Because water is a complex mixture of both organic and inorganic chemicals, there can be a large variation in your water’s general properties. One such general property is the “sodium adsorption ratio” (SAR). While high SAR is not ultimately an issue of human health, it does have some other consequences–namely when it comes to your garden.

Sodium adsorption ratio (SAR) is an important irrigation water quality parameter. It is a measure of the amount of sodium relative to the amount of calcium and magnesium in a water sample. Specifically, SAR is the ratio of the sodium concentration divided by the square root of half the sum of the calcium and magnesium concentration:

Note that the resulting solution to the SAR equation is unitless (because it is a ratio).

SAR indicates the suitability of water for use in agricultural irrigation. High levels of sodium ions in water affect the permeability of soil and can lead to water infiltration issues. While the impact severity of high SAR water depends on many specific soil quality factors (such as soil type, texture, drainage capacity, etc), typically the higher the SAR, the less suitable the water is for irrigation.

If your water has a high SAR, that generally means sodium in your water will cause hardening and compaction of your soil. This will reduce infiltration rates of both water and air. Additionally, the increased salinity reduces the availability of water in storage which can be very important for a plant’s growth and resilience (especially if you’re one who forgets to water sometimes).

Aside from decreased water infiltration and availability, high SAR may also lead to temporary over-saturation of surface soil, high pH, soil erosion, inadequate nutrient availability, and increased risk of plant diseases.

Typically, water with a SAR below 3 is fine for irrigating grass and other common ornamental landscape plants. Alternatively, water that has a SAR greater than 9 may cause significant permeability problems. This is even more true if you have fine-textured soil (like a silty-loam).

Coarse-textured (i.e sandy) soils, on the other hand, are typically less susceptible to permeability problems and SAR-related issues. For those types of soils, the irrigation water's SAR can be slightly higher with no major impact.

While high SAR water can be used for irrigation purposes, soil may require amendments to prevent long-term damage.

An amendment, such as gypsum, when added to soil or water will increase the calcium concentration in the water. Subsequently, this reduces the SAR as the sodium to calcium ratio is also reduced.

However, it’s important to note that if infiltration is poor due to adverse soil texture, compaction, or a high water table–gypsum won’t lead to any major improvements.

Alternatively, you can also install a whole-home reverse osmosis system–although it is a pricier option. We recommend testing your water first in order to ensure that you make the right choice (and don’t spend money unnecessarily).