We believe this may be a major shortcoming of future soil salinity research as specific ion effects may be relevant for improving on plant salt tolerance data , crop salt tolerance breeding as well as for ion effects on soil hydraulic and transport characteristics.To evaluate the effect of different levels of salinity on vegetation and on water fluxes between soil and atmosphere, simulation models are often used.In soil-hydrological models, plant stress effects by both osmotic and matric potentials on the water uptake and plant transpiration are considered.In their review, Hopmans and Bristow defined both type I and type II models of plant root water uptake, to simulate water flow in soil-root systems in a mechanistic manner.Type 1 models are based on computation of water potential gradients along a flow line in the soil-plant system.Macroscopic flow simulation models that describe plant water uptake by type II models compute plant root zone stress by macroscopic values of soil root zone water content and salinity through stress response functions with values between zero and one, representing reduction in plant transpiration relative to potential transpiration.The major advantage of the type I modeling approach is that local processes between the bulk soil and the soil-root interface and their hydraulic connections in the multi-dimensional root architecture are simulated explicitly based on principal laws of water flow in porous media,fodder system acknowledging that plant root tissues can also be described as a porous material.

Type I models therefore avoid empirical parameterizations of root water uptake, uptake compensation, and of stress functions that are used in type II models.In addition to analytical type I models that provide for a simplified representation of the root system, Javaux et al.developed a transient numerical model that considers the 3D detailed root structure and combines flow and transport of the soil and root system.This model was extended to account for salt accumulation at soil root surfaces and its effect on root water uptake.These simulations highlighted the importance of the difference between bulk and root surface water potentials and suggest that salt accumulation at the root surface needs to be considered, necessitating the need for small-scale transport simulations.For type II models, empirical plant water stress response functions were derived from experiments that relate plant response to root zone salinity and water content values.Though in principle simpler, several problems arise with the use of these empirical functions.First, the empirical functions were derived by relating plant responses over an entire growing season, to the averaged soil root zone matric and/or osmotic potentials , irrespective of changing meteorological conditions during the growing season.Yet, soil water flow models resolve flow and root water uptake processes at cm-scale spatial and hourly or smaller scale temporal resolutions.In unsaturated water flow models this is typically accounted for by computing the whole plant response from a composite of local stress responses, derived from local soil matric and osmotic potentials and root distributions.However, local reduction of water uptake may be compensated for by increased water uptake elsewhere in the rooting zone where conditions are more favorable.Another issue arises from the fact that salt and water stress response functions have often been developed independently.Different approaches have been proposed to quantify the combined stresses, but it has been much subject of discussion and remains unresolved , and will be treated in depth in Section 9.Most importantly, type II models use bulk soil potential values for the macroscopic stress response functions, whereas plants respond to potential gradients at the soil-root interface of the rhizosphere.

Consequently, salts are expected to accumulate in the rhizosphere, thus resulting is total soil water potentials that are different from those of the bulk soil.Estimation of the stress response function parameters for type II models can be obtained from in situ measurements of soil water content, salinity, root distributions, plant transpiration and root development.However, since root water uptake cannot be measured directly in the soil, the parameters of these functions are derived using inverse modeling, by which model parameters are optimized such that simulated and the measured variables are sufficiently close.For combined matric and osmotic potential stresses, Cardon and Leteycompared the sensitivity of type I and type II models.They used the type I water uptake model of Nimah and Hanks and concluded that it was insensitive to osmotic stress, while the type II model produced more reasonable results when compared with experimental data.Models that tend to focus on plant water relations for saline soil environments include SWAT and ENVIRO-GRO.The latter model was used by Feng et al.to simulate relative yields of corn and compared with experimentally measured yields for a range of irrigation water salinity and irrigation frequency values.Ben-Asher et al.applied the SWAT model to evaluate its ability to account for soil salinity effects for grapevines using both fresh and saline irrigation waters.Many conventional salinity management practices have focused on ensuring adequate leaching of salts imported by irrigation water while maintaining sufficiently deep groundwater tables, mostly to mitigate crop yield losses by accumulated salts in the rooting zone.However, recent research has focused much more on alternative options as dictated by limited available irrigation water resources.Most prominently, this has been the development of micro-irrigation systems, allowing accurate control of water application volumes and frequency.For example, Hanson et al.showed that subsurface micro-irrigation can be used even for relatively shallow groundwater table conditions, when properly managed and if seasonal rainfall is adequate to leach the accumulated salts above the dripline.Ramos et al.evaluated the threat of increasing soil salinity when using deficit irrigation.

