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United States Department of Agriculture

Agricultural Research Service

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Frequently Asked Questions About Salinity
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Salinity in Agriculture
 
 

Why do we irrigate?

Irrigation is an ancient and important agricultural practice. Crop yields are higher under irrigation and less dependent on the effects of weather. While only 15% of the world's cultivated land is irrigated, it accounts for 35-40% of the global food harvest. Projected population growth rates for the next 30 years will require an increase in food production equal to 20% in developed countries and 60% in developing countries to maintain present levels of food consumption. Expansion of irrigated agriculture was in large part responsible for the "green revolution" in food production and will continue to play an essential role in providing the needed increases in food and fiber production, especially in developing countries.
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What happens when you irrigate?

Irrigation inevitably leads to the salinization of soils and waters. In the United States yield reductions due to salinity occur on an estimated 30% of all irrigated land. World wide, crop production is limited by the effects of salinity on about 50% of the irrigated land area. In many countries irrigated agriculture has caused environmental disturbances such as waterlogging, salinization, and depletion and pollution of water supplies. Concern is mounting about the sustainability of irrigated agriculture.
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Where does all the salt come from?

Application of irrigation water results in the addition of soluble salts such as sodium, calcium, magnesium, potassium, sulfate, and chloride dissolved from geologic materials with which the waters have been in contact. Evaporation and transpiration (plant uptake) of irrigation water eventually cause excessive amounts of salts to accumulate in soils unless adequate leaching and drainage are provided. Excessive soil salinity reduces yields by lowering plant stand and growth rate. Also, excess sodium under conditions of low salinity and especially high pH can promote slaking of aggregates, swelling and dispersion of soil clays, degrading soil structure and impeding water and root penetration. Some trace constituents, such as boron, are directly toxic to plants.
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What problems does salinity cause?

Over the course of history, thriving civilizations declined in part due to their inability to sustain food production on lands that had been salinized. It is estimated that 10 million hectares are now being lost every year as a result of salinity and/or waterlogging. Many of these problems are caused by excessive use of water for irrigation due to inefficient irrigation distribution systems, poor on-farm management practices, and inappropriate management of drainage water. Inefficient on-farm irrigation practices cause local salinity problems. Local problems increase as a result of poor on-farm drainage. Excessive irrigation increases salt loading in water tables and downstream aquifers which causes regional salinization. Lack of local and regional drainage systems results in lands being put out of agricultural production.
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Why is research on salinity so important?

In the future, global food needs will continue to increase while the soil and water resources available for new crop production will be limited and of diminished quality. The need to protect soil resources as well as to conserve water will continue to increase. Water must be utilized more efficiently and its quality protected. World agriculture must expand its base of production and increase production on lands currently under cultivation. Appropriate management practices to control salinity must be implemented on irrigated fields, in irrigation projects, and for geohydrologic systems. In order to meet the ever increasing demands for food and utilizing ever decreasing and more marginal soil and water resources, the nation and much of the world community will continue to look to the U. S. Salinity Laboratory for expertise and leadership in salinity and water quality research and applications to solve these problems.
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General Questions About Salinity and Water
 
 

How do you measure the salinity of water?

Salts in water can be measured by relatively simple methods.

One is by Electrical Conductivity (the reciprocal of electrical resistance) using an appropriate conductivity meter for the measurement. instruments are used by farmers and can be purchased relatively cheaply compared to most scientific instruments. For a historic background on this method there is a century-old paper by Whitney and Means, 1897 (USDA, Div. of Soils Bul. 7, 15 pp.) and our own USDA Handbook 60, 1954.

Another method involves weighing water in a weighing container, and evaporating the water, then re-weighing and determining weight/volume (by difference). This is a little tricky because a sensitive balance must be used, and high temperatures my volatilize some salts.
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When is water too salty to drink?

The Environmental Protection Agency sets the standards for drinking water that involves total salts and specific chemical component limitations.
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How do I find out about the importance of water in California?

