Soil Properties of Reclaimed Coastal Saline–Alkali Farmland in a Chinese Province: Spatial Variability and Soil Profiles

Abstract

Coastal saline–alkali farmland typically experiences poor crop growth and low yields. Clarifying soil quality and identifying the primary constraining factors are crucial for improving productivity. This study systematically investigated the spatial heterogeneity and vertical distribution of soil physicochemical properties in a coastal reclamation area using large-scale field sampling. The results revealed that the plow layer soil in the coastal reclamation zone is characterized by typical saline–alkali conditions, low fertility, and weak nutrient-holding capacity, with a pH range of 8.0 to 9.2. Over 60% of the region had soluble salt (SS) content exceeding 2.0 g/kg, and soil organic matter (SOM), total nitrogen (TN), and cation exchange capacity (CEC) ranged from 7.2 to 24.9 g/kg, 0.45 to 1.42 g/kg, and 1.4 to 15.7 cmol⁺/kg, respectively. Correlation analysis showed significant positive correlations between SOM and TN, available potassium (AK), and CEC, while a strong negative correlation was found between pH and AP. Vertically, the soil demonstrated a notable risk of salt efflorescence and nutrient leaching. Soil salinity and alkalinity increased with depth, while SOM, TN, available phosphorus (AP), and nitrate content decreased. In conclusion, effectively suppressing soil salinization, lowering pH, and increasing organic matter content are essential strategies for improving soil structure, enhancing nutrient retention, and boosting the quality of coastal saline–alkali farmland.

1. Introduction

Coastal saline–alkali land, a distinctive land resource within the land–sea ecotone, represents a vital component and a potential reserve of global arable land. China possesses approximately 3.6 × 10⁷ hm² of saline–alkali land, accounting for 4.88% of its total utilizable land area [1], ranking it the third largest country in the world by such land distribution [2,3]. With rapid socioeconomic development in China, the reclamation and efficient utilization of coastal saline–alkali wasteland have become crucial pathways for ensuring national food security and alleviating the pressure of limited land resources. Soil salinization and alkalization are primary constraints on agricultural productivity [4]. Excessive salt induces osmotic stress and ion toxicity in plants (e.g., sodium ions Na⁺ and chloride ions Cl⁻) [5,6], directly inhibiting seed germination, plant growth, and metabolic processes [7]. Additionally, alkaline soil conditions disrupt ion balance and water uptake, suppress photosynthesis and the synthesis of photosynthetic pigments [8], and exacerbate oxidative and osmotic stress. These factors significantly constrain crop growth and reduce yields [9]. As reported by Haj-Amor et al. [10], highly saline soils severely affect rice growth and yield, with yield reductions for some crops reaching up to 58%. Moreover, soil salinization and alkalization impair microbial functions and nutrient cycling, thus diminishing the efficiency of ecosystem services [11,12]. Therefore, effective strategies for reducing soil salinity and alkalinity are essential for improving crop yields in these regions. Techniques such as drip irrigation, mulching, and the application of organic amendments (including biochar and bio-organic fertilizers) have demonstrated significant benefits in enhancing soil properties and crop productivity. For instance, compared to traditional flood irrigation, drip irrigation can reduce root zone salt content by 37.7% and increase crop yield by 37.4% [13]. Similarly, the combined use of mulch cultivation and organic amendments can decrease soil salinity by up to 20.9% and boost crop yield by over 30% [14]. For alkali soils, gypsum application can improve soil structure and enhance infiltration, thereby creating hydraulic conditions conducive to salt leaching [15,16]. Meanwhile, a well-designed and properly maintained subsurface drainage system provides a continuous pathway for salt export, ensuring the long-term effectiveness of reclamation and salinity control [17]. These findings highlight the importance of salinity and alkalinity management in sustainable agriculture, where effective remediation not only boosts crop yields but also maintains long-term soil health and ecosystem stability. However, coastal reclaimed areas are characterized by a short reclamation history, high water table, and proximity to the sea [18,19]. Under intense evaporation and plant transpiration, salts dissolved in groundwater can readily migrate upward through capillary rise to the soil surface [20], leading to soil resalinization. This process often generates salt patches and localized high-salinity hotspots, making single, stand-alone measures insufficient for stable, long-term remediation. Therefore, it is essential to synergistically integrate the key steps of evaporation suppression, salinity control, soil amelioration, and salt removal into a systematic management framework.

