Spatial Distribution Characteristics and Relationships of Salt-Based Ions and Nutrients in Old Protected Vegetable Fields

Abstract

To achieve a scientific and objective evaluation of soil acidification, secondary salinization, and nutrient imbalance in old protected vegetable fields (OPVs) with over 30 years of cultivation history, a soil surface breeding vigorous moss was investigated. Here, quantitative laboratory analysis and mathematical statistics were employed to explore the spatial distribution of soil salinity and nutrients, as well as their relationships. The results revealed that OPVs exhibited slightly acidified values. The measured anions and cations in the soil salt composition constituted approximately 77% of the total ions. Among which, Ca²⁺ was the dominant cation, while SO₄²⁻ and NO₃⁻ were predominant anions. The total water-soluble salt (TDS) content of the surface soil reached 4.52 g kg⁻¹, exceeding the Chinese Saline Soils standard (1.0 g kg⁻¹) by 350%. In the OPVs, nitrate nitrogen was significantly higher than ammonium nitrogen, and available phosphorus and available potassium were generally abundant. Despite exhibited various soil health concerns, a field visit survey presented consistently high and stable yields in OPVs. We hypothesize that this seemingly contradictory finding may be attributable to several factors, including the abundance of divalent cations (Ca²⁺ and Mg²⁺), the soil fertility and water retention capacity of unsaturated salt-based suitable soil, as well as good soil aggregate structure. These factors had the potential to reduce the stresses on the soil. This study provided a foundational understanding of the nutrient and salinity status of soils in OPVs, offering valuable data and theoretical groundwork for future research endeavors.

1. Introduction

Protected agriculture is an agricultural production method that utilizes modern agricultural technology to manipulate water, fertilizer, gas, heat, light, and other environmental factors inside facilities. This approach mitigates limitations imposed by unfavorable environments, with the goal of achieving optimal plant growth conditions and enabling continuous production [1]. Protected agriculture facilities primarily encompass greenhouses and other simple horticultural engineering structures that function similarly to greenhouses, even those employing only plastic film for protection. Statistics indicated that China’s protected agriculture for vegetables covers an area of 2.567 million hectares, yielding an annual output of 260 million tons of vegetables, which represents 1/3 of the total national vegetable production [2]. Facility types predominantly include winter-warm heliogreenhouses (WHGs), plastic tunnels (PTs), and small tunnels, with WHGs and PTs constituting over 80% of similar structures worldwide [3]. In some old vegetable cultivation regions, WHGs have become the primary mode for overwintering production due to their lower costs and superior insulation properties. PTs, characterized by a relatively simple structure and lower input costs compared to WHGs, offer a less effective heat preservation capability. However, PTs’ cultivation enables multi-vegetable rotation, and remains cost-effective, making it the preferred method for early-spring and late-autumn leafy vegetable production [1].

Compared to open-field conditions, long-term cultivation inside a confined or semi-confined environment, due to the absence of natural rainfall leaching and significantly higher fertilizer application frequencies, resulted in variously complicated soil problems like soil acidification, secondary salinization, and soil-borne disease prevalence [4,5,6]. Soil acidification is a naturally occurring phenomenon, while anthropogenic factors like fertilizer application and irrigation might accelerate this process. This is particularly evident in OPVs characterized by high water and fertilizer inputs and intensive cultivation practices [7]. A single study reported a decline in soil pH of 0.52 units following a decade of planting in OPVs [8]. This observation is consistent with findings of a progressive soil pH decrease within OPVs over extended cultivation periods. The rate of acidification in OPVs surpasses that typically observed in natural soils [9]. Secondary salinization is defined as soil salinization resulting from human activities. Statistical data from Shandong Province, a major producer of protected vegetables in China, indicated that approximately 39.7% of OPVs exhibit varying degrees of secondary salinization [10]. Excessive fertilizer application was identified as a primary contributing factor to this observed increase in soil salinity [10]. The excessive nutrient accumulation in the soil of OPVs stems from the indiscriminate overuse of chemical fertilizers and the lack of natural rainwater leaching [11]. Nutrients and salinity tend to concentrate above the tillage layer, with the most severe accumulation occurring at the surface [5]. This, coupled with the poor ventilation, plant shading, and insufficient light penetration within the facility environment, creates conditions highly conducive to moss growth on the surface soil. Moss, mainly of Pottiaceae [12], a typical indicator of secondary salinization, not only competes with shallow vegetable roots for nutrients but also contributes to soil compaction and the deterioration of soil physical and chemical properties.

