Responses of Spinach (Spinacia oleracea L.) to Acidic Saline Soils as Affected by Different Amendments

Abstract

Soil salinity impedes the normal growth of plants by a number of mechanisms, including osmotic stress and imbalance absorption of essential nutrients. The present study focused on holistic approaches to the production of spinach (Spinacia oleracea L.) in clay loam acidic saline soils. In connection with this, spinach was grown in soils with two salinity levels (hereinafter referred to as soil A: high salinity and soil B: extreme salinity) in the presence of vermicompost (VC), wood ash (WA), and zeolite (ZL) applied at the rates of 1% and 2% (w/w) both alone and in combination along with N-P-K fertilizer. Results indicated better growth as well as the uptake of nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sodium (Na) over control with significant (p < 0.01) differences when VC and WA were applied in combination. The Cshoot/Croot quotient of N, K, Ca, Mg, and Na was found greater than 1, whereas P was observed lower than 1. The Na: K, Na: Ca, and Na: Mg ratios were found to be highest in control that differed significantly (p < 0.01) from the rest of the amended soils. The present study suggests the combined application of VC and WA at the rate of 1% before cultivation to influence soil nutrient dynamics and plant growth in saline soils with acidic soil reactions.

1. Introduction

Saline soils contain substantial quantities of soluble salts giving electrical conductivity (EC) greater than 4 mS cm⁻¹ at saturation extract [1, 2]. More than 1100 million hectares (mha) of lands worldwide are occupied by salt-affected soils, which are expected to rise due to several natural and anthropogenic factors [3, 4, 5, 6]. In Bangladesh, saline soils are now estimated at 1.06 mha, which is about 26.7% higher compared to the past estimate of 0.83 mha in 1973 [7]. Soil salinity impedes the normal growth of plants by a number of mechanisms including osmotic stress, imbalance absorption of essential nutrients such as nitrogen (N), phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg), and toxicities of sodium (Na) and chlorine (Cl) [8, 9]. However, crop production in salt-affected areas is not impacted in the same manner as the degree and extent of salinity are minimum in monsoon during the months of July to August and reaches the maximum in dry periods during the months of March to April. The increase in the degree and extent of soil salinity in most of the periods of the year has raised the challenge of farmers on how to grow high-value and short-duration crops during cool and dry winter months of the year in the coastal regions of Bangladesh. Spinach (Spinacia oleracea L.) is a widely cultivated and consumed popular green leafy dietary vegetable which allows multiple short-duration production cycles (30–48 days) [10, 11]. Besides, spinach is a salt-tolerant and cool-season crop [12, 13] that grows best in slightly acidic to slightly alkaline (pH 6.0–7.5) soil reactions [14].

The application of different amendments has widely been practiced over the few years for the improvement of physico-chemical properties of soils toward sustainable production of crops [15, 16, 17, 18]. Vermicompost (VC) is efficient, cost-effective, environment-friendly and sustainable organic fertilizer [19] that can be prepared by composting a variety of organic substances including animal dung, municipal sewage sludge, and domestic waste [20]. On the other hand, wood ash (WA) is the solid by-product of wood incineration and is utilized in soils as an acid-neutralizing material [21]. Besides, zeolite (ZL), an aluminosilicate, is characterized by large sorption and ion-exchange capacity because of its three-dimensional framework [22]. Although the effects of the aforementioned amendments have been studied separately on plant growth, their combined effects are rarely understood. Moreover, WA has not been studied yet as an acid-neutralizing alternative material to reclaim acidic saline soils. Therefore, the objectives of the present study were to study the effects of VC, WA, and ZL applications alone and in combinations on the performance of spinach in two different saline soils having an acidic reaction.

2. Material and methods

Spinach was grown under the influence of different amendments (e.g., VC, WA, and ZL) applied alone (i.e., VC, WA, and ZL) and in combinations (i.e., VC + WA, VC + ZL, and WA + ZL) at 1% (w/w) and 2% (w/w) rates in soils with high salinity (hereinafter mentioned as soil A) as well as extreme salinity (hereinafter mentioned as soil B) after being subjected to leaching with 2 pore volume (PV) of water.

2.1 Collection and processing of soils and amendments

Bulk soil samples from the top layer (0–15 cm) were collected from two different locations having two different levels of salinity (soil A: high salinity and soil B: extreme salinity). The physical and chemical parameters of soil A and soil B, which were analyzed before setting up for experiments are given in Table 1. After bringing to the laboratory, soil samples were dried at room temperature for few days, hard clods were broken with a wooden a hammer followed by sieving through 4 mm stainless steel mesh for the pot experiment. Representative sub-samples were separated from the 4 mm sieved bulk soil samples and passed again through 2 mm stainless steel mesh for the initial analyses of the physical and chemical parameters.

Parameters	Soil A	Soil B
pH	5.01	5.22
ECe (mS cm^−1)	9.25	37.64
Organic carbon (%)	1.20	1.11
Organic matter (%)	2.07	1.91
Sand (%)	34.0	31.0
Silt (%)	39.0	42.0
Clay (%)	27.0	27.0
Textural class	Clay loam	Clay loam
Available N (mg kg^−1)	63.54	57.86
Available P (mg kg^−1)	2.56	5.99
Available K (mg kg^−1)	145.35	433.64
Available Ca (mg kg^−1)	583.33	850.00
Available Mg (mg kg^−1)	590.00	1610.00
Available Na (mg kg^−1)	782.24	1797.08

Table 1.

Physical and chemical characteristics of soil A and soil B.

Vermicompost of cow manure (hereinafter referred to VC) was collected from a VC producing farm and calcium-type ZL (CaAl₂Si₄O₁₂·nH₂O) was collected from a market distributed by National Agricare, while WA was prepared by burning of woods in mud stoves. All the amendments were passed through a 2 mm stainless sieve. The properties of amendments are given in Table 2.

Parameters	VC	WA	ZL
pH	7.89	11.77	7.47
EC1:10 (mS cm^−1)	1.92	11.27	3.09
Organic carbon (%)†	18.06	0.17	0.17
Total N (%)†	2.01	0.1	0.1
Total P (%)†	0.49	1.16	0.12
Total K (%)†	2.34	4.13	1.81
Total Ca (%)†	2.56	10.4	3.04
Total Mg (%)†	0.42	1.28	0.88
Total Na (%)†	—	0.11	0.47

Table 2.

