Final Report for GNC05-048
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.
Growers with more than two years of experience with high tunnels were sought out in Kansas, Missouri, Nebraska, and Iowa to gather information about high tunnel management and grower perceptions of soil quality. A thirty-six question survey was offered online beginning June 2005 as a link from www.hightunnels.org, a website maintained by Kansas State University. It was also offered in booklet format at the Great Plains Vegetable Growers Conference held in St Joseph, Mo., in January 2006. Contact information of vegetable producers possibly using a high tunnel was provided by research and extension agents in Kansas, Missouri, Nebraska, and Iowa and by other growers. These growers were contacted by telephone and those who had used high tunnels for more than two years were asked to participate in the study by completing the questionnaire and allowing soil samples to be collected from their high tunnels and adjacent field. Because the objective was to investigate the effect of high tunnels on soil quality, high tunnels that had been in use for at least three years and were within easy driving distance for soil collection, were of more interest. Survey participants included growers new to high tunnel production and growers from outside the four-state region, but they were not actively sought out, and were thus purposely under-represented in the survey.
The questionnaire was used to collect information about high tunnel age, size, and number, crop history, nutrient management, organic additions, tillage, irrigation, and perception of soil quality and soil observations such a as surface deposits, crusts, or clods, hardpan formations, and water infiltration. Growers estimated the amount and frequency of organic matter additions made to high tunnels. This was an open ended question with responses given in units preferred by the grower. These estimates were converted to uniform units using conversion factors from Parnes (1990). Organic additions were divided into four categories based on annual application rates. Categories were less than 5000, 25000, 97500 and excess of 97500 kg ha-1 (100, 500, 2000 lb/1000 ft2). Respondents could skip questions or respond to a query as uncertain. Growers were classified as those who self identified as having soil quality problems and those who did not.
Preliminary assessment of soil quality indicators
Soil quality indicators were tested on soil under high tunnels and adjacent fields at the Kansas State University Horticulture Research and Extension Center, Olathe, and at the John C Pair Horticultural Center at Haysville, 8 km south of Wichita, Kansas.
The high tunnels at the Olathe research center were established in 2002, on a Kennebec silt loam soil (fine-silty, mixed, superactive, mesic Cumulic Hapludolls) that was formerly pasture. Soil was collected from six high tunnels and six plots in the adjacent field. The tunnels and field plots had been largely managed with matching crops. Half of the high tunnels and field plots have been managed with organic amendments and half with conventional amendments. Researcher perception of increased clod formation under the Olathe high tunnels indicated the possibility of declining soil quality.
At Haysville, Kansas, four high tunnels and four matching plots in the adjacent field were established in 2002, on a Canadian-Waldeck sandy loam (coarse-loamy, mixed, superactive thermic Udic Haplustolls, and Fluvaquentic Haplustolls) that was formerly used for vegetable production. The tunnels and field plots have been managed with matching crops and conventional amendments.
Soil quality indicator data were analyzed using a mixed analysis of variance procedure (SAS 9.1, Statistical Analysis System Institute, Cary, NC) with a location (high tunnel or field) variable for Haysville data, and location and management variables for Olathe data.
Indicators of soil quality
Soil texture was determined using the Bouyoucos style hydrometer method (Gee and Bauder, 1986). Soil pH was measured in a 1:1 soil and water slurry. Salinity was measured as electrical conductivity in water extracted from a 1:2 soil and water slurry (Rhoades, 1996).
Soil penetration resistance was measured using a cone penetrometer (Soiltest, Inc., 1978). Water infiltration was measured as rate of a volume of water receding in metal rings pushed into the soil.
Modulus of rupture was measured using method and apparatus described by Reeve (1965). Soil was oven dried and ground to pass a 2 mm sieve. Laboratory analysis of modulus of rupture was replicated four times for each sample.
Soil for water stable aggregate (WSA) analysis was air dried before being passed through an 8 mm sieve and caught on a 4.76 mm sieve. A sample of this sieved soil was placed on a nest of four sieves (4.76, 2, 1, and 0.2 mm mesh) in a Yoder (1936) type sieving machine. Samples were submerged for 10 min before being sieved - raised and lowered 3.8 cm about 30 times per minute while submerged – for 10 min. Mean weight diameter, the unit for expression of WSA, was calculated as the sum of products of (1) the mean diameter of each sieve size fraction (6.38, 3.38, 1.5, and 0.6 mm) and (2) the portion of the total dried sample weight in that corresponding size fraction (Kemper and Rosenau, 1986).
