Soil liming: beware of lime alternatives
Over the past few days, farmers have asked about liquid lime. According to the University of Kentucky, a recent (2015) investigation found that what was being marketed as a liquid lime with neutralizing value was in fact calcium chloride (CaCl2). This was bad news because CaCl2 provides no liming value, and is in fact not “liquid lime.” Calcium chloride is used for many purposes, including road salt or tractor tire ballast, however, it cannot be used to neutralize soil acidity. This article is intended to provide some information on liming materials, the lime reaction and explain why CaCl2 is not a lime source. Maintaining soil pH in the proper range is one of the most important parts of soil fertility management. Soil pH is considered the “master variable” because it influences so many of the chemical and biological functions of the soil. Maintaining soils in the optimum pH range often requires liming. East Texas soils are naturally acidic, even though much of the soils sit on top of limestone. Nitrogen fertilizers also contribute acidity when ammonium (NH4+) is nitrified to form nitrate (NO3–). Notice that ammonium has four hydrogens and nitrate has none. When ammonium transforms to nitrate, it releases hydrogen ions to the soil solution and pH is a measure of the activity or concentration of hydrogen ions (H+), which is represented mathematically as pH = –log[H+]. The more hydrogen ions present, the lower the pH value, since it is a negative logarithm. Values below 7 are considered acidic and values above 7 are considered alkaline. For most crops grown in Hopkins County , the target pH is 6.4-6.8. In this range the essential mineral nutrients are most available to growing plants. At pH below 5.0, soluble aluminum, iron and manganese may be toxic to the growth of some plants and phosphorus availability is decreased.
How Lime Works
So what is lime and how does it work? Liming materials are typically oxides (O2-), hydroxides (OH–), carbonates (CO32-) or silicates (SiO44−) of calcium (Ca) or magnesium (Mg). Some examples (with their chemical formulae) are calcitic lime (CaCO3), dolomitic lime (CaMg(CO3)2, quick lime (CaO), and hydrated lime (Ca(OH)2). Many people mistakenly believe it is the calcium or magnesium that works to “lime” a soil (i.e. neutralize acidity), but this is not true. It’s the oxide, hydroxide, carbonate or silicate that removes the H+ ions from solutions and raises the soil pH. The lime reaction is a two-step process, where first the lime dissolves in solution producing an anion. In the second step, the anion reduces the concentration of hydrogen ions (H+) in solution forming a water molecule. The anion produced by dissolving calcitic lime is a hydroxyl (OH–) anion, which then removes an H+ ion from solution forming a water molecule. In this example we can see that the calcium or magnesium actually does not reduce soil acidity, but instead it is the anion associated with the metal that is doing all the work.
What is liquid lime? It is simply a liming product that is dissolved in water. It is typically a high-quality lime with small particle size that dissolves easily in water. Liquid lime usually has a high relative neutralizing value (RNV) so it can quickly modify soil pH. However, since lime is dissolved in water, it typically consists of approximately 50% lime and 50% water by weight. Therefore, per ton of liquid lime applied, you would only be getting ½ ton of lime. If you needed 2 tons per acre of 100% effective lime you would probably need to apply over 4 tons per acre of the liquid lime — well in excess of 700 gallons per acre. That’s a lot of liquid to apply in one application, so you’d probably have to make multiple applications throughout the year to get the amount of effective lime on the field recommended by your soil test. Liquid lime is an actual product with some distinct advantages (fast acting) and distinct disadvantages (takes a lot of material to get the amount of carbonates required). However, CaCl2 is not liquid lime and has no liming ability. When researching the product being marketed in parts of the United States that the farmers were referring to as liquid lime, the product website says “liquid calcium presents growers with an alternative solution to lime applications.” The website also states that “Calcium neutralizes soil acidity.” This is simply not true, because as shown above , it’s not the metal (calcium or magnesium) in lime that reduces the H+activity, but the associated anion.The other sales pitch made was the need for available calcium, an essential plant nutrient. The marketing materials implied that CaCl2 provides calcium you are not getting from your lime, saying “ag lime tends to be insoluble and can take years to break down and show its effectiveness.” It is true that limestone can be moderately soluble, but the finer particles in limestone are soluble and provide available calcium fairly quickly. One manufactured product available in the market is 10% calcium (as CaCl2). At the recommended application rate of 2-5 gallons (11.07 pounds per gallon) per acre, that’s 2.2-5.5 pounds of calcium. There are 800 pounds of calcium in a ton of calcitic lime (assuming 100% effective calcium carbonate). Even low-grade agricultural lime will typically provide a couple hundred pounds of soluble lime in the first year after application. Typically calcium deficiencies are not common for row crop production when proper pH management practices are followed. In summary, as is often the case, the buyer must beware when salesman start making claims that are too good to be true. There is no substitute for lime. The calcium or magnesium in agricultural lime are not what is responsible for its liming action. Furthermore, it is very unusual to need calcium or magnesium fertilizer in Texas soils, which typically have plenty of both available for plant growth with proper pH management. However, if you do need calcium or magnesium, using a calcitic or dolomitic limestone source is a great way to meet those needs.