Characteristics of amendments used in the present study [23].

^†

On dry weight basis.

2.2 Set-up of the pot experiment

In our previous pot experiment without incorporation of any amendment indicated strong inhabitation of germination of spinach seeds by extreme soil salinity (soil B), while leaching with 2 PV of non-saline water resulted in germination and growth of spinach [23]. From the knowledge of previous pot experiments, extreme saline soils (soil B) amended with VA, WA, and ZL as single and in combination (i.e., VC + WA, VC + ZL, and WA + ZL), both at 1% (w/w) and 2% (w/w) rates were subjected to leaching with 2 PV of water using the Eq. (1) [24]. In this regard, the PV was adjusted to 925 cm³ for all the pots. The characteristics of the water used for the leaching of soils are given in Table 3.

Parameters	Properties
pH	6.70
EC (mS cm^−1)	0.12
Total K (mg L^−1)	2.85
Total Ca (mg L^−1)	4.10
Total Mg (mg L^−1)	3.18
Total Na (mg L^−1)	3.37

Table 3.

Characteristics of water used for leaching of soil B and irrigation in pot experiment.

$$PV=V\mathrm{s}\times \Phi \mathrm{s}$$E1

where,

Vs is the volume of the soil in the column and

Φs is the porosity

For the cultivation of spinach, plastic pots with 17 cm height and 13 cm of average diameter were taken. An amount of 2 kg soil (dry weight basis) was placed in each plastic pot having holes in the bottom. The required amount of amendments was weighed in a balance. The amendments were mixed homogeneously with previously weighted soil samples. There was a set of control where neither N-P-K fertilizer nor any amendment was applied. The application of different amendments yielded a total number of 14 treatments (Table 4). All the treatments were taken in triplicates. The pots were arranged in a completely randomized way. The recommended dose of N-P-K (60–18-0 kg ha⁻¹) fertilizer was applied in both soil A and B as basal doses [25].

Treatment legend	Treatment description
T1	Non-amended soil (control)
T2	Soil + N-P-K
T3	Soil + N-P-K + VC (1%)
T4	Soil + N-P-K + WA (1%)
T5	Soil + N-P-K + ZL (1%)
T6	Soil + N-P-K + VC (1%) + WA (1%)
T7	Soil + N-P-K + VC (1%) + ZL (1%)
T8	Soil + N-P-K + WA (1%) + ZL (1%)
T9	Soil + N-P-K + VC (2%)
T10	Soil+ N-P-K + WA (2%)
T11	Soil + N-P-K + ZL (2%)
T12	Soil+ N-P-K + VC (2%) + WA (2%)
T13	Soil+ N-P-K + VC (2%) + ZL (2%)
T14	Soil + N-P-K + WA (2%) + ZL (2%)

Table 4.

Treatment legends and their description for the pot experiment.

2.3 Sowing and harvesting of plants

Spinach seeds were soaked in water for 12 hours before sowing in the soil. Twelve seeds were sown in each pot. The seedlings were later thinned and five vigorous plants were maintained in each pot until harvesting. The soil in each pot was watered with water to maintain soil moisture roughly constant at 60 ± 5% of WHC. It is worth mentioning that the quality of irrigation water is important as irrigating with saline water can worsen the condition of soil salinity [26]. The water used for irrigating plants is classified as non-saline (Table 3) on the basis of EC value [27].

Plants were harvested manually at 35 days of growth after outpouring the soils on the floor so that root damage can be minimized during the uprooting of plants. The plants were cut at the root-shoot junction to separate the plants into two portions. At harvest, some measurements such as fresh weight of shoot and root were taken.

2.4 Processing of harvested plants

The shoot and root were washed first with tap water several times to remove adhering soil particles and then with distilled water. The shoot and root were lapped between tissue papers to absorb excess water adhering to plant parts. After a few days of air drying, the plant parts were separately put in envelopes and allowed to dry in an oven at 65 ± 5°C for 72 hours. After drying to constant weight, plant parts were weighted to determine the moisture content. Oven-dried root parts were ground manually, while shoot parts were ground to a fine powder with an electrical grinder and sieved through a 0.5 mm stainless sieve for chemical analyses [28].

2.5 Methods of analysis

The pH and EC of all the soil samples were measured by pH meter (Seven CompactTM pH/Ion S220) and EC meter (Adwa AD 330) meter after preparing the suspension at 1:5 soil to water ratio (w/v). The pH and EC of VC and WA were measured at a 1:10 ratio (w/v) [29]. The EC_1:5 of soil samples was converted to ECe by multiplying a conversion factor as mentioned in Hazelton and Murphy [30]. The particle size of soil was analyzed by the hydrometer method as described in Huq and Alam [28]. The contents of organic carbon were determined by Walkley and Black wet oxidation method and converted to organic matter by multiplying the organic carbon content by the Van Bemmelon factor of 1.724 [31]. Available N (NH₄⁺-N) and available P in soils were extracted by standard methods as given in Keeney and Nelson [32] and Gupta [33], respectively. The concentrations of exchangeable Na, K, Ca and Mg were extracted with 1 N NH₄OAc (pH 7.0) at 1:5 ratio (w/v) [34]. For the analysis of total concentrations of elements in plants and amendments, a suitable amount of the samples was first digested by a digestion mixture solution as mentioned in Parkinson and Allen [35].

The available and total N concentrations were determined by collecting NH₃ which was liberated by distillation of the extract and/or digest with strong alkali (usually 40% NaOH) containing boric acid- mixed indicator in an Erlenmeyer flask and the titration of the distillate with p < 0.01 N H₂SO₄ [36]. The available P in the extract and total P in the digest was measured by the ascorbic acid blue color method and vanadomolybdo yellow color method using UV–visible spectrophotometer at a wavelength of 880 nm and 490 nm, respectively [28]. The contents of Na and K in extract and digest were measured by atomic absorption spectrometer (Agilent Technologies 200 Series AA), whereas Ca and Mg concentrations were determined by the ethylene di-amine tetra acetic acid (EDTA) method as described in Huq and Alam [28].