In California, there are many publications and countless newspaper articles regarding water needs associated problems. A good summary is a special issue of "California Agriculture", 1984, Vol 38 (10). For current problems and concerns, another resource is a periodical magazine published by the Water Education Foundation, called "Western Water". Your local library can help you obtain copies of all mentioned references.
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Fertilizer and Crop Requirements
 
Is there any practical model available to estimate fertilizer and water requirements of crops in arid regions under irrigated farming?
In Saudi Arabia, both the soil and water are highly saline and all the crops are irrigated via a central pivot system, mainly wheat, barley, alfalfa and forage grasses. Vegetable crops are either grown in greenhouses or in the field using drip (subsurface or above ground) or sprinkler systems. Needed is a basic model to determine fertilizer requirements of crops for a target yield using data on oil fertility, temperature, relative humidity, SAR, water quality etc.
 
Information on irrigation water requirements for various crop species (including grains, grasses, alfalfa, and wheat) is included in a USDA Soil Conservation Service, Part 623 National Engineering Handbook, Chapter 2, Irrigation Water Requirements, printed September 1993. This publication also includes some basic information on salt tolerance and how to compute water needs.
 
We have no specific information on where you can get crop fertilization recommendations, but Extension Service should be a good place to start.
 
Dr. Donald Suarez has been leading a project to develop a generalized crop model for FAO that deals with salinity and crop yield. He is not aware of models for barley, forages, and alfalfa that take in to account and integrate all of the factors in which the question addresses.

Dr. Derrel Martin and others at the University of Nebraska have been working on water and fertilizer requirements under center pivot irrigation. You may want to direct your questions to Derrel.
 
 

Plant Cell and Root Growth, Water and Sodium Chloride
 
 

How does salinity affect root growth? Does it increase or decrease root growth?

Unfortunately, we don't know all the answers! Salt in the root zone decreases root growth. In some plants we call halophytes (literally salt plants) a little bit of salt seems to improve overall growth, in both roots and shoots (if you measure total biomass, i.e. the weight of the plant material). Examples of this are seen in barley and atriplex. Why this is the case is not known, but it is speculated that these plants require Sodium ion (Na+) or Chloride ion (Cl-) for growth.
 
In most cases, however, salinity decreases both root and shoot growth in plants, especially in glycophytes. Glycophytes are plants adversely affected by salts, or literally sweet plants, as opposed to salt plants. Shoot growth is usually decreased more than root growth and as a result the root/shoot ratio changes (the total weight of the roots and divided it by the total weight of the shoot).
 
References:

  • Braun, Y., Hassidim, M., Lerner, H.R., and Reinhold, L. 1986. Studies on H+-translocating ATPases in young plants of varying resistance to salinity. PLANT PHYSIOLOGY. vol. 81, pp. 1057-1061.
  • Munns, R. and Termatt 1986. Whole-plant responses to salinity. AUSTRALIAN JOURNAL OF PLANT PHYSIOLOGY, vol. 13, pp. 143-160.
  • Gilbert, G. A., Wilson, C., Madore, M. A. 1997. Root-zone salinity alters raffinose oligosaccaride metabolism and transport in Coleus. 1997. PLANT PHYSIOLOGY. vol 115, pp. 1267-1276.
 
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Does salinity affect cell division or cell enlargement?

Both. In Halophytes, growth is stimulated by low amounts of salt (equivalent to about 3000 ppm). If you had a lot of time on your hands and bothered to count each and every cell, you would find more cells. Also, you would find that the cells are, on average, larger. Mostly though, plants increase in size by cell enlargement.
 
When Glycophytes are affected by salinity, cells in the roots are smaller and there are fewer of them. Under severe stress, there just isn't a whole lot of root there - fewer cells and smaller cells. Same with the shoot.
 
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Do plants actually take up salt while the water diffuses in?

Ions get into the roots via several mechanisms. Ions that are at lower concentrations outside the plant that inside are taken up by a processes called active transport which requires energy and is mediated by a protein. Ions that exist at higher concentrations outside the plant than inside can diffuse in, but again, a protein is probably involved. These proteins are called transporters, pores or channels depending on their exact nature and how they operate. Both roots and shoots of plants grown in saline environments will have higher salts levels.
 
