It is estimated that the area of salinized soils is expanding with a rate of about 1.0–2.0Mha/year.However, recent data are scarce, and reported data are greatly outdated.As freshwater resources become more scarce, alternative irrigation waters are tapped into, further threatening soil degradation in many arid regions.Furthermore, climatic change is causing sea level rise and more rapid saltwater intrusion in coastal areas, whereas increased evaporative demands require larger irrigation water amounts.Examples of soil salinization by ancient societies are documented widely, caused by over irrigation, flooding and associated rising water tables, specifically in Iraq across the Euphrates and Tigris, but also in Pakistan and India along the Indus plains and in the Americas.In most if not all of these cases, salts have accumulated in the soil rooting zone over hundreds to thousands of years, because of capillary transport from the rising water tables invading the crop’s rootzone, thereby necessitating cultivating increasingly salt-tolerant crops , eventually leading to hunger and wars, ending those early agricultural civilizations.More recently in the last 50 years or so, salinization has degraded lands in the Aral Sea basin in Central Asia, the Yellow River basin in China, the Murray-Darling Basin in Australia, and the San Joaquin Valley in California at a much faster pace.
Although estimates vary widely, salinized lands are growing at an approximate rate of 10%/year or about, mostly by human-induced agricultural practices ,stacking pots according to Szabolics.The intent of writing this synthesis paper was triggered after a literature review on soil salinity over the past few decades.It was found that most recent publications are applied and hardly added new basic research, because most presented concepts dated back to before 2000, when large-scale irrigation projects largely expanded the world’s irrigated area.Moreover, we believe that funding for salinity research has declined as soil research priorities changed.Though not comprehensive but believed to be indicative, a search using Google Scholar on publications that have soil salinity in their title showed that such publications have stagnated over the past 10 years.Similar conclusions were presented in Li et al.for China,that summarizes our mutual findings and discusses a future perspective on the sustainability of irrigated agriculture in the context of societal issues of water and food security.By identifying the most critical knowledge gaps in soil salinity, we intent to accelerate new research funding to generate new knowledge and innovative solutions.We further want to inspire the science community in developing new directions of salinity research that addresses the identified knowledge gaps presented.The purpose of this section is to comprehensively review the soil’s literature on soil salinity and its relevance, and to present proven soil salinity management practices.We will do this through a review of established handbooks and articles.
Specifically, we note the following key references: Salinization of Land and Water Resources , the ASCE Manual on Agricultural Salinity Assessment and Management with revision by Wallender and Tanji , Soil Salinity under Irrigation , and Saline and Sodic Soils.Other references that are relevant are Kamphorst and Bolt and Sposito.Soil salinity issues occur under a wide range of climatic conditions, both under natural and human-induced conditions, but are especially widespread in arid and semi-arid climates where rainfall is inadequate to leach accumulated salts below the plant’s rooting zone, whether irrigated or rainfed.The key factors associated with soil salinity are geology and its chemistry, climate, and local hydrology.It is the main source of salt in seawater and irrigation water taken from streams, lakes, and groundwater.Salts in seawater arrive on land, via atmospheric deposition either by rain or wind, or via seawater intrusion such as by tsunamis or hurricane winds along coastal areas.When formed, soils may already contain high amounts of salts, due to the parent rock material from which it is derived, such as through the weathering of carbonate mineralsor feldspars.Sedimentary rocks typically contain high amounts of carbonates and sulfates, so that their weathering leads to high alkaline soils containing significant amounts of gypsum and/or calcite.In contrast, weathering of granitic rock dominated by primary minerals such as quartz, feldspars, and micas result in more acidic soils.
Climate dictates the rate of both chemical and physical weathering through its temperature regimes, dissolution, and precipitation of salts, leaching of dissolved ions/salts or accumulation of salts.One distinguishes between primary and secondary, human-induced salinization.Primary salinization occurs by natural processes, such as by atmospheric deposition through rainfall or wind or by rock weathering, accumulating soluble minerals in soils, geological deposits and ground waters.For example, fossil ground waters originate from marine depositions, from which salts become available through seepage to near the land surface or through groundwater pumping.Natural soil salinization occurs widely in seawater-submerged soils and geologic formations and in coastal areas with shallow saline ground waters.For example, much of the salinity in US Northern Great Plains is associated with saline seepage through marine shales and derived weathered regolith, originating from a shallow ocean overlying the region some 100 million years ago.The changing salinity of the Plains in recent times is largely attributed to the change in land-use from prairie grassland to cropland and changing weather patterns with extreme summer rains and associated flooding.In the Netherlands, saline seepage from rising seawater in their coastal areas below sea levelal ways threatens its freshwater availability and are causing main concerns for their agricultural land use.Secondary salinization is caused by human activities, principally by irrigation of agricultural crops under poor drainage conditions and while using marginal irrigation waters.In addition, soil salinity can be caused by removal of deep-rooted vegetation and thereby increasing groundwater recharge ,sawtooth greenhouse and by addition of chemicals to soils such as through fertilizers and waste waters.The specific cause of soil salinization depends on local soil and groundwater transport processes relevant to the landscape and thus varies with climate, landscape type, agricultural activities, irrigation method and associated soil and water management practices.Groundwater related salinity occurs when saline ground waters rise to reach close to the plant rooting zone, followed by upwards transport into the near-surface soil through capillary forces that are triggered by soil evaporation and plant transpiration.This can occur through both primary and secondary salinization, for example, through seepage in low-lying areas or when irrigation-induced by rising groundwater tables.In the latter case, either through excess irrigation or native perennial deep-rooted vegetation removal in dryland agriculture.Non-groundwater associated salt accumulation occurs in landscapes with ground waters that are too deep for upwards capillary action to the plant rooting zone.It is prevalent when drainage of rain or irrigation waters is limited, such when largely controlled by soil textural variations in the landscape or with soil depth.Specifically, coarse-textured soils allow for adequate drainage and salt leaching, whereas soils containing low-permeable soil textural layers restrict deep percolation such as in sodic soils, causing water-logging conditions and shallow water tables.Approximately 6% of the world’s terrestrial land is believed to be salinized by primary salinization.In addition, some 20% of all cropland and between 1/4 and 1/3 of irrigated land is salinized by secondary salinization, totaling about 1Bha globally.To quantify soil salinity, one commonly estimates the concentration of total soluble salts through the electrical conductivity or EC, expressed in dS/m or mmho/cm.We note that 1 dS/m corresponds to a salt concentration of approximately 680mg/L of total dissolved solidsin soil solution.However, the effective concentration will depend on the ion activity coefficients, as affected by many factors such as the presence of ion pairs, other complex formations, and temperature.Whereas field measurements of EC represent the bulk soil, the more accepted measurement of soil salinity is using the EC of the extracted solution of a saturated soil paste, defined by ECex.This is so, because plants are dominantly affected by soil salinity through the concentration of salts in the soil’s solution.
