**2. Soil salinity: causes, status, and major impacts**

#### **2.1 Causes and status**

Soils exhibiting the saturation paste extracts (ECe) in the root zone as electrical conductivity (EC) > 4.0 dS m−1 (≈40 mM NaCl) at 25°C and 15% exchangeable Na<sup>+</sup> ion are considered saline. There can be five major classes of soil salinity: nonsaline (EC = 0–2 dS m−1; low saline (EC = 2–4 dS m−1; moderately saline (EC = 4–8 dS m−1; highly saline (EC = 8–16 dS m−1; and extremely saline (EC = ≥ 16 dS m−1) (**Figure 1**) [9–12]. Most saline soils exhibit Na<sup>+</sup> as an anion and Cl<sup>−</sup> as a cation [13]. However, Na+ , Ca2+, and Mg2+ are the major cations component of total soluble salts in soils, whereas Cl− , SO4 2−, and carbonates (CO3 2− and HCO3 − ) are the major anions in total soluble salts in soils. Despite the aforementioned fact, most studies aimed at exploring plant-salinity responses and tolerance have considered Na<sup>+</sup> and Cl− ; and have largely ignored other cations (Ca2+ and Mg2+) and/or anions (SO4 2−, CO3 2−, and HCO3 − ) [14]. Both Na<sup>+</sup> and Cl− ions are the most widespread causes of soil salinity, where Cl<sup>−</sup> is more dangerous than Na+ since it is responsible for many physiological disorders in plants [15].

Notably, the soil salinity may be developed as natural (geological, hydrological, and pedological processes) or induced by human activity (human-caused factors). The long-term natural accumulation of salts (including Cl− of Na+ , Ca2+, and Mg2+

#### **Figure 1.**

*Schematic representation of the major classes of soil salinity [9, 10]. dSm−1, decisiemens per meter.*

and sometimes SO4 2− and CO3 2−) in the soil or surface water contributes to the primary or natural salinity. On the other hand, the disruption of the hydrologic balance of the soil between water applied (irrigation or rainfall) and water used by crops (transpiration), as a result of anthropogenic activities, causes secondary soil salinity [16]. In fact, the secondary salinization (rising NaCl levels in groundwater) impacts irrigated land and eventually leads to the loss of agricultural soils [17]. About 1125 million hectares worldwide have already been impacted by the soil salinization [18]. Huge annual global loss in crop production (≈ US\$27.3 billion) has been reported in saline soils in irrigated areas, representing about 20% of the total salinity-affected soils, mainly in North America, Oceania, and the Middle East. Interestingly, salinity may impact half of the world's irrigated land by 2050 [19–22].

## **2.2 Major impacts**

In terms of the tolerance to salinity levels, plants are grouped into two categories, namely halophytes (salinity-tolerant and adapted to salinized environments) and glycophytes (salinity-sensitive plants). Unfortunately, most agricultural crop plants are glycophytes (salt-sensitive), where soil salinization adversely inhibits growth, metabolism, development, and productivity (yield). Thus, the salinity-sensitivity of most crop plants, increasing rate of land salinization, and salinity-caused serious loss in crop productivity are challenging food security. Soil salinity significantly limits the growth, metabolism, seed germination, flowering, fruiting, and productivity (crop yields) in most crop plants, mainly as a result of osmotic stress and ionic stress [3, 23].
