Soil Salinity: Effect on Vegetable Crop Growth. Management Practices to Prevent and Mitigate Soil Salinization

Abstract

Salinity is a major problem affecting crop production all over the world: 20% of cultivated land in the world, and 33% of irrigated land, are salt-affected and degraded. This process can be accentuated by climate change, excessive use of groundwater (mainly if close to the sea), increasing use of low-quality water in irrigation, and massive introduction of irrigation associated with intensive farming. Excessive soil salinity reduces the productivity of many agricultural crops, including most vegetables, which are particularly sensitive throughout the ontogeny of the plant. The salinity threshold (EC_t) of the majority of vegetable crops is low (ranging from 1 to 2.5 dS m⁻¹ in saturated soil extracts) and vegetable salt tolerance decreases when saline water is used for irrigation. The objective of this review is to discuss the effects of salinity on vegetable growth and how management practices (irrigation, drainage, and fertilization) can prevent soil and water salinization and mitigate the adverse effects of salinity.

1. Introduction

Soil salinization is a major factor contributing to the loss of productivity of cultivated soils. Although difficult to estimate accurately, the area of salinized soils is increasing, and this phenomenon is especially intense in irrigated soils. It was estimated that about 20% (45 million ha) of irrigated land, producing one-third of the world’s food, is salt-affected [1]. Soil salinity affects an estimated 1 million hectares in the European Union, mainly in the Mediterranean countries, and is a major cause of desertification. In Spain, 3% of the 3.5 million hectares of irrigated land is severely affected, markedly reducing its agricultural potential, while another 15% is under serious risk [2]. In the Mediterranean region, land degradation associated with soil alkalization may worsen at increasing rates in the coming decades, owing to the expected increase in irrigated areas and the increasing scarcity of good quality water [3]. The amount of world agricultural land destroyed by salt accumulation each year is estimated to be 10 million ha [4]. This rate can be accelerated by climate change, excessive use of groundwater (mainly if close to the sea), increasing use of low-quality water in irrigation, and massive introduction of irrigation associated with intensive farming and poor drainage. On the other hand, the tendency to increase the efficiency of irrigation water use, as is verified in many regions due to the scarcity of water, and the use of low quality water can lead to the accumulation of salts in the soil, since the leaching fraction is reduced and the salts contained in the irrigation water are not leached enough. It is estimated that, by 2050, 50% of the world’s arable land will be affected by salinity [5]. Soil salinity reduces the productivity of many agricultural crops, including most vegetable crops, which present low tolerance to soil salinity. However, a substantial increase in production and consumption of vegetable crops that include edible portions of herbaceous species (roots, tubers, shoots, stems, leaves, fruits, and flowers) is a global priority. In fact, vegetables play an important role in human nutrition and health, particularly as sources of vitamin C, thiamine, niacin, pyridoxine, folic acid, minerals, and dietary fiber. Some of the world’s most widespread and debilitating nutritional disorders, such as micronutrient deficiencies, are related to low vegetable intake [6]. Generally, vegetables are crops with high productivity per unit of water applied and economic value compared with field crops. This may be a very important advantage for small farmers, because vegetables can grow in small areas, under intensive procedures. Vegetable crops generally require more water and more frequent irrigation than other agronomic crops. Vegetable crop production in arid and semi-arid regions with low rainfall and high temperatures require a larger input of fertilizers and irrigation. However, soil and water salinity increase are closely related to irrigation and fertilization practices. Therefore, the objective of this review is to analyze the effects of salinity on vegetable growth and how management practices (irrigation and fertilization) can prevent soil and water salinization and mitigate adverse effects of salinity.

2. Effects on Vegetable Growth and Nutrition

Salts affect plant growth due to increasing soil osmotic pressure and to interference with plant nutrition. A high salt concentration in soil solution reduces the ability of plants to acquire water, which is referred to as the osmotic or water-deficit effect of salinity. Damage occurs when the concentration is high enough to begin reducing crop growth. The osmotic effect of salinity induces metabolic changes in the plant identical to those caused by water stress-induced “wilting” [7] and shows few genotype differences [8]. Moreover, salt stress reduces plant growth due to specific-ion toxicities and nutritional imbalances [9] or a combination of these factors [10]. Indeed, salinity effects on plant growth reduction are a time-dependent process, and Munns et al. [11] proposed a two-phase model to depict the response of plant growth to salinity. The first phase is very rapid and growth reduction is ascribed to development of a water deficit. The second phase is due to the accumulation of salts in the shoot at toxic levels and is very slow. Despite the fact that this model has been demonstrated in broccoli [12], the relative importance of the two mechanisms on yield reduction is difficult to assess with confidence because they overlap.