Fangcun and Liangzhuang are two towns famous for vegetable production. Fangcun, known as the “Tomato Town”, primarily employs a tomato–kidney bean rotation system cultivated in WHGs. Liangzhuang, conversely, is the “Leafy-vegetable Town”, featuring a diverse crop rotation of spinach, celery, lettuce, and other leafy vegetables in PTs. Notably, over 80% of the vegetables in the two towns have a production history exceeding 25 years. While the planting system exhibited relative homogeneity, and the surface soil was overgrown with moss, most WHGs and PTs maintained remarkably higher yields and stable production. This study endeavors to elucidate the relationship between soil acidification, secondary salinization, and nutrient imbalance in OPVs. Additionally, the potential mechanisms maintaining high-salt tolerance and stable production in OPVs will be explored, aiming to provide a theoretical foundation for soil remediation strategies and the sustainable development of OPVs.

2. Materials and Methods

2.1. Study Area

The study area, shown in Figure 1, was located in the Fangcun and Liangzhuang Townships, suburban districts of Tai’an City, Shandong Province, China (117°11′ E, 35°54′ N). The tested terrain is characterized as an alluvial plain situated in front of mountains. The region experiences a warm temperate monsoon climate with an average annual temperature of 15.0 °C, an average annual precipitation of 680 mm, and an average annual sunshine time of 2759 h. The soil type is Aquic Brown Earths, systematic classification name “Mottlic Hapli-Udic Argosols”. Soil background values are shown in

Table 1. Soil background values for the study area.

Indicators	Content
pH Organic matter	6.4
	0.75%
Alkaline hydrolyzable nitrogen	45.3 mg kg−1
Total nitrogen	0.50 g kg−1
Available phosphorus	4.50 mg kg−1
Total phosphorus	0.80 g kg−1
Available potassium	72.0 mg kg−1
Cation exchange capacity (CEC)	16.9 cmol kg−1
Soil salt base saturation (BS)	98.4%

, sourced from Soils in China (2nd Edition) [13], page 79.

2.2. Design of the Experiment

Soil samples were collected in November–December 2022, following the harvest of fruits and vegetables and prior to sowing the next crop. A total of 107 OPVs were included in this research, encompassing 81 WGHs and 26 PTs. All OPVs exhibited visible moss on the soil surface and a cultivation history exceeding 25 years. Followed the five-point sampling method (Figure 1), which involves designating the midpoint of a diagonal line as the central sampling point and selecting four equidistant points along the diagonal from the central point, soil samples were collected from the surface (defined as the upper 0.5 cm of soil including adhering moss), as well as at depths of 0–20 cm and 20–40 cm among the small inter-row spaces. Then, samples were stratified, thoroughly mixed, and transferred to clean plastic bags. Collected soil samples were cleaned, inactivated as quickly as possible, then dried, smashed, sieved, and finally placed in a valve bag and kept in a dry place. Subsequently, the sieved samples (16 mesh) were used to determine pH, electrical conductivity (EC), total water-soluble salt (TDS), water-soluble anions and cations (CO₃²⁻, HCO₃⁻, Cl⁻, SO₄²⁻, Ca²⁺, Mg²⁺, K⁺ and Na⁺), cation exchange capacity (CEC), soil salt base saturation (BS), and available nutrients. A separate 100-mesh sieve fraction was employed for the analysis of total nitrogen (TN) and total phosphorus (TP).

Figure 1. The location of the study area and sampling methods.

Figure 1. The location of the study area and sampling methods.

2.3. Measurement Indicators

Soil pH was determined using a PHS-3E model instrument (Shanghai INASE Scientific Instruments Corporation, Shanghai, China) with a 1:1 soil-to-water ratio. EC was measured using an AR8011 instrument (Wan Chuang Electronics MFG. Co., Ltd., Dongguan, China) with a 5:1 soil-to-water ratio. TDS was determined gravimetrically by the drying residue weight method. A total of 100 mL of deionized water was added to 20 g of air-dried soil samples. After shaking for 3 min, the mixture was filtered. Subsequently, 50 mL of the filtrate was transferred to an evaporating dish within a water bath. H₂O₂ was then added to remove organic matter. The solution was then oven-dried to constant weight, and TDS was determined by the difference in weight before and after evaporation. Water-soluble CO₃²⁻, HCO₃⁻, Cl⁻, SO₄²⁻, Ca²⁺, and Mg²⁺ were determined titrimetrically [14], while water-soluble K⁺ and Na⁺ were analyzed using a flame photometer (Spectrophotometer 6400A, Cany Precision instruments Co., Ltd., Shanghai, China). Three replications were made for each soil sample within each of the above indicators.

