Medium solution testing is a precise way to monitor the real nutrition status of the rhizosphere with which plant roots intimately take part in exchanging substances. Monitoring the medium solution would thus help in timely adjustment of the fertilization program. Bunt (1986) classified chemical analyses of soilless media into suspensions, saturated media extracts (SME), and displaced soil solution methods; 2 water:1 substrate (v/v) suspension (Sonneveld, 1990), the SME method (Warncke, 1986), and pour-through (PT) extraction (Wright, 1986; Yeager et al., 1983) are the most commonly used medium extraction methods in the three categories, respectively. The procedure of PT described by Wright (1986) is as follows: 1) ensure that the moisture level of the medium is at or near its water-holding capacity; 2) add a sufficient amount of distilled water to the surface of the container so that ≈50 mL of leachate is accumulated; and 3) analyze the leachate collected within 24 h. Compared with other medium extraction methods, PT is much faster to execute and does not require costly equipment. Also, PT does not require physical removal of medium from the container, which can disrupt the root system. It has been used widely on peat-based and bark-based soilless media in laboratories and in nurseries. However, medium testing is rarely performed by Phalaenopsis growers in Japan and Taiwan possibly as a result of the unique chemical and physical characteristics of the substrate—sphagnum moss. Since June 2004, growers in the United States have been allowed to import Phalaenopsis plants grown in sphagnum moss from Taiwan. By importing mature Phalaenopsis in growing medium, the U.S. growers could directly apply cool-temperature forcing without replanting, which resulted in a great saving of labor, potting materials, and greenhouse overhead as well as a faster growth recovery than that of bare-root plants. There is thus a need for a better understanding of the characteristics of sphagnum moss and its fertilization management.
Sphagnum moss is a group of mosses belonging to the Sphagnum genus and could become sphagnum peatmoss after decomposing for years in a wet and cold environment. Dead cells of sphagnum moss are large in volume with thin but firm cell walls, which are excellent for transmitting water and holding shape (Puustjarvi, 1977). These characteristics make it an ideal substrate to retain water and air for epiphytic orchids. The air-filled porosity (v/v) of sphagnum moss is easily influenced by the bulk density with test data exhibiting a range of 15% to 26% under general conditions (Hwang and Jeong, 2007; Yao and Chang, unpublished data). The air-filled porosity of peatmoss (8.1% when moisture is at container capacity) is much lower than that of sphagnum moss, whereas bark has a similar air-filled porosity (20%; Nelson, 2003) as sphagnum moss. The cation exchange capacity (CEC) of sphagnum moss, which ranges from 26 to 120 meq/100 g as the pH of the substrate increases from 3 to 7 (Kubota et al., 1993), is also higher than that of both bark (8 to 60 meq/100 g) and peatmoss (7 to 13 meq/100 g; Nelson, 2003). With properties of both high air-filled porosity and high CEC, the feasibility of applying PT to sphagnum moss is uncertain. The objectives of this study were, first, to ascertain whether PT could be adapted to Phalaenopsis cultivation with sphagnum moss and, second, to develop a standard procedure once feasibility is proved.
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