In its simplest form, a high tunnel is clear plastic covering a frame high enough to walk inside, heated by solar radiation, and cooled by passive ventilation (Wells and Loy, 1993). Construction designs, materials, and other features vary. Producers use high tunnels to modify crop environment. The primary function is to elevate temperatures to allow earlier planting in the spring, earlier ripening, and extend fall harvests (Kadir et al., 2006; Lamont, 2005). Other benefits include wind and rain protection, reduction of some diseases and insects compared with the open field, and typically enhanced crop quality and yield (Lamont, 2005; Wells and Loy, 1993).
The number of vegetable, fruit, and flower growers using high tunnels in the central Great Plains has increased steadily in the past decade (Knewtson et al., 2010). These growers have expressed favorable high tunnel experiences, and there is continued interest in expanding production and improving high tunnel management (Knewtson et al., 2010). Variety and fertility trials in high tunnels in Kansas, Missouri, and Nebraska began in 2002 (Jett, 2004; Kadir et al., 2006; Zhao et al., 2007). However, the effect that cropping under high tunnels has on soil quality is unknown. High tunnel crops and soils are often more intensively managed than field crops, and the growing season is longer. Intensified production may increase soil nutrient removal, tillage, and traffic. There is concern that this combined with covering soil year round will result in a build up of insect pests, soil pathogens, and excess nutrient salt levels (Coleman, 1999). Strategies for soil revitalization have included soil sterilization, soil removal and replacement, removal of the plastic covering for part of the year, pesticide applications, and flushing irrigations (Coleman, 1999). Methods and frequency for physically moving high tunnels were discussed by Coleman (1999). Some growers have expressed concern that soil tilth seems to decline under high tunnels over time (Carey, unpublished data). However, a decline of soil quality has not been confirmed by research.
One strategy to determine if high tunnels alter soil quality is to make paired comparisons of soils from individual high tunnels and adjacent fields. Comparison using high tunnels of varying age would allow evaluation of possible relationships between soil quality and the length of time that the soil was covered. Soil quality comparisons require appropriate indicators to quantify quality. Indicators may include measures of crop productivity or of chemical, physical, or biological soil qualities (Lal, 1994). The use of crop production indicators requires years of data (Dumanski and Pieri, 2000) and so is not useful as a survey tool.
Chemical indicators of soil quality include measurement of salinity. A combination of excessive fertilizer applications, irrigation, and poor drainage can induce salinity (Brady and Weil, 1999), so in some high tunnels, it may be advisable to monitor salinity. Nutrient analysis would not be useful in this study because of potential fertilizer application differences between a high tunnel and the field.
Soil organic matter (SOM) is a commonly used biological indicator of soil quality. 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). Particulate organic matter (POM) is labile organic matter of size fraction 53 μm to 2 mm, and it has the advantage as an indicator of soil quality of faster response to environmental change than SOM (Elliott et al., 1994; Wander, 2004). Changes in POM can be used to predict trends in SOM. Gregorich and Janzen (1996) cited four studies that showed greater resolution and sensitivity in measurements of POM change compared with SOM change. POM has been correlated to microbial biomass (Wander and Bidart, 2000), carbon 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. Evaluation of the portion of the soil carbon in the POM size fraction can be used for comparison of locations or for comparison of changes over time.
The overall objective of the current study was to evaluate soil quality in high tunnels in the central Great Plains. Soil quality was assessed by grower perception and measures of the soil quality indicators of salinity and POM. To assess grower perception, we conducted a survey of producers and asked them about their soil conditions and management practices. We complemented the written questionnaire by sampling soil and comparing quality attributes of soils from established high tunnels and adjacent fields at the farms of survey respondents in four states.
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