Salinity affects photosynthesis by decreasing CO₂ availability as a result of diffusion limitations [13] and a reduction of the contents of photosynthetic pigments [14,15]. Salt accumulation in spinach inhibits photosynthesis [16], primarily by decreasing stomatal and mesophyll conductances to CO₂ [17] and reducing chlorophyll content, which can affect light absorbance [14,18]. In radish, about 80% of the growth reduction at high salinity could be attributed to reduction of leaf area expansion and hence to a reduction of light interception. The remaining 20% of the salinity effect on growth was most likely explained by a decrease in stomatal conductance [19]. Salinity lowers the total photosynthetic capacity of the plant through decreased leaf growth and inhibited photosynthesis, limiting its ability to grow [20].

Salt accumulation in the root zone causes the development of osmotic stress and disrupts cell ion homeostasis by inducing both the inhibition in uptake of essential elements such as K⁺, Ca ²⁺, and NO₃⁻ and the accumulation of Na⁺ and Cl⁻ [21]. Specific ion toxicities are due to the accumulation of sodium, chloride, and/or boron in the tissue of transpiring leaves to damaging levels. Accumulation of injurious ions may inhibit photosynthesis and protein synthesis, inactivate enzymes, and damage chloroplasts and other organelles [22]. These effects are more important in older leaves, as they have been transpiring the longest so they accumulate more ions [7]. Plant deficiencies of several nutrients and nutritional imbalances may be caused by the higher concentration of Na⁺ and Cl⁻ in the soil solution derived from ion competition (i.e., Na⁺/Ca²⁺, Na⁺/K⁺, Ca²⁺/Mg²⁺, and Cl⁻/NO₃⁻ in plant tissues) [23]. Calcium deficiency symptoms are common when the Na⁺/Ca²⁺ ratio is high in soil water. However, lower calcium uptake by tomato plants has been linked with decreased transpiration rate rather than competition effects with Na⁺ [24].

A decrease in plant biomass, leaf area, and growth has been observed in different vegetable crops under salt stress [25,26]. Salt stress effects on root architecture/morphology currently are poorly understood [27]. However, root biomass has been reported to be generally less affected by excess salinity than aboveground organs [10]. Salinity reduced root biomass has been reported in broccoli and cauliflower [26] and root length density (RLD) in tomato [28].

Visual symptoms of salt injury in plant growth appear progressively. The first signs of salt stress are wilting, yellowed leaves, and stunted growth. In a second phase the damage manifests as chlorosis of green parts, leaf tip burning, and necrosis of leaves, and the oldest leaves display scorching [29].

Salt stress decreases marketable yield due to decreased productivity and an increased unmarketable yield of fruits, roots, tubers, and leaves without commercial value. Irrigation with saline water has been shown to enhance the occurrence of blossom-end rot in tomato, pepper fruits, and eggplants, a nutritional disorder related to Ca²⁺ deficiency. However, salinity has some favorable effects on the quality of the edible part of the vegetable crops. In general, salt stress, with the exception of visual appearance (size, shape, and absence of defects), improves the quality of edible part of vegetable crops. In general, salinity increased fruit dry matter content, total soluble solids (TSS), and acid content of melon, tomato, sweet pepper, and cucumber. Salt stress increased carotenoid content and antioxidant activity of tomato [30]. Overall, the nutritional quality (e.g., glucosinolate, polyphenol content, etc.) of the edible florets of broccoli was improved under moderate saline stress [31]. In romaine lettuce, salinity increased carotenoid content [32]. Salt stress increased polyphenol content and decreased nitrate ion and oxalic acid concentration in spinach [33]. The effect of salinity on vegetable yield and quality was also affected by the timing of application of salt stress, which could be important for improved irrigation (e.g., deficit irrigation) and fertilization management strategies. In two melon cultivars (Galia and Amarillo Oro), the application of salt stress from fruiting to harvest did not reduce marketable fruit yield and increased fruit quality (TSS) and maturity index in both cultivars [34].