Some plants exclude toxic ions like Na+ and Cl-. By exclude, it is really meant that they limit the influx of ions. This is accomplished by limiting ion uptake at the level of the roots or by compartmentalizing ions in areas of the plant, even in cells that are away from important metabolic sites and actively growing tissues. In some cases, it appears that salts are sequestered in older leaves that are eventually shed (abscised). Some halophytes have specialized leaf cells called salt glands that excrete salt.
 
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Does water diffuse out of plants?

Water does indeed diffuse out of the leaves by the process called transpiration. Because water molecules cohere to each other via chemical bonds, called hydrogen bonds, water molecules at the top of the plants are connected to water molecules in the soil much like the cars of a train. When water transpires (a diffusion process) from the leaves, other water molecules are brought closer to the root surface. This waterway is actually called the transpiration stream. Ions move in the transpiration stream much like a non-powered boat floats along a river stream. Thus, the transpiration stream brings ions from the soil water, first to the root where they must cross the plasma membrane barrier, and eventually to the leaf. At the leaf the water molecules can escape back into the atmosphere through another specialized leaf cell called a stomata. Ions, however, will be left behind.
 
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Salt Tolerance Criteria
 
What are the criteria used by the Salinity Lab to distinguish between plants that have high, medium, and low salt tolerance?
  • According to pages 65-67 of USDA Handbook 60, the salt tolerance lists are arranged according to major crop divisions; and in each division, crops are listed in three groups.
  • Within each group, crops are listed in the order of decreasing salt tolerance, but a difference of 2 to 3 places in the column may not be significant.
  • ECe values given in the top of the column represent the salinity level at which a 50-percent decrease ion yield may be expected compared to yields on nonsaline soils under comparable growing conditions
 
 

Crop Selection for Saline Soils
 
Web adaptation from Handbook 60 (p. 65-67) originally published in 1954
Because of
  • saline irrigation water,
  • high water table, or
  • low permeability of the soil,
it may not be economically feasible to maintain low salinity. In such instances, the judicious selection of crops that can produce satisfactory yields under saline conditions and the use of special management practices to minimize salinity may make the difference between success or failure.
 
As has already been pointed out, the availability of water to plants is always a factor under saline conditions. For example, suppose alfalfa is being grown on a loam having a salt content of 0.2 percent sodium chloride and a wilting percentage of 6 when the latter is determined on a nonsaline sample. Under such conditions, because the osmotic effect is additive with soilmoisture tension, alfalfa will stop growing when the soil dries to a moisture content of only 13 percent. In other words, if the soil contains 0.2 percent salt, the alfalfa plant cannot use a large part of the soil moisture that is normally available under nonsaline conditions. The presence of even smaller quantities of salt in this soil would cause a fraction of the soil moisture above the wilting percentage to be unavailable to the plant. More frequent irrigation would be required to decrease the inhibitory effect of the salt on the growth of alfalfa.
 
Although it has been shown that crop growth on saline soils is definitely benefited by more frequent irrigation, the need for this irrigation may not be indicated by the appearance of the crop (Richards and Wadleigh, 1952). In nonsaline soils, there is usually a relatively abrupt transition from low moisture stress to high moisture stress conditions, and the wilting of the plant indicates the need for irrigation. In saline soils, changes in moisture stress are more gradual and, although the plants may be subjected to high stress, there is no abrupt transition in the turgor condition of the plant and, hence, no sign of the need for irrigation. Nevertheless, experiments have shown that crop growth is greatly improved by more frequent irrigation under such conditions. Careful leveling of the fields to insure more uniform moisture distribution during irrigation will also improve chances for successful crops on saline soils.
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Germination

In selecting crops for saline soils, particular attention should be given to the salt tolerance of the crop during germination because poor crops frequently result from a failure to obtain a satisfactory stand. This problem is complicated by the fact that some crop species which are very salt tolerant during later stages of growth may be quite sensitive to salinity during germination (fig. 19). Sugar beets, for example, which are very salt tolerant during later stages of growth, are extremely sensitive during germination. On the other hand, barley has very good salt tolerance during all stages of growth, although it is more sensitive during germination than at later stages (Ayers and others, 1952). Under field conditions, it is possible by modification of planting practices to minimize the tendency for salt to accumulate around the seed and to improve the stand of crops that are sensitive to salt during germination (Heald and coworkers, 1950).
 
