We note that most crop salt tolerance data come from field plot studies or greenhouse experiments

The same authors also showed that, while development of wireless networks has focused mostly on integrating sensor technologies, there is limited research done on integrating control systems with sensor data acquisition, aimed at automating smart valve systems at the plant or tree scale.Much of this is required for advanced PI systems, allowing for high resolution control of water and nutrient application such as presented by Coates et al..In addition to ground-based sensors, there is great potential for use of airborne instruments, with the development of commercial airplane based remote sensing and UAVs for agricultural applications.Especially non-contact platforms such as Electro Magnetic Induction could potentially be used with drones for soil salinity monitoring.In addition, hyperspectral and thermal cameras can be used for plant monitoring of water or salinity stress and diseases.In addition to using wireless and new sensor technologies for improved soil salinity management and control, integrating real-time sensor and control data with soil and crop growth simulation models allows for real-time management at the plant/tree scale,potted blueberries when combined with visualization tools and decision support systems.Recent examples of such an approach that integrates sensor information with a combined irrigation application and biophysical crop simulation model was presented by Sperling et al.and Gonzales Perea et al..

However, other applications have shown the successful application of machine-learning, ANN and AI algorithms, training the DDS system using past information to improve forecasting of soil and plant status, as well as for calibration and validation.Advancing PI even further, is to combine sensing and modeling information in a single DDS system , to allow for adaptive irrigation water and soil salinity management.With specific attention to soil salinity, such an integrated management system would allow real-time and plant-scale or zone control of water and fertilizer application, minimizing crop water and salinity stress and optimizing yield and water use efficiency.Summary: The general absence of intensive soil salinity measurements and monitoring prohibits development of improved soil salinity management practices that maintain crop productivity while minimizing soil and water degradation.Knowledge gaps to advance PI are mostly associated with the need for cost-effective technologies that integrate soil moisture, salinity, and nutrient measurements within a cloud-based multi-sensor platform.When included to an IoT cloud-based system that integrates a wireless monitoring and control network with real-time computer simulations of soil water, salinity and crop growth, PI will allow for real-time adaptive management close to the individual plant/tree or zone scale.Soil moisture and salinity monitoring networks are cost prohibitive, as sensors are still too expensive for application at small management zones.Crops vary widely in their tolerance to soil salinity.The physiological response of crops to salinity is related to two processes: osmotic and specific ion effects.

These processes are dependent on each other and often impact the crop collectively.Salinity reduces the osmotic potential of the soil solution thereby requiring the plant to osmotically adjust by concentrating solutes inside their cells to readily extract water via osmosis.This concentration process requires metabolic energy , but its ultimate cost to plant growth depends on ion transport efficiencies across membranes and energy requirements to synthesize organic solutes, which differs among species and varieties within a species.As such, the efficiency of transport processes involving specific ions will affect the overall osmotic response.As a result, salt-stressed plants are stunted, even though they may appear healthy in all other regards.Both adjustment processes, i.e., accumulation of ions and synthesis of organic solutes occur but the extent by which one process dominates over the other is dependent on plant type and level of salinity.At the plant cell level, compartmentalization is critical to keep toxic ions away from locations of sensitive metabolic processes in the cytoplasm.Such compartmentation is controlled by transport processes across the plasma membrane and the tonoplast , as explained in Section 10.Specific ion effects can be directly toxic to the crop, due to excess accumulation of Na, Cl or B in its tissue, or cause nutritional imbalances.While specific ions reduce the osmotic potential of the soil solution, ion toxicities are rarely observed in annual crops grown in the field , provided the ion ratios are not extreme or salinity is too high.However, when Na+ dominates the cations or Cl concentrations are sufficiently high, these constituents can accumulate in older leaves and produce plant injury.Specific ion toxicities are particularly prominent in tree and vine crops and injury becomes more prevalent over the years, sam but can be controlled by root stock selection.Specific ions can also induce nutritional disorders due their effect on nutrient availability, competitive uptake, transport, and partitioning within the plant.

For example, excess Na+ can cause sodium-induced Ca2+ or K+ deficiency in many crops.As indicated earlier, soil salinity adversely affects plants by a combination of mechanisms, including osmotic influences, toxic ion effects and nutritional imbalances.The most relevant one depends on the crop, its growth stage, duration of salinity exposure and environmental conditions , so that salt tolerance is difficult to quantify.For example, ion toxicity in tree and vine crops becomes more pronounced over the years with foliar injury particularly prominent later in the season.Because of the many factors affecting soil salinity tolerance, there is considerable uncertainty regarding the yield threshold values, as they lack physiological justification.Despite investigators controlling salinity and minimizing all other stresses that could affect yield, the standard errors associated with the ‘threshold’ values can be 50–100%.Obviously, these large percentages represent considerable uncertainty and suggest that true “threshold” values do not exist.Instead, it has been suggested by van Straten et al.to substitute it with a soil salinity parameter, ECe90, that equates to 90% yield.Others have developed non-linear expressions to improve on the physiological response of plants to salinity stress.Crops in most these studies were irrigated frequently, using high leaching fractions to avoid crop water stress.This was done intentionally to create a uniform soil salinity profile across the rooting zone that remained approximately constant during the growing season.In this way, one could compare salinity tolerances among crop species and rank their sensitivity and explains why most salt tolerance models fit such data very well.While creating uniform, steady state root zones experimentally, such uniform profiles are uncharacteristic for an irrigated field.Field soils develop characteristic salt distribution patterns that vary with soil depth and irrigation type.These patterns are a result of water movement via gravitational and capillary action and subsequent root water extraction and soil evaporation.Under sprinkler or border irrigation, the salinity increases with soil depth while under furrow or drip, salinity increases horizontally in the direction of water flow in addition to their increases in the depth direction.Furthermore, soil salinity is affected by rainfall patterns during the growing season,square plastic pot whereas crop salt tolerance can be affected by soil structural changes due to sodic conditions.Under such conditions, three-fold variations in wheat yield were determined for similar soil profile.The accumulation of salts vis-a`-vis their osmotic effects is further modified as a function of soil texture, agro-climatic conditions, ionic constituents of salinity, and soil-irrigation-crop management strategies which impact salt tolerance limits of crops under field conditions.

