- Production Systems: organic agriculture
- Soil Management: soil analysis, soil quality/health
Comparison of soils from high tunnels and adjacent fields determined that soil quality was not negatively impacted by high tunnel structures over time. Soil quality, measured by grower perception, soil salinity, water stable aggregates, and particulate organic matter carbon, did not generally decline in high tunnels. Eighty-one growers in Kansas, Missouri, Nebraska, and Iowa participated with questionnaire responses and seventy-nine allowed soil collection at their farm. The questionnaire collected information about grower management practices and soil quality observations. Analysis showed that soil quality is due to factors more complex than the duration of high tunnel use or single management practices.
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. Horticulturists use high tunnels to modify crop environment. The primary function is to elevate temperatures a few degrees. This allows earlier planting in the spring, early ripening and extended fall harvests. Other benefits include wind and rain protection, reduction of some diseases and insects compared to open field, and typically, enhanced crop quality and yield (Lamont et al., 2005; Wells and Loy, 1993).
Much of the research and published high tunnel experience in the US has been from the northeastern states (Lamont et al., 2002). University researchers in Kansas, Missouri, and Nebraska began doing variety and fertility trials in high tunnels in 2002 (Jett, 2004, Kadir et al., 2006, Zhao et al., 2007). The number of growers using high tunnels in the central Great Plains has increased steadily in the past decade. Midwest vegetable, fruit, and flower growers report favorable high tunnel experience and with each passing year the number of high tunnels in use has increased.
The effect of cropping in high tunnels over time on soil quality is uncertain. 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. Some growers are concerned that covering soil year round will result in a buildup of insect pests, soil pathogens, and excess nutrient salt levels (Coleman, 1999). Soil revitalizing options have included soil sterilization, soil removal and replacement, removal of plastic covering for part of the year, pesticide applications and flushing irrigation (Coleman, 1999). Methods and frequency for physically moving high tunnels were discussed by Coleman (1999). However, the necessity of moving the high tunnel because of declining soil quality has not been confirmed by research.
Indicators of soil quality
Soil quality comparisons require appropriate indicators to quantify quality. Indicators may include measures of crop productivity or of physical, chemical, or biological qualities (Lal, 1994). The use of crop production indicators requires years of data (Dumanski and Pieri, 2000) and so may not be useful as a survey tool. To determine if high tunnels alter soil quality, paired comparisons can be made of soils from individual high tunnels and adjacent open fields. Comparison using high tunnels of varying age would allow evaluation of possible relationships between soil quality and time of soil covering.
Physical indicators considered included: water infiltration, penetration resistance, modulus of rupture, and analysis of water stable aggregates. Penetration resistance measures the mechanical impedance plant roots may experience in soil. Quantitative measurements of resistance have been correlated to crop yields and tilth (Davidson, 1965). Modulus of rupture is a measurement used to evaluate the cohesion of dry soil. Cohesion forces relate to soil surface crusting and clod formation (Reeve, 1965). The stability of soil aggregates will determine the existence of soil macropores. Large pores in the soil generally favor good infiltration rates, aeration, and tilth (Kemper and Rosenau, 1986). A combination of soil drying, wetting, and sieving can be used to measure aggregate stability.
Chemical indicators considered included: pH and salinization. Soil nutrient analysis would not be useful because of potential fertilizer application differences between high tunnel and field. pH is closely correlated to base saturation and may be used as an indication of nutritive quality (Singh and Goma, 1995). A combination of irrigation and poor drainage can induce salinity (Brady, 1999), so in some high tunnels it may be advisable to monitor salinity.
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). Soil organic carbon is used to estimate organic matter (Nelson and Sommers, 1996). In non-calcareous soils total carbon is equivalent to organic carbon (Loeppert and Suarez, 1996). Particulate organic matter (POM) is labile organic matter of size fraction 53 microm – 2 mm, and 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 to SOM change. Particulate organic matter has been correlated to microbial biomass (Wander and Bidart, 2000), C and N 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. The ratio of POM C : total C can be used for comparison of locations or for comparison of changes over time.
The purpose of the current study was to evaluate soil quality in high tunnels in the central Great Plains, to determine if problems increase with tunnel age.
To achieve this our specific objectives were:
(1) determine suitable indicator of soil quality for comparison of high tunnels and adjacent fields on farms,
(2) assess grower perceptions of soil quality,
(3) quantify and compare soil quality in high tunnels and their adjacent fields,
(4) determine if soil quality declines over time under high tunnels, and
(5) investigate possible relationships between soil and crop management and soil quality.