Crop Response to Soil and Water Salinity

The tangible effects of climate change are bringing to the fore the need for the world’s most fragile areas affected by salinity and water scarcity to find ways to mitigate the effects of salinity and other abiotic stresses. Moreover, the demand for food will grow in the next decades, spurring the cultivation of marginal areas affected by primary or secondary salinity. The factors that determine salinity are both natural (primary salinity) and anthropogenic (secondary salinization) [1].

According to the last FAO report [2], the area of salt-affected soil is estimated to be 1 381 million ha (Mha), or 10.7 percent of the total global land area.

In the most affected countries, stress can lead to yield losses of up to 70% of crops. Ten percent of irrigated cropland and ten percent of non-irrigated cropland are affected by salinity. Models of aridity trends based on temperature growth indicate that the affected area could increase to between 24% and 32% of the total land area, the vast majority of which is in developing countries, and, today, 10 countries (Afghanistan, Australia, Argentina, China, Kazakhstan, Russia, the United States, Iran, Sudan, and Uzbekistan) account for 70% of the world’s salt-contaminated land.

Under salinity stress, plants respond with a decrease in crop production as a consequence of adverse effects on germination, growth, and reproduction. Plants activate physiological, biochemical, and molecular mechanisms under salinity stress to cope with salt stress. Salinity modifies soil’s hydraulic conductivity, affecting water movement and retention. Soils affected by salinity undergo a reduction in total porosity with the loss of larger pores and an increase in smaller pores. Some agronomic practices can mitigate salinity stress, such as leaching, the use of salt-tolerant plants or genotypes, irrigation management improvement, the use of mulching to mitigate soil evaporation, and controlling the water osmotic effects by avoiding extreme soil dry condition of the root zone.

In these scenarios, it is crucial to mitigate the impact of soil and water salinity on crop production and the fragile environment. We must shift towards sustainable agriculture by merging the preservation of fragile ecosystems. We foresee something very useful emerging from this Special Issue: a new toolkit to make this process work, namely, sustainable agriculture in a saline environment. We can look at water, soil, and bio-agents for solutions. Salt leaching becomes feasible if high-quality water is available. If not, a viable option is salt-tolerant species, such as halophytes for non-food use. Reducing tillage and the recovery of waste material can help improve saline soils. Saline environments, with their spontaneous flora capable of tolerating high salt concentrations, are a source of potential tools for our toolkit. Rhizosphere microorganisms such as plant-growth-promoting bacteria (PGPB) and arbuscular mycorrhizal fungi (AMF) have documented potential as beneficial agents for increasing plants’ adaptability to stress. To get the most out of our toolkit, we need to start with a broad knowledge of the territory in order to use the right toolset in the right place.

This goal can be achieved by gaining a profound knowledge of soil–plant–microorganism–atmosphere interactions and by using eco-friendly agronomic practices.

For this Special Issue, we invited authors to submit articles addressing the sustainability of saline land by improving or proposing innovative irrigation technology and management schemes. We also encouraged authors to submit work on the use of alternative crops, e.g., halophytes that better tolerate saline environments. Finally, we promoted the submission of studies on soil–plant–rhizosphere interactions, with a focus on increasing the adaptability of crops under salinity and water scarcity conditions. This volume contains 13 original research articles.

One of the presented articles helps highlight all the aspects related to salinity, as it is a review related to the types of salinity, salinity’s effects on soil and plants, and the potential water, environmental, and mitigation management solutions for salinity

Three of the submitted articles propose solutions for irrigation management on saline soils or with waters of different qualities when good water resources are scarce.

One contribution shows that it is possible to irrigate halophyte crops grown on non-sodic soils by using concentrated wastewater produced by reverse osmosis. Their conclusions underscore the sustainability of such irrigation practices when applied to halophyte forage plants (Atriplex) and how they do not compromise the environment. Moreover, this study suggests applying leaching irrigation with high-quality water when feasible to mitigate environmental impacts and preserve soil quality.

Another contribution documents the impact of irrigation with water of different sodic compositions, i.e., NaCl or NaHCO₃, on a calcareous clay soil. The factors that must be considered in the management of such soils with these types of water interact with each other; i.e., pH, SAR, cations, and anions are released into soil solutions when applying the above types of water. The authors conclude that physical and chemical soil degradation can be limited to a certain extent when using sodic water to balance the deficit of irrigation water. They emphasized that site-specific conditions associated with soil properties and water quality must be considered.

