RandomGemini originated this thread with some questions. Among them were "What does water quality mean with respect to orchids?" It refers to how pure the water is and how acid the water is.
Minerals, mostly calcium and magnesium salts, are dissolved in ground water. Rain has few of these minerals, unless there was a lot of dust in the air. Minerals in water are commonly described with measurements called Total Dissolved Solids or TDS, hardness and alkalinity. Hard and/or alkaline water tends to have more calcium and magnesium in it. TDS does not measure which specific minerals are in the water, but it is almost always calcium and magnesium salts.
Epiphytic orchids tend to prefer water with low to very low TDS. Most of them are not adapted to water with higher salt content.
Ocean shore orchids, and a few from semi-desert areas, often can handle higher TDS in water, because they're adapted to being sprayed with salt water, or having alkaline and hard desert ground water.
Cloud forest orchids, and terrestrials from near streams, often need water even lower in TDS than some other orchids. Examples are many Pleurothallids, Paphs and Phrags. When watered with water high in TDS, these plants begin getting brown leaf tips, which spreads up the leaf until the leaf dries up and dies. This represents excessive salt accumulation in the leaves.
The other factor in water quality is the acidity of the water, measured in pH units. The pH scale runs from 1 to 14 and refers to solutions of things in water. You cannot measure the pH of a solid; only something dissolved in water. Lower numbers are more acid, and higher numbers more alkaline. Your stomach produces hydrochloric acid with a pH of 1. Lye, or sodium hydroxide, when dissolved in water, has a pH of around 14.
Pure water, with no carbon dioxide dissolved, has a pH of 7, and is considered neutral. Distilled water sitting on the shelf is slightly acid because of the atmospheric carbon dioxide dissolved in the water.
Most orchids (most plants in general) do best watered with a pH not too far from neutral. Well water, and most municipal water, has a relatively high pH, and is not always good for orchids. Fertilizer intended for use with tap water often brings the pH down and closer to neutral. Often the many dissolved minerals in water cause the pH to be high. There are plants with definite preferences for alkaline or acid water. Acid-loving plants tend to come from areas with a lot of humus on or in the soil, or in very wet, marshy areas. Alkaline-loving plants tend to come from deserts, like cacti and most succulents. Minerals don't wash away when rainfall is low.
The pH scale is logarithmic, meaning it does not increase at a constant rate: the difference in acidity from 2 to 3 pH units is not the same as the difference between 3 and 4 pH units.
The acidity of water increases 10 times when you move from one pH unit to the next lower number. Moving two pH units lower increases the acidity by a factor of 100. That is what is meant by a logarithmic scale. So, water with a pH of 6 contains 10 times the acid of water with a pH of 7, 100 times the acid of water with a pH of 8, and 1,000 times the acid of water with a pH of 9.
Dendrobium cuthbertsonii... I haven't grown it. But it seems to be a cloud forest orchid. This means it gets rain with very low total dissolved solids. Rain is normally slightly acidic due to dissolved carbon dioxide. It likely grows on patches of moss, which means it has an acid environment. So I would expect this plant to do poorly with most tap water, which is high in TDS and alkaline. Some municipalities have purer tap water, but this is not common.
You can read the water quality report for your tap water utility online. Here you will find the pH and mineral content of your tap water. If you are hoping to grow orchids with tap water, you need to be familiar with your tap water. I can't use my tap water for most of my orchids.
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