The formation of coastal saline–alkali land primarily results from seawater immersion, tidal incursions, and salt-laden parent material. As a result, soils in newly reclaimed areas typically exhibit a combination salinity and alkalinity, along with significant fertility depletion. In addition to the saline–alkaline stress, these soils are characterized by inherently low background levels of key fertility factors such as soil organic matter (SOM), total nitrogen (TN), and available potassium (AK) [21,22]. Micronutrient deficiencies, including a lack of available iron and manganese, are also commonly observed [23], further limiting normal crop growth and development. Furthermore, the coastal region is primarily composed of sandy or sandy loam textures [24]. While these soils benefit from favorable permeability and aeration due to their loose structure and macro-porosity, they exhibit inherently low cation exchange capacity (CEC), resulting in poor nutrient and water retention. In addition, the region receives substantial annual precipitation (562–1100 mm) [25], which promotes high nutrient leaching, particularly of mobile nitrogen compounds and certain forms of phosphorus. Under irrigation or rainfall conditions, these nutrients migrate to deeper soil layers or groundwater [26,27]. This nutrient leaching represents a significant waste of agricultural resources and poses risks of non-point source pollution, while also making nutrients largely inaccessible to crop roots. Additionally, in coastal sandy soils, soil biota can influence both organic matter inputs and nutrient redistribution. However, the elevated salinity typical of coastal saline–alkali soils suppresses the abundance and activity of soil fauna and microbial communities, thereby constraining organic matter incorporation into the soil matrix and nutrient cycling processes [28,29].

Under these conditions, the sustainable utilization of coastal saline–alkali farmland faces a dual challenge of salt accumulation and nutrient leaching. Salts accumulate in surface soils due to evaporation, while nutrients are leached to deeper layers through water movement. This opposing dynamic results in a spatially compounded negative effect of soil saline–alkaline stress coupled with fertility depletion. In typical coastal reclamation regions, such as Yancheng in Jiangsu Province, natural conditions including high water tables, sandy soil textures, and abundant rainfall create bottlenecks that hinder the effectiveness of saline–alkali land improvement. These conditions facilitate vertical nutrient leaching and the surface accumulation of salts. However, there is still a lack of systematic understanding regarding the spatial correlations between salinization–alkalization and nutrient indicators, as well as their distribution patterns in vertical soil profiles at varying depths. Therefore, to mitigate the compounded adverse effects of salt upward return and nutrient leaching losses, it is necessary to address both irrigation and drainage regulation and nutrient management. Sustainable farmland management practices, such as optimized irrigation and drainage, mulching to suppress evaporation, improved fertilization strategies, and organic matter amendment, should be implemented to jointly regulate water and salt dynamics, as well as nutrient cycling.

To systematically analyze the aforementioned issues, this study conducted an integrated sampling strategy in the coastal reclamation zone of Yancheng, Jiangsu Province. This approach combined extensive horizontal spatial sampling with vertical stratified sampling of typical soil profiles. Soil samples were collected from 110 horizontal sampling points and four typical profiles at depths ranging from 0 to 140 cm. The study systematically analyzed soil salinity–alkalinity and nutrient indicators. The objectives were to clarify the spatial distribution patterns and vertical variation characteristics of physicochemical properties in saline–alkali farmland soils, uncover the intrinsic relationships between salinity, pH and nutrient availability, and explore their governing mechanisms on nutrient leaching behavior. The findings aim to provide scientific basis for the targeted amelioration of coastal saline–alkali land, optimization of fertilization strategies, and the sustainable enhancement of soil productivity.

2. Materials and Methods

2.1. Study Area

The study area is located in Sheyang County, Yancheng City, Jiangsu Province (33.93° N–33.99° N, 120.38° E–120.42° E), with ground elevations ranging from 0.8 to 2.2 m above sea level, exhibiting a general topographic trend of higher elevation in the west and lower elevation in the east. The region experiences a warm temperate monsoon climate, with an average annual temperature of approximately 14 °C and annual precipitation around 1000 mm. The water table typically ranges from 2.0 to 2.5 m. Based on the analysis of 55 soil samples randomly collected across the study area (at a density of approximately one sampling point per 13.3 hectares), the soil texture was identified as sandy loam. Prior to sampling, the reclaimed area had been under continuous cultivation for approximately 30 years, primarily following rice–wheat or rice–rapeseed rotation systems. The fertilization regime consisted of compound fertilizer (15-15-15) applied at 450 kg/ha and urea at 300 kg/ha per crop season. Irrigation in the study area is conducted using surface irrigation, with water supplied from branch canals of a north–south-oriented river system. The irrigation coverage is 100%, and the annual variation in irrigation-water mineralization ranges from 1.0 to 3.0 g/L.

