Should I Add Lime To My Soil

By: Beaule Agerter

In the eastern United States soil is often 'limed' because it is naturally acidic. Folks relocate to the western states and want to lime their soils there. This can help destroy western soils because of excess soluble salts that are often present therein. Excess soluble salts such as chlorides and sodium can seriously interfere with the growth of most plants. Soil with toxic levels of sodium are sometimes called saline or sodic soils. Endeavoring to leach salts out of such soils with only water can markedly raise the pH, and interfere even more with plant growth, especially if the water source is high in sodium chloride content. This is because the exchangeable sodium, (once the neutral soluble salts are removed), readily hydrolyzes and thereby sharply increases the hydroxyl ion concentration of the soil solution. Thus sodium toxicity to the plants increases. It is common knowledge that when a water solution containing a large amount of sodium chloride is brought into contact with a plant cell, it will cause a shrinkage of the protoplasmic lining, a process called plasmolysis, which increases with increasing amounts of sodium chloride. The more sodium chloride loaded water that is applied to the soil, the more plasmolysis increases. The plant cells then collapse.

Retardation of evaporation is an important feature of the control of sodium chloride and other salts in soil. This will retard the translocation upward of soluble salts into the root zone. Plastic covers, mulch, even sawdust can help retard evaporation in home gardens. However, there are no inexpensive methods of reducing evaporation from large acreages. For such other control methods must be practiced. These can include avoiding excess water when irrigating- water deep but not so much that there is runoff and just a little less often as necessary without jeopardizing yield goals. The timing of irrigations is extremely important too, especially during the spring planting season: irrigation should precede planting and should be limited as much as possible until after the seedlings (which are especially sensitive to the salt content of the soil solution), have become more established. It may also be wise to plant a salt tolerant crop such as barley, rye, sweet clover, wheatgrass, alfalfa, sunflower, asparagus, beets, artichoke, lima bean, cow pea, turnips, or squash. If the alkaline condition of the soil can be at least temporarily alleviated prior to planting, such crops may maintain themselves in spite of the sodium concentrations that may redevelop later.

To at least temporarily alleviate the alkaline condition of the soil the excess sodium needs to be replaced with calcium. Sulfur can be used to advantage because it helps lower the pH of the soil solution, which aids in dissolving any limestone (calcium carbonate) or gypsum (calcium sulfate) in the soil, thus freeing any calcium therein to exchange, or displace the sodium, which then can be leached out with water. Upon oxidation, sulfur yields sulfuric acid, which not only changes sodium carbonate to less harmful sulfate but also tends to reduce the intense alkalinity, which is sometimes referred to as black alkali. The carbonate radical is entirely eliminated. For application recommendations you can contact the Agricultural Agent at the local Extension Service. Approximate rates of application of sulfur are useful for small areas for planting. However, if you plan to apply sulfur to large acreages, then using approximations can be unnecessarily costly. By calculating the amount of sulfur needed to exchange with the sodium ions to alleviate the alkalinity of the soil, you can determine nearly the exact amount of sulfur you will need.

First you determine the cation exchange capacity of the soil, which is expressed in terms of milliequivalents/100 grams. The term "equivalent" is defined as: "1 gram atomic weight of hydrogen or the amount of any other ion that will combine with or displace the amount of hydrogen." The milliequivalent weight of a substance is 1000th of its atomic weight. So the term "milliequivalent", or meq means 1 milligram of hydrogen or the amount of any other ion that will combine with or displace it. Thus, if a soil has a cation exchange capacity of 1 meq per 100 grams, that soil is capable of exchanging 1 mg of hydrogen, or its equivalent, for every 100 grams of the soil. This is 10 parts per million. Therefore, an acre-furrow slice of such a soil weighing 2 million pounds could absorb 20 lbs of exchangeable hydrogen or its equivalent, such as sulfur.