2.6 Analysis of data

The uptake of elements in spinach was determined by multiplying the concentration of that element with the dry mass of plant parts [37].

$$\mathit{Uptake}=\mathit{Concentration}\mathit{in}\mathit{shoot}\mathit{or}\mathit{root}\times \mathit{dry}\mathit{weight}$$

The shoot-root quotient (C_shoot/C_root quotient) was calculated to estimate the transfer of the element from the underground part to the aerial part by using the following formula [38].

$$\mathit{Cshoot}/\mathit{Croot}\mathit{quotient}=\frac{\mathit{Cshoot}}{\mathit{Croot}}$$

Where,

C_shoot = Concentration of the element in shoot.

C_root = Concentration of that element in root

Pearson’s correlations and Duncan’s Multiple Range Test (DMRT) were performed by using statistical packages for social sciences (SPSS) to measure significant differences between pairs of means of the obtained results. Standard deviations were determined using Microsoft Excel 2016.

3. Results and discussion

The performance of spinach grown in soil A and soil B in the presence of different organic and inorganic amendments applied alone and in combination at two different rates (i.e., 1% and 2% w/w) was evaluated in response to yield, concentration, and uptake of several elements. In addition, the C_shoot/C_root quotient and different ratios among Na, K, Ca, and Mg are presented sequentially.

3.1 Yield of spinach

The fresh weight of shoot and root of spinach grown in soil A and soil B is shown in Figures 1 and 2, respectively. The application of amendments had a significant effect (p < 0.01) over control on the growth and yield of spinach. The fresh weight of shoot and root of spinach grown in both soil A and soil B increased significantly (p < 0.01) when VC and WA were applied in combination either at a 1% or 2% rate. The fresh weight of shoot and root of spinach grown in soil A was found 212.14% and 350.81% higher in T6 in comparison to T1. In soil B, the fresh weight of shoot in T12 increased by 622.67%, whereas of root in T6 increased by 235.17% compared to T1. When the biomass of spinach growing in soil A and soil B was compared (Figures 1 and 2), the adverse effect of salinity was found to be highest in control and lowest in amended soils especially when VC plus WA were applied in combination. The fresh weight of the shoot decreased by 2.30–62.08%, while of root decreased by 12.02–33.74% when grown in soil B compared to soil A.

Figure 1.

Fresh weight (g pot⁻¹) of different parts of spinach grown in soil A. Mean (s) followed by the same letter within the bar do not differ at 5% level of significance. Treatment legend description is given in Table 4.

Figure 2.

Fresh weight (g pot⁻¹) of different parts of spinach grown in soil B. Mean (s) followed by the same letter within the bar do not differ at 5% level of significance. Treatment legend description is given in Table 4.

3.2 Concentration, uptake and C_shoot/C_root quotient

The concentrations of N, P, K, Ca, Mg and Na in spinach grown in soil A and soil B are shown in Tables 5 and 6, respectively. There was a significant variation (p < 0.01) among the treatments in the concentration of N and K in shoot and root for both soil A and soil B. The application of amendments also increased the concentration of P, Ca, and Mg with significant differences both for the shoot (p < 0.01) and root (p < 0.01) of spinach grown in both soil A (Table 5) and soil B (Table 6). The maximum concentrations of P, Ca and Mg in both shoot and root were found when VC along with WA was incorporated in soils. The application of amendments resulted in significant variation in the concentration of Na in the shoot (p < 0.05) and root (p < 0.01) of spinach grown in soil A (Table 5). In soil A, the concentration of Na in the shoot decreased by 24.61% in T3 relative to T1, whereas root concentration decreased by 41.25% in T6 compared to T1. The maximum concentration of Na in both parts of spinach grown in soil B was also observed in control which was significantly higher (p < 0.01) from the rest of the treatments (Table 6). The concentration of Na in T12 and T6 decreased by respectively 50.31% and 65.46% compared to T1 for shoot and root.

Treatment	N		P		K		Ca		Mg		Na
	Shoot	Root	Shoot	Root	Shoot	Root	Shoot	Root	Shoot	Root	Shoot	Root
T1	2.02^d	1.11^h	0.35^e	0.85i	1.89^g	1.11^h	0.45^g	0.27^g	0.87^h	0.35^g	1.39^ab	1.09^a
T2	3.01^abc	2.19^cd	0.57^cd	0.94^ghi	2.74^f	1.41^g	0.51^fg	0.30^fg	1.05^fgh	0.40^fg	1.35^abc	0.81^de
T3	3.34^a	2.14^d	0.72^bc	0.92^hi	3.41^abcde	1.86^f	0.54^f	0.38^d	1.21^cdef	0.50^e	1.05^d	0.71^fg
T4	3.38^a	2.63^a	0.60^cd	1.02^fgh	3.40^abcde	2.49^ab	0.55^ef	0.42^c	1.15^defg	0.65^ab	1.25^abcd	0.97^bc
T5	3.44^a	2.21^cd	0.79^ab	1.19^cde	3.06^def	2.09^de	0.56^def	0.34^ef	1.12^defg	0.56^cde	1.24^abcd	0.88^d
T6	2.53^c	1.56^g	0.92^a	1.19^cd	3.80^a	2.13^de	0.76^ab	0.56^a	1.28^bcde	0.70^a	1.14^cd	0.64^g
T7	3.15^ab	2.23^c	0.72^bc	1.06^efg	3.70^abc	1.98^ef	0.53^f	0.33^ef	1.37^abc	0.48^ef	1.12^cd	0.81^de
T8	3.22^a	1.97^e	0.67^bcd	1.14^def	3.23^bcde	2.11^de	0.63^cde	0.37^de	1.18^cdef	0.50^e	1.28^abcd	0.84^de
T9	3.16^ab	2.36^b	0.87^a	1.38^ab	3.44^abcde	2.16^de	0.53^f	0.33^ef	1.50^a	0.51^e	1.16^bcd	0.70^fg
T10	2.95^abc	2.30^b	0.57^cd	1.28^bc	3.21^cde	2.53^ab	0.58^def	0.44^c	1.07^efg	0.56^cde	1.26^abcd	1.04^ab
T11	3.38^a	2.17^cd	0.60^cd	1.08^def	2.98^ef	2.20^cd	0.63^cd	0.32^f	1.15^defg	0.53^de	1.08^d	0.90^cd
T12	2.62^bc	1.85^f	0.92^a	1.42^a	3.73^ab	2.37^bc	0.72^ab	0.49^b	1.28^bcd	0.63^abc	1.14^cd	0.75^ef
T13	2.93^abc	2.15^d	0.68^bc	1.04^fg	3.85^a	2.09^de	0.78^a	0.53^a	1.44^ab	0.52^de	1.19^bcd	0.86^d
T14	2.94^abc	1.85^f	0.53^d	1.10^def	3.52^abcd	2.58^a	0.69^bc	0.41^cd	0.97^gh	0.61^bcd	1.43^a	1.07^a
p value	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.05	0.01

Table 5.

