There is a wealth of scientific research that has been conducted on high tunnel production in the last several years. NE SARE funded Cornell Vegetable Specialist Robert Hadad’s “How to Use Season Extension Effectively for Winter Market Sales: Investigating Planting Date, Types of Cover, and Fertility” study in 2010 to assess the temperatures and fertility dynamics in unheated low-tunnel winter production. Many of the findings from the varietal trial component of that study have been adopted by growers. However, the current proposal aims to more accurately assess nitrogen uptake by submitting tissue samples for analysis instead of using Brix readings, which only provide an aggregate sugar and mineral content analysis. Similarly, Ruth Hazzard’s SARE funded multi-year study “Expanding Winter Harvest and Sales for New England Vegetable Crops” provided valuable information regarding varietal selection and post-harvest handling for low-tunnel and unheated high-tunnel production. However, the study of similar subject, specifically nitrogen dynamics and yield, remain understudied in winter production systems in heated high tunnels.
The information that does exist on minimum temperature thresholds in heated high tunnels for winter production comes from growers. A group of experienced farmers came together for the first time in 2014 at the Frozen Ground Conference hosted by the University of Vermont. Notes were taken by Ruth Hazzard of University of Massachusetts Extension and demonstrate the wide range of temperatures used by winter growers and the strictly anecdotal nature of impacts on yield. While some farmers insist that the additional cost of heating above a thermostat setting of 25° F is not economical, others argue for a base temperature of 45° F. Attendees also discussed their intuitions and inferences on nitrogen dynamics under different temperature treatments, but eventually settled on the conclusion that winter systems are nitrate-deficient due to a lack of biological activity at low temperatures. Given the divergence in grower experiences with perceived costs and benefits of heating to different temperatures, it seems clear that there is a need for a more rigorous scientific approach to assessing yield response and nitrogen availability impacts at different temperatures.
While this proposal aims to answer some of these uncertainties, other researchers are currently studying the yield impacts of different nitrogen fertilizers in winter growing systems (personal correspondence with Amy Ivy, Cornell Cooperative Extension).
The Poughkeepsie Farm Project erected two identical 42’ x 196’ side-by-side high tunnels in 2016 that are equipped with high efficiency Modine 93 propane heaters. By using identical planting dates, fertilization rates, and crop mixes in the two houses, it is possible to measure and compare the impact on marketable yield and nitrogen availability and uptake in two different temperature zones. One house will operate with the thermostat set at 32-
degrees Fahrenheit while the second house operates at 40-degrees Fahrenheit. Installing in-line propane meters in each house will allow for precise monitoring of fuel use and allow for a cost comparison at the end of the study.
Soil nitrate level tests and plant tissue sampling paired with soil temperature monitoring will demonstrate any difference in nitrogen availability in the soil and actual plant uptake in the two temperature zones. This information will allow farmers using season extension techniques to make more informed decisions about the costs and benefits of heating to two different thresholds and impacts on nutrient management strategies under different
temperatures in high tunnels.
Seeding and Planting
Winterbor kale was seeded in the greenhouse on 8/21/17 and transplanted into the tunnels on 9/18/17 in 4 rows per bed with 18 inch in-row spacing. The Green Incised Salanova® lettuce was seeded in the greenhouse 9/18/17 and transplanted into the tunnels on 10/9/17 in 6 rows per bed with 8 inch in-row spacing. Kolibri spinach was direct seeded in both tunnels on 10/02/17 in 12 rows per bed; however, additional spinach was transplanted to fill in areas of poor germination on 10/16/17. Both tunnels are irrigated on the same schedule using three lines of drip tape per bed.
