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-3 N solution at each irrigation. The nutrient solution applied to half the pots contained a wetting agent (WA), and the remaining pots received no WA. Core samples were removed at 15-cm increments to a depth of 90 cm from the soil beneath the pots. The average leaching fraction (LF) from pots receiving a WA was 0.29 but was 0.26 from pots receiving no WA. However, WA did not affect the leachate NO 3 -N concentration or the total NO 3 -N deposited on the soil beneath; these were most influenced by LF. After week 2, NO 3 -N concentration in the upper 15 cm soil layer was 3.4 times higher with a high LF than with a low LF (30 and 8.8 g·m -3 respectively). At week 10, the NO 3 -N concentration in the 30 to 45 cm soil layer averaged 71.9 g·m - 3 under the high LF and 35.5 g·m Greenhouse crop management practices that contribute to NO 3 -N loading to the soil profile are regarded as a threat to groundwater quality and are of concern to the commercial industry and the public. Surveys conducted in Europe (Molitor, 1990) and the United States (Walker, 1990) report that excessively high N concentrations (>200 g·m -3 ) can be found in the top meter of soil underlying commercial greenhouses. More recently, NO 3 -N con- centrations >230 g·m -3 in the top meter of soil have been measured under decades-old greenhouses in Connecticut (McAvoy, 1991). The leaching fraction (LF) or the portion of the irrigation volume that leaches from a pot greatly influences the electrical conductivity (EC) in the potting medium and the leachate (Ku and Hershey, 1991, 1992; Yelanich and Biernbaum, 1990). Ruter (1992) reported a direct, positive linear relationship between EC and NO 3 -N level in the potting medium extract. One of the few studies to characterize the fate of NO 3 -N in the soil profile underlying a containerized greenhouse crop reported that LF also exerted a direct, positive effect on NO 3 -N accumula- tion and movement in the soil profile (McAvoy et al., 1992). With poinsettia stock plants irrigated with a high LF, NO 3 -N accumu- lated at higher concentrations and moved deeper into the soil profile than with a low LF. Although limited data are available to relate crop management practices to soil loading of NO 3 -N in the greenhouse, many factors are known to affect the water-holding properties of potting media and subsequent leachate quality. For example, irrigation method and medium amendments affect the water-holding capacity of potting media. In one study (Elliott, 1992), the highest effective water-holding capacity for various peat-lite media was observed when overhead irrigation, hydrophilic gels, or wetting agents

journal of the american society for horticultural science

Nitrate Nitrogen Movement through the Soil Profile beneath a Containerized Greenhouse Crop Irrigated with Two Leaching Fractions and Two Wetting Agent Levels