Concentration (%) of different elements in shoot and root of spinach grown in soil A.

Mean (s) followed by the same letter within the column do not differ at 5% level of significance. Treatment legend description is given in Table 4.

Treatment	N		P		K		Ca		Mg		Na
	Shoot	Root	Shoot	Root	Shoot	Root	Shoot	Root	Shoot	Root	Shoot	Root
T1	1.92^c	1.01^g	0.18^g	0.45^c	2.61^h	1.91^g	0.36^c	0.22^h	0.72^f	0.29^e	4.32^a	3.87^a
T2	2.71^ab	1.97^ab	0.32^bcde	0.52^c	3.38^defg	2.34^de	0.38^c	0.26^gh	0.83^ef	0.35^de	2.97^b	2.77^b
T3	2.59^ab	2.07^a	0.34^bcde	0.47^c	4.04^a	2.65^abc	0.48^ab	0.28^fg	1.16^abc	0.42^bcd	2.32^de	1.70^fg
T4	2.55^ab	1.79^d	0.34^bcde	0.49^c	3.27^efg	2.06^fg	0.42^bc	0.31^def	1.04^bcde	0.61^a	2.65^bcd	1.99^d
T5	2.93^a	2.03^ab	0.24^fg	0.47^c	3.72^cde	2.38^cde	0.47^ab	0.32^cde	1.04^bcde	0.42^bcd	2.44^cde	1.72^fg
T6	2.40^b	1.51^f	0.44^a	0.88^a	3.66^cde	2.24^def	0.48^ab	0.37^b	1.20^abc	0.40^bcd	2.18^e	1.34^h
T7	2.62^ab	1.74^de	0.39^abc	0.75^b	3.78^abcd	2.19^ef	0.53^a	0.32^cde	1.34^a	0.40^bcd	2.51^cde	1.61^g
T8	2.58^ab	1.91^bc	0.42^ab	0.89^a	3.28^efg	2.47^bcde	0.46^ab	0.34^bcd	1.10^bcd	0.44^bc	2.81^bc	2.19^c
T9	2.47^b	1.82^cd	0.37^abcd	0.92^a	3.92^abc	2.52^bcd	0.43^bc	0.31^cdef	1.36^a	0.48^b	2.29^de	1.80^ef
T10	2.49^b	1.68^e	0.28^def	0.97^a	3.17^g	2.35^de	0.46^ab	0.34^bcd	1.00^cde	0.43^bc	2.54^bcde	1.94^de
T11	2.79^ab	1.80^d	0.26^ef	0.68^b	3.53^cdef	2.69^ab	0.42^bc	0.30^ef	1.08^bcd	0.41^bcd	2.35^de	1.89^de
T12	2.47^b	1.72^de	0.33^bcdef	0.89^a	3.49^cdef	2.65^abc	0.47^ab	0.42^a	1.16^abc	0.38^cd	2.15^e	1.55^g
T13	2.70^ab	1.98^ab	0.30^cdef	0.91^a	3.97^ab	2.82^a	0.52^a	0.33^cde	1.24^ab	0.45^bc	2.46^cde	1.98^d
T14	2.78^ab	1.82^cd	0.25^fg	0.94^a	3.22^fg	2.41^bcde	0.48^ab	0.35^bc	0.87^def	0.46^b	2.58^bcde	2.01^d
p value	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01

Table 6.

Concentration (%) of different elements in shoot and root of spinach grown in soil B.

Mean (s) followed by the same letter within the column do not differ at 5% level of significance. Treatment legend description is given in Table 4.

When the concentration of N of spinach grown in soil A (Table 5) was compared with that of spinach grown in soil B (Table 6), it was found that the concentration of N increased by as highest as 41.33% in shoot and 46.67% in the root of spinach grown in the soil A. The concentration of P, Ca, and Mg in both shoot and root decreased in response to increased soil salinity for all the respective treatments (comparison between Tables 5 and 6). The concentration of P increased by as highest as 227.80% in shoot and 151.57% in the root of spinach when grown in soil A compared to soil B. Similarly, the concentration of Ca and Mg respectively decreased by as highest as 37.19% and 26.77% in shoot and 38.19% and 73.91% in the root of spinach grown in soil B compared to soil A. However, the concentration of K in response to salinity showed a variable trend depending on the types of the treatments. The K concentration in the shoot of spinach grown in soil B was found higher than soil A for all treatments excluding T4, T6, T10, T12, and T14. The concentration of K in root was also found higher in soil B in comparison to soil A for all treatments except T4, T10, and T14. On the other hand, it was evident that the concentration of Na decreased in all the respective treatments in soil A compared to soil B. The Na concentration decreased by as maximum as 67.80% in shoot and 71.86% in root when spinach was grown in soil A relative to soil B (comparison between Tables 5 and 6).

The uptake of N, P, K, Ca, and Mg in both aerial and underground parts of spinach grown in both soil A (Table 7) and soil B (Table 8) was found to be lowest in control and highest when VC and WA were incorporated in combination, with significant differences among the treatments both for the shoot (p < 0.01) and root (p < 0.01). The uptake of Na in both shoots was also found lower in control and higher in soils amended with VC along with WA. The uptake of Na in T1 decreased by 66.10% and 59.12% respectively in shoot and root for soil A (Table 7) and 77.64% and 33.88% for soil B relative to T12 (Table 8).

