Sea Salinity: Ocean Salinity, Soil Salinity & Management

Pour a glass of seawater and it looks like any other water, yet roughly 3.5 percent of its weight is dissolved salt (NOAA National Ocean Service, U.S. ocean agency) — a chemistry that drives ocean currents, shapes marine life, and separates drinkable water from water that can ruin a crop. Understanding sea salinity means knowing how it is measured, which seas are saltiest, and why the same salt that flavors the ocean can quietly degrade farmland.

Average ocean salinity: 35 parts per thousand (‰), or 35 grams per kilogram · Highest surface salinity: Atlantic Ocean, ~36.5‰ · Dead Sea salinity: ~340‰, up to 10× the ocean average · Drinkable water threshold: below 500 mg/L total dissolved solids (≈0.5‰) · Salinity unit: practical salinity unit (PSU) or ‰ · Major dissolved salt: sodium chloride, ~85% of total salts

Five numbers, one pattern: open-ocean readings cluster near 35‰, enclosed seas swing to extremes, and farmland operates on a completely different, far smaller scale.

What is salinity?

Salinity is the total amount of dissolved salts in water, and in the ocean that mix is dominated by sodium chloride. No single probe weighs the salt directly; every method measures something related — conductivity, evaporation residue, or light refraction — and converts it to a salinity value.

How is salinity measured?

• Oceanographers measure conductivity, because dissolved salts make seawater a better electrical conductor; PSU values come from that indirect reading, according to the FAO Soils Portal (UN food and agriculture agency).
• Soil scientists use the same conductivity principle on a water extract taken from the ground and report results in deciSiemens per meter (dS/m) (Utah State University Extension, agricultural research unit).

There is no practical way to weigh the salt in a moving ocean, so indirect methods dominate. The trade-off is that “how salty” depends on which probe and which scale you are reading.

What units are used for salinity?

• ‰ (parts per thousand) — grams of dissolved salt per kilogram of seawater; the classic public-facing unit.
• PSU (practical salinity unit) — a conductivity-based scale, not a direct mass measurement, according to the FAO Soils Portal (UN food and agriculture agency).
• dS/m (deciSiemens per meter) — the standard unit for soil salinity assessment (Utah State University Extension, agricultural research unit).

The two most common mistakes in salinity reporting are treating PSU as a weight measurement and mixing up ocean units with soil units. The difference matters: a soil at 4 dS/m is already “slightly saline” in Utah State University Extension’s classification, while open-ocean seawater holds far more salt than any soil reading.

The pattern: salinity is not one number but a family of measurements, and each answers a different question — about ocean currents, about drinking water, or about the next planting season.

Which ocean has the highest surface salinity?

The Atlantic Ocean takes that title. Its surface runs measurably saltier than the Pacific or Indian Oceans because warm subtropical skies pull water vapor out of the surface faster than rainfall and river inflow replace it.

Salinity of the Atlantic Ocean

• Average surface salinity is about 36.5‰, the highest among the major ocean basins (NOAA National Ocean Service, U.S. ocean agency).
• The subtropical Atlantic is an evaporation-dominated zone: water leaves as vapor, salts stay behind (NOAA National Ocean Service, U.S. ocean agency).

Salinity of the Pacific Ocean

• The Pacific averages about 34.5‰ — lower than the Atlantic, in large part because its precipitation and runoff balance differs across the basin (NOAA National Ocean Service, U.S. ocean agency).

Salinity of the Indian Ocean

• The Indian Ocean shows sharper seasonal swings: monsoon rains pour freshwater over parts of the basin and temporarily dilute the surface, which is exactly the evaporation-precipitation effect NOAA National Ocean Service describes.

Six bodies of water, one pattern: the more enclosed the basin and the hotter the evaporation, the saltier the water — unless rivers dilute it first.

The implication for anyone reading a salinity map: “sea level” is not one number. A crew crossing the Atlantic, a swimmer in the Baltic, and a farmer drawing brackish irrigation water are all looking at completely different salt loads.

What are the factors that influence the salinity of the ocean?

