Phyto-toxic ions
The most common toxic ions present in sewerages waters are:
- Boron (B)
- Chloride (Cl)
- Sodium (Na)
Sodium and chloride are usually absorbed by the roots; When the absorption is through the leaves the rate of accumulation is bigger. The direct absorption usually occurs in sprinkler irrigation systems at high temperatures and low humidity conditions. The concentration suitability of these anions depends on the type of crop, state of growth, concentration of the toxic ion or ions and combination, climate and soil conditions.
Toxic levels of specific ions (meq/L)
|
|
Boron |
Chloride |
Sodium |
| None |
< 1 |
1 – 3 |
> 3 |
| Slight to Moderate |
< 4 |
4 – 10 |
> 10 |
| Severe |
< 3 |
3 – 9 |
> 9 |
Source: Robert Morris and Dr. Dale Devitt, "Sampling and interpretation of landscape irrigation water", University of Nevada
Boron can be toxic at very low concentration levels. Boron concentration less than 1mg/L is essential for plant development, but higher levels can cause problems in sensitive plants. Most plants exhibit toxicity problems when the concentration of boron exceeds 2mg/L (see chart below).
The main source of anthropogenic boron comes from domestic effluents (average level of 1mg/L) for the use of products such as perborate as bleaching agent (i.e. boron can be found in urban wastewater at concentration levels as high as 5mg/L in dry countries and concentrated sewage) with an average level of 1mg/L. Both soil and water testing can provide analyses to discover any constituents that might be toxic.
Relative tolerance of agricultural crops to boron
| ToleranceNote |
Concentration of boron in soil water (mg/L)Note2 |
Agricultural crop |
| Very sensitive |
<0.5 |
Blackberry |
| Sensitive |
0.5-1.0 |
Peach, cherry, plum, grape, cowpea, onion, garlic, sweet, potato, wheat, barley, sunflower, sesame, strawberry |
| Moderately sensitive |
1.0-2.0 |
Red pepper, pea, carrot, radish, potato, cucumber |
| Moderately tolerant |
2.0-4.0 |
Lettuce, cabbage, celery, turnip, oat, corn, artichoke, tobacco, mustard, squash |
| Tolerant |
4.0-6.0 |
Tomato, alfalfa, purple, parsley, sugar-beet |
| Very tolerant |
6.0-15.0 |
Asparagus |
Relative tolerance of agricultural crops to chloride ion
| Sensitivity |
Chloride (mg/L) |
Affected crop |
| Sensitive |
<178 |
Almond, apricot, plum |
| Moderately sensitive |
178-355 |
Grape, pepper, potato, tomato |
| Moderately tolerant |
355-710 |
Alfalfa, barley, corn, cucumber |
| Tolerant |
>710 |
Cauliflower, cotton, safflower, sesame, sorphum, sugar beet, sunflower |
Source: Extracted from the Australian Water Quality Guidelines for Fresh & Marine Waters (ANZECC)
Note. Tolerance will vary with climate, soil conditions and crop varieties
Note2. Maximum concentration tolerated in irrigation water without reduction in yield are approximately equal to soil water values
Trace elements
Trace elements are chemical compounds which are required, usually in tiny quantities, for the growth, development and physiology of plants and animals.
Fortunately, most irrigation supplies and sewage effluents contain low concentrations of trace elements not posing a risk for irrigation with recycled water.
However, more than 85% of the applied trace elements are likely to accumulate in the soil, and may be leached to groundwater and cause pollution. The toxicity limit will depend on the type of plant. For example, fluoride added to drinking water may be toxic at low levels for interior plants (e.g. Dracaena).
It will also depend on the type of soil. When an element is added to the soil from irrigation, it may be inactivated by chemical reactions or it may build up in the soil until it reaches a toxic level. For example some soil structures may retain these elements making them available at the root area.
The irrigation system also can affect the absorption of toxic elements by the plant. Example, sprinkler irrigation may pose a higher risk of absorption of these toxic elements through the leaves.
Phyto-toxic threshold levels of some trace elements.
Source: Extracted from the Australian Water Quality Guidelines for Fresh & Marine Waters (ANZECC)
Free Chlorine
Free chlorine (Cl2) is highly reactive and unstable in water. Therefore, a high level of residual chlorine rapidly dissipates when the water is store in tanks or reservoirs for a couple of hours. A concentration of residual free chlorine below 1mg/L is not likely to affect plant foliage.
Related pages
Bicarbonate hazard of irrigation water
Irrigation water lab analysis
Irrigation water quality
Nutrients in irrigation water
Salinity hazard irrigation
SAR hazard of irrigation water
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