2.2. Soil Sample Collection

To investigate the spatial patterns of soil salinization–alkalinization and fertility status, an intensive large-scale soil sampling campaign was conducted across the approximately 733 ha reclamation land. Due to the uneven spatial distribution of soil properties in coastal saline–alkali areas, particularly the high variability of saline–alkali indicators, sampling density was increased in areas with low productivity and obvious saline–alkali constraints based on preliminary survey results, in order to improve the accuracy of spatial distribution analysis of soil properties. Specifically, one sampling point was established per approximately 13.33 ha in areas without obvious constraints and with relatively balanced crop productivity, and one sampling point per approximately 3.33 ha in areas with low productivity and obvious saline–alkali constraints. A total of 110 sampling points were established (Figure 1c). The sampling campaign took place in June 2024 during the wheat–rice rotation interval. Each sampling point featured a 5 m × 5 m sampling unit, with topsoil collected from 0 to 20 cm using the five-point sampling method. Soil samples from five sub-sampling points (spaced 2–3 m apart) were thoroughly mixed in equal proportions to form a composite representative sample for subsequent analysis of soil pH, SOM, TN, available phosphorus (AP), AK, soluble salt (SS), and CEC.

To further investigate the vertical variation patterns of soil properties, four representative sites were selected for soil profile excavation (Figure 1c). Given the uniform soil texture and stable topography and groundwater table in the study area, the distance to surrounding water bodies became the primary factor controlling salinity. Therefore, representative sampling sites were comprehensively determined using distance to water as the primary stratification criterion, in combination with typical crop growth conditions in the field. At each profile, soil samples were collected at depths of 0, 20, 40, 60, 80, 100, 120, and 140 cm, with three replicates obtained at each depth interval. The profile soil samples were analyzed for bulk density (BD), pH, electrical conductivity (EC), SS, SOM, TN, AP, AK, ammonium nitrogen (NH₄⁺-N), and nitrate nitrogen (NO₃⁻-N).

2.3. Soil Sample Analysis

The collected soil samples were sieved through a 2 mm mesh after removing plant roots and stones, then air-dried for physicochemical analysis. The determination of various indicators followed the Soil Agrochemical Analysis Methods [30]. Soil pH was measured using a pH meter (Mettler S220, Mettler Toledo AG, Columbus, OH, USA) with a soil-to-water ratio of 1:2.5. Soil EC was measured with a conductivity meter (Mettler S700, Mettler Toledo AG, Columbus, OH, USA) at a soil-to-water ratio of 1:5. Soil SS was quantified using the evaporation method after extraction at a 1:5 soil-to-water ratio and filtration. Soil TN was determined through the Kjeldahl method. SOM was measured using the potassium dichromate oxidation method. Soil AP was measured by the sodium bicarbonate extraction–molybdenum–antimony anti-colorimetric method, and AK was determined through the flame photometer method. Soil NH₄⁺-N and NO₃⁻-N concentrations were determined using a continuous flow analyzer (Skalar SAN⁺⁺, Skalar Analytical B.V., Breda, The Netherlands) after extraction with 2 mol/L KCL solution at a 1:5 soil-to-solution ratio. Soil BD was measured by the core method. Soil texture was determined using the hydrometer method. With reference to the United States Department of Agriculture (USDA) particle-size classification standard, the soil textural class was identified based on the percentages of sand, silt, and clay.

2.4. Data Processing

The soil contents of SS, SOM, TN, AP, AK, NH₄⁺-N, NO₃⁻-N per hectare (ha) were calculated using Equation (1):

$$C\left(X\right)=C\left(x\right)\times B\left(x\right)\times 0.2\times 0.01$$

(1)

where C(X) is the content of SS, SOM, TN, AP, AK, NH₄⁺-N, NO₃⁻-N, per ha (t/ha or kg/ha); C(x) is the content of SS, SOM, TN, AP, AK, NH₄⁺-N, NO₃⁻-N (g/kg or mg/kg); B(x) represents the bulk density for each soil layer (kg/m³). The thickness of the corresponding soil layer (m) is 0.2. 0.01 is the conversion factor for g/kg or mg/kg to t/ha or kg/ha.

Soil nutrient indicators and salinity levels were evaluated and classified according to the criteria outlined in the Chinese Soil Survey Technology [31] and The Third National Soil Survey Tentative Specifications [32], respectively (Appendix A (

Table A1. Classification methods for soil nutrient indicators.

Grading	Classify	SOM (g/kg)	TN (g/kg)	AP (mg/kg)	AK (mg/kg)	CEC (cmol+/kg)
Level 1	Sufficient	>40	>2.00	>40	>200	>20.0
Level 2	Moderately Sufficient	30–40	1.50–2.00	20–40	150–200	15.4–20.0
Level 3	Moderate	20–30	1.00–1.50	10–20	100–150	10.5–15.4
Level 4	Marginally Sufficient	10–20	0.75–1.00	5–10	50–100	6.2–10.5
Level 5	Insufficient	6–10	0.50–0.75	3–5	30–50	<6.2
Level 6	Highly Deficient	<6	<0.50	<3	<30	-

and

Table A2. Classification methods for soil salinity and alkalinity.

