment and Management of Irrigation and Drainage Projects, pp: 125-138, Awan, N.M. and M. Latif (Eds.). Centre of Excellence in Water Resources Engineering, Univ. Eng. Technol., Lahore.

Shainberg, I. & Letey, J. (1984). Response of soils to sodic and saline conditions. Hilgardia, 52(2): 1-57, DOI:10.3733/hilg.v52n02p057

Shainberg, I., Sumner, M.E., Miller, W.P., Farina, M.W.P.., Pavan, M.A. & Fey, M.V. (1989). Use of gypsum on soils: a review. In: Advances in Soil Science, B.A. Stewart (ed.). Springer-Verlag, New York, pp. 2-111.

Steel, R.G., Torrie, J.H. & Dickey, D.A. (1997). Principles and procedures of statistics: a biometrical approach. 3rd Edition, McGraw Hill book Co. Inc., New York, USA., 400-428.

US Salinity Lab Staff (1954). Diagnosis and improvement of saline and alkali soils. USDA Handbook 60, Washington DC, USA.

Van-Camp, L., Bujarrabal, B., Gentile, A.R., Jones, R.J.A., Montanarella, L., Olazabal, C. & Selvaradjou, S.K. (2004). Reports of the technical working groups established under the thematic strategy for soil protection, EUR 21319 EN/3, Office for Official Publications of the European Communities, Luxembourg. 872 p.

Zaka, M.A., Rehman, O.U., Rafa, H.U. & Khan, A.A. (2005). Integrated approach for reclamation of salt affected soils. J.Agric.Soc.Sci., 1(2): 94-97.

 

Abbreviations used: EC_e (electrical conductivity of soil extract); pHs (pH of soil saturated paste); GR (gypsum require-ment); SAR (sodium absorption ratio); BD (bulk density); HC (hydraulic conductivity).