Soil carbon was measured after sample combustion with a TruSpec CN 2000 (Leco Corp, St. Joseph, Mich.). The particulate organic matter fraction (POM) was separated by moist sieving soil samples dispersed in 0.5% sodium hexametaphosphate through a 53 um sieve (Gregorich and Ellert, 1993). Sieves were rinsed with distilled water so clay and silt size particles drained out. Sand and POM were retained on the sieve. This was dried at 55 °C and ground with a mortar and pestle. Carbon was measured in POM after combustion with a TruSpec CN 2000.
Soil analysis for farms
Soil was collected from high tunnels and adjacent fields on 79 farms in Kansas, Missouri, Nebraska, and Iowa in the autumn of 2006 (Fig. 1). Soil collection was focused on high tunnels that had been in place at least three years. A few high tunnels in use for less than three years were included in the soil collection (Fig. 2). These were mainly from farms with high tunnels erected over a series of years. Locations where soil under the high tunnel was not that of the adjacent field (e.g. a creek bottom soil had been brought in) were not included in the data set. Soil samples were bulked after at least five random collections within crop rows. Soil pH, texture, total carbon, and POM were determined in soil collected to 15-cm depth with a soil probe. Soil was collected with a trowel from the surface 2-cm for salinity analysis. Soil samples collected with a trowel to a 15-cm depth and held by an 8 mm sieve were used for WSA analysis.
Results were analyzed using SAS 9.1 program for correlations between the ratio of quantified quality indicators for soil samples from under high tunnels and adjacent fields, and tunnel age, soil characteristics (pH and texture), and information about management practices and observations of soil conditions as reported for that location in the grower questionnaire. Statistical analysis was done using t-tests with binomial data, Chi-Square test of independence with categorical data, and correlations with numeric data.
Results and Discussion
Determination of quality attributes for comparing high tunnel and open field soils
Preliminary evaluation was conducted at university research plots at Olathe and Haysville, Kansas. Particulate organic matter carbon and water stable aggregates were indicators that could measure differences between high tunnel and field soil quality. Results from testing soil quality indicators at the KSU research stations showed that some of the quantification methods were not practical for our purpose. Management is not uniform across a high tunnel as several crops may be grown in a season. Areas with different crops may be irrigated on different schedules and tilled at different times. Tillage differences can be overcome when sampling research station plots that have matching crops, but soil water content under the high tunnels and in the adjacent fields may differ depending on irrigation schedules or precipitation in the fields. Comparison of indicators strongly influenced by moisture and tillage is practically impossible on farms where little attempt is made at identical management for high tunnels and adjacent fields.
Summary of preliminary studies in quantification of soil quality indicators used to compare high tunnels and field soils.
- pH - Effect significant statistically, but not for practical purposes.
- Electrical conductivity - Significantly affected by HT at Olathe and Haysville.
- Penetration resistance - Dependent on soil moisture and tillage.
- Water infiltration - Dependent on soil moisture and tillage.
- Modulus of rupture - Not significantly affected by HT or organic management at Olathe.
- Water stable aggregates - Not significantly affected by HT or organic management at Olathe, but significantly affected by HT at Haysville.
- Particulate organic matter - Significantly affected by HT presence at Olathe and Haysville.
- POM C : Total C - Significantly affected by HT presence at Olathe and Haysville and by management (organic vs. conventional) at Olathe.
Soil quality in high tunnels on farms in the central Great Plains: Grower perception of soil quality
High tunnel age in situ ranged from one to fifteen years among survey respondents. The median grower experience with high tunnels was four years. Growers were asked in the survey about their production experience in their oldest high tunnel compared to outside in the adjacent field.
Fifty-six percent of respondents were of the opinion that they did not have soil quality problems in their high tunnels compared to adjacent fields. Fifteen percent perceived problems. The remainder were uncertain if they had experienced soil quality problems.
Growers who did and did not perceive their soil to have quality problems reported observation of increased mineral deposition, clod, crust and hardpan formation in high tunnels. Hardpans were reported by 32% of respondents. Mineral surface deposits were seen in 30% of high tunnels. Clod formation was reported to be worse in high tunnels compared to outside by 12% of respondents, and surface crusting by 13%. Water infiltration was a concern for 13% of growers. Among growers also reporting general soil quality problems, there was a larger portion also reporting adverse soil observations.
Mineral surface deposits reported by growers may indicate salt accumulation near the soil surface. To verify this we measured electrical conductivity (EC) in the upper 5 cm of soil at 63 farms, which included 93 high tunnels. Soil with EC greater than 4 dS m-1 is considered saline (Brady and Weil, 1999). The highest EC measured in field soil was 2 dS m-1 (Fig. 3). From this it can be concluded that soils at the farms evaluated were not inherently saline.
