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PublicationsIrrigation Water Quality Guidelines For Mississippi

Levels and specific makeup of dissolved substances in irrigation water affect crop productivity and soil structure. They also determine if water is suitable for irrigation. This publication considers several areas of concern, including salinity and toxicity and other specific items that make up water.


Sampling Irrigation Waters

Samples must represent the water supply being sampled. Water well samples should be taken after water is pumped at least 30 minutes. Collect the sample in a clean plastic or glass bottle. Wash or rinse the bottle at least three times with the water being sampled. Send the sample to

Mississippi State Chemical Laboratory
112 Hand Laboratory, Box CR
Mississippi State, MS 39762


What To Test For

How you plan to use the water determines specific testing of samples. If you will use the water to irrigate field crops or garden purposes, request the following tests:
  • Electrical conductivity (total salts), millimhos per centimeter
  • Sodium, calcium, magnesium parts per million and milliequivalents per liter
  • Bicarbonates and carbonates, parts per million and milliequivalents per liter
  • pH
  • Calculations for sodium adsorption ratio (SAR) and residual sodium carbonate (RSC)
  • Sulfate sulfur, parts per million
  • Chlorides, parts per million
If you are concerned about other dissolved substances, such as nitrates, iron, and ammonia, request these tests as well.

Effects of Poor Water Quality on Soils

If levels of calcium, magnesium, and sodium, as well as chlorides, sulfates, and bicarbonates, as a group or alone, are too high, crop growth can be hurt. High levels can even cause crop failure. Often it is associated with poor soil structure.

Crop growth reductions because of dissolved substances in the soil are similar to drought-stressed effects. An osmotic gradient on salty soils is formed. Water uptake by plant roots is increasingly restricted as the concentration of soil salts increases. Because of this, as soil salts build up in the soil, more frequent irrigation is necessary to help flush out salts and reduce water stress.

Crop species differ in their abilities to withstand salt stress. Fortunately, some of the major field crops grown in Mississippi are moderately to highly tolerant of elevated soil salts. Table 1 outlines the relative tolerances of crops to salts.

A breakdown of soil structure is a major effect of elevated sodium. Soil aggregates are bonded by calcium and magnesium. High levels of dissolved sodium tend to displace these bonding elements and disperse the aggregates. As sodium increases, dispersion increases and soil tilth declines. Soil dispersion caused by sodium can make soils run together, crust easier, and can limit water infiltration.

Table 1. Relative tolerance of selected crops to salinity of irrigation water.

Salt Tolerance
High
(8-12 mmhos/cm)
Medium
(3-8 mmhos/cm)
Low
(1-3 mmhos/cm)
FIELD CROPS
Cotton
Rye, wheat, oats, sorghum, corn, and soybeans
Field beans, peanuts
FORAGE CROPS
Bermudagrass
Sweet clover, dallisgrass, sudan grass, alfalfa, fescue, wheat and oats for hay, vetch
White clover and landino clover
VEGETABLE CROPS
Garden beets, kale, asparagus, and spinach
Tomatoes, broccoli, cabbage, peppers, cauliflower, lettuce, sweet corn, potatoes, carrots, onions, peas, squash, and cucumbers
Radishes, celery, and green beans
FRUIT CROPS

Figs, grapes, and cantaloupes
Pears, apples, oranges, plums, apricots, and peaches

Table 2. Guidelines for interpreting irrigation water quality tests.

Constituent Water Quality Hazard
Low
Medium
High
Electrical Conductivity
(mmhos/cm)
0.00-0.75
0.76-3.00
3.01+
Sodium Adsorption Ratio
(SAR)
0-6
6-12
(potential hazard with extended use on light-textured soils)
12+
12-18
(potential hazard with extended use on all soils)
18+
Residual Sodium Carbonate
(RSC)
0-1.24
1.25-2.50
2.51+
Chlorides, ppm (root absorption)
0-142
143-355
356+
Chlorides, ppm (foliar absorption)
0-106
107+

Iron, ppm (foliar staining potential)*
0-3.00
3.01+

Bicarbonates, ppm (overhead sprinkler systems, foliar staining)*
0-90
91-520
521+
pH (corrosivity)
6.5-8.4
5.5-6.4
less than 5.5
* Data applicable to nursery production only.

Interpretation of Test Results

Use Table 2 as a guide for interpreting your water test results. You should make interpretations, as related to specific crop response, after considering specific plant and soil conditions. Following are the major constituents of irrigation waters.

