Installing a stationary high tunnel (HT) is a significant, long-term commitment to the parcel of soil beneath it, especially if the crops will grow directly in that soil. Maintaining, and preferably enhancing, the health, quality, or productivity of that soil for as long as possible should be a high priority beginning at HT installation.
Soils in HTs are less well understood than uncovered soils in “open sky”/open field production. However, the HT farming, extension-research, and industry communities are aware that HT soils are prone to specific issues and require specific care to remain commercially viable. These issues and preventative or reclamation tactics are the subject of much research and extension. Therefore, HT growers are encouraged to stay tuned for more information, including on how they can participate directly in identifying concerns and developing solutions. Examples of concerns and working solutions were summarized in a recent presentation (https://www.youtube.com/watch?v=XpUl0IwaDFI). Choosing one concern, in a summary of a presentation given at the 2013 New England Fruit and Vegetable Conference (https://newenglandvfc.org/sites/newenglandvfc.org/files/content/proceedings2013/Hoskins%20High%20Tunnel.pdf), Bruce Hoskins of the University of Maine’s Analytical Lab and Soil Testing Service mentions that the buildup of nutrient salts over time is “one of the most common problems in a continuously covered HT system,” that HT soil management can be similar to “irrigated desert production in the west and southwest,” and that growers familiar with open-field production can “fail to realize this potential problem or take steps to remediate it.” He also mentions that nitrate may carryover from one HT crop cycle to the next more readily than in open field production.
We heard from Bruce Hoskins and John Spargo during recent conversations about HT soil management. They direct soil testing and analytical labs at the University of Maine (https://umaine.edu/soiltestinglab/) and Penn State University (https://agsci.psu.edu/aasl), respectively. Each of these labs receives soil samples from hundreds of HT growers (conventional, organic) each year and have been actively helping improve soil management recommendations and cropping outcomes for HT growers. They have been joined in that work by others, including farmers, across the Northeast and Mid-Atlantic regions for years.
Take-aways from these recent conversations include that routine soil testing is essential, along with accounting for potential nutrient salt buildup when collecting soil samples. Normally, samplers: 1) use a soil probe or spade to retrieve a column of soil about twelve inches deep, 2) drop the soil in a bucket, 3) repeat the process one or more times from other areas, 4) mix the soil in the bucket, and 5) submit a portion of it for analysis. Listening to testing and other experts, the best approach appears to include “stratified” sampling; that is, submitting samples taken from 0-4 inches deep (upper layer of the rooting zone) separately from samples taken from four inches and deeper (lower layer of the rooting zone). Salts tend to accumulate in upper layers, especially if soil is heavy-textured and irrigation is frequent but brief. So, standard “mixed” samples may either: (a) underestimate salt levels in upper layers of soil experienced by roots of transplants and more mature plants or (b) overestimate salt levels if samples include only the upper level. Stratified sampling, mindful that soil characteristics can change with depth, equips growers and others with information to better manage HT soils. Regarding the costs of soil testing, especially of stratified samples, input from soil testing labs suggests that few of the growers they work with mention it as a significant concern. Instead, most growers appear to have done their math and concluded that soil analysis offers a significant return on investment, given that its cost is more than offset by gains in crop yield and quality in the current and subsequent years.