AGRICULTURAL SERVICE LABORATORY
GUIDELINES1 FOR INTERPRETATION OF WATER QUALITY FOR IRRIGATION
Irrigation Problem Degree of Problem
No Problem Increasing Problem Severe Problem
TDS (mg/L) 480 480-1920 1920
Specific Ion Toxicity
SAR - sodium toxicity - See "*" at the bottom of the analysis report
Chloride (mg/L) <142 142-355 >355
Boron (mg/L)3 <0.75 0.75-2.0 >2.0
HCO3 (meq/L)2 <1.5 1.5-8.5 >8.5
pH2 - Normal range 6.5-8.4
Abbreviations and Symbols:
EC= Electrical Conductivity
TDS = Total Dissolved Salts
SAR = Sodium Adsorption Ratio - A ratio of the sodium vs. calcium and magnesium in the irrigation water
NO3-N = Nitrate nitrogen
mmhos/cm = millimhos per centimeter
mg/L = milligrams per liter - also the same as ppm or parts per million
<= less than
>= greater than
1These guidelines are a management tool developed to assist in the preliminary evaluation of the suitability of a water supply for irrigation. The user must guard against drawing unwarranted conclusions based strictly on laboratory results. Laboratory data should be related to field conditions or tested and confirmed by field trials and experience.
2The error in these values increases with an increase in temperature and transport time from sampling to analysis.
3The value for Boron may be erroneously high if the sample was transported in a glass container.
These guidelines are based on information from: Ayers, R. S. and D. W. Westcot, 1976.
Water Quality for Agriculture. Food and Agriculture Organization of the UN, Irrigation and
Drainage Paper 29, Rome.
SALINITY PROBLEM EVALUATION
EC and TDS
The presence or absence of a potential salinity problem is evaluated from the electrical conductivity of the irrigation water (EC) and by the Total Dissolved Salts (TDS). These two values by themselves are usually an adequate measure of a potential salinity problem.
SPECIFIC ION TOXICITY
A toxicity problem is different from a salinity problem in that a toxicity occurs within the crop itself as a result of the uptake and accumulation of certain constituents from the irrigation water and may occur even when the salinity is low. The toxic constituents of concern are sodium, chloride, and boron. They can reduce yields and cause crop failure. Not all crops are equally sensitive but most tree crops and other woody perennial-type plants are sensitive. Toxicity problems of sodium and chloride, however, can occur with almost any crop if concentrations are high enough. Toxicity problems often accompany and are a complicating part of a salinity problem. Sprinkler irrigation may cause special toxicity problems due to sodium and chloride being absorbed through the leaves.
SAR - Sodium Toxicity
Most tree crops such as deciduous fruits and nuts and other woody-type perennial plants are particularly sensitive to low concentrations of sodium. Most annual crops, with some exceptions, are not as sensitive but may be affected by higher concentrations.
Use of irrigation water high in sodium will usually result in a soil high in sodium but it may take several irrigations to cause the change. The crop takes up sodium with the water and it is concentrated in the leaves as water is lost by transpiration. Damage (toxicity) can result if sodium accumulates to concentrations that exceed the tolerance of the crop. Leaf burn, scorch, and dead tissue along the outside edges of leaves are typical symptoms.
Sodium toxicity is often modified and reduced if calcium and magnesium are also present. Moderate amounts of calcium and magnesium may reduce sodium damage and higher amounts even prevent it. Since the effect of sodium is dependent on the sodium, calcium and magnesium concentrations, a reasonable evaluation of the potential toxicity is possible using the adjusted sodium adsorption ratio (SAR), which is a ratio of the sodium vs. calcium and magnesium in the irrigation water.
The symptoms of sodium toxicity occur first on the oldest leaves since a period of time (days or weeks) is normally required before accumulation reaches toxic concentration. Symptoms usually appear as a burn or drying of tissue at the outer edges of the leaf and as severity increases, the symptoms progress inward between the veins towards the center of the leaf.
