High tunnels (sometimes called hoophouses) in their simplest form are constructed with a framework tall enough to walk under and are covered by clear plastic film, heated by solar radiation, and cooled by passive ventilation. Construction design, materials, and other features vary. High tunnels are used to modify the crop environment allowing season extension (early or late), some exclusion of rain, wind, and insects as well as enhanced crop quality and yield (Lamont, 2005). Protected agriculture under plastic film structures began in the 1950s and has since continued to expand worldwide. There was a 50% increase in the area under high tunnels around the Mediterranean between 1985 and 1995 (Baudoin, 1999). Also during this time, there was a growing interest in high tunnel use in the United States. Although plastic-covered tunnels are widely used for overwintering in the nursery industry in the United States, it is only more recently that the possibility of their use began to be realized by vegetable and small fruit producers (Lamont, 2009). In the early 1990s, research and extension professionals in the Northeast began reporting the potential that high tunnels hold for vegetable producers in the United States (Wells and Loy, 1993). It is estimated that of the 800,000 ha under high tunnels worldwide (Enoch and Enoch, 1999), only ≈5000 ha are in the United States (Carey et al., 2009). Increased high tunnel use in the United States is sure to continue as high tunnels were reported in 45 states in 2007 with ongoing research and demonstration projects underway in 36 states (Carey et al., 2009).
High tunnel crops and soils are often more intensively managed than field crops. Intensified production may increase soil nutrient removal, tillage, and traffic. The effect that this may have on soil quality is uncertain. In a 2006 survey of vegetable, fruit, and flower growers using high tunnels in the central Great Plains, 14% of growers were of the opinion that they had soil quality problems in their high tunnels compared with adjacent fields (Knewtson et al., 2010b). There is concern that covering a soil year-round will result in a buildup of insect pests, soil pathogens, and excess nutrient salt levels (Coleman, 1999). Coleman (1999) discusses soil revitalization options that include soil sterilization, soil removal and replacement, removal of the plastic covering for part of the year, and moving the high tunnel to a new location. Publications circulated among vegetable growers such as Small Farm Today have stated that imbalances occur where soil is covered and that movable tunnels allow “wind, rain, and sun to improve soil health and pest management” (La Mar, 2010). However, the decline of soil quality under high tunnels has not been confirmed by research. University research and extension studies have mainly focused on crop production methods (Carey et al., 2009). Also, most research done under high tunnels in the United States is still fairly new. The question of sustainability of soil quality under high tunnels becomes more important as existing tunnels age and growers ponder whether to maintain structures in their current location or construct new high tunnels at different locations or as growers plan to use high tunnels on a larger scale where frequent structure shifting is less feasible.
It is the objective of this research to determine if the presence of a high tunnel affects soil quality in a silt loam soil after eight years. Because of the design and management of the experimental plots, we were able to investigate soil quality under high tunnels compared with adjacent fields under both conventional and organic management. Measures of soil quality were: pH, salinity, total soil C, and POM C.
Chemical indicators of soil quality include pH and salinization. pH is closely correlated to base saturation and may be used as an indicator of nutritive quality (Singh and Goma, 1995). Exclusion of rainfall that allows leaching makes high tunnels suspect for salinization, so it is advisable to monitor high tunnel salinity (Knewtson et al., 2010b).
Because organic matter influences soil structure, nutrient storage, water-holding capacity, biological activity, tilth, water and air infiltration, erosion, and even efficacy of chemical amendments made to soil (Dumanski and Pieri, 2000), it is commonly used as a biological indicator of soil quality. Soil organic C is used to estimate organic matter (Nelson and Sommers, 1996), and in non-calcareous soils organic C is equivalent to total soil carbon (Loeppert and Suarez, 1996). In this study total soil C was used to indicate soil quality.
Particulate organic matter as an indicator of soil quality has the advantage of a faster response to environmental change than soil organic matter as a whole (Elliott et al., 1994; Wander, 2004). Particulate organic matter is the labile organic matter of size fraction 53 μm to 2 mm. Gregorich and Janzen (1996) cited four studies that showed greater resolution and sensitivity in measurements of POM change compared with organic matter change. Particulate organic matter has been correlated to microbial biomass (Wander and Bidart, 2000), C and nitrogen mineralization (Bremer et al., 1994; Janzen et al., 1992), and soil aggregate formation and stability (Waters and Oades, 1991), demonstrating that increased POM indicates improved soil quality.
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