Figure 19
 

Relative Salt Tolerance of Crop Plants

The salt tolerance of many species and varieties of crop plants has been investigated at the Laboratory. Previously published lists (Magistad and Christiansen, 1944, and Hayward and Magistad, 1946) have been modified on the basis of recent findings and are presented in Table 8.
 
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The salt tolerance of a crop may be appraised according to three criteria:
  • Ability of the crop to survive on saline soils
  • Yield of the crop on saline soils
  • Relative yield of the crop on a saline soil as compared with its yield on a nonsaline soil under similar growing conditions.
Many previous observations on salt tolerance have been based mainly on the first criterion, ability to survive; but this method of appraisal has very limited practical significance in irrigation agriculture. Although it is recognized that the second criterion is perhaps of greater agronomic importance, the third criterion was used in compiling the present salt-tolerance lists because it provides a better basis of comparison among diverse crops.
 
The salt-tolerance lists are arranged according to major crop divisions; and, in each division, crops are listed in three groups. Within each group, the crops are listed in the order of decreasing salt tolerance, but a difference of 2 or 3 places in a column may not be significant. EC, values given at the top of a column represent the salinity level at which a 50 percent decrease in yield may be expected as compared to yields on nonsaline soil under comparable growing conditions. For example, for crops with high salt tolerance in the division of field crops, EC, values of 16 mmhos/cm occur at the top of the column and 10 mmhos/cm at the bottom. This indicates that crops near the top of this column will produce about 50 percent as well on a soil having an EC, of 16 mmhos/cm. as on a nonsaline soil under similar conditions, and crops near the bottom of this column will produce about 50 percent as well on soils having an EC, of 10 mmhos/cm. as on a nonsaline soil. EC, values having similar significance have been shown for each group of plants for which such data are available.
 
In most instances, these data are based on a field-plot technique in which crops are grown on soils that are artificially adjusted to various salinity levels after the seedlings are established. By this method, crop yields were related to EC, values for comparable saline and nonsaline soils, and the salinity level associated with a 50 percent decrement of yield was determined graphically. In many of these studies, a number of varieties of a given crop were compared. Significant varietal differences were found for cotton, barley, and smooth brome, while for truck crops such as green beans, lettuce, onions, and carrots varietal differences were not of practical significance.
 
In applying the information in the following table, it is important to remember that climatic conditions may influence profoundly the reaction of plants to salinity. The choice of suitable salt-tolerant varieties and strains will depend on local climatic factors; and, consequently, information on salt-tolerant varieties should be evaluated with reference to the conditions under which the crops are to be grown. The position of each crop in this table reflects its relative salt tolerance under management practices that are customarily employed when this crop is grown under irrigation agriculture and not the inherent physiological ability of the crop to withstand salinity under some given set of conditions that is uniform for all crops.
 
A salt-tolerance list for some important crops of Holland has recently been prepared by Van den Berg (1950) . Based on field-plot studies in areas which had been inundated by salt or brackish water in 194.4.-45, the salinity values ("salt index," expressed as grams NaCl per liter of soil water) associated with 75 percent of normal yields for 14 crops were determined. Despite obvious differences in climate and cultural practices, Van den Berg's results for relative salt tolerance are in good agreement with those in Table 8.
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Relative Boron Tolerance of Crop Plants

Plant species differ markedly in their tolerance to excessive concentrations of boron. In sections where boron tends to occur in excess in the soil or irrigation water, the boron-tolerant crops may grow satisfactorily, whereas sensitive crops may fail. The relative boron tolerance of a number of crops was determined by Eaton (1935), and his results are reported in Table 9 with minor modifications based on field observations. The boron tolerance lists are analogous to the salt-tolerance lists and subject to much the same limitations in interpretation. Differences in position of a few places may or may not be significant, and there is no sharp division between successive classes. Climate and variety may also be factors in altering the indicated tolerance of a given species under specific conditions.
 