While other extraction methods may quantitatively be more reproducible and have shown good correlations with the chemistry of the saturated paste for Cl dominated systems , the US Salinity Laboratory promoted using ECex becausethe chemistry of the saturated soil extract is close to that of the soil waterandthe chemistry could vary due to dissolution and precipitation of sulfate and carbonate minerals, should larger soil water dilutions be employed.The widely accepted classification of what constitutes different levels of soil salinity was defined by the US Laboratory Staff , with ECex values smaller than 2 dS/m classified as non-saline soils, whereas ECex values between 2 and 4, 4–8, 8–16 dS/m are defined as slightly, moderately, and strongly saline soils, respectively.Though widely accepted, there are limitations to its use.Firstly, it is a laboratory measurement that underestimates the in situ salinity for unsaturated soils.Second, soil wetting in the laboratory will often lead to dissolution of precipitated salts , thereby overestimating EC of the natural soil.The other relevant soil salinity property is related to the amount of sodiumin soils, as expressed by the Exchangeable Sodium Percentage or the Sodium Adsorption Ratio.The weathering of primary rock minerals results in the generation of individual soil particles that are negatively charged, thereby leading to the electrostatic adsorption of cations from soil solution to counterbalance the total charge along the particle’s interstitial hydrated surfaces.Much of the ability to adsorb cations will depend on soil mineral type and varies widely between clay minerals.However, all soils do adsorb ions at a certain level with the type of cations absorbed largely controlled by the composition of the soil solution.The magnitude of adsorptive capacity and level of negative charge is quantified by the soil’s Cation Exchange Capacity and varies between near zero for pure sands to 100meq/L or larger for smectite clay soil minerals.Much of the impact by on soils is caused by relatively high exchangeable sodium levels , through its adsorption from soil solution thereby largely affecting soil physical properties such as the bulk density and the water retention and hydraulic conductivity characteristics.As compared with divalent cations such as Ca and Mg, the sodium ionis less strongly adsorbed to soil particle surfaces.When hydrated, soil particles surrounded with sodium dominated water film tend to repel each other, thereby leading to soil dispersion.This causes soil aggregates to break down into individual soil particles, thereby clogging interstitial pore spaces and forming depositional soil crusts upon drying.Particle dispersion is further accelerated by soil swelling, driven by osmotic gradients, forcing pore water into the interlayers of clay minerals, especially pronounced at low salinity.Upon soil drying, these soils will shrink, creating soil cracks that can go very deep into the soil profile.When wetting, these types of soil structural degradation will largely reduce water infiltration and soil drainage, causing water logging and flooding and making soil prone to water and wind erosion.Much more detail will be presented in Section 12.Plants—Increasing levels of total salinity in the soil water solution will reduce the ability of plants to take up water from the soil because of osmotic effects, whereas specific ions such as Na, Cl, or B negatively impact plant physiological processes and can become toxic when adsorbed by the plant.In addition, saline soils can reduce plant nutrient uptake or cause ion imbalances as specific ions such as Na can compete with other essential plant nutrients, causing mineral nutrition disorders and further the plant’s ability to survive and produce.Any of these effects vary among plant species and crops.For that reason, empirical crop salt tolerance response functions have been developed , defining yield reduction as a function of total soil solution salinity based on ECex data.However, such data for developing salt tolerance parameters were collected assuming constant and high soil moisture conditions, both during the growing season and with crop rooting depth.However, in real field conditions soils wet and dry depending on irrigation frequency so that soil salinity conditions are typically non-uniform with time and soil depth.These factors along with the exclusion of specific ion effects on plant stress and yield limits their applicability.Yet, more detailed additional information is often not available.The osmotic effect on crop growth, as quantified by the soil water osmotic potentialis often considered simultaneously with soil water stress, as determined by soil water matric potential.Both are abiotic stresses that can be added, such as illustrated in Fig.2 , to reflect the combined additional energy required for plant root water uptake , as a function of soil water content and soil solution salinity.