CEC was determined using the BaCl₂-MgSO₄ forced exchange method with three replicates for each soil sample [14]. Briefly, 2 g of air-dried soil was weighed and extracted with 20 mL of 0.1 M BaCl₂ solution for 2 h with shaking. The suspension was then centrifuged, and the supernatant was discarded. The residue was washed three times with 20 mL aliquots of 0.002 M BaCl₂ equilibrium solution using the same centrifugation and supernatant discard procedure. Subsequently, 10 mL of 0.005 M MgSO₄ solution was added to the residue for forced exchange. The mixture was stirred for 1 h and the electrical conductivity of the supernatant (EC_susp) was measured. The solution was then titrated with 0.005 M MgSO₄ solution until the EC_susp matched the electrical conductivity of the reference solution (EC_ref) containing 0.0015 M MgSO₄ solution. The volume of the titrant used was recorded.

The soil exchangeable salts base (SEB) was determined using the NH₄OAc exchange neutralization titration method with three replicates for each soil sample. Subsequently, the BS was calculated. Briefly, 50 mL of 1.0 M NH₄OAc-extracted soil leachate was transferred to an evaporating dish and evaporated to dryness in a water bath. The residue was then ashed in an induction furnace at 500 °C for 15 min. After cooling, the residue was re-dissolved in 10 mL of 0.1 M HCl standard solution, followed by the addition of methyl red indicator (1 drop). The solution was then titrated with 0.05 M NaOH solution until the endpoint (color change) was reached. Other basic nutrient indexes were determined as described in

Table 2. Experimental measuring items and methods.

Measuring Items	Measuring Methods
Alkaline hydrolyzable nitrogen	NaOH hydrolysis—boric acid absorption—acid titration
NO3−-N, NH4+-N	KCl extraction–spectrophotometric method
Available phosphorus	NaHCO3 extraction, Mo-Sb Anti-spectrophotometric
Available potassium	NH4Ac extraction–flame photometry
Total nitrogen	H2SO4 digestion–Kjeldahl method
Total phosphorus	H2SO4-HClO4 digestion, Mo-Sb Anti-spectrophotometric

[14].

2.4. Studies Analysis

$$\mathrm{Soil}\mathrm{salt}\mathrm{base}\mathrm{saturation}\left(BS,\%\right)={\displaystyle \frac{\mathrm{Cation}\mathrm{exchange}\mathrm{capacity}\left(CEC\right)}{\mathrm{Soil}\mathrm{exchangeable}\mathrm{salts}\mathrm{base}\left(SEB\right)}}\times 100$$

$$\mathrm{Comprehensive}\mathrm{nutrient}\mathrm{evaluation}\mathrm{score}\left(Y\right)=W_1\times Y_1+W_2\times Y_2+W_3\times Y_3$$

$$Y_i=U_{i1}\times Z\left(TN\right)+U_{i2}\times Z\left(HN\right)+U_{i3}\times Z\left(N{O}_3^{-}-N\right)+U_{i4}\times Z\left({NH}_4^{+}-N\right)+U_{i5}\times Z\left(TP\right)+U_{i6}\times Z\left(AP\right)+U_{i7}\times Z\left(AK\right),\left(i=\mathrm{1,2},3\right)$$

$$U_{ij }={\displaystyle \frac{A_{ij}}{\sqrt{{\lambda }_i}}}\left(i=\mathrm{1,2},3. j=\mathrm{1,2},\mathrm{3,4},\mathrm{5,6},7\right)$$

where λ_i is “Total” (eigenvalue of the matrix), A_ij is the component matrix output by IBM SPSS Statistics 26 (26.0.0.0.0), U_ij is the principal component loading matrix, the symbol Z denotes the value after standardization of an indicator, and W₁, W₂, and W₃ are the “Percentage of variance”.