The current salt tolerance data are based on crop response to saturated soil extract measurements, whereas the crop is responding to the salinity of the soil water in situ, which is continuously changing over space and time.Over the past several decades it is noted that agricultural irrigation is increasingly shifting from conventional surface irrigation methods to pressurized systems that are more efficient.Studies have shown that crops with high-frequency irrigation are more tolerant to salinity than using conventional irrigation methods.Though the wetted root zone is typically much smaller than for low frequency surface irrigation, under high frequency drip irrigation, the salinity of the soil water near the dripper is close to that of the irrigation water with the water content close to field capacity.Therefore, the roots are exposed to a lower soil salinity than for conventional irrigation practices.While the wetted soil volume is smaller, high frequency irrigation allows the crop to maintain its crop water needs.The more recent change to pressurized irrigation puts into question the current validity of historical soil salinity tolerance data using the concepts of Eq.developed for conventional surface irrigation systems , as the root-accessible soil water is near that of the irrigation water salinity using high frequency irrigation.Alternatively, one may think about measuring real-time salinity in situ at multiple soil depths based on the depth-dependent root distribution.The non-uniform conditions of the irrigated soil complicates how best to characterize the root zone in their response to soil salinity, as the roots are exposed to changes in soil water content and salinity in different parts of the profile.It has been recognized for decades that the major root activity is found in the least saline portions of the soil profile.Consequently, it has been shown that shoot biomass can be 3–10-fold higher in heterogeneous soil profiles than under equivalent homogeneous salinity conditions, equal to the average root zone salinity of the heterogeneous soil.Experiments with alfalfa indicated that root water uptake rate reacts to soil salinity, but that additional factors such as root activity and evaporative demand can become more important in controlling uptake patterns.Roots will grow and develop in the most favorable portions of the root zone considering factors such as salinity, water content, nutrients, pH, oxygen availability, soil strength, and disease pressure.For example, soil salinity may be low in the upper portion of the soil profile but soil water content will vary widely due to higher root length density there.In the lower portion of the soil profile the salinity can be substantially higher but water content is higher and fluctuates less due to lower root activity.Other experimental and modeling studies have shown that the sensitivity of plants to salinity depends on the evaporative demand.When multiple stresses occur simultaneously, the dominant stress largely controls crop growth and response.Likewise, release of the most dominant stress will promote the most growth.The root’s developmental response to a combination of variable stresses is remarkable , yet there is considerable uncertainty how the plant integrates multiple stresses over space and time and it remains a huge knowledge gap.More research is needed to better understand the physiological mechanisms underlying plant water relations and shoot ion regulation in plants under heterogenous salinities and how roots can adapt over the growing season with changing soil conditions.While there will likely be complex interactions, it is nonetheless an important area of future research.Summary: Crop salt tolerance data are urgently needed for micro-irrigated crops, rather than using historical information developed for surface irrigation.Though of tremendous value in the past, soil saturation extracts do not necessarily represent in-situ root zone salinity.In addition, there is considerable uncertainty how the plant integrates multiple stresses across the rooting zone and during its growing season and it is a huge knowledge gap.As new cost-effective sensor technologies are being developed, they may be applied across field trials, thereby much better representing real-time and in situ information on the plant’s response to soil salinity, together with other relevant abiotic and biotic soil and plant measurements.Drought and salinity are the two most common abiotic stressors in agricultural crops and their simultaneous occurrence is relatively common in irrigated fields.In addition, the use of saline or brackish water or the reuse of treated effluents for irrigation is expanding , particularly in arid and semi-arid regions with an increased pressure on water resources.Although the proper understanding of crop response to combined water and salinity stressors is a key question for hydrological and crop modeling, little is known about how the combination of these two stressors affects plant health and crop development, transpiration, dry matter accumulation and yield.In this section we will focus primarily on plant root water uptake, whereas the associated impacts of soil salinity on crop yield is treated in Section 10.Plant root water uptake is controlled by potential gradients across the soil-root interface, and is generally described by a Darcy-type flow equation, with flow into the roots driven by a combination of matric potential and osmotic potential gradients , multiplied by a conductance coefficient.Water fluxes into the plant root will reduce both because of decreasing potential gradients, for example due to salt accumulation in the rhizosphere, and by decreasing soil and plant conductance, for example, as the soil dries.At low soil matric potentials, Gardnerand Cowan showed that water uptake is reduced by a local drop in soil hydraulic conductivity at high water potential gradients.