Salinity in the high tunnels was found to be slight and mostly superficial. None of the high tunnel soils were saline in the upper 15 cm. All but five had an EC less than 2 dS m-1 in the upper 15 cm.
Analysis did show salt accumulation in the surface 5 cm. Of the high tunnels sampled 26% had an EC greater than 2 dS m-1, and 3% had an EC greater than 4 dS m-1 in the upper 5 cm. This surface accumulation could potentially have deleterious affects on seed germination or transplanted seedlings. A 10 % yield reduction may occur in tomato crops with soil EC 4 dS m-1 and in lettuce at 2 dS m-1 (Bernstein, 1964). Because the salt accumulation we found was in the surface 5-cm, most growers can avoid yield reduction by leaching salts deeper into the soil profile with heavy irrigation before planting.
The surface mineral deposit reported by 30% of the survey respondents was not a cause for alarm. Electrical conductivity was not different (p = 0.34) in high tunnels with and without visible surface minerals. The mineral deposition at the surface could be carbonates, or salts, but as both pH and salinity were within acceptable limits at nearly all locations the presence of a surface mineral deposit is fine.
High tunnel salinity was correlated to soil clay (p = 0.04) and total carbon (p = 0.05) content. These soil components are responsible for most cation exchange, so this is reasonable. Fertility management significantly affected salinity (p = 0.01). Growers who self identified as using only organic soil amendments had a mean EC of 1.16 dS m-1 in the soil upper 5 cm, compared to conventional fertilizer utilization with a mean EC of 1.85 dS m-1. The higher salinity of conventional fertility management is acceptable for vegetable production. It is interesting to note that the chance of having increased salinity was lower with organic nutrient amendments. Salinity was not significantly correlated to high tunnel age (p = 0.96).
Water stable aggregates
Analysis of water stable aggregates (WSA) was done on soil from twelve farms. This included nineteen high tunnels ranging in age from 2 to 12 years. Five of the growers (representing eight high tunnels) self identified as experiencing soil quality problems.
An increase in the unit mean weight diameter (MWD) indicates increased aggregate stability. Aggregate stability was similar between high tunnel and adjacent field for four high tunnels (giving a HT:Field ratio close to 1), declined under two high tunnels and was greater under the remaining thirteen high tunnels.
The high tunnel MWD and MWD high tunnel : adjacent field were not significantly correlated to the age of the high tunnel structure (p = 0.917 and 0.713, respectively). We concluded that soil aggregate stability may differ between high tunnel and field, but this difference is due to factors more complex than just the duration of high tunnel use.
The average MWD of water stable soil aggregates was higher in high tunnels and fields of growers who self identified soil quality problems. However, it is interesting that the MWD ratio of high tunnel to adjacent field was higher (p = 0.059) for the group that did not identify with soil quality problems. The perception of soil quality was based on problems under a high tunnel compared to the adjacent field, so analysis of this subsample of 12 growers may indicate a correlation between WSA and grower perception of soil quality. But the MWD ratio of high tunnel : adjacent field was not significantly related to reports of specific quality problems like clod formation (p = 0.317), surface crusting (p = 0.508), or hardpan formation (p = 0.349).
Total C measured under high tunnels and in adjacent fields ranged from 10.7 to 125 g C kg-1 soil. Eighty percent of high tunnels were found to have higher total C, with 16% having double the amount of C, compared to adjacent fields. Many growers give high tunnels priority when making organic additions. This generalization is supported by statistical analysis that showed significant correlation between total C in high tunnels to total C in the adjacent field (p = 0.001) and the amount of organic matter growers estimated as having added in the high tunnel (p = 0.005). Total C in the high tunnel is a function of both the original base level of soil C, thus the correlation to C in the adjacent field, and the amount of organic matter added during high tunnel production. Comparison of the ratio of total C in high tunnels and fields, with sets of varying high tunnel age, would not indicate a high tunnel effect over time, but would rather reflection grower management. Rate of organic decomposition could be affected by the presence of a high tunnel and be less dependant on grower management. This would be reflected by POM C:total C ratio comparison.
Particulate organic matter carbon
Particulate organic matter made up 10 to 67% of the total C under high tunnels. In 78% of the high tunnels it made up more than a quarter of the total C. In the fields, the POM was 10 to 89% of the total C. Particulate organic matter made up 25% or more of the total C in 48 % of the fields. Particulate organic matter has been observed to make up 10% of total soil C in long-term arable soil and 40% under grassland (Christensen, 1996).