Data were statistically analyzed using Microsoft Excel 2013. Duncan’s multiple comparison method on SPSS for significance using at a level of significance of 0.05. Pearson’s correlation coefficient calculations were conducted on SPSS and Pearson’s correlation heatmaps were made on Origin 2024b (10.1). On SPSS, principal component analysis was calculated the scores of nutrient composite evaluation [15], and fitted graphs were made on Origin 2024b. The soil nutrient grading evaluation referred to the second soil census of China and related standards [13].

3. Results

3.1. Soil pH at Different Depths in OPVs

As shown in

Table 3. Soil pH and percentage of pH range in winter-warm heliogreenhouses and plastic tunnels.

Items	Winter-Warm Heliogreenhouses			Plastic Tunnels
	Surface	0–20 cm	20–40 cm	Surface	0–20 cm	20–40 cm
Mean (SD)	6.13 ± 0.56	6.24 ± 0.53	6.29 ± 0.58	6.35 ± 0.44	6.08 ± 0.61	5.78 ± 0.70
pH < 5.5	16.0%	7.41%	9.88%	7.69%	19.2%	50.0%
5.5 ≤ pH < 7.0	77.8%	84.0%	80.2%	88.5%	77.0%	42.3%
pH ≥ 7.0	6.17%	8.64%	9.88%	3.85%	3.85%	7.69%

, soil pH in OPVs exhibited an acidic tendency, with some sampling points exceeding recommended levels. The percentage of soil samples with pH below 5.5 were 11.1% and 21.8% in WHG and PT, respectively, indicating a more severe acidification problem in PTs. The average soil pH of WHGs in the surface, 0–20 cm, and 20–40 cm layers were 6.13, 6.24, and 6.29, respectively, and there was no significant difference between different layers in vertical direction. The average soil pH of PTs in the surface, 0–20 cm, and 20–40 cm layers were 6.35, 6.08, and 5.78, respectively, a gradual decreasing trend with increasing depth. Among the sample points of investigated OPVs, those with pH < 5.5 in the surface, 0–20 cm, and 20–40 cm soil layers accounted for 13.1%, 9.34%, and 18.7%, respectively.

3.2. Soil Salinity Conditions at Different Depths in OPVs

As shown in Figure 2, OPVs exhibited generally high total water-soluble salt (TDS) concentrations, with surface soil reaching values as high as 5.0 g kg⁻¹. Both the soil electrical conductivity (EC) and TDS of WHGs were significantly higher than PTs (p < 0.05). The average soil TDS of WHGs exceeded 1.0 g kg⁻¹ at all sample points in the investigated OVFs, while 50% of PTs exceeded the standard (

Table 4. Conductivity of soil saturated leachate in relation to salinity and crop growth.

Saturated Leachate EC25 (dS m−1)	Salinity (g kg−1)	Degree of Salinity	Plant Response
0–2	<1.0	Non-Saline Soils	No salt damage to crops
2–4	1–3	Saline Soils	Crop yields for species highly susceptible to salinity stress are likely to be negatively impacted
4–8	3–5	Moderately Saline Soils	Yields of salt-sensitive crops are affected, but salt-tolerant crops (e.g., alfalfa, cotton, sugar beets, sorghum, cereals) are not significantly affected
8–16	5–10	Heavy Saline Soils	While only salt-tolerant crops were successfully harvested, seed germination was negatively impacted, resulting in seedling deficits and substantial yield reductions
>16	>10	Extremely Saline Soils	Plant growth is restricted to highly salt-tolerant species, such as salt-tolerant pasture grasses, shrubs, and trees

(Source: Bao Shidan. Soil agrochemical analysis [M]. China Agricultural Press, 2000, page 187) [14].

). Both soil EC and TDS exhibited a significant (p < 0.05) fall with increasing depth, highlighting a characteristic pattern of surface accumulation.