Treatment	N		P		K		Ca		Mg		Na
	Shoot	Root	Shoot	Root	Shoot	Root	Shoot	Root	Shoot	Root	Shoot	Root
T1	3.79^f	0.29^d	0.65^f	0.22^f	3.53^i	0.29^h	0.83^h	0.07^g	1.61^f	0.09^f	2.59^h	0.28^f
T2	7.26^e	0.99^c	1.38^ef	0.43^def	6.56^hi	0.64^g	1.23^gh	0.14^efg	2.51^ef	0.18^e	3.21^gh	0.37^def
T3	15.80^ab	1.43^b	3.36^c	0.61^cde	16.08^cd	1.24^cde	2.56^cd	0.26^c	5.67^c	0.33^cd	4.95^cdef	0.47^cd
T4	9.43^de	1.08^c	1.68^e	0.42^def	9.51^fgh	1.03^ef	1.52^efg	0.18^def	3.22^de	0.27^de	3.48^fgh	0.40^def
T5	9.58^de	0.88^c	2.20^de	0.47^de	8.41^gh	0.84^fg	1.54^efg	0.14^efg	3.12^de	0.22^e	3.42^fgh	0.35^def
T6	14.72^ab	1.59^ab	5.34^ab	1.21^ab	22.00^b	2.16^a	4.42^ab	0.57^a	7.41^b	0.71^a	6.62^ab	0.65^ab
T7	13.06^bc	1.11^c	2.93^cd	0.53^de	15.30^de	0.98^efg	2.21^cde	0.17^def	5.73^c	0.24^e	4.67^cdefg	0.40^def
T8	10.78^cd	1.06^c	2.27^de	0.62^cd	10.97^fg	1.14^def	2.14^cde	0.20^cde	4.00^d	0.27^de	4.33^defg	0.46^cde
T9	16.11^ab	1.85^a	4.49^b	1.09^b	17.64^cd	1.70^b	2.74^e	0.26^c	7.58^ab	0.40^c	5.97^bc	0.55^bc
T10	9.55^de	0.94^c	1.85^e	0.52^de	10.42^fg	1.03^ef	1.87^def	0.18^cdef	3.46^de	0.23^e	4.09^efgh	0.42^cde
T11	10.13^cde	0.80^c	1.80^e	0.39^ef	8.97^gh	0.81^fg	1.91^def	0.12^fg	3.42^de	0.20^e	3.21^gh	0.33^ef
T12	17.59^a	1.74^ab	6.22^a	1.34^a	25.15^a	2.23^a	4.87^a	0.46^b	8.64^a	0.59^b	7.65^a	0.70^a
T13	14.21^b	1.57^ab	3.33^c	0.76^c	18.68^c	1.52bc	3.81^b	0.39^b	7.00^b	0.38^c	5.86^bcd	0.62^ab
T14	10.63^cd	1.05^c	1.91^e	0.62^cd	12.75^ef	1.46bcd	2.48^cd	0.23^cd	3.51^de	0.35^cd	5.18^bcde	0.61^ab
p value	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01

Table 7.

Uptake (mg plant⁻¹) of different elements in shoot and root of spinach grown in soil A.

Mean (s) followed by the same letter within the column do not differ at a 5% level of significance. Treatment legend description is given in Table 4.

Treatment	N		P		K		Ca		Mg		Na
	Shoot	Root	Shoot	Root	Shoot	Root	Shoot	Root	Shoot	Root	Shoot	Root
T1	1.39^h	0.19^g	0.13^f	0.09^f	1.90^f	0.36^g	0.26^i	0.04^i	0.52^g	0.05^g	3.12^h	0.74^cd
T2	5.89^g	0.46^f	0.70^e	0.12^ef	7.33^e	0.55^fg	0.83^h	0.06^hi	1.79^f	0.08^fg	6.47^fg	0.64^cde
T3	9.38^cd	0.93^bc	1.22^c	0.21^e	14.63^d	1.20^c	1.74^de	0.13^def	4.16^cd	0.19^cd	8.33^de	0.77^cd
T4	5.89^g	0.54^ef	0.78^e	0.15^ef	7.62^e	0.62^fg	0.98^h	0.10^fgh	2.44^f	0.19^cd	6.12^g	0.60^de
T5	7.08^efg	0.64^def	0.58^e	0.15^ef	9.17^e	0.78^ef	1.16^fgh	0.10^efg	2.60^f	0.14^de	6.00^g	0.55^e
T6	12.64^b	1.44^a	2.35^a	0.84^a	19.32^b	2.14^a	2.51^b	0.35^a	6.35^ab	0.39^a	11.47^b	1.28^a
T7	10.69^bc	0.88^c	1.58^b	0.38^d	15.43^cd	1.11^cd	2.16^bc	0.16^cd	5.46^bc	0.20^c	10.25^bc	0.81^c
T8	6.06^fg	0.46^f	0.96^cde	0.21^e	7.62^e	0.59^fg	1.08^gh	0.08^ghi	2.57^f	0.11^ef	6.54^fg	0.52^e
T9	11.72^b	1.10^b	1.73^b	0.55^c	18.64^bc	1.52^b	2.03^cd	0.18^c	6.45^ab	0.29^b	10.82^bc	1.08^b
T10	7.48^defg	0.69^de	0.83^de	0.39^d	9.51^e	0.96^cde	1.39^efg	0.14^de	3.00^ef	0.18^cd	7.65e^fg	0.79^c
T11	8.07^def	0.62^ef	0.76^e	0.23^e	10.22^e	0.93^cde	1.22^fgh	0.10^efg	3.11^ef	0.14^de	6.78e^fg	0.65^cde
T12	16.01^a	1.27^a	2.12^a	0.66^b	22.84^a	1.95^a	3.04^a	0.31^b	7.53^a	0.28^b	13.97^a	1.13^ab
T13	10.61^bc	0.82^cd	1.18^cd	0.38^d	15.61^cd	1.17^cd	2.04^cd	0.14^def	4.84^cd	0.19^cd	9.64^cd	0.83^c
T14	8.76^cde	0.70^de	0.77^e	0.36^d	10.11^e	0.92^de	1.51^ef	0.14^def	2.74^f	0.18^cd	8.10^de	0.77^cd
p value	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01

Table 8.

Uptake (mg plant⁻¹) of different elements in shoot and root of spinach grown in soil B.

Mean (s) followed by the same letter within the column do not differ at a 5% level of significance. Treatment legend description is given in Table 4.