The high percent of POM in many of the locations we sampled probably indicates recent additions of organic matter not yet decomposed. Soil C and organic amendments added to soil were correlated to POM. Particulate organic matter in the high tunnel was significantly correlated to total C in the high tunnel (p=0.0001), as well as to POM (p = 0.004) and total C (p = 0.04) in the field. Particulate organic matter was also correlated to the estimated amount of organic matter added to the high tunnel (p = 0.0003).
High tunnel POM was not correlated to the number of years a high tunnel had been in location (p = 0.61).
High tunnel : adjacent field particulate organic matter carbon
The ratio of POM C : total C under high tunnels was compared to that in the adjacent field. The high tunnel : adjacent field ratio of POM C fraction (POM HT:Field) ranged from 0.38 to 3.2. At 80% of locations sampled, the POM C fraction was higher in the high tunnel than the adjacent field. As representative of labile organic matter, increased POM C is usually considered an indication of improved soil quality. There, thus, seems to be a general trend toward improved soil quality in high tunnels. This POM trend was not limited by soil texture. The POM C fraction in high tunnel compared to field was not correlated to soil sand percent (p = 0.33) or clay percent (p = 0.75).
There is poor correlation between the POM high tunnel : adjacent field ratio and high tunnel age (p = 0.33). Particulate organic matter fraction size did differ between high tunnel and adjacent field, but this difference was not because of the length of time a high tunnel covered the soil. This was still true when the data was separated in to two categories based on POM C fraction in the high tunnel being higher, or lower, than in the field. Age of high tunnel did not effect POM HT:Field in either category.
The high tunnel : field ratio of POM fraction did not differ between growers who did and did not consider their high tunnel to have declining soil quality (p = 0.45). It was also not correlated to observations of soil clods, surface crusts, surface mineral deposits or hardpan formation.
Influence of management practices on soil quality
Organic matter additions in the high tunnels affected POM in the high tunnel, but it was not correlated to POM HT:Field (p = 0.98). The increase in POM C fraction in high tunnels compared to fields may be related to a reduced rate of organic decomposition in high tunnels. Organic decomposition could be retarded by environmental factors that adversely affect the soil microbial population. A high tunnel fallow period without irrigation would be such an example. However, most fallow coincided with the cold winter months, so the effect of fallow time was not strong. Fallow months were not well correlated with POM HT:Field (p = 0.791), nor the high tunnel POM fraction (p = 0.494).
Comparisons were made with the soil quality indicator POM HT:Field and various high tunnel management practices as reported by the growers in the questionnaire. Management practices as single factors did not significantly affect POM HT:Field. Soil quality as measured by POM HT:Field was analyzed for correlation with organic management, organic amendment amount, frequency of organic amendment application, months a high tunnel is cropped annually, tillage depth, tillage frequency, crop rotation, cover crop use and irrigation leaching. It seems that the high tunnel system is so interrelated that the effects of single management practices are diluted.
Observation of adverse soil conditions (i.e. increased crust, mineral deposit, clod or hard pan formation) influenced grower perception of soil quality, but was not correlated with measures of soil quality. Management practices were poorly correlated to measures of soil quality and observations of adverse soil characteristics.
Only the frequency of organic matter application was possibly correlated to adverse soil characteristics. Observation of adverse soil characteristics increased with more frequent organic amendment applications. However, frequency categories were skewed toward annual application and combined with the low number of adverse soil reports statistical results were mathematically suspect. With six categories of organic application frequency (never, once in four years, once in two years, annually, twice a year, more frequently) and binomial soil observations (observed: yes or no) there is a total of twelve categories for this statistical computation. Half the growers were in the category with annual organic matter application and no adverse soil observation and eight categories had five or fewer growers.
Although we can not account for the effect of single management practices, with whole farm variability there is evidence that age does not cause a decline in measured soil quality. This is true for older high tunnels (in use more than seven years) and newer high tunnels, and with organic and conventional management. The possible exception to this is in the measure of salinity. However, salinization was generally found to be manageable in high tunnels.
This research was conducted with a limited number of soil quality indicators. It may be found that high tunnel age influences soil quality as measured by some other indicator. Pathologic or pest problems may also become factors that influence high tunnel management in the future.
Particulate organic matter carbon was a good indicator of soil quality for comparison of high tunnel and field soils based on literature and our analysis of research locations with matched high tunnel and open field plots. Soil carbon in a high tunnel was dependent on field C and organic amendments. Water stable aggregate analysis was also a potentially good indicator for high tunnel and field location comparison.