Salt content (‰)	1.0–2.0	2.0–4.0	4.0–10		-	-
Classify	Non-Saline	Low-Saline	Moderate-Saline	High-Saline	-	-
pH	<4.5	4.5–5.5	5.5–6.5	6.5–7.5	7.5–8.5	8.5–9.5
Classify	Strongly Acidic	Acidic	Slightly Acidic	Neutral	Slightly alkaline	Slightly alkaline

)). Data organization and descriptive statistical analysis were performed using SPSS 22.0 and Excel 2024. Correlation analysis and regression fitting among various indicators were conducted using Origin 2021, while one-way analysis of variance (ANOVA) was performed with SPSS 22.0. Additionally, the vertical variation patterns of soil properties along the profiles were visualized employing both SPSS 22.0 and Origin 2021. The spatial distribution patterns of soil properties were analyzed through Inverse Distance Weighting (IDW) interpolation in ArcMap 10.8 software. The IDW method is a simple deterministic interpolation approach in which unknown values are estimated as a weighted average of nearby observations, with weights determined by a power function of the Euclidean distance between sampled and unsampled locations. In this study, IDW was selected for spatial interpolation based on the following considerations: (1) previous studies showing no significant accuracy difference between IDW and Ordinary Kriging in comparable coastal environments [33,34]; (2) the suitability of IDW for our non-uniform adaptive densification sampling strategy and its computational efficiency [35]; and (3) the advantage of IDW in preserving local extremes, which is essential for characterizing spatial heterogeneity in saline–alkali constraints in coastal tidal flat areas [36,37]. The IDW estimation of values is given by [38]:

$$\theta {(x}_0)={\displaystyle \frac{\sum _{i=1}^n\theta \left(x_i\right)d_i^{-r}}{\sum _{i=1}^n{d}_i^{-r}}}$$

(2)

where θ(x₀) is the estimated value at the unsampled location x₀; θ(x_i) is the observed value at the i-th sampled location x (i = 1, 2, …, n); d_i is the Euclidean distance between the unsampled location x₀ and the sampled location x_i; r is the power parameter controlling the influence of distance on the weighting; and n is the number of neighboring sample points used for interpolation. In this study, the power parameter $r$ was set to 2, and 12 neighboring points were used for the IDW interpolation.

3. Results

3.1. Spatial Distribution Patterns of Soil Physicochemical Properties in the Coastal Reclamation Zone

3.1.1. Spatial Distribution Patterns of Soil Salinity and Alkalinity Indicators

The coastal reclamation zone exhibited significant spatial heterogeneity in soil pH and soluble salt content (Figure 2). Overall, soil pH was lower in the west and higher in the east (Figure 2a). The pH ranged from 8.0 to 9.2, indicating alkaline soil conditions throughout the area. Specifically, slightly alkaline soils (pH 7.5–8.5) accounted for 39.1% of the samples, while alkaline soils (pH 8.5–9.5) constituted 60.9% of the total. The spatial distribution of soil soluble salts showed limited regularity, with salt patches distinctly present in localized areas (Figure 2b). The salt content ranged from 0.7 to 14.9 g/kg. According to the salinization classification criteria, non-saline soils (<1.0 g/kg) accounted for merely 5.5% of samples, while lowly (1.0–2.0 g/kg), moderately (2.0–4.0 g/kg), and highly (4.0–10 g/kg) salinized soils represented 33.6%, 49.1%, and 11.8% of the total, respectively.

3.1.2. Spatial Distribution Patterns of Soil Nutrient Indicators

The soil nutrient indicators exhibited complex spatial distribution patterns (Figure 3). The SOM content was lower in the northern and southern sectors and higher in the central region (Figure 3a), with values ranging from 7.2 to 24.9 g/kg. Specifically, 33.6% of samples were classified as Marginally Sufficient (10–20 g/kg), while 14.5% were classified as Insufficient (6–10 g/kg). The remaining 51.8% of samples had Moderate SOM levels (20–30 g/kg). The spatial distribution of TN followed a pattern similar to SOM, with lower values in the northern and southern sectors and higher concentrations in the central region (Figure 3b), ranging from 0.45 to 1.42 g/kg. Specifically, 54.5% of samples demonstrated Moderate TN levels, while 30.0% and 13.6% of samples were classified as Marginally Sufficient (0.75–1.0 g/kg) and Insufficient (0.5–0.75 g/kg), respectively. The spatial distribution of CEC showed no clear spatial pattern, with generally low levels observed across the coastal reclamation zone (Figure 3c). The CEC ranged from 1.4 to 15.7 cmol⁺/kg, with only 22.7% of samples showing Moderate CEC levels (10.5–15.4 cmol⁺/kg), while 42.7% and 31.8% of samples were classified as Marginally Sufficient (6.2–10.5 cmol⁺/kg) and Insufficient (<6.2 cmol⁺/kg), respectively.