3. Alkalization

Salinity can affect plant growth indirectly by sodium’s effect on the degradation of the soil’s physical condition and by increasing the soil’s pH. In normal soils with some organic matter content, exchangeable cations such as Ca²⁺ and Mg²⁺ link clay particles to humic acids of the organic matter, generating stable micro-aggregates which are the basis for soil structure, porosity, and internal drainage. In soils with high concentrations of sodium, calcium and magnesium adsorbed on the soil exchange complex will be replaced by sodium, which has low flocculating power (Table 1), causing dispersion of soil particles. The damage to soil structure is accompanied by an increase in the compactness of soils and a decrease in infiltrability, hydraulic conductivity, and the oxygen availability in the root zone. Another effect of a high concentration of sodium is increased pH (alkalization), which is produced by the presence of HCO₃⁻ and CO₃²⁻. There is a linear relationship between the exchangeable sodium percentage (ESP) and the pH of the soil [35]. Excess sodium (Na⁺) in the soil competes with Ca²⁺, K⁺, and other cations to reduce their availability to crops. Therefore, soils with high levels of exchangeable sodium (Na⁺) may impact plant growth by dispersion of soil particles, nutrient deficiencies or imbalances, and specific toxicity to sodium sensitive plants.

[ Table omitted. See PDF. ]

4. Vegetable Tolerance to Salinity

The salinity tolerance of any crop is defined as the ability to endure the effects of excess salt in the root zone. Salt tolerance is described by models that relate the decrease in relative production with the increase in soil salinity [37,38,39]. In the model of Maas and Hoffman [37], relative crop yield is not affected until a salinity threshold (EC_t) is exceeded, according to the following equation: Y = 100 − (EC_e–ECt) S. In this equation, Y is the relative crop yield, 100 is the maximum yield, EC_e is the salinity of soil saturation extract, EC_t (dS m⁻¹) is the threshold, the value of the electrical conductivity that is expected to cause the initial significant reduction in the maximum expected yield, and S is the slope that represents the percentage of yield expected to be reduced for each unit of added salinity above the EC_t. Salt crop tolerance is rated by salinity threshold (EC_t) and the percent of reduction of relative yield per unit increase in soil salinity above the threshold. The majority of vegetable crops have a salinity threshold that is ≤2.5 dS m⁻¹ [28] (Table 2). Thus, the area of soils with restrictions for vegetable crop production is certainly greater than the area of salinized soils, since a saline soil is generally defined as showing an electrical conductivity (EC) value of the saturation extract (EC_e) in the root zone that exceeding 4 dS m⁻¹ (approximately 40 mM NaCl) at 25 °C and having an exchangeable sodium level of 15% [1]. The Maas-Hoffman model only considers soil salinity and species; however, salt tolerance depends on many factors such as plant growth stage, climate, and salt type [29,40], soil properties [38], root-zone temperature [41], air concentration of CO₂ [42], and cultural practices (e.g., leaching fraction), etc. Therefore, the salt tolerances of different vegetable crops presented in Table 2 serve only as a guideline to assess relative tolerances among the crops. Concerning plant salt sensitivity relative to the growth stage, a general observation is that plants at earlier growth stages (seedling, establishment) are more sensitive to salt stress than plants at later stages. During germination and emergence, determination of tolerance is based on percent survival, while during the later developmental stages, tolerance is usually measured as relative growth reductions [8]. Salinity affected cauliflower growth mainly when imposed in the first growth phase [43]. The EC of irrigation water also affected salt tolerance [44] (Table 2). The lowest threshold level of irrigation water EC_w not restricting crop growth was 0.7 dS m⁻¹, lower than EC_e (1 dS m⁻¹) (Table 2). The majority of vegetable crops present low tolerance to saline water applied continuously (Table 2). The classes of salt tolerance are: sensitive, moderately sensitive, moderately tolerant, tolerant, and unsuitable for crops. The majority of vegetable crops are sensitive or moderately sensitive [38,45] (Table 2). Asparagus has been considered the most salt-tolerant vegetable crop.