Fifteen percent of growers were of the opinion that there was a soil quality problem in their high tunnel compared to the adjacent field. Fifty-six percent did not perceive soil quality problems. The remainder of growers surveyed were uncertain about their soil quality.
The particulate organic matter fraction increased under most high tunnels but decreased under some. Water stable aggregates were measurably different between high tunnel and field, but the difference was due to an increase under high tunnels in some cases and a decrease in others.
Soil quality as measured by grower perception, salinity, water stable aggregates, and particulate organic matter carbon as a fraction of total carbon were not effected by age of a high tunnel in comparisons between high tunnel and adjacent field. Particulate organic matter C : total C was not correlated to high tunnel age, soil type, organic input quantity, or fallow time.
Soil pH was not negatively affected by high tunnel structures. Salinity can be a problem in high tunnels, but in the geographic region of our study it is manageable.
Soil quality as measured in this study was not negatively impacted by high tunnel structures over time. High tunnels in fixed locations for up to fifteen years continued to maintain acceptable quality. We conclude that soil quality can be successfully managed in stationary high tunnels on the central Great Plains.
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[There are figures and tables associated with this report. Copies of the report that include those figures and tables may be obtained by contacting the north central regional SARE office.]
Educational & Outreach Activities
Knewtson, S. 2008. Soil management in high tunnels of the central Great Plains. A presentation at the Great Plains Vegetable Growers Conference, 10-12 January 2008, in St Joseph, Missouri.
Knewtson, S. and E. Carey. 2007. Soil Quality in High Tunnels: Producer perception and reality in the central Great Plains. A presentation at the annual meeting of the American Society for Horticultural Science, 16-19 July 2007, in Scottsdale, Arizona.
Knewtson, S. 2007. Letter sent to each grower who participated in soil collection. The letter included analysis results of soil pH and salinity for the growers high tunnel and adjacent field and general comments on salinity management.
Knewtson, S.J.B. 2008. Studies in vegetable and high tunnel production on the central Great Plains. A Dissertation from Kansas State University.
Publications in academic journals are anticipated from this research.
The conclusion of this research was that the presence of a high tunnel does not out weigh other management factors in effects on soil quality in the central Great Plains. Soil quality can be successfully maintained, and may even improved, in a high tunnel. It is therefore unnecessary to move a high tunnel to a new location as a preventative measure to maintain physical soil quality.
Publication of our results demonstrating sustainable soil quality in high tunnels may encourage the use of high tunnels. High tunnels were formerly a novelty on the central Great Plains. Our research may be part of the process of high tunnels being accepted as a standard practice among growers of horticultural crops in soil.
A secondary impact was the opportunity during farm visits and informal discussions to increase awareness among growers of the services that university research and extension personnel have to offer.
The economic benefit of this research was that it assured growers of the sustainability of soil quality under high tunnels. This may be an added incentive to a grower contemplating the addition of a high tunnel to his farm system.
Eighty-one growers managing 185 high tunnels in Missouri, Kansas, Nebraska, and Iowa participated in a survey about their high tunnel management practices. The survey of growers was conducted from 2005 to 2007. The average grower had four years of high tunnel experience. The oldest tunnel still in use was fifteen years old. Seventy-nine farms with high tunnels were visited in Missouri, Kansas, Nebraska, and Iowa in the autumn of 2006.
Soil quality as measured in this study was not negatively impacted by high tunnel structures over time. High tunnels in fixed locations for up to fifteen years continued to maintain acceptable soil quality. We conclude that soil quality can be successfully managed in stationary high tunnels on the central Great Plains.
We encourage the use of high tunnels as part of horticulture production systems on the central Great Plains.
Areas needing additional study
In our original research proposal we planned to compare the effect of high tunnel production on crop quality as measured by mineral content in plant tissue using spinach as a model crop. This research would be of interest to growers. If it were found that high tunnels improve the nutritive value of crops, growers who sell “locally grown produce” from high tunnels would benefit from this additional selling point. Unfortunately we were not able to pursue this area of research. Twice our spinach crops failed in fields adjacent to high tunnels, so we were unable to compare high tunnel and field spinach. This research was part of a graduate student’s program of study and time schedules did not allow further pursuance of this portion of the project. It is our opinion that such a study would be of interest to growers in the central Great Plains.
Analysis of soil quality during this project did not include pathologic or pest measurements. It may be of interest to include these in a study in the future. High tunnel users in Ohio have reported increased incidence of charcoal rot in tomato crops. Symphilans were a pest problem known to plague in soil green house production in the past. It would be interesting to assess soil quality in high tunnels of the central Great Plains in ten years with an expanded set of quality indicators.