The AP content was generally high across the coastal reclamation zone, exhibiting a spatial trend of lower concentrations in the northern sector and higher concentrations in the southern region (Figure 3d), with values ranging from 4.8 to 85.5 mg/kg. Only 2.7% of samples were classified as Marginally Sufficient (5–10 mg/kg), while the remaining samples reached Moderate or Sufficient levels. The spatial distribution of AK demonstrated a pattern of lower values in the northern and southern sectors and higher concentrations in the central area (Figure 3e). The overall AK content was abundant, ranging from 90 to 263 mg/kg, with only 3.6% of samples classified as Marginally Sufficient (50–100 mg/kg).

3.2. Interrelationships of Soil Physicochemical Properties

Correlation analysis revealed significant correlations among multiple soil physicochemical properties (Figure 4). SOM showed extremely strong positive correlations with TN, AK, and CEC (p < 0.001). TN also exhibited highly positive correlations with both AK (p < 0.01) and CEC (p < 0.001). CEC demonstrated extremely strong positive correlations with AK and pH (p < 0.001), while showing significant negative correlations with AP (p < 0.001) and SS (p < 0.05). Additionally, pH was strongly negatively correlated with AP (p < 0.001).

Further linear regression analysis revealed clear linear relationships between key indicators (Figure 5). A highly significant positive linear correlation was observed between SOM and TN (r = 0.756, p < 0.001). Similarly, pH and CEC also showed a highly significant positive linear correlation (r = 0.789, p < 0.001). Meanwhile, CEC and AP exhibited a significant negative correlation (r = −0.522, p < 0.001).

3.3. Vertical Distribution Characteristics of Soil Physicochemical Properties

3.3.1. Vertical Variation Patterns of Soil Physicochemical Indicators

The physicochemical properties of the four soil profiles exhibited broadly similar vertical variation trends (Figure 6 and Figure 7). Soil BD generally increased with increasing depth. Two of the four profiles showed relatively low BD (1.15–1.17 g/cm³) in the 0–20 cm surface layer, whereas the remaining two profiles exhibited comparatively higher BD (1.37–1.49 g/cm³). Across all profiles, BD reached its maximum at a depth of 40 cm (1.79–1.81 g/cm³), followed by a slight decrease at 60 cm depth and subsequent stabilization at approximately 1.65 g/cm³ in deeper layers (Figure 6a). Soil pH also exhibited an increasing trend with depth. The surface layer exhibited the lowest pH (7.36–7.87). In all four profiles, soil pH reached its maximum at the 40 cm depth (8.21–8.60), then showed a slight decrease at 60 cm depth and subsequently stabilized at approximately 8.2 in deeper layers (Figure 6b). Both soil EC and SS content increased progressively with soil depth across all profiles (Figure 6c,d). In the three profiles characterized by relatively low surface salinity, EC increased from 217–300 μS/cm in the surface layer to 676–971 μS/cm at a depth of 140 cm, while SS increased from 0.11–0.46 g/kg to 0.71–1.78 g/kg. In contrast, the remaining profile, which exhibited higher initial surface salinity, showed an increase in EC from 692 μs/cm to 982 μs/cm and in SS from 0.89 g/kg to 1.67 g/kg over the same depth interval. At the unit area scale, soil salinity at depths of 120–140 cm and 140–160 cm (4.14–4.44 t/ha) was significantly higher than that in the 0–20 cm topsoil layer (1.20 t/ha) (p < 0.05) (Figure 8a).

3.3.2. Vertical Distribution Characteristics of Soil Nutrient Indicators

Soil nutrient contents exhibited clear vertical stratification patterns along the soil profiles (Figure 7). Across all four profiles, both SOM and TN contents decreased progressively with increasing depth, reaching their minimum values at a depth of 40 cm. Among the profiles, three exhibited relatively high SOM and TN contents in the surface-layer, with mean values of 18.29 g/kg and 1.13 g/kg, respectively. These values declined to 7.23 g/kg for SOM and 0.64 g/kg for TN at the 40 cm depth. In contrast, the remaining profile showed lower initial surface values, with SOM and TN contents of 13.57 g/kg and 0.79 g/kg, respectively, which decreased to 7.49 g/kg and 0.68 g/kg at the 40 cm depth (Figure 7a,b). When expressed at the unit area scale, SOM reserves in the 40–60 cm and deeper soil layers (18.80–24.05 t/ha) were significantly lower than those in the 0–20 cm and 20–40 cm layers (43.68–44.72 t/ha) (p < 0.05) (Figure 8b). Similarly, TN reserves in the 120–140 cm and 140–160 cm depth layers (2.11–2.14 t/ha) were significantly lower than those in the 0–20 cm and 20–40 cm layers (2.73–2.97 t/ha) (Figure 8c).

