Lime, Lime, and More Lime

In a related article, many growers suggested the use of lime to raise the pH (low is acid, high is alkaline) for some ferns. Different growers suggested that Adiantums, Aspleniums, and even Pyrossias could benefit from added lime. But, it is imortant to know why lime is good and what lime is best. Improperly used, lime can actually inhibit plant growth.

The pH of a soil is a measure of acidity or alkalinity. The numbers reported from a soil test generally range from 5.0 to 8.5. A pH of 7.0 is neutral. Those below 7.0 are acidic and those above 7.0 are alkaline. A pH of 6.5 is ideal for many plants; the range from pH 6.0 to 7.0 is good for vegetables. Below 5.5 and above 7.5, soil treatment or modification often is necessary.

Most soils contain appreciable amounts of acidic components (hydrogen H+ and aluminum Al+++) that can be toxic to plants. Lime neutralizes those acidic components, making it a great soil anti-acid. Furthermore, plants utilize applied fertilizers more efficiently on soils that are properly limed.

You will find reference to at least four kinds of lime in agriculture: crushed limestone [Calcium carbonate (CaCO3)], dolomitic lime [Calcium-magnesium carbonate (CaCO3--MgCO3)], burned or quick lime [Calcium oxide (CaO)], and slake or hydrated lime ]Calcium hydroxide (Ca(OH)2)]. Of these, dolomitic lime appears to be the best additive to raise pH and promote growth.

This is because dolomitic lime includes manganese and because dolomite lime acts slowly and continuously, and hydrated lime acts almost instantly. Use hydrated lime instead of dolomite lime for faster results, but don't use too much; you can create toxicity problems, which won't arise with dolomite lime, by using too much hydrated lime. Always use fine dolomite lime, since coarser grades take years to have a substantial effect. Dolomite lime is available at most nurseries.

Do not over-lime. It's best to apply lime according when a soil test shows that the ph for your fern is too low. Excess lime is likely to result in iron chlorosis of plant. If plants appear chlorotic after a lime application, low manganesecan also be the problem. Applying lime raises the soil pH and reduces the availability of manganese. In such cases, manganese should be applied to the soil. Soils with adequate manganese levels should not show symptoms following lime application.

One last note. Gypsum cannot be used instead of lime. Gypsum, also known as landplaster, is calcium sulfate. It is an economical source of calcium and sulfate but cannot be used as a liming material because it does not have the capability to neutralize soil acidity. In fact, applying gypsum to an acid soil (pH less than 5.5) can have adverse effects on certain plants by displacing soil aluminum, which is toxic to plant roots. Gypsum is frequently applied to provide supplemental calcium. But, there is no substitute for lime in neutralizing soil acidity.