Four main forces move salt into and out of seawater: evaporation, precipitation, freshwater inflow from rivers, and the freezing and melting of sea ice. NOAA National Ocean Service (U.S. ocean agency) groups them under the same logic — remove water and salts concentrate, add water and they dilute (NOAA National Ocean Service, U.S. ocean agency).

Evaporation and precipitation

• Evaporation removes water vapor but leaves dissolved salts behind, so surface salinity climbs in hot, dry zones (NOAA National Ocean Service, U.S. ocean agency).
• Rainfall adds freshwater to the surface layer and lowers salinity; the same happens where large rivers pour into the sea.

Ocean currents and mixing

• Saltier water is denser, and those density differences help drive the thermohaline circulation — the global conveyor belt of deep and surface currents (NOAA National Ocean Service, U.S. ocean agency).
• Currents also redistribute salt between basins, which is why local salinity cannot be predicted from local weather alone.

River runoff and ice melt

• Rivers and meltwater add freshwater, lowering salinity near coasts, estuaries, and ice margins (NOAA National Ocean Service, U.S. ocean agency).
• Sea-ice formation does the reverse: ice excludes salt, so the water around new ice becomes saltier.

The same four forces explain the extremes on a world map. The Atlantic is saltier than the Pacific because its evaporation balance leans that way; the Baltic stays brackish because rivers pour in faster than evaporation can concentrate them.

Why this matters: ocean salinity is a map, not a single value, and scientists treat its shifts as signals of changing rainfall, evaporation, and ice melt across the planet.

What is the major source of sea salinity?

The ocean’s salt came from the land. Over millions of years, rain weathered continental rocks and rivers carried the dissolved ions — mostly sodium and chloride — into the sea, in the same weathering-and-runoff cycle described in FAO guidance on water quality for agriculture.

Chemical weathering of rocks

• Rainfall slowly dissolves minerals in continental rocks, releasing sodium and chloride ions into streams and rivers (FAO Water Quality for Agriculture, UN guidance).
• Rivers then deliver those dissolved salts to the sea, and sodium chloride accounts for roughly 85 percent of the total salts dissolved in seawater (FAO Water Quality for Agriculture, UN guidance).

Hydrothermal vents

• Deep-sea vents release mineral-rich fluids at mid-ocean ridges and add some salts to the ocean.
• Their contribution is minor compared with continental weathering, which remains the dominant long-term salt source.

Atmospheric deposition

• Sea-salt aerosols travel through the atmosphere and return to the ocean in rain, creating a loop that moves salt from sea to land and back.

The takeaway: today’s ocean salt is mostly ancient terrestrial rock, dissolved by rain and carried by rivers — which is why the same water cycle that feeds crops also keeps the sea salty.

What are the different types of salinity?

Salinity appears in several forms, and the labels matter: natural or human-caused, marine or soil-based, and an extreme range of regional readings that look like exceptions until you trace the cause.

Primary vs secondary salinity

• Primary salinity is natural: it develops from geology, climate, and groundwater patterns without human help, and it explains why parts of Australia have extensive naturally saline soils (FAO Soils Portal, UN food and agriculture agency).
• Secondary salinity is human-driven, most often from irrigation that raises water tables and leaves salts behind as water evaporates.
• The FAO Soils Portal classifies salt-affected soils as saline, saline-sodic, or sodic, depending on salt quantity, salt type, sodium content, and soil alkalinity (FAO Soils Portal, UN food and agriculture agency).

Ocean salinity vs soil salinity

• Ocean salinity is measured in PSU or ‰ and sits near 35 for the open ocean.
• Soil salinity is measured as electrical conductivity in dS/m; the scale runs from nonsaline (0–2 dS/m) to strongly saline (above 16 dS/m) (Utah State University Extension, agricultural research unit).

The gap between the two environments is enormous. Typical seawater at 35‰ is roughly 70 times saltier than the 500 mg/L threshold usually treated as the upper limit for fresh drinking water (FAO Water Quality for Agriculture, UN guidance).

