The Importance of pH to Orchid Growth

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A monthly growers advice column by Courtney Hackney.

Few successful orchid growers can successfully explain why pH is important to growing orchids.  If one looks up pH in the dictionary, the definition has to do with the number of hydrogen ions in water; a fact that has little meaning to most orchid hobbyists.

The pH of water used to grow orchids is important and so is the pH of the medium on which orchids grow.  The vast majority of hobbyists use the medium available plus whatever water they have and never understand pH because the combination of medium and water they are using is well within the ideal range for most orchids. 

Many years ago, Ralph Wasdon, was known as one of the best orchid growers in Eastern North Carolina.  He was noted for using only K-Mart generic fertilizer, one of the cheapest around.  Other growers, noting his technique tried to duplicate his growth without success.  Ralph knew nothing of pH, but by trial and error had found the right combination of growing medium, which combined with his water source and fertilizer provided an almost perfect pH balance for the absorption of nutrients.  He used very dilute solutions of fertilizer, but his perfect 6.2 pH was ideal for nutrient uptake.

There are a few hobbyists who have extremely poor water or who decide to try a new type of fertilizer, growing medium, or pesticide/fungicide who do need to understand pH.  Some water sources are extremely basic or acidic and there are a few fertilizers that contain excess micronutrients.  Micronutrients can be toxic under extreme water pH.  

Most hobbyists that try to improve their growing by testing water for dissolved solids or pH fail to appreciate that it is the pH of the medium combined with water that is most important.  While water source is one component, most water and nutrient uptake occurs where roots are in contact with the medium.  Here, the pH may change dramatically from that of the applied water.

In peat based media for example, the breakdown products of peat lead to acid conditions that may become extreme.  If micronutrients are in the water source or applied as fertilizer they may become so soluble under acidic conditions that the orchids receive toxic levels.  This can be exacerbated by using some of the high nitrogen Cal-Mag fertilizers, especially blended for orchids.  These fertilizers when mixed with water lower the pH.  Typically this is not a problem if the water source is full of minerals. If the source is rainwater or DI water the acidity can be so extreme that orchid roots can be killed.  Adding a solution that raises pH can produce fantastic growth in orchids, but requires an understanding of pH.

Many pesticides are most effective at a specific pH, usually slightly acidic.  One popular fungicide, Kocide can be toxic under a very acidic pH, but very effective if the pH is over 7.  Kocide contains copper that is soluble and taken up by plants at lower pH values.

So what should the average orchid hobbyist know and do about pH.  If your orchids are growing well, the answer is nothing.  If you begin experimenting with new fertilizers, media or pesticides a simple pH meter may prevent you from damaging your orchids and make you a better grower.

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