As illustrated in Figure 3, the soil cation exchange capacity (CEC) in OPVs ranged from 17.0 to 19.0 cmol kg⁻¹, indicating a moderately high capacity for fertilizer retention. Notably, the CEC of PTs was significantly higher than WHGs. Vertically, CEC exhibited a significant increase with increasing soil depth in both WHGs and PTs. Soil exchangeable salts base (SEB) levels ranged from 10.0 to 15.0 cmol kg⁻¹, reflecting moderate levels. While WHGs exhibited significantly higher SEB compared to PTs, the SEB difference between these facilities was not statistically significant among each vertical layer. The average soil salt base saturation (BS) in the surface, 0–20 cm, and 20–40 cm layers of WHGs (75.2%, 75.3%, and 73.6%, respectively) was generally higher than PTs (67.5%, 59.9%, and 59.6%, respectively) in the investigated OPVs, and all of them were at the unsaturated level.

As shown in Figure 4, the combined proportion of anions and cations in OPVs constituted approximately 77% of total water-soluble ions across all investigated layers, suggesting a significant presence of other ion species. Sulfate (SO₄²⁻) and nitrate (NO₃⁻) were the dominant anions, while Ca²⁺ was the primary cation, and CO₃²⁻ was not detected. In the soil surface, 0–20 cm, and 20–40 cm layers, SO₄²⁻ accounted for 23.4%, 21.3%, 19.8%, NO₃⁻ accounted for 19.2%, 17.3%, 16.3%, and Ca²⁺ accounted for 15.7%, 13.8%, 13.7% of total ions, respectively. The percentage of SO₄²⁻, NO₃⁻, and Ca²⁺ on the surface was higher than in deeper soil, showing a decreasing trend from top to bottom. There was a typical characteristic of upward movement with water and accumulation on the surface. Conversely, Na⁺ and Cl⁻ exhibited an opposite distributional tendency, with a rising trend from top to bottom, showing a distinct leaching process. The higher proportion of NO₃⁻ coincided with the elevated nitrate-nitrogen content of OPVs depicted in Figure 5. In addition, the sum of anions was about 50%, while the sum of cations was only about 27%, implying the potential presence of unmeasured water-soluble cations in OPV soil.

3.3. Soils Base Nutrient Conditions at Different Depths in OPVs

Soil total nitrogen (TN) content in WHGs ranged from 0.50 to 3.0 g kg⁻¹, whereas PTs exhibited content from 0.25 to 1.25 g kg⁻¹. The soil alkaline hydrolyzable nitrogen (HN) in WHGs (average 270 mg kg⁻¹) was classified as abundant, while PTs displayed a moderately abundant level (142 mg kg⁻¹). TN, HN, and nitrate nitrogen (NO₃⁻-N) of WHGs were significantly (p < 0.05) higher than PTs (Figure 5A,B,D), while the ammonium nitrogen (NH₄⁺-N) of soil indicated no significant difference between the two facilities (Figure 5C).

Vertically, both WHGs and PTs displayed significant (p < 0.05) decreases in soil TN, HN, and NO₃⁻-N with increasing depth. Soil NH₄⁺-N in WHGs exhibited a significant (p < 0.05) decrease with increasing depth. In PTs, surface soil NH₄⁺-N was significantly higher (p < 0.05) compared to the deeper layer, while no significant difference was observed between the 0–20 cm and 20–40 cm depths. Importantly, soil NO₃⁻-N in OPVs was significantly higher than NH₄⁺-N at all layers.

In contrast to nitrogen availability, soil phosphorus and potassium nutrients in OPVs were highly abundant, with some sites exhibiting exceptionally high levels (Figure 6). Soil available phosphorus (AP) and available potassium (AK) displayed average concentrations of 263 mg kg⁻¹ and 487 mg kg⁻¹, respectively, exceeding the highest nutrient levels by 558% and 144%, respectively. Notably, soil AP and AK in WHGs were significantly higher than in PTs (p < 0.05). Vertically, both soil AP and AK in WHGs and PTs decreased significantly (p < 0.05) with increasing depth.