[ Table omitted. See PDF. ]

5. Management Practices

The key to producing vegetable crops is to control salinity levels in the root zone to values equal to or smaller than the EC_t of a crop. In order to control salinity levels, management must include soil reclamation of the saline and sodic soils, and the practices of the fertilization and irrigation should aim to prevent soil salinization and to mitigate the effect of soil salinization and/or use of saline irrigation water in the growth and development of vegetable crops.

5.1. Soil Reclamation

Soil salinity and sodicity are problems too difficult to overcome, requiring salt removal from the root zone (reclamation). This is perhaps the most effective and long-lasting way to minimize or even eliminate detrimental effects of salinity [7]. However, in addition to being slow and expensive, the process requires large quantities of quality water and effective soil drainage. It is not always easy to obtain enough quality water, because the possible water sources next to the soils to be treated may already themselves be highly saline. If soil drainage is poor and the water table is shallow, an artificial drainage system must be installed. Consequently, it is not always possible or feasible to carry out a “true reclamation” technique. The reclamation of sodic soils may, in addition to leaching, require the application of amendments to increase soil permeability and reduce the exchangeable sodium levels. Sodic soils reclamation involves substituting sodium in the soil with calcium ions, through applying large quantities of gypsum (CaSO₄). The released sodium ions are then leached deep beyond the root zone using excess water and finally moved out of the field through drainage. Gypsum, when slowly mixed with water, releases calcium ions, which replace sodium ions from the soil into the downward moving water. Sulfuric acid and elemental sulfur (S⁰) can also be used as alternatives to gypsum, because soil microbes convert sulfur into sulfuric acid (S⁰ + ½O₂ + CO₂ + 2H₂O − H₂SO₄ + CH₂O). The effect of S⁰ amendment could be slower, because sulfur oxidation depends on soil temperature, humidity, and aeration, etc. Sulfuric acid and elemental sulfur addition also contribute to soil pH reduction, due to an increase of the H₃O⁺ in soil solution.

5.2. Fertilization

Crop fertilization is one of the sources of salinization of soils. To reduce this negative impact, the fertilizer characteristics, the method of fertilizer application, irrigation water quality, and fertilization scheduling, etc., must be considered. Excessive nutrient applications must be avoided, and high-purity, chloride-free, low-saline fertilizers should be selected. In irrigated vegetable crops the crop nutritional requirements must by supplied by the soil, fertilization, and the nutrient content in the irrigation water. Irrigation waters could contain high nutrient levels (e.g., nitrate-N, calcium, magnesium, sulfur, and boron) sufficient to partially or completely satisfy crop needs [44,47]. Many agricultural regions in the world have high amounts of N in the groundwater due to NO₃ leaching from fertilizers [47]. Ca²⁺, Mg²⁺, and SO₄²⁻ concentrations in irrigation water may easily exceed apparent uptake concentrations [48].

The application of fertilizers through irrigation water (fertigation) can reduce soil salinization and mitigate salt stress effects because it improves the efficiency of fertilizer use, increases nutrient availability and timing of application, and the concentration of fertilizers are easily controlled. Fertigation allows frequent applications of very low fertilizer rates which adjusts nutrient supply to plant requirements. Nutrient supply rate must take into account the rates of nutrient uptake and of evapotranspiration and irrigation water quality. The solutions applied in fertigation should generate low additions of EC_w and should not exceed the EC_t (electrical conductivity threshold) tolerated by the crops, which varies with the irrigation water and with the fertilizer used [49,50]. The application of fertilizers in irrigation waters with EC_w values of >0.7 dS m⁻¹ (Table 2) must be made carefully. Nitric acid and sulfuric acid fertigation represent rapid ways to reduce or minimize salinity and sodicity in arid regions. Nitric acid applied with fertigation reduces soil pH and increases Ca²⁺ dissolution in clay soils, thereby minimizing salinity injury due to Ca²⁺/Na⁺ competition. It may also reduce chloride salinity in the root zone, because the nitrate can counterbalance the excess of chloride [51]. In arid regions, soils are commonly alkaline, with high concentrations of free calcium carbonate (CaCO₃). In this case, sulfuric acid can be applied by fertigation, with a consequent release and leaching of the Na⁺ existing in the soil profile [52]. Iron must be supplied in chelated form (Fe-DTPA Fe-EDDHA) to increase it availability to plants.

