Nutrients necessary for plant growth are optimally available to most plants, including orchids, in a general pH range of 5.5 to 6.8. Most commercial water-soluable fertilizers are made to ensure that the resulting dilute solution falls in this optimal pH range. Well water and most municipal water sources have a pH range of 7.0 to 8.0 and are buffered by naturally occuring dissolved calcium carbonate. Distilled or Reverse Osmosis (RO) water usually has a pH of 7.0, but it contains no buffering agent, as well as any other mineral content. Rainwater also has little mineral content but is usually acidic. The addition of soluable fertilizer to tap water usually causes a mild move of the pH into the acidic range of 6.0 to 6.5. However, the same amount of fertilizer added to reverse osmosis or rainwater can make a drastic swing into the acidic range to pH of 4.5 and lower.

Orchid potting mediums such as fir bark, sphagnum moss, and coconut husk chips have their own mild pH buffering action. Nevertheless, making a fertilizer solution using rainwater or RO water will usually require a special fertilizer or a modification of the pH.

 
     
 

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