3.4. Correlation Analysis of Soil pH, Nutrients, and Salinity in OPVs

As shown in Figure 7, among salinity indicators, soil water-soluble HCO₃⁻ was significantly positively correlated with other salinity and nutrient indicators, but the correlation degree was the lowest, followed by Na⁺. Among nutrient indicators, soil TP was significantly positively correlated with other salinity and nutrient indicators (except Na⁺), but the correlation degree was the lowest, followed by soil AP. Conversely, salinity indicators like soil EC, TDS, and water-soluble salt-base ions were significantly positively correlated with HN, TN, NO₃⁻-N, NH₄⁺-N, and AK, and the correlation degree was higher. They were also significantly positively correlated with TP and AP, but the correlation coefficient was lower. In addition, soil pH was significantly negatively correlated with salinity and nutrient indicators (except HCO₃⁻, Na⁺, and TP), but the degrees of correlation were all low. Collectively, these findings suggest a positive correlation between nutrients and salinity within the soil of OPVs. Rising nutrient levels coincided with an increase in salinity. The negative correlation between soil pH and most salinity and nutrient indicators implies that, within a specific pH range, increasing acidity might be accompanied by higher nutrient levels. This observation aligns with the co-occurrence of mild soil acidification and high fertility observed in OPVs.

3.5. Principal Component Analysis of Soil Nutrients in OPVs

In principal component analysis (PCA), the selection of principal components typically relies on an eigenvalue (λ) greater than 1.0 or a high cumulative contribution rate of variance (often the cumulative exceeding 85%). While the eigenvalue of the third component in

Table 5. Total variance explanations for principal component analysis.

Components	Initial Eigenvalues			Extract Sums of Squared Loadings
	Total	Percentage of Variance (%)	Cumulative (%)	Total	Percentage of Variance (%)	Cumulative (%)
1	3.849	54.99	54.99	3.849	54.99	54.99
2	1.129	16.13	71.13	1.129	16.13	71.13
3	0.916	13.09	84.21	0.916	13.09	84.21
4	0.600	8.568	92.78
5	0.346	4.947	97.73
6	0.152	2.170	99.90
7	0.007	0.101	100.0

Note: significance of Bartlett’s test of sphericity is less than 0.001, suitable for principal component analysis.

is 0.916, and the cumulative of the first three components reaches 84.2%, it falls short of the conventional thresholds. However, given its close proximity to the standard criteria, three principal components were retained for further analysis.

A scatter plot was generated to visualize the relationship between the nutrient comprehensive evaluation score (horizontal axis) and total water-soluble salt (vertical axis). The data for the three soil depths (surface, 0–20 cm, and 20–40 cm) were clearly distinguished. As shown in Figure 8, nutrient scores exhibited a gradual increase from deeper to surface layers, mirroring a similar trend for salinity. This observation further reinforces the positive correlation between soil salinity and nutrients within OPVs. The fitted regression line yielded a determination coefficient (R²) of 0.412. Notably, 95% of sample points fell inside the confidence ellipse, and those that fell outside the ellipse mostly were surface samples, which showed that there was a certain degree of specificity in surface soil. Both nutrient and salinity in surface soil were higher, the data points were the most dispersive, and the data fluctuated most.

4. Discussion

4.1. Soil Acidification in OPVs

The analysis of soil samples revealed significant disparities in NH₄⁺ and NO₃⁻-N concentrations, suggesting an elevated rate of nitrification within soil of OPVs. Nitrification, however, generates H⁺ as a byproduct, contributing to soil acidification [16]. Moreover, most crop species exhibit a preferential uptake of ammonium (NH₄⁺) and potassium (K⁺) over anions. To maintain charge balance within plant tissues, H⁺ are subsequently released, further exacerbating soil acidification [17]. The higher prevalence of acidic conditions in deeper soil layers of OPVs might be attributed to the higher salinity at the surface, which potentially limits the rate of acidification in upper layers [8]. Unlike natural saline soils, the soil of OPVs did not take on a high percentage of salinogenic ions, and the eight ions only accounted for about 60%. Consequently, these soils exhibit high salinity but acidity, as evidenced by the negative correlation observed between pH and salinity. Previous studies have reported that soil colloids in soils with high BS tend to preferentially adsorb H⁺ (active acid), converting them into potential acids [18]. In this present study, the BS ranged from 60% to 75%, indicating an unsaturated state. This unsaturated condition allows for more reactive H⁺, contributing to the observed soil acidity.

Soil acidification was not defined an exact threshold value. It was a process, namely a decrease in pH relative to the original level. Most acidified soil had a pH below 5.5, and severely acidified soil fell to 4.5. It is generally assumed that when soil pH is less than 5.5, it will constrain the productivity of agroecosystems [19]. In this study, aquic brown earths exhibited a slightly acidic response [13]. The combined results indicated that a portion of the soil in OPVs exit potential acidification, with 13.7% and 2.80% of sampling sites displaying pH ≤ 5.5 and pH ≤ 5.0, respectively. These results suggest that as long as soil acidity fluctuation remains within a moderate range, there may not be a substantial impact on plant growth. This phenomenon could be a key factor underlying the absence of significant yield reductions observed in acidified soil of OPVs.

