Guest Blog – Frederick Dobbs On Soil Salinity

Norman Borlaug died recently (12 September 2009) at the age of 95. Borlaug began life as an Iowa farm boy, was trained as a plant pathologist at the University of Minnesota, and went on to direct some of the most important plant breeding efforts of the 20th century. He was awarded the Nobel Peace Prize in 1970 for those efforts.

Borlaug is father of the “Green Revolution”, which began after World War II. Green Revolution agriculture is often criticized because of its reliance on pesticides, hybrid technology, and intensive water use, but it is also widely credited as having saved more human lives, particularly in the Third World, than anything else in the history of the human species. Later in life, Borlaug conceded that there might be some merit in these criticisms but that the Green Revolution was a good start in the right direction.

A good start it was, but we still face many of the same problems that Borlaug encountered. One of these is excess soil salt, or salinity. Salinity is a soil condition characterized by a high concentration of soluble salts that inhibit plant growth. Excess soil salt is a problem as old as agriculture. The civilizations of the Fertile Crescent, the area centered around modern-day Iraq, are thought to have dissipated as a result of climate change and excess soil salt that destroyed their agriculture.

Soil salinity is one of the primary abiotic stresses affecting plant growth and quality. As much as 6% of the earth’s total land area is affected by excess soil salt. Much of this arises from natural causes. Rock weathering releases soluble salts, and rainwater itself contains 6–50 ppm sodium chloride. Clearing land for cultivation and irrigation are two other causes of increased soil salinity; both raise the water table and salts are then concentrated in the root zones of plants.

Salinity is a common element of arid and semiarid lands, but it is also found in regions with moderate rainfall such as the U.S. Midwest and Northeast, particularly where irrigation is used. Poor quality irrigation water and poor drainage can make it worse. And irrigation is important in agriculture. Only about 15% of all cultivated land is irrigated, but irrigated land is about twice as productive as rained land and accounts for about 30–40% of the world’s food production. Breeding salt tolerance in plants is an important goal for plant scientists.

Now, what’s a salt? Salts are ionic compounds, and ionic compounds are characterized as having an electrostatic bond between metal and nonmetal ions. Ions are charged atoms. In water, salt dissolves as the ions composing the salt disassociate. If the water evaporates and the concentration of salt in water (in solution) becomes too great (saturated), the salt precipitates out of solution and becomes solid once more. Sodium chloride (table salt) is the primary salt involved in soil salinity; the primary ions responsible for salinization are sodium, potassium, calcium, magnesium, and chlorine.

Sensitivity to salt differs in plants. Some are tolerant while others are quite sensitive. Plants that grow in salt marshes and estuaries where the salt concentration may vary diurnally are (not surprisingly) able to thrive at much higher salinities than can woodland plants; this is easily demonstrated. But salt is so common in soils that all plants have evolved the ability to cope with and adapt to some degree of salinity.

How does salt affect plants? There are two basic ways. First, high salt concentrations in soil make it harder for plant roots to extract water from the soil. This is purely the result of osmosis, the movement of water across a semipermeable membrane, as in a plant cell, from an area of high water potential (low salt concentration) to an area of low water potential (high salt concentration). When the concentration of soil-water salt rises above a threshold, water will tend to flow out of the plant. If plants had no way of regulating this process, they would quickly dehydrate and die. Second, in a saline environment, salt enters the plant and accumulates. With time, it can reach toxic concentrations.

Both can be exacerbated by environmental factors such as sunlight, air temperature, and humidity, but of the two, osmotic stress has the most impact, and after soil-water salt exceeds a certain threshold its effect on plant growth is more or less immediate. Salt accumulation, on the other hand, has a more gradual effect. Stress from salt accumulation occurs later in the plant’s life cycle, and only at very high levels of salinity does its effect dominate.

How do plants adapt to increased salinity? Traditionally, plants have been described as either “excluders” or “includers” of salt, those that select against its uptake or those that regulate its accumulation. In most plants, a little of both strategies is seen. Other plants adapt to salinity by completing their life cycles rapidly and avoiding the toxic effects of accumulated salt altogether. These are worthwhile summaries but trivial answers to complex processes.

All plant functions ultimately result from the genes that plants possess that control and coordinate growth in concert with the constraints of the environment, and that plants mount a coordinated response to their environment is easily demonstrated. The physiological manifestations of salt tolerance and the salt-stress response have been pretty well described. Traditional plant breeding of the type that Borlaug directed has produced quite stress-tolerant crops, mainly by introducing traits from stress-adapted wild relatives. So great progress has been made, but our understanding on a molecular and cellular level is only piecemeal.

Teasing answers from several issues will provide insights into the processes that cause salt tolerance and toxicity in plants. For example, what molecular processes control salt (actually ion) compartmentalization in plants, and what accounts for tissue tolerance and osmotic adjustment? How is salt transported once inside the plant? A gene family responsible for initial entry of ions into plants has been identified and gives us some insights. One fascinating question is how do the leaves know the roots are in salty soil? Clearly, they do because leaf growth rate is reduced proportionally to the concentration of salt in the soil solution and not to the salt concentration within the leaves. What accounts for this long-distance communication within plants?

