The reliance on freshwater for agriculture is a growing global concern, especially as water scarcity becomes widespread. The planet has an abundance of water in its oceans, yet this massive supply remains largely unusable for growing the crops that feed humanity. The fundamental barrier preventing the use of seawater for large-scale irrigation is its high concentration of dissolved salts. Applying ocean water directly to most crops causes immediate and cumulative harm, both to the plants and to the soil itself.
The Concentration Problem: Why Seawater is Too Salty
The difference between suitable irrigation water and seawater is one of magnitude. Acceptable water for irrigating most conventional crops generally has a total dissolved solids (TDS) level below 500 to 700 parts per million (ppm). Water with a TDS over 5,120 ppm is considered too saline for irrigation, even for highly tolerant crops.
Seawater, by contrast, has an average salinity of about 35,000 ppm, or 3.5% salt by weight. This concentration is approximately 50 to 70 times higher than the limit for safe agricultural use. While sodium chloride accounts for about 85% of the dissolved minerals, seawater also contains high levels of other ions. These include magnesium, sulfate, calcium, and potassium, all contributing to the salinity hazard for plants.
How Salt Disrupts Plant Biology
High salt concentrations harm crops through two primary and interconnected mechanisms: osmotic stress and ion toxicity. Osmosis is the passive movement of water across a semipermeable membrane from an area of low solute concentration to an area of high solute concentration. Plant roots normally absorb water because the salt concentration inside the root cells is slightly higher than the concentration in the surrounding soil.
When a plant is exposed to highly saline water, the external salt concentration in the soil becomes much higher than the concentration inside the root cells. This reverses the natural osmotic gradient, causing water to be drawn out of the plant roots and back into the soil. The result is physiological drought, where the plant experiences severe water stress and wilting, even though the soil is saturated with water.
Beyond this water stress, plants also absorb the individual salt ions, leading to toxicity. Sodium and chloride ions are taken up by the roots and then transported and accumulated in the leaves. High concentrations of these ions disrupt the plant’s cellular machinery, interfering with essential functions like photosynthesis and enzyme activity.
This accumulation of toxic ions in the leaves can cause cell death, visibly manifesting as leaf burn or necrosis, starting at the tips and margins. Over time, this damage reduces the plant’s ability to produce energy, stunts growth, and ultimately leads to crop failure.
The Impact on Agricultural Land
The use of saline water for irrigation causes long-term, structural damage to the agricultural land itself. When the irrigation water evaporates, the dissolved salts remain behind in the soil, leading to a process called salinization. The continuous buildup of these salts over successive growing seasons can render once-fertile land unproductive.
A related problem is sodification, which occurs when a high proportion of the remaining salts is sodium. Sodium ions displace other beneficial ions like calcium and magnesium, causing the soil particles to disperse. This dispersion destroys the soil’s natural structure, leading to a loss of permeability and aeration.
Sodified soil loses its ability to infiltrate water effectively, becoming dense and hard, which restricts root growth and reduces available water. The surface often forms a hard crust, impeding water absorption and seedling emergence. Reversing this degradation is extremely difficult and requires costly management practices, such as applying soil amendments like gypsum.
Pathways to Using Saline Water
While direct irrigation with seawater is infeasible, researchers are exploring several pathways to utilize saline water sources. Desalination technology, which removes salt from water, is technologically proven but remains an impractical solution for most agriculture. The process requires massive energy and has an extremely high operating cost, making the resulting freshwater too expensive for growing low-value food crops.
A more biologically focused approach involves developing crops that can tolerate higher salinity levels. This includes studying naturally salt-tolerant plants, known as halophytes, and using selective breeding or genetic engineering to transfer tolerance mechanisms into conventional food crops. Progress is slow, but it offers the potential for crops that can thrive in brackish water environments.
Specialized irrigation methods also help manage salinity by minimizing the salt’s immediate impact on the plant. Drip irrigation, for instance, delivers water directly to the root zone. This reduces surface evaporation that concentrates salts and minimizes contact between the saline water and the plant’s leaves. These methods, combined with careful water management, allow for the use of slightly brackish water that would otherwise be too damaging.