Electrical Conductivity -- Electrical conductivity, also called salinity, arises from weathering of rocks and soils. Saltwater intrusion into water supplies located near coastal areas also may contribute to electrical conductivity. This usually is expressed in millimhos per centimeter (mmhos/cm) and may be converted into total dissolved-salt concentration by multiplying mmhos/cm by 640 or 700. The soluble-salt level should normally be less than about 1.00 mmhos/cm for most irrigation situations.

Calcium and Magnesium -- These elements are results of rock weathering. Calcium usually is higher than magnesium in groundwaters, but where there is seawater contamination, magnesium concentrations may be greater than calcium. These elements are the main ones causing water hardness and the scale-forming properties of waters. As these elements increase, the tendency for sodium to be toxic decreases.

Sodium -- Sodium arises from rock and soil weathering, seawater intrusion, and sewage and irrigation waters. Large amounts of sodium, combined with chloride, give water a salty taste. If the water is for a sprinkler system, and calcium and magnesium are low, medium to high levels of sodium can defoliate sensitive plants. When the sodium in water is high relative to calcium and magnesium levels, and precipitation of Ca and Mg bicarbonates and carbonates is high, a sodium problem could develop on some soils.

The sodium adsorption ratio (SAR) and residual sodium carbonate (RSC) are useful for evaluating sodium hazard in water applied directly to the soil. In these calculations, the potential for precipitation of calcium, magnesium bicarbonates, and carbonates is considered. If these constituents precipitate out of the water, relative amounts of sodium will increase in the soil solution.

SAR is calculated using sodium, calcium, and magnesium expressed in milliequivalents per liter (meq/L) and is written as the following:

Formula:

SAR = Amount of Sodium divided by the square root of ((Amount of Calcium + Amount of Magnesium) divided by 2)

Residual Sodium Carbonate (RSC) is calculated using milliequivalents of calcium, magnesium, carbonates, and bicarbonates:
RSC = (Carbonates + Bicarbonates) - (Calcium + Magnesium)

Once a high buildup of sodium and salt occurs, water in excess of that needed for irrigation is necessary to leach these salts below the root zone of the crops. If levels of sodium are elevated, applying high rates of gypsum on alkaline soils or lime on acid soils (depending on pH) may eliminate some of the sodium problem. Consult your county agent or Extension soil testing specialist for more specific recommendations. Salt-tolerant crops, such as bermudagrass, may be useful in soil stabilization and erosion control. Soils with a high salt content but low levels of exchangeable (clay-fixed) sodium may require only leaching.

Chlorides, ppm: Chlorides arise from dissolved rocks, seawater intrusion, and sewage. The presence of sodium carbonates is suspected if the ratio of sodium to chloride is greater than 0.648. This constituent is most harmful in overhead sprinkler systems. Chloride should not be confused with chlorine (Cl2), which indicates the level of dissolved gaseous chlorine in water.

Iron, ppm: Iron is dissolved from practically all rocks and soils and also may arise from plumbing, pumps, and tanks. Iron in groundwater quickly oxidizes to a reddish-brown product when exposed to air. Iron at greater than one-third part per million can cause clogging in drip-irrigation systems and could stain foliage in overhead applications.

Bicarbonates and carbonates, ppm: These constituents most often are associated with calcium, magnesium, and sodium. White residues on plant foliage usually are because of high bicarbonate content of water. When calcium and magnesium bicarbonates precipitate out of irrigation water before use, sodium hazard may be increased.

pH: Low pH in water is caused by acids, acid-generated salts, and dissolved carbon dioxide. High pH is from carbonates, bicarbonates, hydroxides, phosphates, silicates, and borates. You should check water samples with less than 5.5 or greater than 8.5 pH to determine cause of abnormal values. Check water with a pH lower than 6.5 for corrosion potential on plumbing, pumps, or storage tanks.

Nitrates and Ammonium Nitrogen, ppm: Generally, levels of these constituents should not be a problem if kept at 5 ppm or lower. Problems can occur at 6 to 30 ppm. At greater than 30 ppm, severe toxicity is seen in some plants.


By James G. Thomas, Extension Specialist, Agricultural and Biological Engineering

Publication 1502
Extension Service of Mississippi State University, cooperating with U.S. Department of Agriculture. Published in furtherance of Acts of Congress, May 8 and June 30, 1914. Ronald A. Brown, Director

(500-06-01)


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