High Tunnels – What Does Removing the Covers for a Winter Do?

Soil in high tunnels isn’t exposed to the elements like soil in the field is, and if the plastic is kept on the tunnels for multiple winters, little leaching takes place. Thus, nutrients and salts can accumulate. How much difference does taking the covers off for one winter make?

At “Tunnel Town” at Penn State’s Horticulture Research Farm, in preparation for work that will be done as part of a USDA-NIFA Specialty Crops Research Initiative project, we had a chance to answer this question.

We wanted to find out how much variability there was among tunnels before planting (so we knew whether soil variability might have a bigger effect than our treatments), and also whether taking the covers off for the winter would help to “even things out”. Soil samples were collected on November 24, 2014 from each of the existing tunnel locations on the site (each designated by a number and letter in the figure below) shortly after plastic was removed from the tunnels in the fall.

A second set of samples was collected on April 18, 2015. The samples were analyzed for pH, phosphorus, calcium, magnesium and soluble salts at Penn State’s Ag Analytical Lab.

Over the course of the winter, calcium, and magnesium generally increased slightly, while phosphorus and potassium decreased very slightly, probably due to natural variability (see table). Soluble salts levels decreased to nearly half of their original levels on average.

As might be expected, the tunnels with the highest salts levels in the fall had the largest decrease (see graph), except for one puzzling tunnel where we might have hit a pocket with higher salts in the spring. I was wondering why the soil pH would have increased over the course of the winter from 7.4 in the fall to 7.7 in the spring.

According to John Spargo, Director of Penn State’s Ag Analytical Lab, this was most likely due to the decrease in salt levels, as pH readings will be lower when salt levels are higher, and as the salts leach out, the pH readings increase. Mystery solved. Regarding the original questions, it turned out that nutrient levels, especially phosphorus and potassium, were very variable among the tunnels.

In addition, soil pH, phosphorus, calcium and magnesium were in the excessive range for all tunnels at this site for growing small fruit crops, and taking the plastic off for one winter didn’t make all that much difference – the only big change was in salt levels. Our conclusion on how we want to grow our plants is that we’ll be growing them in a soilless mix in grow-bags (raspberries) or grow-slabs (strawberries) for the project, so I’m sure we’ll be learning a lot from the experience—stay tuned.

(by Kathleen Demchak, Senior Extension Associate, Penn State University)