Soil AP content across all depths of the soil profiles was generally low, with all four profiles exhibiting extremely low levels (<1 mg/kg) below 20 cm depth (Figure 7c). When expressed at the unit area scale, AP reserves in the 20–40 cm and deeper soil layers (0.80–1.26 kg/ha) were significantly lower than those in the 0–20 cm surface layer (4.74 kg/ha) (Figure 8d). For the AK content, three profiles showed an increase from a mean value of 165 mg/kg at the surface layer to 312 mg/kg at the 140 cm depth. In contrast, the remaining profile showed a decreasing trend with depth, attributable to an exceptionally high AK content in the topsoil (590 mg/kg) (Figure 7d). Overall, however, no significant differences were observed in AK reserves at the unit area scale among soil layers (p < 0.05) (Figure 8e).

The vertical distribution of soil NH₄⁺-N generally exhibited an increasing trend with depth. Soil NH₄⁺-N content in the surface ranged from 1.9 to 2.1 mg/kg and gradually increased to 2.7–4.7 mg/kg with increasing depth. Peak NH₄⁺-N concentrations across all profiles were primarily observed at depths of 100–120 cm (Figure 7e). In contrast, NO₃⁻-N exhibited distinct surface enrichment, with a mean content of 18.70 mg/kg in the 0–20 cm layer. However, it decreased sharply to 7.40 mg/kg at 20 cm depth and remained relatively stable below this depth (<5 mg/kg) (Figure 7f). At the unit area scale, the NH₄⁺-N content reserves in the 0–20 cm soil layer (4.99 kg/ha) were significantly lower than those in the 20–40 cm and deeper layers (7.24–10.82 kg/ha). Conversely, NO₃⁻-N reserves in the 0–20 cm layer (47.03 kg/ha) were significantly higher than those in the 20–40 cm and deeper layers (7.14–20.83 kg/ha) (Figure 8f,g).

4. Discussion

4.1. Spatial Distribution Characteristics of Soil Physicochemical Properties

The results of this study indicate that the saline–alkali farmland soils in the coastal reclamation area of Jiangsu Province are characterized by elevated pH and high salt content. These coastal saline–alkali soils primarily originate from seawater-saturated saline silt, and their pedogenesis is accompanied by long-term salinization, resulting in generally elevated soil salinity across the region [39]. Consistent with our findings, previous studies have also demonstrated that this coastal area generally maintains a high water table, with the annual groundwater depth ranging between 2.0 and 3.0 m and exhibiting high mineralization (1.0–3.0 g/L) [40]. Such shallow, highly mineralized groundwater can readily migrate upward through capillary rise, thereby promoting salt accumulation in the soils [41,42]. Farmland soils in coastal reclaimed areas also commonly exhibit high pH, which is largely associated with excessive Na⁺. Previous studies have shown that exchangeable sodium percentage (ESP) in coastal saline–alkali soils is generally high, often exceeding 15%, and in some areas reaching values as high as 90% [43,44]. Elevated ESP indicates the dominance of Na⁺ over calcium ions (Ca²⁺) and magnesium ions (Mg²⁺) in the soil solution, which promotes soil alkalization and may further induce soil crusting and compaction [45]. Therefore, during the desalinization of reclaimed soils, particular attention should be paid to the potential risk of excessive Na⁺ incorporation into the soil exchange complex, as this may exacerbate alkalization and structural degradation.

In contrast to salt accumulation, soil SOM, TN, and CEC contents in the coastal reclamation zone were generally low. This pattern can be primarily attributed to the dual inhibitory effects of saline–alkali stress on vegetation growth and soil microbial activity. High salinity limits plant growth, thereby reducing organic matter inputs derived from plant residues, while simultaneously suppressing microbially mediated processes of organic matter decomposition and nutrient cycling. Together, these effects result in slow accumulation of SOM and TN [46,47,48]. In addition, excessively high Na⁺ concentrations promote soil dispersion, which disrupts soil structure and further enhances the leaching loss of organic matter and associated nutrients [49,50]. The low CEC observed in the study area is closely related to the low clay content and sandy texture of the soils [24,51].