The uptake in both parts of spinach decreased in all the respective treatments in soil B compared to soil A (comparison between Tables 7 and 8). The uptake of N, P, Ca, and Mg decreased by as highest as 63.36%, 80.51%, 68.71%, and 67.50% respectively in shoot and 57.11%, 71.75%, 65.20%, and 60.59% respectively in root when spinach was grown in soil B. However, the uptake of K in both shoot and root with respect to salinity did not found in any definite trend. The shoot K uptake decreased in all treatments except for T2, T5, T7, and T9 whereas root uptake decreased in all treatments except for T1, T7, and T11 when spinach was grown in soil B. By contrast, the uptake of Na in both shoot and root of spinach was observed higher in soil B for all the respective treatments. The uptake of Na decreased by as highest as 54.42% and 61.38% respectively in shoot and root when grown in soil A.

The C_shoot/C_root quotient of N, K, Na, Ca, and Mg was found greater than 1, whereas P was observed lower than 1 both in soil A (Table 9) and soil B (Table 10). The C_shoot/C_root quotient of N, K, Na, Ca, and Mg were found in the range of 1.28–1.89, 1.27–1.95, 1.20–1.79, 1.29–1.97, and 1.61–2.96 for soil A, whereas 1.26–1.90, 1.32–1.73, 1.08–1.64, 1.11–1.74, 1.70–3.03 for soil B. On the contrary, the C_shoot/C_root quotient of P ranged from 0.41 to 0.78 and 0.27 to 0.72 for soil A and soil B, respectively.

Treatment	N	P	K	Ca	Mg	Na
T1	1.83^a	0.41^f	1.70^abcd	1.66^abc	2.47^abc	1.28^cd
T2	1.38^bcdf	0.61^cd	1.95^a	1.71^ab	2.74^ab	1.66^ab
T3	1.57^bcd	0.78^a	1.84^ab	1.42^bcd	2.44^abcd	1.48^abcd
T4	1.29^ef	0.60^cd	1.37^ef	1.29^d	1.78^ef	1.29^cd
T5	1.55^bcde	0.67^abc	1.47^def	1.65^abc	2.02^cdef	1.41^bcd
T6	1.62^abc	0.77^ab	1.78^abc	1.35^cd	1.82^def	1.79^a
T7	1.41^bcdef	0.68^abc	1.87^a	1.59^bcd	2.85^a	1.39^bcd
T8	1.64^ab	0.59^cd	1.54^cdef	1.70^ab	2.38^abcde	1.52^abcd
T9	1.34^def	0.63^bc	1.59^bcde	1.63^bc	2.96^a	1.67^ab
T10	1.28^f	0.45^ef	1.27^f	1.33^cd	1.92^cdef	1.22^cd
T11	1.56^bcde	0.56^cde	1.36^ef	1.97^a	2.17^bcdef	1.20^d
T12	1.42^bcdef	0.65^abc	1.58^bcde	1.48^bcd	2.04^cdef	1.53^abc
T13	1.36^cdef	0.66^abc	1.85^ab	1.48^bcd	2.84^a	1.40^bcd
T14	1.59^abcd	0.48^def	1.37^ef	1.69^ab	1.61^f	1.34^bcd
p value	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01

Table 9.

C_shoot/C_root quotient of different elements for spinach grown in soil A.

Mean (s) followed by the same letter within the column do not differ at a 5% level of significance. Treatment legend description is given in Table 4.

Treatment	N	P	K	Ca	Mg	Na
T1	1.90^a	0.39^cde	1.37^cde	1.65^ab	2.51^bcd	1.12^de
T2	1.38^bc	0.62^ab	1.45^bcde	1.51^abc	2.34^bcde	1.08^e
T3	1.26^c	0.72^a	1.52^abcde	1.74^a	2.77^abc	1.36^bc
T4	1.42^bc	0.72^a	1.59^abc	1.35^bcd	1.70^e	1.33^cd
T5	1.46^bc	0.51^bc	1.57^abcd	1.46^abc	2.51^bcd	1.42^bc
T6	1.59^b	0.51^bc	1.64^ab	1.29^cd	2.96^abc	1.64^a
T7	1.50^b	0.52^bc	1.73^a	1.64^ab	3.34^a	1.56^ab
T8	1.35^bc	0.47^bcd	1.33^de	1.34^bcd	2.50^bcd	1.29^cde
T9	1.36^bc	0.40^cde	1.56^abcde	1.40^bcd	2.88^abc	1.27^cde
T10	1.48^bc	0.29^e	1.35^cde	1.34^bcd	2.30^cde	1.31^cd
T11	1.55^b	0.39^cde	1.32^e	1.41^bcd	2.65^bc	1.24^cde
T12	1.44^bc	0.37^cde	1.33^de	1.11^d	3.03^ab	1.40^bc
T13	1.37^bc	0.33^de	1.41^bcde	1.59^abc	2.78^abc	1.24^cde
T14	1.53^b	0.27^e	1.34^cde	1.36^bcd	1.89^de	1.28^cde
p value	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01

Table 10.

C_shoot/C_root quotient of different elements for spinach grown in soil B.

Mean (s) followed by the same letter within the column do not differ at 5% level of significance. Treatment legend description is given in Table 4.

3.3 Na: K, Na: Ca and Na: Mg ratios

The Na: K, Na: Ca, and Na: Mg ratios of shoot and root were found to be highest in control that significantly differed (p < 0.01) from the rest of the treatments for both soil A (Table 11) and soil B (Table 12). The Na: K, Na: Ca, and Na: Mg ratios of shoot increased by 146.02%, 107.91%, and 105.87%, whereas of root increased by 226.29%, 256.53%, and 244.05% in T1 compared to the lowest values of the respective parts of spinach grown in soil A. On the other hand, in the case of soil B, the Na: K, Na: Ca, and Na: Mg ratios increased by 191.75%, 169.46%, and 261.22% in T1 compared to the lowest values for the shoot, while the Na: K, Na: Ca and Na: Mg ratios increased by 247.88%, 389.11% and 319.93% in T1 compared to the lowest values for root. The Na: K, Na: Ca and Na: Mg ratios of both shoot and root of spinach grown in soil B was observed higher in comparison to corresponding ratios of the concerned part of plants when grown in soil A (comparison between Tables 11 and 12). The Na: K, Na: Ca, and Na: Mg ratios increased by as highest as 126.82%, 295.95%, and 227.32% for the shoot, while 146.49%, 335.27%, and 334.74% for root when grown in soil B relative to soil A.

