by Stephen Early

When pH, lime and calcium are discussed at orchid meetings, there is often considerable confusion about the different terms and effects. Hopefully this article will explain the differences and remove some of the confusion. pH is a measure of the acidity of a liquid. It is a measure of the number of hydrogen ions in the solution. A neutral solution has a pH of 7, while an alkaline solution has a pH greater than 7 and an acidic solution has a pH less than 7. Each change of 1 in a pH number results in a change of 10 in the number of hydrogen ions in the liquid. Thus a change from pH 7 to pH 5 means that the number of hydrogen ions in the solution has increased 100-fold and the solution has become 100 times more acidic.

Lime The term lime is often used for a number of products that are actually quite different. Lime comes from limestone rock, which is basically calcium carbonate (CaCO3), its most common impurity being magnesium carbonate (MgC03). No purification is done so the exact composition depends on what hill the limestone is dug from. If the limestone is crushed and bagged it is then sold as Garden Lime. If it contains a significant amount of magnesium carbonate, then it is sold as Dolomite Lime.

Calcium carbonate is a stable, relatively insoluble chemical until added to an acidic solution, when the calcium carbonate dissolves and carbon dioxide gas is released; the calcium carbonate only stops reacting with the solution when it has become neutral (pH 7). Hence adding Lime (or Dolomite Lime) to a potting mix will counteract any acidity and will continue to react to maintain neutrality (pH 7) until it is washed out or consumed (much less likely).

The first step in processing limestone is to crush it and heat it. This produces calcium oxide (CaO), a highly alkaline product sold as Lime. I would never use this product as it is dangerous to handle and quite likely to damage plants because of its high alkalinity. It may make the pH of a potting mix far too alkaline for safe use on orchids. A further step is to add water to the Lime to produce Hydrated Lime or Slaked Lime. This is calcium hydroxide (Ca(OH)2), an alkaline solid that is slightly soluble in water. It is not as strongly alkaline as Lime and I know some growers use it. However, it can produce a pH much higher than 7 if added in too large an amount. Both Lime and Hydrated Lime will dissolve in water and over time react with carbon dioxide (a trace component of air) to produce calcium carbonate, particularly when in contact with water. If you wish to add lime I would choose Dolomite Lime, as it can cause little damage and will react only if the potting mix is acidic.

Lime or Dolomite Lime provides calcium only to acidic potting mixes – it provides no calcium if the mix is neutral or alkaline. Lime and Hydrated Lime may add more calcium but only by making the mix very alkaline, which would damage the plants. Calcium nitrate is another source of calcium that is often used. It’s a highly desirable additive, as it provides nitrogen without simultaneously increasing the amount of potassium or sodium present. However, most calcium salts are fairly insoluble so when adding calcium nitrate I would expect much of the calcium to end up as insoluble material not available to the plant.

Although I intend to add lime to my plants, I often forget, so I have looked for another method of simultaneously controlling pH and adding small amounts of calcium. I therefore add a small amount of shell grit {Canunda Shell) to my potting mix. This is a good source of calcium carbonate, which, being fairly coarse, will not wash through the pot when I water. In conclusion, to grow orchids well you need to keep your potting mix neutral (pH 7). This is readily and safely achieved by adding Dolomite Lime to the potting mix. Another method is to add shell grit (Canunda shell) or marble chips to the mix.

from OSCOV Website