Soil samples to a depth of 6 inches were taken from each tunnel on 9/22 and submitted to the Agro-One Soil Analysis laboratory for a Pre-Sidedress Nitrate Test (PSNT). The samples were processed on 9/29. Tunnel 1 (33° F) had been used to produce summer tomatoes and was found to have 17.2 ppm NO3-N (approximately 34.4 lbs/acre) available and soil organic matter of 5.2%. Tunnel 2 (40° F) was dry fallowed for the summer after being used to grow late spring/early summer greens and was found to have 2.7 ppm NO3-N (approximately 5.4 lbs/acre) available and soil organic matter of 4.5%.
According to the Cornell Guidelines, the total recommended range for nitrogen for field-grown spinach is 100-125 lbs/acre, lettuce is 100 lbs/acre, and 100-120 lbs/acre for brassicas. Given the farmer-cooperator’s experience with winter production, supplemental nitrogen fertilizer in the form of feathermeal was added to each tunnel to achieve a total of approximately 70 lbs/acre of nitrogen for the kale, Salanova®, and spinach taking into consideration the PSNT results in each tunnel. An additional PSNT analysis was performed on 10/10 by Waypoint Analytical following the incorporation of the feathermal and both tunnels were found to have 17 ppm, or approximately 34 lbs/acre of available nitrate.
Depending upon the results from foliar samples submitted over the course of the winter, additional nitrogen in the form of soluble Chilean nitrate injected into the drip irrigation system may be added equally in the two tunnels.
Temperature Control and Monitoring
Supplemental heat in the tunnels is provided by propane-fueled overhead Modine 93 high efficiency heaters. Ambient air temperature is controlled by a wireless digital sensor in the middle of each tunnel suspended approximately 2 feet above ground level. The thermostat in Tunnel 1 is set to a minimum of 33° F while Tunnel 2 is set to a minimum of 40° F. Each tunnel is also equipped with an auto-ridge vent that is set to open when ambient air temperature reaches 60° F. Variation in ambient air temperature is recorded by the integrated Monnit sensor system as well as a HOBO V2 temperature/relative humidity data logger set to record every hour located in each tunnel. Additionally, soil temperature is being logged every hour at both 1-inch and 3-inch depths in each tunnel using WatchDog B100 2K button loggers.
Analog propane meters were installed in early September at each of the four supply lines for the Modine 93 heaters (two per tunnel). To convert the measured units of propane from cubic feet to liquid gallons, the reading from the meter is divided by 35.97. An average rate of $2.75/gallon is used to compute costs based on the data available for the Middle Hudson Region from the New York State Energy Research and Development Authority (NYSERDA).
Soil Nitrate Availability
After submitting the PSNT to Agro-One in late September, weekly soil nitrate availability tests were conducted beginning the week of 10/23. Most weekly tests were conducted using the Horiba LAQUA twin nitrate meter (reproducibility of ± 10 ppm) and soil nitrate extraction kit. A minimum of 15 samples taken to a depth of 6 inches were mixed in each tunnel from the beds growing the crops in the study. The NO3-N values are converted from ppm to lbs/acre by multiplying by a factor of 2. Additional PSNT test results from Waters Agricultural Laboratory in Camila, GA were used to provide the soil NO3-N levels for the weeks of 12/25/17 and 1/8/18. No nitrate reading was taken the week of 1/1/18.
Nitrogen Uptake Measurement
Foliar samples of all three crops were collected and submitted to Waters Agricultural Laboratory every other week beginning the week of 11/6/17. Total N concentration in the leaf tissue is reported on a percent basis. Foliar samples were not taken for the Salanova® lettuce during the week of 12/25/17 because plants had been harvested and did not have enough re-growth for sampling.
The harvest crew at the Poughkeepsie Farm Project recorded fresh harvest weights from sub-plots within each tunnel. For both the Winterbor kale and the Green Incised Salanova® lettuce, each subplot was 25 bed feet. Due to excessive leafminer damage, much of the spinach in Tunnel 2 was lost, so the subplots used to record yield are only 7 bed feet long.