The salt tolerance of the crops could be improved by the addition of different nutrients [53]. Plant response to fertilizers depends on severity of salt stress in the root zone [46] the species, cultivar, nutrient source, and fertilizer application method. However, the application of fertilizers to saline soils also may exacerbate soil salinization [46]. The strategy used in the addition of inorganic fertilizers is mainly based in competition between ions (one ion limits the uptake of another ion).

The addition of NO₃⁻, Ca²⁺, K, P, salicylic acid, and silicon (Si) to the saline medium or in foliar application has improved salt tolerance of numerous vegetable crops such as tomato, pepper, eggplant, melon, bean, strawberry, etc. (Table 2). Increasing the nitrate content in a nutrient solution would decrease chloride uptake and its accumulation [54]. However, several studies have shown that under salt stress conditions the effects of salinity can be alleviated by application of nitrate and ammonium compared to growth on only nitrate or ammonium [55]. The ratio of NO₃⁻/NH₄⁺ most appropriate to improve salt tolerance depends on the crop [56,57]. In tomato, the deleterious effect of salinity on biomass production can be minimized by the use of nutrient solutions containing higher NH₄⁺ concentrations [56]. Although deemed a “non-essential” mineral nutrient, Si has been shown to be effective in mitigating salinity effects on several vegetable crops (Table 3). Si decreased the root-to-shoot translocations of Na⁺, Cl⁻, and boron in tomato plants grown on a sodic-B toxic soil [58]. The majority of these results were obtained under controlled conditions. Therefore, it is necessary to study the effect of these substances in salt tolerance of vegetable crops in field conditions.

[ Table omitted. See PDF. ]

Humic substances can ameliorate the deleterious effects of salt stress by increasing root growth, altering mineral uptake, and decreasing membrane damage, thus inducing salt tolerance [59]. The addition of humic acids to the saline medium improved salt tolerance of different crops (Table 3). Applications of humic acids enhanced K⁺/Na⁺ and Ca²⁺/Na⁺ ratios in pepper [60].

The use of biofertilizers can also mitigate salinity effects on vegetables and reduce soil salinization. A biofertilizer could be defined as a formulated product containing one or more microorganisms that enhance the nutrient status (and the growth and yield) of the plants by either replacing soil nutrients, by making nutrients more available to plants, and/or by increasing plant access to nutrients. Plant growth promoting rhizobacteria (PGPRs), endo- and ectomycorrhizal fungi, and many other useful microscopic organisms led to improved nutrient uptake, plant growth, and plant tolerance to salt stress. The inoculation of seeds of various crop plants, such as tomato, pepper, bean, and lettuce, with PGPRs can result in increased root and shoot growth, dry weight, fruit, and seed yield and enhanced tolerance of plants to salt stress [61]. PGPR and Si synergistically enhanced salinity tolerance of the mung bean [62]. The use of arbuscular mycorrhiza (AM) has been shown to be able to alleviate salt stress in tomato, onion, and lettuce [63,64,65]. Biofertilizers can reduce soil salinization by reducing application of fertilizers, improving soil fertility by fixing atmospheric N₂, both in association with plant roots and independent of roots, solubilizing insoluble soil phosphates, and producing plant growth substances in the soil.

5.3. Irrigation

Irrigation method, management (irrigation scheduling and leaching fraction), and artificial drainage can prevent and mitigate the effects of soil and water salinity by influencing water-use efficiency (WUE) and nutrient-use efficiency, salt accumulation and distribution, and salt leaching. Where foliar damage by salts in irrigation water is a concern, irrigation methods such as surface drip irrigation (DI) and subsurface drip irrigation (SDI), furrow irrigation, and low energy precision application (LEPA) irrigation must be used. DI and SDI, compared with other irrigation methods, allow for better salinity management by increasing water-use efficiency and nutrient-use efficiency [49,83,84]. Additionally, soil inside the wet bulb, where root density is the highest, is mostly salt leached, which creates a suitable root-zone salinity (EC_e < EC_t). Under drip irrigation, water moves in a more or less radial pattern around the emitter and the ions eventually mirror this pattern [45]. In the wet bulb, the ions tend to accumulate in the interface between the dry soil and wetting front due to the difference in osmotic potential [85], mainly next to the soil surface [45].

