Salinity Management

Why do Salinity Management?

While salts are found in virtually all natural water sources, excessive concentrations can significantly effect irrigation practices. Some plants can tolerate more salts than others, but all plants are affected.

Salinity In Your Irrigation Water

An acre-foot of water weighs about 2,720,000 pounds; therefore, 1 ppm of salt in an acre-foot of water weights 2.72 pounds. This means that 1 acre-foot of typical San Diego county water containing only 735 ppm (ECw = 1.15 dS/M) carries one ton of salt! High levels of salt in the soil have a similar effect as droughtiness by making water less available for uptake by plant roots.

Salinity in Your Field

When you use saline water to irrigate, salts will accumulate over time in the irrigated root zone (saline water is added to the soil, the water is removed by the plants and evaporation, leaving the majority of the salts behind). Salt accumulation is largely dependent on the salt load of the irrigation water. To counteract this build-up, sufficient amounts of water need to be applied (in the form of irrigation and rain), over and above the normal needs of the crop. This extra water will pass through the root zone and carry salts with it. Applying excess irrigation water to move salts through the soil profile is known as leaching. The amount of leaching necessary is determined by 1. soil texture, 2. the amount of salt in your irrigation water and 3. the tolerance of your crop to salt.

Several approaches can be taken regarding leaching. Leaching can be accomplished by adding extra water during each irrigation session. Other methods include leaching monthly, quarterly or annually.

The Chemistry Behind It All

When salts dissolve in water, they separate into electrically charged particles known as ions. Ions exist as positively charged cations or negatively charged anions. Because of their charge, cations bind with soil particles (soil carries a negative charge), and tend to be immobile in the soil profile. Anions on the other hand, do not bind readily with soil particles and are thus very mobile.

Common cations found in irrigation water include calcium (Ca⁺⁺), magnesium (Mg⁺⁺), sodium (Na⁺) and potassium (K⁺). Calcium, magnesium and potassium are usually considered beneficial cations. Sodium is considered a non-beneficial cation. Excess amounts of sodium can destroy soil structure and severely reduce crop production. Fortunately, most water in San Diego County is high enough in both calcium and magnesium to counteract the ill effects of sodium. 

Common anions found in irrigation water include bicarbonate (HCO₃^-), carbonate (CO₃^2-), chloride (Cl^-) and sulfate (SO4^-). Of all the anions, chloride gives us the most problems here in San Diego County. It is common in both district water and most well water. High chloride concentrations can cause severe problems in both avocados and citrus. Because it is an anion, chloride moves through the soil profile readily.

Signs of Salinity

• increased soil wetness in semiarid and arid areas to the point that the soil does not support equipment
• irregular patterns of crop growth and lack of plant vigor Advanced signs:
• white crusting on the surface
• a broken ring pattern of salts adjacent to a body of water
• white spots and streaks in the soil, even where no surface crusting is visible

How To Manage Salinity

• Irrigate to maintain salts at a level below the root zone in the soil, i.e. leaching. Leaching can be done as frequently as every irrigation or as infrequently as every 6 months.
• Promote adequate infiltration and permeability by building organic matter for soil aggregation and avoiding compaction.
• Install artificial drainage systems in severely affected areas only.

Leaching Table

Pure water does not conduct electricity, but water containing dissolved solids does. ECw is used to estimate a water sample’s Total Dissolved Solid (TDS) content. ECw is expressed in dS/M. When you receive an ECw reading for your irrigation water sample, follow these calculations:

(ECw/((5*EC factor)-ECw))*100 = Leaching Requirement (%)

Let’s do an example: Your water is metered at 1.5 dS/m. You use it to irrigate avocados:

    (1.5/((5*1.3)-1.5))*100 = 30%

If you normally give trees 300 gallons per week, once a month you should leach the soil with 30% more water (90 gallons more), irrigating 390 gallons during the set.

                                                Last Updated: June 10, 2008