In another study, Skaggs et al.studied the effects of reusing saline drainage waters on alfalfa yield, exemplifying much recent focus of the need to apply soil salinity models to better understand the long-term effects of using marginal irrigation waters on soil salinity and plant growth.Others, such as Assouline and Shavit and Lyu et al.evaluated the use of reclaimed irrigation water on groundwater quality.In addition to effects of marginal waters such as treated waste waters on soil salinity, specific prevalent solution ions can interact with the soil matrix.Specifically, Na is affecting soil pore distribution, soil structure and thus the flow-controlling hydraulic properties such as soil water retention and permeability.Such effects were simulated by Russo , showing that exchangeable Na in treated wastewater may considerably reduce the soil’s hydraulic conductivity, thus impacting infiltration rates of irrigated soils.Other needs for detailed soil salinity modeling include the evaluation of remediation of saline-sodic soils as presented by Chaganti et al..Though one can likely refer to many different soil salinity management models, those most widely used are HYDRUS and SALTMED.Particularly because of its extensive documentation, the modeling environment of the HYDRUS software packages is widely used and offers diverse use of its computer simulation tools,fodder system for sale with one- and multi-dimensional codes, integrated with other modules such as UNSATCHEM, PHREEQC, MODFLOW, and WOFOST, among others, also allowing for evaluation of irrigation, salinization and sodification management practices.The electromagnetic field produced generates smaller current loops with magnitude depending on soil EC.These smaller currents produce an induced secondary magnetic field of which the voltage is measured through a receiver coil.A significant advantage of the EMI as well as the Wenner array probe is that the representative depth interval of the measurement can be varied by changing resistor or coil configurations.Alternative sensors are based on measurement of the soil’s dielectric permittivity , such as TDR and capacitance sensors, mostly used for measurement of soil moisture but can be used for soil EC information as well.In Time Domain Reflectometry , a voltage signal is propagated along a set of soil-inserted wave guides, with both soil moisture and salinity affecting the shape, duration, and magnitude of the reflected wave forms.Capacitive soil salinity sensors are based on the measurement of the imaginary component of the complex permittivity.Both TDR and capacitive sensors require good contact between the soil and the sensor probes with no air gaps.Geophysical methods offer the possibility to image non-invasively three-dimensional subsurface structures of soil properties and associated flow and transport processes at spatial scales ranging from soil columns to field scale.

Using electrical methods such as electrical resistivity tomography , images of the spatial distribution of the bulk soil electrical conductivity can be derived non-invasively.ERT methodology is based on the same principle as the Wenner array described above, however, consisting of large electrode arrays and are DC-based or at low frequency AC.As defined in Eq., bulk soil electrical conductivity is strongly related to water content, so that ERT can also be used to map root water distributions.Resulting current flows are computed from numerical models, after which the soil electrical resistance distribution is mapped after model inversion, so that soil EC or other soil characteristics can be determined.Coupling the ERT data inversion with a process-based hydro-salinity model to inversely estimate process model parameters instead of spatial distributions of bulk soil ECs offers a way to improve this approach.Elaborate reviews on the application of ERT are presented by Furman et al.and Vanderborght et al..Lysimeters are tools for accurately calculating water and solute balances and are successfully used in research as well as to guide decision-making for soil reclamation, fertilization or irrigation with low quality water.Under certain conditions, including those often found when irrigating with high salinity water in dry climates, changes in soil water storage inlysimeters are negligible for a fixed time period, so that the water balance can be calculated from irrigation and drainage only.When relying on lysimeter data for salinity management with low quality water, either drainage volumes or drainage concentration can be used.The drainage amount allows estimation of crop evapotranspiration, while the drainage EC measurement enables calculation of actual LF.The use of such lysimeter data has been attractive for decision-making purposes for hydroponics and in greenhouses using water recycling.Although salt-affected soils are widespread and are increasingly listed as a major threat for a food-secure world, the core data still widely used originate from an outdated soil map with data collected in the 1970s.Derived from it, the Global Assessment of Soil Degradation was the first attempt to publish a world map on the status of human-induced soil degradation.It led to a global map at a scale of 1:10 million, defining physiographic units, themselves based on expert judgment, in which type, degree, extent, rate and main causes of degradation were characterized.Among the four soil degradation classes, it included chemical deterioration defined by loss of nutrients and/or organic matter, salinization, acidification, and pollution.The GLASOD map was primarily intended as a guide for policymakers to illustrate regions of concern, and not as a highly accurate technical product.The GLASOD data suggested that about 1Bha of the world’s soils are salt affected.These and other available estimates suggest that about 412 millionha are affected by salinity and 618 million ha by sodicity , but these figures do not distinguish areas where salinity and sodicity occur simultaneously.Estimates of secondary salinization vary and range from 45 to 80Mha, comprising some 20–30% of all irrigated land and 5–10% of the global salinized area, with about half of it located in the four countries of India, Pakistan, China and the United States.The global irrigated area is estimated to be around 300Mha.In addition, 2% of dryland agricultural area , equal to about 30Mha is estimated to be salt-affected.Similar numbers on the extent of salt-affected soils are widely used by various reports , listing that 25–30% of all irrigated lands are salt-affected and that 10% of all the world’s arable land is affected by soil salinity and/or sodicity.A more recent regional review of salt-affected soils was provided by Shahid et al., reporting global areas of salt-affected soils ranging from 45 to 77Mha.It is not clear whether these numbers also include the area of agricultural land that has been permanently lost to salinization, which was estimated to be 76Mha.Although salt-affected soils are widespread and occur in more than 100 countries, recent statistics on their global extent are absent.To support the development of national strategies for food and water security, economic development and resource conservation, the need for updated soil information on global degradation was widely recognized.For this purpose, the Harmonized World Soil Database was developed to improve on the FAOUNESCOsoil map.The new map comprised over 15,000 different soil mapping units that combined updated regional and national updates of soil information world-wide but was nevertheless largely based on the outdated FAO-UNESCO Soil Map of the World.