Soils in the coastal reclamation zone exhibited relatively abundant levels of AP and AK. In general, the high soil pH typical of coastal saline–alkali lands promotes the formation of insoluble compounds between Ca²⁺ and phosphate (PO₄³⁻), thereby reducing phosphorus availability [26]. In the present study area, the relatively high AP content in the surface soil is mainly attributed to the long-term application of phosphorus fertilizers to enhance crop yields. Potassium deficiency is generally uncommon in coastal regions due to the inherently high potassium content of the parent material [52]. Soil profile analysis further showed that AK contents remained above 150 mg/kg even below a depth of 40 cm, which is unaffected by tillage, providing additional evidence that the parent material serves as an abundant source of potassium in this region.

4.2. Analysis of Interrelationships Among Soil Physicochemical Properties

Correlation analysis further revealed the intrinsic relationships among soil physicochemical properties in the coastal reclamation zone. Consistent with previous studies [53,54,55], SOM showed strong positive correlations with TN, AK, and CEC, highlighting its pivotal role in ameliorating saline–alkali soils and enhancing soil fertility. More than 90% of soil nitrogen exists in organic forms, such as proteins, amino acids, and humus, resulting in synchronous accumulation of SOM and TN [56,57]. Therefore, the soil TN content is positively correlated with the SOM content. Soil CEC primarily originates from the negatively charged colloidal surfaces, including inorganic colloids (e.g., clay minerals) and organic colloids (e.g., humus) [58]. Owing to their large specific surface area and abundance of negatively charged functional groups, organic colloids generally contribute much more to CEC than inorganic colloids [59]. Consequently, increasing SOM directly enhances soil CEC, thereby effectively improving nutrient retention capacity. Because SOM itself contains a certain amount of potassium, microbial decomposition and mineralization of SOM release potassium into the soil in the form of potassium ions (K⁺), directly increasing the content of AK [60]. In addition, the enhanced nutrient retention capacity associated with higher SOM levels favors the preservation of available potassium in the soil [61].

A notable negative correlation between soil AP and pH was observed in the study area, consistent with findings from saline soils of the Songnen Plain reported by Jiang et al. [62]. Under high-pH conditions, phosphorus readily forms insoluble phosphates with Ca²⁺ and Mg²⁺, leading to its immobilization in soil and a consequent reduction in availability [63,64].

Soil CEC exhibited a strong positive correlation with pH but a negative correlation with salinity, in agreement with previous studies by Van Erp et al. [65] and Solly et al. [59]. Under alkaline conditions, increased hydroxide ion (OH⁻) concentrations promote deprotonation of functional groups on colloidal surfaces, thereby increasing negative charge density and enhancing cation adsorption capacity [66]. In contrast, elevated salinity, particularly increased Na⁺ concentrations, competes with other cations for exchange sites on colloids, resulting in reduced CEC [67].

4.3. Vertical Distribution Characteristics of Soil Physicochemical Properties and Environmental Effects

The physicochemical properties of coastal saline–alkali soils exhibited significant stratified heterogeneity along the vertical profile. Soil pH, EC, and SS content increased progressively with depth, indicating a notable risk of surface salt accumulation. Driven by intense evaporation, salts from deeper layers are continuously transported upward through capillary action and accumulate in the surface soils [68,69]. In coastal areas, soluble salts are primarily composed of sodium chloride (NaCl), and Na⁺ accumulation increases ESP, exacerbating soil alkalization [70,71]. Although the rice–wheat rotation system and annual rice irrigation help leach salts from surface soils, salts still accumulate in deeper layers.

In contrast, nutrient indicators exhibited vertical distribution patterns distinct from those of soil salinity and alkalinity. Due to inputs from crop residues and fertilizer application, SOM, TN, AP, and NO₃⁻-N contents in the 0–20 cm plough layer were higher than those in deeper soil layers [72,73,74]. However, coastal soils are predominantly sandy loams with good aeration and permeability but weak nutrient-retention capacity, making nutrients susceptible to leaching or surface runoff. Under conditions of high rainfall and shallow groundwater tables in coastal areas [75], long-term excessive fertilizer use can readily lead to agricultural non-point source pollution and groundwater contamination [76,77].

4.4. Existing Problems and Integrated Amelioration Strategies for Coastal Saline–Alkali Soils

Based on the results of this study, saline–alkali farmland in the coastal reclaimed area of the study region is constrained by two opposing processes, namely readily upward salt return and pronounced nutrient leaching. On the one hand, shallow, highly mineralized groundwater supplies salts upward through capillary rise under evaporation-driven fluxes, promoting salt enrichment in the surface layer and increasing the risk of resalinization. On the other hand, sandy or sand-dominated soils in the study area have low CEC and weak nutrient-retention capacity. Under abundant rainfall and irrigation-induced disturbance, mobile nutrients, particularly nitrogen in the form of NO₃⁻–N, are prone to vertical leaching losses, making it difficult to maintain topsoil fertility. This “bidirectional constraint” leads to a spatially compounded penalty in which salinity and alkalinity stress co-occurs with fertility depletion. Consequently, reliance on a single salinity-control measure rarely sustains stable, long-term improvement. Effective amelioration of coastal saline–alkali soils therefore requires integrated management, including salinity regulation, soil-structure improvement, and sustained fertility enhancement.

