Dealing with salty irrigation water
by Ryan Dickson - ryan.dickson@unh.edu
This e-GRO Alert discusses the importance of water quality, sources of salt contamination and their effects on plant growth, and strategies to mitigate risks associated with “salty” irrigation water.
Figure 1. Lower leaf necrosis and burn resulting from chloride toxicity in Astilbe.
Water quality and dissolved salts
Dissolved salts are a major aspect of irrigation water quality. Dissolved salts can be plant nutrients, such as calcium and magnesium, or non-essential and potentially harmful elements such as sodium and chloride. Sources of irrigation water often contain naturally dissolved salts, where the type and concentration depends on the water source and location. For example, water from deep artesian wells often contain calcium, magnesium, and carbonate ions that dissolve in varying concentrations as a result of the natural mineralogy of the aquifer system. Surface water sources (ponds, lakes, rivers) typically have relatively low amounts of salts, and rainwater is usually one of the purest sources of water.
Figure 2. Lower leaf burn and necrosis in snapdragon cut flowers resulting from sodium toxicity.
We often measure soluble salts in irrigation water and soil solutions as electrical conductivity (EC), where high EC values equate to higher dissolved salt concentrations. Water-soluble fertilizers and mineral acids are also salts, and contribute to EC when dissolved into the irrigation water. Poor water quality is typically characterized by high concentrations of total dissolved salts (high EC values) or having high concentrations of individual elements that can be toxic, such as sodium, chlorine, and boron.
Potential issues when irrigating with “salty” water include (a) plant stress caused by salt accumulation in the growing substrate and (b) toxicity of harmful elements such as sodium and chloride.
Figure 3. Stunting and lower leaf necrosis resulting from sodium toxicity in snapdragon. Symptoms occurred in a location where the soil was dry fro poor irrigation tube uniformity.
High salts supplied by the irrigation water can build up in the growing media and cause high substrate-EC and salt stress, which can appear as dark stunted, wilting, and burning. Roots take up non-essential elements such as sodium and chloride, but these elements are not essential for plant growth and can accumulate to toxic levels in the older and more mature leaves. In addition, sodium and chloride compete for uptake with other essential plant nutrients, such as potassium and chloride, and can increase the risk of deficiency symptoms.
Salt contamination of groundwater is a major problem in certain regions. For example, aquifers near the seacoast are more susceptible to seawater intrusion, resulting in higher sodium and chloride concentrations. In Northern regions, salt used to de-ice roads during winter can seep into groundwater, spiking salt concentrations for growers during spring production. In areas of intense agricultural production, fertilizer nutrients can leach and accumulate to harmful concentrations in groundwater over time, particularly with trace elements such as boron.
Dealing with salty irrigation water
Both grower operations in our example were located in the north and near a highway, where road salt applied during the winter contaminated the groundwater with sodium and chloride in early spring. In addition, both operations were “dry” growers with very little leaching at each irrigation, allowing sodium and chloride to build up to toxic concentrations and enhancing salt stress on the roots. In-house soil testing showed that substrate-EC was within an acceptable range, but both substrate and irrigation water EC were higher than normal because of the extra salts. In these scenarios, too much sodium and chloride (i.e. toxicity) were the likely causes of burning symptoms.
The growers were able to solve the problem and salvage portions of the crop by first leaching heavily with clear (no fertilizer) irrigation water to wash away the sodium and chloride that had accumulated over time, and then reapplying water soluble fertilizer to re-establish the nutrient balance in the root zone. They continued to leach more at each irrigation and monitored irrigation water EC. Later in spring, sufficient snowmelt and rainfall replenished the aquifers, which diluted the sodium and chloride to low concentrations and dropped the water EC. Once water EC returned to normal, the growers were able to resume normal cultural practices.
If possible, the best option for dealing with poor quality irrigation water is usually to switch to a better water source. However, in some cases this may not be practical. There is no guarantee a new, or even deeper well, will always result in better quality water.
Switching to municipal water may be expensive or not be an option in some locations, and reverse osmosis and water de-ionization equipment also come with a high capital cost. Another option for dealing with poor water quality is to leach more heavily at each irrigation, preventing the accumulation of harmful salts by washing them from the root zone. Since leaching also washes away fertilizer nutrients, leaching heavily also requires increasing the fertilizer application rate. This practice is more wasteful, but in many cases is a more practical option.
A good practice is to conduct frequent in-house monitoring of irrigation water quality using a hand-held meters, where changes in water EC and sometimes pH indicate changes in water quality. In addition, work with a commercial testing laboratory to test your water at least twice a year to measure the concentrations of all essential plant nutrients, alkalinity, pH, EC, and potential contaminants such sodium (Na), chloride (Cl), and fluoride (F). Setting up an in-house substrate and water monitoring program will help you identify problems before they become too severe, allowing you to take preventative action.
Source: e-GRO