Regional sea salinity examples

• Dead Sea: roughly 340‰; Red Sea: about 40‰; Mediterranean: about 38‰ — all three exceed the open-ocean average because evaporation concentrates salts (NOAA National Ocean Service, U.S. ocean agency).
• Baltic Sea: as low as about 7‰, kept brackish by heavy river inflow (NOAA National Ocean Service, U.S. ocean agency).

The pattern worth remembering: “salinity” is a spectrum, and the label changes depending on whether you are standing on a ship, a riverbank, or a farm.

What is a good salinity level for soil and how is it managed?

For most crops, the target zone is 0–2 dS/m. Utah State University Extension classifies soil extracts in that range as nonsaline and suitable for most crops, and it treats readings above 16 dS/m as strongly saline and generally unsuitable for crops (Utah State University Extension, agricultural research unit).

What is a good salinity level for soil?

• 0–2 dS/m: nonsaline, suitable for most crops.
• 2–4 dS/m: very slightly saline.
• 4–6 dS/m: slightly saline.

Six classes, one boundary worth memorizing: the climb from 2 to 16 dS/m is the difference between healthy yields and fields that will not grow most crops (Utah State University Extension, agricultural research unit).

What are 5 methods to manage salinity soils?

• Leaching: apply water beyond the crop’s evapotranspiration need so a net amount passes through the root zone (FAO Soils Portal, UN food and agriculture agency).
• Drainage: install or maintain drainage so leached salts actually leave the field; FAO water-quality guidance is blunt that leaching alone does not fix the problem (FAO Water Quality for Agriculture, UN guidance).
• Salt-tolerant crops: choose species and varieties that hold up at higher EC levels.
• Organic amendments: improve soil structure and help water move through the root zone.
• Gypsum: a practical tool on sodic soils, where sodium needs to be displaced before salts can be leached away.

The order matters as much as the methods. Drainage comes first; leaching then flushes the salt somewhere it will not return; crops and amendments buy resilience while the system stabilizes.

What reduces soil salinity?

• Rainfall and good-quality irrigation water dilute the salt in the root zone.
• Well-drained soils allow the diluted salt to move downward and out of the root zone.
• Salt-tolerant plants keep the ground covered and draw down shallow water tables.

The catch is timing. Salt control is not a one-off fix: every irrigation event adds a little more salt, so monitoring and drainage have to operate every season, not just during a crisis.

Which crop is most tolerant to salinity?

Barley sits at the top of the salt-tolerance list among grain crops. It can keep producing on soils that push most other cereals out of the field, which makes it the default answer for growers facing saline paddocks.

Salt-tolerant crops: barley, cotton, sugar beet

• Barley is among the most salt-tolerant cereal crops, with reported tolerance up to 10–12 dS/m.
• Cotton and sugar beet also rank high on salt-tolerance lists compared with other commodity crops.

By comparison, Utah State University Extension classifies soil above 16 dS/m as strongly saline and generally unsuitable for crops (Utah State University Extension, agricultural research unit).

What happens if soil salinity is too high?

• High salt pulls water against the plant: roots have to work harder to take up moisture, and nutrient balance is disturbed.
• Most crops begin to lose yield above roughly 4 dS/m (Utah State University Extension, agricultural research unit).
• Yield loss gets severe in the 8–16 dS/m range, which the same classification describes as generally unsuitable for crops.

The consequence is slow and easy to underestimate: salt builds quietly in the root zone, leaves brown at the edges, yields shrink, and eventually a field can become unprofitable. In Australia, where naturally saline soils are widespread, choosing salt-tolerant crops is part of standard farm planning rather than a last-minute rescue.

These numbers point one direction: salinity is a management problem, not just a chemistry fact. For ocean researchers, salinity is a live signal of evaporation, rainfall, and melting ice. For growers — especially in Australia, where naturally saline soils are already widespread — the practical decision is clear: test soil electrical conductivity before planting, install drainage before salts accumulate, or watch yields shrink as the root zone turns against the crop.

Related reading: Everest Peak: Height, Cost, Deaths, and Climbing Rules Explained and Kilimanjaro Peak: Facts, Climbing Difficulty, K2 vs Everest.

Frequently asked questions