The importance of quantifying the response of a crop to salinity and water stress is well known in the scientific community. One study highlighted the usefulness of a better comprehension of the macroscopic root water uptake model under saline conditions. Such knowledge was applied to model irrigation and salt/sodic reclamation strategies in saline agriculture [3] to evaluate plant root water uptake under various water deficits. The sink term in the Richards equation for drought and salinity stress was determined via a garlic crop pot experiment under controlled conditions. The authors demonstrated the good accuracy and cost-effectiveness of this simplified method. The values found can be used in automated irrigation systems as threshold values at which to start irrigation based on continuous measurements of soil water potential.

Two articles deal with classical germination tests in order to evaluate the tolerance to salinity of some plants in the germination phase. A germination test was performed on two invasive varieties of amaranth. The germination test was followed by the growth of the seedlings in pots in a controlled environment. The performance of the two varieties was different, but both appeared to tolerate moderate NaCl concentrations. In particular, one performed better in the germination phase, while the other exhibited better performance in the growth stage. Due to their presence in several regions as an invasive species and their remarkable salt tolerance, amaranth could be introduced as a vegetable crop in saline environments.

Several priming agents were used to evaluate the salt tolerance at germination of a pepper crop. Hydropriming, proline priming, and salicylic acid were tested. The two cultivars of pepper responded with greater germination when primed with proline and salicylic acid. In contrast, the Herkules cultivar had a higher salt tolerance when proline was used as a priming agent. Priming with proline and salicylic acid at the seed stage led to better performance in the germination and seedling stages.

In a saline area in coastal India, the potential of tillage management was evaluated. Sustainable practices, i.e., zero tillage planting, crop residue recycling, and crop rotations, increased yield with less input. This crop management strategy reduced soil salinity, accumulated soil organic carbon, reduced the water footprint, and was more profitable than tillage-intensive conventional practices.

The remaining articles can be classified into two groups: a group in which the application of amendment agents mitigated salinity, and another group in which the relationships between growth-promoting bacteria and/or mycorrhiza fungi were studied. In the latter, the bacteria/fungi interacted with the plant, improving water uptake, reducing ion toxicity, and inducing its resilience to osmotic and oxidative stress, thereby enhancing crop performance and productivity.

In the first group, the application of selenium via foliar spraying on a bean crop in saline soil improved plant water status, growth, and yield. The use of selenium is a potential method of improving the drought stress tolerance of plants grown in saline soils.

Another contribution concerned the use of different organic and inorganic amendments mixed with Technosol (Technosols consist of engineered soils specifically designed for a specific environmental problem), gossan waste from the São Domingos mine, and salic Fluvisol (Eutric) from the Sobralinho salt marsh area to remediate saline soil or mining-waste soil. This study also evaluated whether mixtures of such amendments could be useful to improve marginal soils in order to grow energy crops or for livestock grazing. The Technosol/fluviosol combination reduced salinity by 65% and exchangeable sodium by 60% compared to the treatment with fluvisol only. In the soil treated with gossan waste, high concentrations of heavy metals were observed. The treatment with Technosol/gossan (TG), and Technosol/Fluvisol (TVF) yielded high values of Cu in pasture plants.

One submission concerned a pot experiment conducted on saline soil to evaluate an amendment of Technosol in combination with the inoculation of PGPB and AMF to support the growth of halophyte Limonium algarvense. The hypothesis was that this treatment could increase plants’ growth in the vegetative and reproductive stages when irrigated with estuarine water. Technosol has been proven useful for improving soil characteristics and increasing plant vitality in the long term. AMF and PGPB help further improve plant survival, with better reproductive and vegetative development. The crucial point of this and the other studies related to PGPB and AMF is that the beneficial effects observed under controlled conditions and in pot experiments should be validated through field trials, thereby providing a better understanding of the effective costs of these promising treatments.

The articles in the second group deal with the use of bacterial strains to improve the performance of plants under salt stress conditions. In this group, the first contribution was on the effect of rhizobia on improving the tolerance of Medicago plants to salt stress. Significant differences in the responses of different cultivars were observed. This study contributes further evidence to the specificity of microorganism–plant associations and their influence on the local environment.

This study evaluated the inoculation of a bacillus strain to improve rice tolerance to salt stress when irrigated with saline water and to increase the nutritional supply of zinc to rice. The experiment clearly demonstrated the bacillus improved salt tolerance but did not provide evidence of improved zinc nutrition.

The importance of the symbiosis of rhizomes and legumes for nitrogen fixation and phosphorus mobilization has been documented. The latter can be further improved through the presence of PGPB to increase adaptability to salt stress. This study further stressed the potential of the legume–rhizobium–PGPB partnership for sustainable agriculture.

We would like to thank all the contributing authors in this Special Issue on “crop response to soil salinity stress” and all the reviewers who dedicated their time and constructive efforts to improving the quality of science during the review process.