Treatment	Na: K		Na: Ca		Na: Mg
	Shoot	Root	Shoot	Root	Shoot	Root
T1	0.74^a	0.98^a	3.12^a	4.05^a	1.61^a	3.13^a
T2	0.49^b	0.57^b	2.61^b	2.69^bc	1.30^bc	2.08^b
T3	0.31^c	0.38^c	1.94^cde	1.86^fg	0.88^fg	1.43^def
T4	0.37^bc	0.39^c	2.29^bc	2.29^de	1.09^def	1.49^def
T5	0.41^bc	0.42^c	2.23^c	2.60^bcd	1.10^cde	1.58^cde
T6	0.30^c	0.30^d	1.50^f	1.14^i	0.89^efg	0.91^g
T7	0.30^c	0.41^c	2.11^cd	2.42^cde	0.82^g	1.67^cde
T8	0.39^bc	0.40^c	2.04^cd	2.27^e	1.09^def	1.68^cde
T9	0.34^c	0.32^d	2.18^cd	2.13^ef	0.78^g	1.39^ef
T10	0.39^bc	0.41^c	2.19^cd	2.36^cde	1.18^cd	1.85^bc
T11	0.37^bc	0.41^c	1.73^def	2.81^b	0.95^efg	1.71^cde
T12	0.31^c	0.32^d	1.59^ef	1.53^h	0.89^efg	1.19^fg
T13	0.31^c	0.41^c	1.51^ef	1.62^gh	0.82^g	1.71^cde
T14	0.41^bc	0.42^c	2.10^cd	2.63^bcd	1.48^ab	1.78^bcd
p-value	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01

Table 11.

Na: K, Na: Ca and Na: Mg ratios in shoot and root of spinach grown in soil A.

Mean (s) followed by the same letter within the column do not differ at a 5% level of significance. Treatment legend description is given in Table 4.

Treatment	Na:K		Na:Ca		Na:Mg
	Shoot	Root	Shoot	Root	Shoot	Root
T1	1.68^a	2.02^a	12.35^a	17.65^a	6.08^a	13.61^a
T2	0.90^b	1.18^b	7.78^b	10.91^b	3.68^b	7.93^b
T3	0.58^d	0.64^fg	4.82^c	6.15^c	2.00^d	4.08^cd
T4	0.81^bcd	0.96^c	6.27^bc	6.36^c	2.55^cd	3.24^d
T5	0.66^bcd	0.73^ef	5.21^c	5.33^c	2.42^cd	4.15^cd
T6	0.60^d	0.60^g	4.58^c	3.61^d	1.86^d	3.31^d
T7	0.67^bcd	0.74^f	4.83^c	4.99^cd	1.90^d	4.01^cd
T8	0.86^bc	0.89^cd	6.13^bc	6.37^c	2.58^cd	5.07^c
T9	0.58^d	0.71^f	5.32^c	5.87^c	1.68^d	3.80^cd
T10	0.80^bcd	0.83^de	5.50^c	5.66^c	2.54^cd	4.49^cd
T11	0.66^bcd	0.70^f	5.54^c	6.33^c	2.18^cd	4.64^cd
T12	0.62^cd	0.58^g	4.61^c	3.68^d	1.87^d	4.05^cd
T13	0.62^cd	0.70^f	4.73^c	6.07^c	1.99^d	4.45^cd
T14	0.80^bcd	0.84^d	5.41^c	5.66^c	2.98^bc	4.38^cd
p-value	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01

Table 12.

Na: K, Na: Ca, and Na: Mg ratios in shoot and root of spinach grown in soil B.

Mean (s) followed by the same letter within the column do not differ at a 5% level of significance. Treatment legend description is given in Table 4.

3.4 Correlation co-efficient (r) of plant nutrients

Tables 13 and 14 show the correlation matrix among different parameters in shoot and root of spinach grown in soil A and soil B, respectively. In the shoot of spinach grown in soil A, the relationship of Na with all other elements was negative. Similarly, the relation of Na with all other elements except K was also found negative in the root. In both shoot and root of spinach grown in both soil A and soil B, the relations among K, Ca, and Mg were found positive. In both shoot and root of spinach grown in soil B, the concentration of Na was negatively correlated with all other elements. Similar to spinach grown in soil A, the relationships between K-Ca, K-Mg, and Ca-Mg were found positive for both shoot and root of spinach grown in soil B.

	N	P	Na	K	Ca	Mg
N	1	0.149	−0.216	0.241	−0.103	0.262
P	0.203	1	−0.428**	0.619**	0.424**	0.616**
Na	−0.119	−0.311*	1	−0.324*	−0.116	−0.387*
K	0.490**	0.562**	0.035	1	0.614**	0.617**
Ca	−0.012	0.324*	−0.236	0.524**	1	0.302
Mg	0.185	0.459**	−0.159	0.726**	0.636**	1

Table 13.

Correlation coefficient (r) values for shoot and root of spinach grown in soil A.

^*

Correlation is significant at the 0.05 level (2-tailed).

^**

Correlation is significant at the 0.01 level (2-tailed).

Values above 1 indicate for shoot.

Values below 1 indicate for root.

	N	P	Na	K	Ca	Mg
N	1	0.042	−0.420**	0.211	0.365*	0.052
P	0.061	1	−0.426**	0.375*	0.294	0.527**
Na	−0.542**	−0.414**	1	−0.674**	−0.535**	−0.584**
K	0.560**	0.347*	−0.407**	1	0.387*	0.720**
Ca	0.165	0.639**	−0.668**	0.287	1	0.392*
Mg	0.399**	0.142	−0.411**	0.006	0.209	1

Table 14.

Correlation coefficient (r) values for shoot and root of spinach grown in soil B.

^*

Correlation is significant at the 0.05 level (2-tailed).

^**

Correlation is significant at the 0.01 level (2-tailed).

Values above 1 indicate for shoot.

Values below 1 indicate for root.

3.5 Discussion

The soil samples collected for the present experiment were saline in nature and acidic in reaction. Soils having EC greater than 4 mS cm⁻¹ at saturation extract are regarded as saline soils [1, 39]. Saline soils with acidic character are rare in nature which could be ascribed to the dominance of S [40]. The acidic character of the soils collected from the area was also reported by LRUG [41].