Deviations from Proprosal
Though the original proposal was to assess the nitrogen dynamics at 35° F and 42° F, the thermostat settings were reduced to 33° F and 40° F for two main reasons. First, after further conversations with winter tunnel growers, there was more interest expressed in seeing results at a temperature just above freezing than at 35° F. Second, the grower-cooperator was concerned about the cost of additional propane to keep the second tunnel at 42° F, so that temperature was also decreased by 2° F.
The first date with observed propane use was 11/17/17. Through the month of November, the propane use in gallons for each tunnel was 55.602 for Tunnel 1 (33° F) and 216.847 for Tunnel 2 (40° F). As of 12/26/17, the totals were 177.926 gallons for Tunnel 1 and 578.260 for Tunnel 2. Assuming an average cost of propane in the Mid-Hudson Valley region of New York of $2.75, it has cost $1,100.92 more to heat Tunnel 2 ($1,590.21) than to heat Tunnel 1 ($489.30) through 12/26/17.
Soil Nitrate Availability
No clear relationship has emerged so far between the different temperature thresholds and measured soil nitrate availability. Though the results have been quite variable week by week, two observations are worth noting. First, Tunnel 1 (33° F) experienced a flush of nitrate measured on 11/9/17 (prior to any supplemental heating in either tunnel) much higher than the highest measured soil nitrate level in Tunnel 2. One hypothesis to explain this is the cropping history difference between the two tunnels. Tunnel 1 was used for high tunnel tomato production into the late summer and, as a result, was irrigated regularly and supported actively growing plants with only a short time gap in the transition to winter greens. Conversely, Tunnel 2 was bare fallowed for over two months prior to being prepared for winter greens. Since Tunnel 2 was allowed to dry out completely and had no plants growing in it, it can be hypothesized that the soil microbial community was adversely impacted and, as a result, less able to mineralize the equivalent input of nitrogen when compared to Tunnel 1.
Second, nitrate availability dipped and has been lower in Tunnel 1 than in Tunnel 2 after the onset of consistently cold temperatures and greater dependence on supplemental heat to maintain ambient air temperatures in the tunnels. This trend matches the growing difference in total nitrogen uptake between the two tunnels discussed below.
Despite measured differences in soil nitrate availability between Tunnels 1 and 2 beginning in early November, nitrogen uptake (as measured by total foliar nitrogen concentration) was very similar in all three crops. However, an emergent trend toward higher uptake in Tunnel 2 (40° F) became apparent in December in all three crops, but especially in spinach (see graphs for more detail: SARE-2017-Annual-Report-N-uptake-graphs).
Thus far, there is no observable difference in yield in the subplots of kale, lettuce, and spinach between the two tunnels. However, as of 12/26/17, it appeared as if the Salanova® lettuce in Tunnel 2 (40° F) would be ready for a second cut 1-2 weeks ahead of the same variety harvested for the first time on the same day in Tunnel 1 (33° F). Similarly, the spinach in Tunnel 2 was observed to be much larger after the first harvest on 12/26/17 than in Tunnel 1. No obvious difference in kale regrowth has been noted.
Pest and disease pressure have skewed yield data slightly in all three crops. Brassica powdery mildew was severe on the kale early on, but mostly controlled with a sulfur application. However, the heavy residue left from the sulfur rendered some of the earliest mature leaves unmarketable, so they were not included in yield data. Lettuce powdery mildew has become more severe as the winter has progressed; yield has not yet been impacted, but the infestation appears more severe in Tunnel 2 than in Tunnel 1 and may impact yield comparisons going forward. It is not apparent that the higher temperature threshold is singularly responsible for the more severe presence of lettuce powdery mildew in Tunnel 2. Tunnel 2 has more total lettuce being grown in it, had lettuce grown in it while tomatoes were rotated into Tunnel 1, and is also physically closer to the field where lettuce was grown in the late fall where inoculum may have developed. Finally, beet or serpentine leafminer damage was excessive in the spinach in both tunnels. As a result, the first cutting of spinach was discarded completely and not accounted for in the yield data.