 
Available information on boron tolerance does not permit the establishment of definite permissible limits of boron concentration in the soil solution. Irrigation waters are classified on the basis of boron content in table 14, chapter 5, with reference to sensitive, semitolerant, and tolerant crops. The effect of a given concentration of boron in the irrigation water on the boron content of the soil solution will be conditioned by soil characteristics and management practices that influence the degree of boron accumulation in the soil. In the discussion of saturation extracts of soils (ch. 2), 0.7 ppm. boron in the saturation extract was indicated as the approximate safe limit for sensitive crops.
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Measurement of Electroconductivity
 
 

Theory of operation

The term Conductance refers to the readiness of materials to carry an electric current. Liquids which carry an electric current are generally referred to as electrolytic conductors. The flow of current through electrolytic is accomplished by the movement of electric (positive and negative ions) when the liquid under the influence of an electrical field. The conductance of a liquid can be defined by its electrical properties - the ratio of current to voltage between any two points within the liquid. As the two points move closer together or further apart, this value changes. To have useful meaning for analytical purposes, a dimension needs to be given to the measurement; i.e., the parameters of the measurement.
 
By defining the physical parameters of the measurement, a standard measure is created. This standard measure is referred to as specific conductance or conductivity.
  • It is defined as the reciprocal of the resistance in ohms, measured between the opposing faces of 1 cm cube of liquid at a specific temperature.
  • The units used to define conductance are:
    • 1/ohm = 1 mho = 1000 mS = 1,000,000 uS.
  • S.I. units may be used in place of mhos
    • 1 mho = 1 Siemen (S)
  • Conductivity units are expressed as
    • µS/cm (1.0 dS/m = 1. 0 µS/cm) or mS/cm
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Design of the conductivity cell

In theory, a conductivity measuring cell is formed by two 1-cm square surfaces spaced 1-cm apart. Cells of different physical configuration are characterized by their cell constant, K. This cell constant (K) is a function of the electrode areas, the distance between the electrodes and the electrical field pattern between the electrodes. The theoretical cell just described has a cell constant of K = 1.0. Often, for considerations having to do with sample volume or space, a cell's physical configuration is designed differently. Cells with constants of 1.0 cm-1 or greater normally have small, widely spaced electrodes. Cells with constants of K = 0. 1 or less normally have large closely spaced electrodes. Since K (cell constant) is a "factor" which reflects a particular cell's physical configuration, it must be multiplied by the observed conductance to obtain the actual conductivity reading.
 
For example, for an observed conductance reading of 200 µS using a cell with K 0. 1, the conductivity value is 200 x 0. 1 = 20 µS/cm.
In a simplified approach, the cell constant is defined as the ratio of the distance between the electrodes, d, to the electrode area, A. This however neglects the existence of a fringe-field effect, which affects the electrode area by the amount AR. Therefore K = d/(A + AR). Because it is normally impossible to measure the fringe-field effect and the amount of AR to calculate the cell constant, K, the actual K of a specific cell is determined by a comparison measurement of a standard solution of known electrolytic conductivity.
 
The most commonly used standard solution for calibration is 0.01 M KCl. This solution has a conductivity of 1412 µS/cm at 25oC
Note: Some sources in literature quote this value at 1409 or 1413 µS/cm at 25oC. Differences exist due to the use of kilogram of water rather than liters, as well as changes in assigned molecular weights, definitions of the Siemen, the use of different temperature scales, and whether or not the inherent conductivity of water was subtracted out. Regardless, for normal laboratory calibration the use of 1409 µS/cm versus 1413 µS/cm is insignificant.
 
In summary, the calibration of a conductivity probe is to compensate for the fact that:
  • K is not specifically known
  • K changes as the electrode ages
Calibration simply adjusts the measured reading to the true value at a specified temperature.
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The effect of temperature

The conductivity of a solution with a specific electrolyte concentration will change with a change in temperature. By definition, temperature compensated conductivity of a solution is the conductivity which that solution exhibits at the reference temperature. This temperature is chosen to be either 25o C or 20oC. A measurement made at reference temperature, therefore, needs no compensation. Generally for most aqueous samples, a coefficient of 2.1% per degree Celcius is used in temperature compensation, with the apparent value being 2.1% high for each degree C above 25oC or conversely the apparent value being 2.1% low for each temperature for measurement is 25oC. A useful algorithm for temperature correction is:
 
CT = C25 [1 + 0.021 (T - 25)]
where CT = the measured conductivity of a solution at sample temperature; C25 = the conductivity of the solution at 25oC and T = the sample temperature (oC)
 
Many conductivity meters today automatically compensate for temperature if the conductivity probe includes a thermistor. However, as will be explained later, this can be a major source of error in analysis if the thermistor is not accurate or if the instrument is improperly calibrated.
 