Most tree crops and other woody perennial plants are sensitive to low concentrations of chloride while most annual crops are not, though less sensitive crops may be affected at higher concentration. Chloride is not adsorbed by soils but moves readily with the soil water. It is taken up by the roots and moves upward to accumulate in the leaves. The toxicity symptom for chloride is a leaf burn or drying of leaf tissues which typically occurs first at the extreme leaf tip of older leaves and progresses back along the edges as severity increases. Excessive leaf burn is often accompanied by abnormal early leaf drop and defoliation.
Boron is one of the essential elements for plant growth but is needed in relatively small amounts. If excessive, boron becomes toxic. The sensitivity to boron appears to affect a wide variety of crops while sodium and chloride toxicities are most common with tree crops and woody perennials.
Boron is taken up by the crop and is accumulated in the leaves and other parts of the plant. Toxicity symptoms typically show first on older leaf tips and edges as either a yellowing, spotting, or drying of leaf tissues (or these in combination). The yellowing or spotting in some cases is followed by drying which progresses from near the tip along the leaf edges and toward the center between the veins (interveinal). A gummosis or exudate on limbs or trunk is also sometimes very noticeable on seriously affected trees such as almonds.
Many sensitive crops show toxicity symptoms when boron concentrations in leaf blades exceed 250 to 300 ppm (dry weight). Some crops, however, are sensitive but do not accumulate boron in leaf blades. Stonefruits (peaches, plums, almonds, etc.), and pome fruits (pear, apple, and others) even though being damaged by boron, may not accumulate boron in leaf tissue to the extent that leaf analysis is a reliable test.
Nitrogen in the irrigation water acts the same as fertilizer nitrogen and excesses will cause problems just as fertilizer excesses cause problems. Production of nitrogen sensitive crops may be affected at nitrogen concentrations above 5 ml/L (5 ppm). Sugar beets, for example, under excessive nitrogen fertilization grow to a large size but with low purity and low sugar content and the amount of sugar produced per hectare may actually be reduced. Grapevines, in some instances, grow too vigorously and yields are reduced, or the grapes are late in maturing. Maturity of stonefruits may also be delayed and the fruit may be poorer in quality. For many grasses and grain crops, lodging may result from excessive vegetative growth.
At more than 30 mg/L (30 ppm), severe problems are expected with nitrogen sensitive crops. For crops not sensitive, more than 30 mg/L nitrogen may be adequate for high crop production and little or no fertilizer nitrogen may be needed. Less than 5 mg/L nitrogen has little effect even for the nitrogen sensitive crops. However, algae and aquatic plants in streams, lakes, ponds, and canals are often affected. When temperature, sunlight, and other nutrients are optimum, very rapid growth or algae blooms can occur. This excessive growth may result in plugged pipelines, sprinklers, and valves.
Bicarbonate, even at very low concentrations, has been a problem primarily when fruit crops or nursery crops are sprinkler irrigated during periods of very low humidity (RH < 30%) and high evaporation. Under these conditions, white deposits are formed on fruit or leaves, which are not washed off by later irrigation. The deposit reduces the marketability of fruit and nursery plants.
A toxicity is not involved but as the water on the leaves partially or completely evaporates between rotations of the sprinkler, the salts are concentrated and CO2 is lost to the atmosphere. If the concentration effect and CO2 loss is great enough the less soluble constituents in the water, such as lime (CaCO3), will precipitate and deposit on fruit and leaves.
pH is a measure of the acidity or alkalinity of water. It is of interest as an indicator but is seldom of any real importance by itself. The main use of pH is a quick evaluation of the possibility that the water may be abnormal. If an abnormal value is found, this should be a warning that the water needs further evaluation. The pH scale ranges from 1 to 14, with pH 1 to 7 being acid, 7 to 14 being alkaline, and pH 7.0 being neutral. A change in pH, as from pH 7 to pH 8, represents a 10-fold decrease in acidity or a 10-fold increase in alkalinity. The normal range for irrigation water is from pH 6.5 to pH 8.4. Within this range, crops have done well. Irrigation waters having pH values outside this range may still be satisfactory but other problems of nutrition or toxicity become suspect.