During the 1980s, many geranium producers observed a sporadic, unexplained decline in substrate pH. During the same time period, they also reported the occurrence of toxic concentrations of Fe or Mn in leaf tissue (Bachman and Miller, 1995). In organic-based soil-less substrate, pH has a large affect on nutrient availability (Nelson, 2003). As pH decreases, most micronutrients become more available and can become toxic. In a study with ‘Ringo Scarlet’ geraniums, dry tissue Fe, Mn, and Zn concentration increased from 82 to 951, 118 to 9260, and 56 to 1340 mg·kg−1, respectively, as final substrate pH decreased from ≈6.5 to ≈5.5 (Lee et al., 1996). The symptoms of micronutrient toxicity in geranium include reduced leaf and plant size; black spots on young leaves; dark green color, large purplish or black spots, leaf cupping, and interveinal, marginal, or speckled chlorosis on the entire plant; cupping, and interveinal, marginal, or speckled chlorosis or necrosis on older leaves; and in severe cases, complete necrosis of older leaves (Bachman and Miller, 1995; Broschat and Moore, 2004; Lee et al., 1996; Smith et al., 2004).
Sudden pH decline (SPD) describes the situation where crops growing at an appropriate pH rapidly (within 1–2 weeks) cause the substrate pH to shift downward one to two units. Within a greenhouse firm or geographic area, one geranium crop may be affected, whereas the subsequent crops are not, although all crops appear to be grown similarly. Geraniums are the highest valued bedding plant of the $2.53 billion bedding plant industry (USDA, 2005) and SPD can cause severe economic loses or lead to tedious pH adjustments, such as application of flowable lime. The cause of SPD is unknown.
Apparently, an unknown signal(s) triggers an acidification process by roots. Three nutrient deficiencies known to cause plants to acidify substrate include: Fe (Welkie and Miller, 1993), Zn (Cakmak and Marschner, 1990), and P (Schjorring, 1986; Hinsinger, 2001). Pea (Pisum sativum L.), sugar beet (Beta vulgaris L.), and bean (Phaseolus vulgaris L.) lowered root substrate pH from around 7.0 to 4.0 in 6 to 10 h when Fe was missing in the nutrient solution (Landsberg, 1981). The drop occurred in a NO3 − N solution that normally causes the substrate pH to rise. Zinc deficiency has also been shown to cause acidification in dicotyledonous species even when NO3 − was the sole source of N (Cakmak and Marschner, 1990). However, it is unlikely that Fe or Zn deficiency is the cause of SPD because both nutrients are applied sufficiently by most growers and the syndrome typically leads to Fe toxicity, not deficiency. On the other hand, P deficiency seems a likely contributor to the induction of SPD because growers rely heavily on alkaline fertilizers to offset pH decline, particularly in geranium, and P is low or absent in these alkaline fertilizers; i.e., 13N–0.9P–10.8K, 15N–0P–12.5K, and 14N–0P–11.6K (Nelson, 2003).
In addition to low P, there is evidence high temperature might also contribute to the occurrence of SPD. Micronutrient toxicity symptoms have been reported to be more common after high temperature stress (Smith et al., 2004). Taylor et al. (2007) showed that geraniums grown at 26 °C day/22 °C night required more base to maintain hydroponic solution pH than plants grown at 22 °C day/18 °C night, indicating increased acidification. The objectives of this research were to determine if high temperature causes geraniums to acidify the substrate (Expt. 1), if the temperature effect is independent of tissue P concentration (Expt. 2), and to quantify the amount of acidity produced due to high temperature and P stress in hydroponics (Expt. 3).
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