Toxicity of specific ions
Toxicity of specific ions
High levels of boron, chloride, and sodium in irrigation water are potentially harmful to plants. Boron is by far the most likely element to harm plants irrigated with reclaimed wastewater. Small amounts of boron (i.e. <0.5 mg/kg) are essential for plant growth, however, at only slightly higher concentrations (> 0.5 mg/l in irrigation water), it may become toxic to plants. Plant tolerance to boron in soils varies widely. The threshold is established based on B concentrations in soil saturation extracts; it may be as low as 0.5 mg/l for sensitive plants or greater than 16 mg/l for B tolerant plants. (For more information about B plant toxicity, see the FAO Irrigation and Drainage Paper 48, 1992, http://www.fao.org/docrep/T0667E/T0667E00.htm.) Concentrations of boron in reclaimed wastewater principally originate from household detergents and cleansing agents, and are not expected to be high enough to cause immediate harm to plants. However, boron may accumulate in the root zone through long-term use of reclaimed wastewater.
Chloride and sodium ions are major dissolved constituents of the water. In addition to their role in salinity, both chloride and sodium may be harmful to plants at high concentration (FAO Irrigation and Drainage Paper 48, 1992).
The following tables contain the boron and chloride tolerance limits for various crops.
Boron tolerance limits for agricultural crops (adapted from Maas, 1990).
1Maximum permissible concentration in soil water without yield reduction. Boron tolerances may vary, depending upon climate, soil conditions and crop varieties. 2Tolerance based on reductions in vegetative growth.
Boron tolerance limits for ornamentals (adapted from Maas, 1990).
1 Species listed in order of increasing tolerance based on appearance as well as growth reduction. 2Boron concentrations exceeding the threshold may cause leaf burn and loss of leaves.
Chloride tolerance limits for agricultural crops (adapted from Maas, 1990).
1 Cl- concentrations in saturated soil extracts samples in the rootzone. To convert Cl' concentrations to ppm, multiply threshold values by 35. To convert % yield decreases to % per ppm, divide slope values by 35. 2Less tolerant during emergence and seedling stage. 3Values for paddy rice refer to the Cl" concentration in the soil water during the flooded growing conditions.
Chloride tolerance limits for some fruit cultivars and rootstocks (adapted from Maas, 1990).
1For some crops, these concentrations may exceed the osmotic threshold and cause some yield reduction. 2Data from Australia indicate that rough lemon is more sensitive to Cl" than sweet orange. 3Data available for one variety of each species only.