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Is Soil Salinity a Threat to Our Food Security?

Soil is a vital resource for feeding the burgeoning global population which is expected to reach over 9.8 billion people by 2050. The presence of excess salts in the soil make it saline, and pose a significant threat to farm productivity, environmental health and financial welfare. Based on the FAO/UNESCO soil map of the world, the total area of saline soils is 397 million hectares (Mha) which is approximately 3.1% of the world’s land area (FAO, 2005). Moreover, future projections of climate change and human population growth suggest that the extent of saline soils will grow accordingly.

Is Salinity a Threat to our Soil?

Salt accumulation in the root zone or soil surface results in loss of soil fertility and alters the soil properties, thus harmfully impacting the soil’s environmental functions and its ability to support plant growth. 

Salinity restricts water intake and the soil water capacity limit, which causes surface runoff and erosion, leading to soil degradation. 

Soil salinization prompts the change or even disturbance of the characteristic natural biochemical and erosional properties. Therefore, elevated salinization levels would result in the loss of the available soil resources, affecting agricultural development and ecological well-being. If left unattended, this condition could develop into a socio-economic and human health problem in the long run.

What Causes Soil Salinity?

There are two major causes of soil salinity: natural (primary salinization), and human-made (secondary salinization). The occurrence of parent materials and physical or chemical weathering of minerals and seawater intrusion is the leading natural cause of soil salinization. The use of low-quality water for crop irrigation because of prolonged dry spells, in conjunction with heavy chemical fertilizers is the principal human-made practice bringing about soil salinization. These conditions are even worse where drainage is poor. Irrigated areas are more susceptible to land degradation, and over 14 km2 of fertile areas are lost per day due to soil salinization.

Dry Areas Have Their Own Issues

Raised soluble salts such as sodium and chloride in soils are a significant risk to agriculture globally, predominantly in dry areas. In these areas, salts are inclined to build up in the soil profile because of elevated evaporative conditions that raise the osmotic stress and negatively influence soil water availability to plants. A surplus of salt in the rootzone relates to a decreased capacity to extract water. This affects the plants ability to grow well and yields are reduced.

Can You See the Damage in Plants?

Different plants vary in their response to saline conditions, also within the same species. The two main mechanisms of salinity impact on plants are the osmotic and toxicity effects. The osmotic effect occurs when salt concentrations outside the root membrane exceed the in-root concentration, which limits the plants’ ability to extract water from the soil, which in turn leads to wilting. On the other side, some of the water extraction by the plant is an active action by specific water channels in the root cells. This active water extraction along with active ion extraction pumps on the root cells, increases the total ion concentration within the plant tissues. Plants have different methods to adapt to increasing ion concentrations, by storing the excess ions in specific cells or flushing the salts through the water vascular to the leaf tips. This is why in many cases the effect of salt can be seen by burnt leaf tips.

Even Microorganisms are Affected

Salinity affects microorganisms mainly by decreasing osmotic potential, which reduces their activity and alters the composition of the microbial community. These changes have implications for soil organic matter decomposition. Recent studies projected the loss of soil organic carbon (SOC) from saline soils across the globe due to a reduction of plant growth and reduction in microbial activity which will reduce the SOC inputs and increase the emission of the remaining carbon from these soils. This research suggests that saline soils can lose up to 17% of their SOC stock, which can reach up to 3.99 tons.

What Does the Future Hold?

Looking forward, the future looks salty. Recent studies show various trends in salt accumulation across the globe where some gain and some lose. Those trends threaten the capacity of agriculture to supply the future food demand for the rapidly increasing population. In addition, as a result of climate changes, water availability and soil degradation, the main cropping areas of today might not be as fertile tomorrow, while other areas would be the main global food suppliers. That would affect the entire global political and economic map as we know it today.

Drip Irrigation to the Rescue

While we can’t eliminate soil salinity and increasing land salinization, we can control and mitigate its impact through irrigation management. Under each irrigation system, a salinity develops at a specific location at the soil profile, according to water flow.  When using flood irrigation, the maximum salinity is found in deeper layers due to the wetting front, with the lowest salinity being at the surface. In addition, at the end of each irrigation cycle, the soil dries out and the salts are concentrated, adversely affecting the crop yield.  In furrow irrigation, the salt tends to accumulate on the soil ridges and affect the plant's roots immediately.

But, when using drip irrigation, water is delivered precisely to the roots of the plant in small measured doses. When water is provided in controlled quantities, it can be absorbed more efficiently, leaving less water to evaporate and form salty layer on the soil surface and preventing the accumulation of salt in the soil. With drip irrigation, salts accumulate along the edges of the expanding wet soil zone. The lowest salt concentrations are found close to the water source, while the highest are at the soil’s surface and in the center of the volume of wetted soil between two drippers. In other words, the drip pushes aside the high-constated salts out of the root zone, and provides crops access to salt-free water and favorable conditions for development.  Additionally, drip irrigation systems can be designed to deliver fertilizer directly to the root zone of the plants, reducing the overall amount of fertilizer needed and preventing further salinization of the soil. 

The bottom line? By using drip irrigation to manage soil salinity, farmers can improve crop yields and reduce their water use, making it an ideal solution for sustainable agriculture and a food secure future.