Study Finds Root Causes of pH Drop in Orchid Substrate:

Taiwan: Phalaenopsis roots thought to be “major contributor” to pH decline

Phalaenopsis has become one of the most coveted and economically viable floral crops in the world. In Taiwan alone, the export value of Phalaenopsis grew from $64 million in 2009 to $82.6 million in 2010—a 29% increase in just 1 year.

Sphagnum moss has traditionally been used as the main substrate in Phalaenopsis production, but commercial growers are challenged by decreasing pH levels of the moss and worry that the pH drop affects plant health. Scientists Wan-Yi Yen and Yao-Chien Alex Chang from the Department of Horticulture at National Taiwan University, along with colleague Yin-Tung Wang from the Department of Horticultural Sciences at Texas A&M University, investigated the causes of pH decline in Phalaenopsis production in a study published in HortScience. "Some orchid growers consider that the drop in pH may be caused by the decomposition of the moss. However, the decline in substrate pH during production may be the result of the characteristics of the moss, fertilizer applications, and the presence of roots," they explained.

The team designed five experiments that explored the effects on pH decline of fertilizer application, characteristics of the substrate, light exposure, and the physiological nature of Phalaenopsis itself. In four experiments the researchers used mature Phalaenopsis Sogo Yukidian 'V3' plants planted in sphagnum moss in clear plastic pot; an oncidium hybrid was used in the fifth experiment to determine if there was a difference among orchid species in causing pH decline.

Results showed that, although increasing fertilizer concentration resulted in a low initial pH, fertilization itself was not the primary cause of the long-term pH decline. Regardless of whether the sphagnum moss was fertilized, the pH of the substrate without plants increased as time progressed, whereas the pH of the substrate in which living Phalaenopsis plants were growing declined with time.

Other results showed that the magnitude and course of pH decline were different in various substrates, but the pH of sphagnum moss, artificial textile fiber, and pine bark substrates in which Phalaenopsis plants were growing declined with time. Whether the substrate was exposed to light (when planted in clear pots) or not (when planted in opaque pots) had no effect on substrate pH, indicating that algae were not a factor in pH decline.

The researchers concluded that the roots of the plant may be the major contributor to substrate pH decline during production. "The results of our study show that Phalaenopsis roots may be the main cause of substrate acidification, which may have something to do with how originally the epiphytic roots absorb mineral nutrients in their natural habitat."

The study concluded that growth of Phalaenopsis is normal in a substrate with low pH; therefore, raising substrate pH may not be needed for production

The complete study and abstract are available on the ASHS HortScience electronic journal web site:

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