In the next few decades, we will answer these questions. And in the process, we will have taken more steps in the right direction.

       —Frederick Dobbs

This entry was posted on Monday, October 12th, 2009 at 9:47 pm and is filed under Original Posts. You can follow any responses to this entry through the RSS 2.0 feed.
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16 Responses to “Guest Blog – Frederick Dobbs On Soil Salinity”

  1. laura perkins said:

    Thank you for this – It was beautifully written, informative and relevant.

  2. Frederick Dobbs said:

    Thank you, Laura.

  3. Cindy said:

    Interesting article. I was looking for solutions to the problem, but I guess this one was mostly about the history of the issues of salinity. What about our home gardens? What about putting too much chemical matter on the soils and changing its chemistry?

  4. Frederick Dobbs said:

    Dear Cindy,

    One can certainly put too much chemical (including fertilizers) on a garden. Fertilizers can suppress the natural microbial-plant associations in a garden. This is particularly true in relatively arid areas with relatively infertile soils.

    Roses certainly need fertilizer, but my belief is that most garden flowers do not need much in the way of fertilizer. Mulch your garden heavily in the fall and turn it under in the spring. See if that doesn’t have a good effect.

  5. Patricia said:

    This is fascinating. My Santa Rosa, CA neighbor and I have been trying to figure out what is killing our gardens, and it just may be excessive salinity. I thought my zinnias might be dying from “damping off” in yet still I wondered about what appeared to be salt crystals on the surface of the soil… Now I can pursue this with a soil testing lab. Thank you.

  6. Frederick Dobbs said:

    Dear Patricia,

    Salinity is certainly a possibilty. If after you irrigate, and the irrigation water pools, you see a crystalline precipitate on the soil surface when the water evaporates, that sounds like salt.

    The key to controlling soil salinity is adequate soil drainage. Drainage allows salts to be leached below the root zone. Some amendments can improve soil drainage, and indirectly help control soil salinity.

    A soil test sounds like a good idea.

  7. ben lewis said:

    for many years i lived and worked in agriculture in imperial valley, california and all the problems with salt and alkaline soil. some times the salt got so bad that d 8 crawlers had to have 8 or 10 ft chisels to break up the hardpan so the accumulated salt would finally be able to drain and allow crops to grow;. a very facinating article.

  8. Frederick Dobbs said:

    Thanks for your kind words, Ben.

  9. Richard said:

    Great information here, relevant for me on my Florida marsh island, but what do I do about it? Is there a strategy other than plant selection and how can I know which plants to select other than trial and a lot of error?

  10. Frederick Dobbs said:

    Dear Richard, As I said above and as Ben notes, the key to controlling soil salinity is adequate soil drainage. Now, how deep is your water table? Since you’re on an marsh island, it many not be too deep and adequate drainage may not be possible. As for plant selection, there should be a lot of information about this. Your county ag agent should have information about salt tolerant plants or should at the least be able to tell you where to get it. If you’re interested in citrus, Florida State and Univ. of California at Riverside have active citrus breeding programs.

    Thanks

  11. Susan Wallace said:

    Thank you for this useful information for non-chemists or soil scientists!

  12. Frederick Dobbs said:

    You’re quite welcome, Susan.

  13. Deafpetr said:

    Like I always write.. because it is always true..
    Stop driving your car.. We humans are devouring the earth resources. We intellectually justify DESTRUCTION IN THE NAME OF EDUCATION.
    Until we can return to the stream of life.. forgetting the internal combustion engine and its meanings.. we shall yes return to your “green rock” and be even the destruction of that incredible habitat.
    So..
    go ahead..
    keep on chattering..
    The band played on whilst the Titanic sank..
    Blessings..
    Peter

    • Frederick Dobbs said:

      Thanks for your thoughts, Peter.

  14. Charlotte Fanders said:

    I have been aware of this problem since the ’50s when the Ag. Engineers from Pakistan and India with whom I was acquainted were trying to desalinate the Punjab–or at least trying to find a way to do so at my university. I also have read that the desert which the Isrealis caused to”bloom” is now salty. And that the Imperial Valley of California is becoming saline.
    It seems to me that a different approach is necessary–perhaps a departure from synthetic chemicals and an introduction of organic elements.—-Also a selection of plants compatable with the existing soil conditions.
    Charlotte Fanders (fanders9@wmconnect.com)

    • Frederick Dobbs said:

      You bring up some good points, Charlotte. All of the areas that you mention are around 30 degrees north, blest with favorable climates, and are prime agricultural lands. One of the limiting factors in all these areas is water, all are under irrigated agriculture, and the amount of evapotranspiration exceeds rainfall, which leads to salinization. There is lots of agricultural research being conducted to deal with these situations; plants compatible is one of the prime approaches.

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