An appropriate irrigation scheduling with DI and SDI methods can also reduce the effects of salinity by continuously maintaining moist soil around plant roots and providing steady leaching of salt to the edge of the wetted zone. SDI, in comparison with DI, increased water use-efficiency in tomato [49,86] and reduced sodium and chloride accumulations in tomato plant tissues on a silty clay soil in Tunisia [87]. Under SDI irrigation, water and ions flow in spherical manner and the salts accumulate near the soil surface, which may constitute a significant constraint for vegetable crops sown and/or transplanted, because most crops at an early juvenile development stage are more susceptible to soil salinity. This can reduce plant population density to suboptimal levels and consequently impact the yield. With furrow irrigation, soluble salts in the soil move with the wetting front, concentrating at its termination or at the convergence with another wetting front. When adjacent furrows are irrigated, salts concentrate in the middle spaces between furrows. Manipulating bed shape and planting arrangements are strategies often used to ensure that the zones of salt accumulation stay away from germination seeds and plant roots. Sprinkler irrigation and an appropriate leaching fraction generally move salts below the root zone. However, when saline water is used with irrigation, the crops are potentially subject to additional damage caused by salt uptake into the leaves, and burn from spray contact with the leaves. The degree of injury depends on weather conditions: it is most severe during hot dry conditions, because evaporation concentrates the salts at the leaf surface. Therefore, sprinkler irrigation with saline water must be done when temperatures are coolest.

When irrigation water is scarce, as due to the occurrence of a drought, the irrigation schedule may include deficit irrigation strategies. Deficit irrigation (DI) is an optimization strategy in which the application of water is smaller than the full crop evapotranspiration requirements. Water restriction is applied, outside of drought-sensitive growth stages of a crop, during which yield loss due to water stress may be compensated by the value of saved water. Deficit irrigation may increase WUE and vegetable quality, but imposes some degree of yield reduction and increases the risk of soil salinization due to reducing leaching. Partial root-zone drying (PRD), a modified form of deficit irrigation, in which the two halves of the root are alternately irrigated, increased WUE and did not affect yield in tomato [88] and in potato [89].

5.4. Maintenance Leaching

To ensure long-term land use with irrigated vegetable crops, it is necessary to do a maintenance leaching. The volume of water applied with irrigation must include a water amount that drains down the root zone, which is in addition to the amount required for normal irrigation. This additional water is defined as the leaching fraction (LF) [90]. Leaching is absolutely necessary to achieve long-term successful irrigation [90,91]. A LF of 15 to 20% is commonly recommended [21]. The required frequency of leaching varies with the degree of salinization and evaporative demand [92] and salt sensitivity of the crops [84]. In arid regions, LF must be included in each irrigation event [52]. The frequency of leaching when drip irrigation is used could be two or three times a week or daily for moderately sensitive and sensitive salt crops, respectively [84].

6. Conclusions

Soil salinity is becoming a major constraint to vegetable crop production. Vegetable crop production requires a high input of fertilizers and water, each possibly increasing soil salinity. Fertilization and irrigation management strategies must consider the effects of salinity on vegetable growth, crop salt tolerance, soil proprieties, and effects on water use efficiency and soil salinity. Drip irrigation and subsurface drip irrigation, compared with other irrigation systems, increase water use efficiency and create a suitable root-zone salinity (EC_e < EC_t). Fertigation increases nutrient use efficiency and allows fertilizer application without provoking excessive increases in soil salinity. Salt tolerance of vegetable crops can be enhanced by applying some nutrients (e.g., silicon, humic acid, etc.). Biofertilizers also have the potential to increase salt tolerance of vegetable crops and reduce soil salinization.

Acknowledgments

This work was funded by the FEDER Funds through the Operational Programme for Competitiveness Factors—COMPETE, and National Funds through FCT (MCTES).

Author Contributions

Rui Manuel Almeida Machado and Ricardo Paulo Serralheiro conceived and wrote the paper.

Conflicts of Interest

The authors declare no conflict of interest.

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