4.2. Secondary Soil Salinization in OPVs

The study revealed a high prevalence of secondary salinization within the investigated OPVs. Notably, WHGs exhibited significantly higher levels of secondary salinization compared to PTs, a pattern observed across all soil layers. This phenomenon can be attributed to the study area’s location within the Huanghuaihai Plain, characterized by heavy reliance on irrigation, with approximately 40% of water resources sourced from groundwater [20]. In this study area, the groundwater reveals a mineralization level of about 2.0 g L⁻¹, dominated by Ca²⁺ cations, and HCO₃⁻ and SO₄²⁻ anions [21]. Decades of continuous irrigation have inevitably led to the accumulation of corresponding salt ions in soil. However, the acidic soil promotes the conversion of HCO₃⁻ to CO₂, resulting in a lower HCO₃⁻ content compared to natural saline soils, which typically exhibit a synchronous high in HCO₃⁻ and pH [18]. Beyond the influx of salt ions via irrigation, the extensive use of compound fertilizers containing calcium superphosphate and ammonium sulfate also contributes to soil salinization. Furthermore, the relative abundance of divalent cations (Ca²⁺ and Mg²⁺) over monovalent cations (Na⁺ and K⁺) promotes soil coagulation, leading to increased soil viscosity and reduced particle dispersion [22]. A portion of soil Ca²⁺ was adsorbed by soil colloids, with the remaining fraction existed in the soil solution [23]. This soluble Ca²⁺ played a critical role in the formation and maintenance of soil aggregate structure, as well as the structural integrity of root cells.

4.3. Nutrient Imbalance in OPVs

The results of the study revealed that soil nitrogen levels were moderate, which contradicts with the typical nitrogen accumulation data observed in vegetable field soils reported in previous studies [24]. Two potential explanations for this phenomenon are considered: firstly, the sampling coincided with the late harvest period for vegetables. During this stage, most of the soil nitrogen has been absorbed by the plants to support yield formation, resulting in a depletion of residual soil nitrogen. Secondly, soil available nitrogen actives chemically. The ammonium (NH₄⁺-N) forms readily volatilize, while the nitrate (NO₃⁻-N) form is prone to leaching. These properties make it challenging to retain sufficient bioavailable nitrogen around the root zone [25].

The available phosphorus (AP) content within the soil of OPVs was found to be high, constituting 9.42% of the total phosphorus pool. This elevated AP poses a significant risk of water pollution. Furthermore, excessive phosphorus availability can antagonize the uptake of other essential micronutrients, such as Zn and Ca, potentially leading to physiological disorders in plants [26]. Notably, all sampling sites in the surface and 0–20 cm soil layers, and 93.4% of the sampling sites in the 20–40 cm soil layer, exceeded the established available phosphorus of the fluvo-aquic soil threshold (58.4 mg kg⁻¹) for OPVs [27], indicating a serious phosphorus accumulation within OPVs. This elevated AP can primarily be attributed to the frequent application of phosphorus fertilizers coupled with the low mobility and subsequent chemical fixation of phosphorus within the soil environment [28].

In this study, the dominant crops cultivated within WHGs were tomato and kidney beans, while PTs primarily consisted of spinach, celery, lettuce, and leek. These crops all have high potassium requirements [29]. Consequently, due to the high level of concern regarding potash fertilizer application, coupled with the inherently low mobility of potassium in soil, available potassium exhibits high level accumulation in OPVs.

This study revealed significant differences in soil fertility between WHGs and PTs. WHGs generally exhibited higher fertility levels compared to PTs, and a similar trend was observed for salinity. Two potential explanations are proposed: firstly, WHGs are predominantly cultivated with fruits or vegetables like tomato or cucumber, and plants need more fertilizer supply compared to those typically grown in PTs. Secondly, PTs often employ a high frequency of rotational intercropping, planting 3–4 types of leaf vegetables per year. The cultivation of diverse plant species promotes a more complete and balanced utilization of inherent soil nutrients [29].