A soil high in excess soluble salts is alkaline, with a pH between 7.5 and 14. Soil with a pH less than 6.5 is said to be acidic. Sulfur is a good choice for 'changing' a soil from an alkaline condition (with high excess soluble salts) to a neutral pH (with a pH between 7 and 7.5) or even to an acidic condition. You can purchase sulfur at hardware stores and nurseries in such forms as Ammonium sulfate, sulfur-coated urea, elemental sulfur, or Aluminum sulfate. However, to acidify the soil, sulfur has to become sulfuric acid, and it takes a long time for bacteria and water to effect that change, even in warm soils. Therefore, straight sulfuric acid works faster. It requires 2.6 lbs of sulfuric acid to supply 1 lb of soluble calcium to displace the sodium. However, you still need to know how much excess sodium salts are present in your soil and use that ratio to determine how much sulfuric acid to apply. Now, determine how much calcium in the form of lime stone or such is already present in your soil. If there is a sufficient amount, sulfuric acid can lower the pH which in turn helps to dissolve the limestone, (and other forms of calcium that may be present) to free up the calcium to displace the sodium. For example, if you have 12 inches of soil with 4 meq of sodium (Na/100g), we know that it takes 1.28 tons of elemental sulfur (S) per acre foot of soil to do the job, and we know that it takes 3.06 tons of sulfuric acid to supply 1 ton of elemental sulfur, and by multiplying 3.06 by 1.28 (the amount of elemental S required) we find that we need 3.9 tons of sulfuric acid per acre. --Managing Salt-affected Soil for Crop Production, D.A. Horneck, J.W. Ellsworth, B.G. Hopkins, D.M. Sullivan, and R.G. Stevens, Oregon State University, 2007

If you want to strictly avoid applying any excess sulfuric acid you need to take the meq of sodium, determine the amount of CaCO3 and sodium in your soil, find the soil pH to determine how much you want to lower the pH, take into consideration the texture of the soil and the climatic conditions of your area, and plug in the lbs of sulfuric acid it will take to free up the specific amount of calcium already present in the soil. If you are concerned with saving $ by applying no more sulfur than is absolutely necessary, make a date of it and you and your wife do the calculations together.

Now sulfuric acid is dangerous to handle- after all it is battery acid. It takes specialized equipment and specific safety methods to use it properly. As stated above, Ammonium sulfate, sulfur-coated urea, elemental sulfur, or Aluminum sulfate can be used to help acidify the soil to free up calcium to displace the sodium. We also noted above that sulfuric acid does the job faster. However, if you want to avoid the safety issues inherent with sulfuric acid and if you have the time (sometimes months or even years) you might want to utilize such products while growing salt tolerant crops or letting the soil go fallow.

Another product useful for managing saline and/or sodic soils but slow in helping to displace sodium is gypsum. It takes allot more gypsum to do the job than it does sulfuric acid, and takes a long time because gypsum does not dissolve very easily in water. I once added gypsum to some soil and found some of it still undissolved 5 years later. However, it is safer to use than sulfuric acid in that it does not require specialized equipment and it can be handled far easier. Gypsum is readily available at many hardware stores, plant nurseries, and even department stores. Application rates are easily available from knowledgeable suppliers. You can also find application rates for gypsum at The Story About Gypsum, The Funny Thing About Gypsum And The U.S. Clean Air Act, and Gypsum- First Aid For Lawns. There are many other Web sources for information about Gypsum, as a simple search will no doubt produce.

Whatever you are determined to use, winter application to your soil will save you $ because the snow melt will water it in. Since snow melt is low in sodium, the combination of that clear water and the sulfur or gypsum or whatever you decide to use should be very effective in at least temporarily alleviating your alkalinity problems, and you should be able to grow a good crop in the spring even if you continue to use the same water with a large amount of sodium chloride, as long as you follow the recommendations above. However, a water source with less sodium chloride eventually must be found and utilized.

The text above was largely taken from notes in Soils class at Mesa Community College in 1977. This article is not all inclusive- use it at your own risk. It definitely fails to supply all the information available for managing saline and/or sodic soils. The web is a great source for more extensive information on the subject- especially University and Agricultural Extension websites.

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