While salts are found in virtually all natural water sources, excessive concentrations can significantly affect irrigation practices. Some plants can tolerate more salts than others, but all plants can be impacted by high salt concentrations.
Salinity in Irrigation Water
An acre-foot of water weighs about 2,720,000 pounds; therefore, 1 part per million (ppm) of salt in one acre-foot of water weighs 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 drought by making water less available for uptake by plant roots.
Salinity in Your Field
When saline water is used to irrigate, salts will accumulate over time in the irrigated root zone as the water in the soil is removed by the plants and normal evaporation, leaving the majority of the salts behind. Salt accumulation is largely dependent on the salt load (or salinity) 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 push through the soil past the root zone and carry the salts down with it. Applying excess irrigation water to move salts through the soil profile and below the root zone is known as leaching. The amount of leaching necessary is determined by:
- Soil type and texture;
- The amount of salt in your irrigation water; and
- 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 on a monthly, quarterly or annual basis.
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 particles are negatively charged), and tend to be immobile in the soil profile. Anions on the other hand, do not bind readily with soil particles and are 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 yields. Fortunately, most of the imported water in San Diego County is high enough in both calcium and magnesium to counteract the negative effects of sodium.
Common anions found in irrigation water include bicarbonate (HCO3-), carbonate (CO32-), chloride (Cl-) and sulfate (SO4-). Of all the anions, chloride is the most problematic for soils and crops in San Diego County. It is found 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 High Salinity in Soil
- Increased soil wetness in semiarid and arid areas to the point that the soil does not support the weight of equipment
- Irregular patterns of crop growth
- Lack of plant health and vigor
- White crusting on the surface of the soil
- 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 visibleHow To Manage Salinity
- Irrigate to maintain salts at a level below the root zone in the soil, i.e. leaching. Leaching frequency will depend on soil and crop type and economic factors.
- Promote adequate infiltration and permeability by building organic matter for soil aggregation and avoiding soil compaction.
- Testing for the salinity in the irrigation water is highly recommended. For agricultural producers in its district, the District offers a water test on non-municipal (well and pond) irrigation water.
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 the ECw reading for the irrigation water sample is determined, the follow these calculations are used to determine the leaching requirement:
(ECw/((5*EC factor)-ECw))*100 = Leaching Requirement (%)
The water salinity was measured at 1.5 dS/m. It is used to irrigate avocados:
(1.5/((5*1.3)-1.5))*100 = 30%
If the trees are normally irrigated with 300 gallons per week, once a month the soil should be leached with 30% more water (90 gallons more), irrigating 390 gallons during the irrigation set.