The growth of spinach in regards to the fresh weight was adversely inhibited by extreme soil salinity which is in agreement with Panuccio et al. [42]. Furthermore, the fresh weight for all the respective treatments was found lower at relatively higher levels (soil B) of soil salinity in the present study. The reduction in the growth of plants in saline soils might be due to the result of the deficiencies of essential nutrients as well as toxicities of Na and Cl [43, 44]. However, in both levels of soil salinity (soil A and soil B), the application of amendments especially VC alone and VC in combination with WA had been found to increase the growth and uptake of nutrients in spinach. Several authors observed maximum growth and yield of plants when VC was applied in soils [45, 46]. The positive response of plants to salt stress had also been observed as a result of the application of amendments [21, 47]. However, instead of a high concentration of K, Ca, and Mg in WA, spinach growing in WA treated soils did not show considerable growth in the present study which may be due to a lack of N in the ash. In an experiment, Augusto et al. [48] stated that the absence of N in the WA did not increase plant growth growing on mineral soils of Nordic countries.

As a consequence of increased soil salinity, the uptake of N was inhibited which is in support of the findings of other authors [49, 50], and might be due to the antagonistic relation of Na with NH₄⁺ [51]. The high concentration of N in different parts of spinach amended with VC either alone or in association with other amendments might be due to the high inherent N concentration and its release through decomposition of VC. A similar result was found by Erenoglu et al. [52], where the concentration of N in the shoot of wheat increased as the N content in soil was enhanced.

The concentration and uptake of P also decreased with increasing soil salinity. The reduced uptake of P in spinach with increasing salinity of soils might be due to high Cl⁻ concentration which is a common problem in saline soils. Several authors reported that high soil salinity is attributed to the reduction in P uptake by plants due to the antagonistic relationship between H₂PO₄⁻ and Cl⁻ [9, 51, 53, 54].

A high concentration of total K in spinach for most of the treatments with increasing soil salinity might be due to high K content in the growth medium. The result was consistent with the findings of other authors [55, 56], where the concentration of K in plants increased with the increase of salinity. Al-Karaki [57] also reported that the concentration of K in the growth medium determines the net uptake of K and observed higher translocation of K from root to shoot in plants to a greater extent in the saline environment in comparison to non-saline conditions with the increase of K concentration in the medium. However, low uptake of total K by spinach for most of the respective treatments at relatively higher salt stress could be ascribed to a lower amount of total dry matter.

Both the concentration and uptake of both Ca and Mg in different parts of spinach decreased with the increase of soil salinity. A similar trend was found for saltmarsh grass, where the concentration of Ca and Mg decreased significantly with increasing salinity [53]. Sahin et al. [9] also observed significant negative correlations of Na with that of the concentrations of Ca and Mg. Consistent with the present findings, similar results were reported by other authors where the salt stress resulted in an increased accumulation of Na and decreased amounts of Ca and Mg in plants [51, 58]. The increase in the concentrations of Ca and Mg in treated soils compared to control might be due to the greater affinity for these cations relative to Na under different amendments.

The concentration of Na in shoot and root increased as the salinity of soil increased which is in agreement with other findings [9, 53, 59]. However, the decline in Na concentration in both shoot and root of spinach growing in soils especially amended with VC and VC plus WA can be explained by a dilution effect, that is, an increase in dry matter accumulation. A similar finding was reported by Kaya et al. [60]. Moreover, the significant increase in the uptake of Na in VC plus WA amended soils over control could be due to higher biomass content under amended conditions. In an experiment, Ferreira et al. [61] also found that the accumulation of Na in spinach increased by 1.3–3.0 times and increased tissue Na was neither a hindrance nor a benefit for the growth of spinach.

The C_shoot/C_root quotient of N, K, Ca, and Mg greater than 1 indicated their mobility from root to shoot, while P less than 1 indicated its immobility_. In the present study, the ratios of Na: K, Na: Ca, and Na: Mg were found higher in the control. Hadi et al. [62] also stated that the uptake and transport of K, Ca, and Mg can be adversely affected by the high concentration of Na in saline soils, resulting in higher Na: K, Na: Ca, and Na: Mg ratios in plants. Consistent with the present findings, in another experiment, the ratios of Ca: Na and Mg: Na in cabbage were found lower with the increase of salinity levels [9]. Furthermore, the addition of amendments decreased Na: K, Na: Ca, and Na: Mg ratios which could be the consequence of better uptake of K, Ca, and Mg under different amendments.

4. Conclusion

The production of the crop is restricted due to soil salinity throughout the world possessing one of the greatest threats to food security. Soil salinity is also one of the most severe environmental constraints limiting the production of crops in the coastal areas of Bangladesh. Soil salinity coupled with acidity is a major challenge in coastal areas all over the world. The present experiment provided approaches toward sustainable use of saline soils with acidic reaction through the integrative approaches of leaching and application of different amendments. The growth and uptake of nutrients by spinach decreased with increasing the levels of soil salinity irrespective of the treatments. The response of spinach varied depending on several factors such as salt content in the soils, types, and rates of amendments. It can be summarized from the present experiment that the application of VC alone or in association with WA or ZL can be well suited for better performance of spinach, whereas the application of WA and ZL either alone or in combination (i.e. WA plus ZL) had the least effects in enhancing the yield of spinach in a saline environment. The application of VC and WA in combination resulted in maximum yield and uptake of nutrients at different levels of soil salinity while the ratios of Na: K, Na: Ca, and Na: Mg of shoot and root were found to be highest in un-amended soils. Moreover, the application of VC in association with WA with acid-neutralizing capacity presents an interesting source to enhance the performance of spinach by improving the physico-chemical properties of acidic saline soils. It can be suggested from the present work that the application of VC plus WA at 1% of each can be practiced before cultivation to influence soil nutrient dynamics, thereby augmenting growth and uptake of nutrients by plants in acidic saline soils.

Acknowledgments

This work is a part of the research project funded by Research and Publication Cell, University of Chittagong (Grant No. 6760/Res/Con/Pub/Cell/C.U./2019). The authors are also grateful to the Department of Soil Science, University of Chittagong for providing laboratory facilities to conduct this work.

Conflict of interest

The authors declare no conflict of interest.