Note the two following examples to explain the effect and compensation of the fringe-field effect and temperature.
 
Example #1 - Manual Temperature Compensation:
An analyst wishes to calibrate a conductivity probe and measure an unknown sample. The conductivity probe is specified to have a cell constant of 1.0. The analyst is calibrating in a 0.01 M KCI (EC = 1412 µS/cm at 25oC) solution at a temperature of 22oC. Automatic temperature compensation (ATC) is not available.
  1. Determine the conductivity of the 0.01 M KCI at 22oC.
    • EC KCI 22oC = 1412[l + 0.021(22-25)]
    • EC KCI 22oC = 1412 [0.937]
    • EC KCI 22oC = 1323 µS/cm
  2. Immerse the conductivity probe into the standard and adjust the value to 1323 µS/cm. adjustment being made is compensating for the difference the specified cell constants and the true cell constant.
  3. The analyst now measures an unknown sample whose temperature is at 19oC and obtains a value of 967 µS/cm. How is this value adjusted to 25oC
    967 µS/CM = C25[1 + 0.021(19-25)]
    C25 = 967 µS / [1 + 0.021(19-25)]
    C25 = 967 µS / [1 + 0.021(-6)]
    C25 = 967 µS / 0.874
    C25 = 1106 µS/cm
 
Example #2 - Automatic Temperature Compensation:
An analyst wishes to calibrate the conductivity probe and measure a sample. The conductivity probe is specified to have a cell constant of 1.0. The analyst is calibrating in a 0.01 M KCI (EC = 1412 µS/cm at 25oC) solution at a temperature of 22oC. Automatic temperature compensation (ATC) at 25oC is available.
  1. Immerse the conductivity probe into the standard and adjust the value to 1412 µS/cm. Any adjustment being made is compensating for the difference between the specified cell constants and the true cell constant. NOTE: On most modern instrumentation, the true temperature is displayed along with the temperature compensated conductivity value. In this case the display would show a conductivity of 1412 µS/cm and of 22oC.
  2. Once the electrode has been calibrated, it is cleaned, placed into the unknown sample at 19oC. Once temperature is stable, the correct conductivity value (1106) µS is displayed.
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Sources of error in measurement

Temperature Compensation
Since many conductivity probes now include a thermistor for ATC it is important to determine if the thermistor reading is accurate at the temperatures that samples are being measured. If not, then the automatic temperature corrected value will be inaccurate. Compare the measured value from the thermistor with that of a quality laboratory thermometer. If the values differ significantly, contact the manufacturer as to the defect or consider manual temperature compensation.
 
Improper Calibration
Too often, calibration standards have been sitting around a laboratory for extended periods. Standards should be fresh and known to be correct within at least ± 1% before attempting a calibration. Since the conductometric response is not perfectly linear at all ranges it is best to calibrate the probe in the same magnitude of range as the samples being measured. In other words don't calibrate your conductivity probe in a 100 µS/cm standard if your samples are typically in the >1000 µS/cm range. Standard conductivity solutions:
 
KCI Concentration Conductivity (mS/cm)1
0.001 N 0.147
0.010 N 1.413
0.020 N 2.767
0.050 N 6.668
 
1temperature KCl solutions 250C
 
Condition of Probe
Probes can become inaccurate when they become coated with interfering substances on the probe element. During normal use, rinse the probe thoroughly with laboratory grade water between each measurement. This will help to minimize the buildup of the coating substances. If the probe needs cleaning first try ethanol which is good for removing most organics. If this isn't successful, clean the probe with a strong detergent solution. Rinse thoroughly with demineralized water.
 
The cells may occasionally need replatinization to refresh the cell plates and return them back to the original cell constant. The cell constant changes when the platinum black layer becomes partially removed or contaminated. Follow the manufacturer's directions on this procedure.
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Last Modified: 10/18/2005