The susceptibility of coastal saline–alkali soils to resalinization is jointly controlled by water table depth and groundwater mineralization, surface evaporation intensity, and soil capillary pore structure. To address this issue, the core amelioration concept can be summarized as “zoned management, salinity control and blocking, and bioremediation.” “Zoned management” refers to conducting targeted soil surveys, producing spatial salinity distribution maps, accurately delineating “salt patches” as well as slightly, moderately, and severely salinized zones, and then implementing zone specific technical measures accordingly [78]. “Salinity control and blocking” can be achieved by establishing an effective drainage system, combined with water saving irrigation to enable periodic leaching and salt flushing [13]. In addition, straw incorporation should be promoted across the area to form a biomass barrier layer that physically interrupts upward salt accumulation pathways [4]. “Bioremediation” involves planting salt tolerant species in severely saline–alkali areas, such as Suaeda salsa and Tamarix spp., to facilitate biological desalination [79,80]. In slightly to moderately salinized zones, salt tolerant rice, wheat, and rapeseed can be cultivated [81], thereby developing a utilization model integrating “pioneer plant restoration plus salt tolerant crop production” to achieve co benefits for ecological restoration and agricultural productivity [82]. Moreover, salinity management in coastal saline–alkali soils should emphasize calcium supplementation. Regular application of calcium-based amendments, such as gypsum or calcium superphosphate, can increase soil Ca²⁺ availability, displace exchangeable Na⁺ from soil colloids, promote aggregate formation, and alleviate soil crusting and compaction [45].

Nutrient leaching in coastal saline–alkali soils is closely associated with a sand-dominated texture, low CEC and SOM, as well as fertilization practices and irrigation and drainage regimes, which collectively result in soil impoverishment and weak nutrient-retention capacity. To address this issue, the core amelioration concept can be summarized as “deep tillage to expand the effective rooting zone, organic matter building, and nutrient balancing” [83]. “Deep tillage to expand the effective rooting zone” refers to implementing conservation deep tillage (to a depth of 30–35 cm) combined with subsoiling to break the plow pan and thereby increase effective topsoil thickness [84,85]. “Organic matter building” involves increasing organic fertilizer inputs and promoting straw incorporation and green manure cropping, which sustainably enhance SOM and organic colloid contents, strengthen CEC, improve soil structure, and increase nutrient and water retention [86,87]. “Nutrient balancing” refers to applying zone-specific management based on soil survey data and gradually correcting imbalances in N, P, and K through integrated application of mineral N and P fertilizers and organic amendments [88], while optimizing nutrient ratios [89], thereby enabling a fundamental shift toward balanced and sustainable soil fertility under saline conditions.

5. Conclusions

Farmland soils in coastal regions are simultaneously subjected to salinity–alkalinity stress and fertility imbalance. Soils in the study area generally exhibit severe saline–alkali conditions, characterized by high pH and elevated salinity, together with low SOM, TN, and CEC. These properties indicate an overall status of poor fertility, degraded soil structure, and weak nutrient-retention capacity. Although AP in the topsoil is relatively high due to long-term fertilizer inputs, saline–alkali soils have a strong capacity to fix phosphorus, and a potential risk of phosphorus deficiency may still exist. In addition, vertical profile results reveal a clear tendency toward surface salt accumulation and an enhanced risk of nutrient leaching in this region.

Therefore, amelioration and management of coastal saline–alkali farmland should adopt a systematic strategy that gives equal priority to suppressing salt rise and conserving soil fertility. For water and salt regulation, optimized irrigation and drainage management, together with effective drainage outlets and evaporation-suppression measures, is required to enhance salt export efficiency and reduce the risk of resalinization. Meanwhile, timely calcium supplementation should be implemented to improve soil structure and hydraulic conductivity. For fertility enhancement, increasing SOM and strengthening CEC should be the central objectives, supported by an optimized fertilization regime to reduce nutrient leaching losses. This approach can mitigate the risks of non-point source and groundwater pollution at the source, thereby promoting high-yield, high-efficiency, and sustainable agricultural development. In future work, multiple interpolation methods should be integrated and compared to enhance the reliability of spatial evaluation. In addition, building on the obstacle factors identified herein, further efforts should focus on assessing the long-term effects of improvement measures and introducing innovative remediation technologies, thereby providing stronger theoretical support for the precise management of coastal saline–alkaline cropland.