4.4. Soil Multi-Factor Barrier and Outlook for OPVs

Unlike natural saline soils, secondary salinization in OPVs exhibited acidic pH values, some even reaching a strong degree of acidity (pH ≤ 4.5). This finding suggests that maintaining a relative balance in soil salinity may play a significant role in mitigating acidification in OPVs [4]. While the vast majority of the soil samples exceeded established saline soil thresholds several times, and soil secondary salinization theoretically would be severe, a considerable proportion of OPVs maintained good and stable yields. This seemingly paradoxical observation might be attributable to several key soil characteristics: an unsaturated salt base, high CEC, and exceptional soil fertility retention [18]. The presence of adequate divalent cations and a relatively rich humus (4.0% organic matter content) in the soil of OPVs likely contributes to the formation of a favorable granular structure. This structure may partially compensate for the negative impacts of high salinity ions on soil properties. Furthermore, the mildly acidic soil environment may help maintain most nutrients within a relatively bioavailable range [30], potentially serving as a critical factor in ensuring sustained vegetable yields.

This study revealed a positive correlation between nutrients and salinity in the soil of OPVs, while soil pH exhibited a negative correlation with both salinity and nutrients. A strong interrelationship existed between nutrients and salinity. For instance, excess soil phosphorus could combine with Ca²⁺ or Mg²⁺ to form precipitates, thereby reducing their bioavailability [31]. Furthermore, an increase in water-soluble cations such as K⁺, NH₄⁺, Na⁺, and other monovalent cations could disrupt the soil aggregate structure, potentially promoting the formation of compacted soil [32]. The Na⁺ content in this study approached 5.0%, and both soil salinity and nutrients were generally high. While secondary salinization significantly exceeded established thresholds for saline soils, the plants did not exhibit typical symptoms of salt stress. Additionally, the soil did not display invisible compaction or harden. The underlying physicochemical mechanisms required further investigation.

In addition, relative to the saline soil criteria for field soil, the evaluation criteria for secondary salinization in OPVs need to be revised and improved urgently to adapt to the current situation [33]. As illustrated in Figure 8, the majority of data points from soil samples fell within a specific range, with only a few surface sample points exhibiting deviations that did not significantly alter the overall trend. These observations suggested that, in the soil of OPVs, as long as the interactive relationship between nutrients and salinity was within a moderate range, there would be no substantial yield reductions occurring. However, if a particular soil exhibited both high overall salinity and concurrently low nutrients, the corresponding data points would be positioned within the upper portion of the confidence ellipse. This scenario could potentially lead to plant wilting and yield limitations, as exemplified by continuous brackish water irrigation. Such irrigation practices elevate soil EC, a proxy for salinity, and have been documented to cause a relative maize yield reduction of 16.1% [34]. Conversely, if a soil exhibited high nutrients accompanied by low salinity, the data points would be located on the right side of the confidence ellipse, potentially resulting in excess nutrients and reduced economic benefits [35].

5. Conclusions

Old protected vegetable fields exhibited a generally high level of soil fertility, characterized by abundant phosphorus and potassium. However, this nutrient enrichment was concentrated in the surface layer, potentially contributing to the observed prevalence of moss. The soil BS was unsaturated, and soil salinity exhibited a significant decrease with depth, with the highest levels measured at the surface. Furthermore, WHGs displayed a greater degree of secondary salinization compared to PTs. Interestingly, soil acidification was confined to the deeper layers (20–40 cm) in OPVs around the suburban area of Tai’an. In contrast to the high alkalinity typically observed in saline–alkaline soils, OPVs presented a unique characteristic of exhibiting both high salinity and acidic soil conditions simultaneously. Our observations suggest that crops cultivated in old protected environments could tolerate a certain level of soil quality issues without experiencing significant yield reductions. This tolerance may be attributed to the abundance of divalent cations (Ca²⁺ and Mg²⁺), unsaturated BS, good soil fertility and water retention capacity, and favorable granular structure. These combined factors likely mitigated the negative effects of soil acidification, secondary salinization, and other potential stressors on plant growth. Therefore, a key strategy for improving soil quality would be to implement a reduction in fertilizer application, particularly focusing on phosphorus-based fertilizers. This approach could deduce economic input costs while maintaining crop yield, thereby indirectly promoting soil fertility in a comprehensive manner.