If you grow with high tunnels, have you noticed that for the first few years your tunnel crops have amazing yields with healthy plants, but after four or five years the crops just don’t seem to do as well? Maybe you keep watering, adding compost or fertigating with fish emulsion, but it’s never quite as awesome as the first years. There’s a story about soils in this dynamic that has come from NOFA-NY’s collaboration with the Cornell Vegetable Program’s Judson Reid and Cordelia Machanoff on a two-year NYFVI-funded high tunnel project. Judson Reid designed the research project to investigate what goes on in the high tunnel soils that ends this fertility honeymoon period, before a collaborative effort with participating growers determined the best strategies to maintain high tunnel soil quality for the long haul.

What essentially happens in tunnels is that we create a warm, protected environment that in turn increases crop yield. However, to support this higher yield we need a lot more nutrition from the soil for those bigger, more demanding plants. Over time, this gets tricky to manage without getting your soil nutrition out of balance. Additionally, traditional organic soil management practices like crop rotation and adding animal-based compost aren’t necessarily a good fit for tunnel production, and the removal of precipitation in the form of rain and snow from the whole equation causes all sorts of consequences for the soil chemistry.

Before we go any further, if you are a newer grower or need a refresher on soil science, I always like SARE’s book Building Soils for Better Crops, which you can download here. For something that’s a shorter read, many extensions have good articles on soil basics, including University of Minnesota Extension, Michigan State University Extension, and Cornell University. For audio-focused folks, I just listed to this neat little four-part podcast on organic matter on soil health—Priming for Production. If you want more intermediate resources or ideas for staff training, UC Santa Cruz has the full curriculum of some of their soil courses on line.

Back to the tunnels...
One of the challenges for organic growers is many of us learned that adding compost is a great way to boost fertility. This works well in the field (as long as you aren’t adding compost to the point where you get nutrient runoff), but a high tunnel is not a natural environment. There isn't precipitation to leach out nutrients in animal-based compost, that can start building up in the soil. In addition, rather than more neutral water of natural precipitation, many NY growers live in regions with high calcium soils and/or highly alkaline water. Irrigating with this alkaline ground water can rapidly impact tunnel soil nutrient levels.

Some soil fertility issues that can arise in high tunnels include:

• Increases in soluble salt levels
• Rising soil pH
• Excessive nutrient buildup of calcium and phosphorus
• Issues with compaction and loss of soil structure.

(Here’s a little non-organic, but helpful, primer from Cornell on soluble salts in soil.)

When phosphorus and especially calcium levels rise the soil preferentially holds onto those nutrients, which fills all the nutrient holding spots on the soil particles. This limits the soil from taking hold of other necessary nutrients, meaning the plants have too much access to phosphorus and calcium, but deficiencies of nitrogen and potassium. Meanwhile, watering with alkaline ground water raises pH levels of the soil, as they creep higher into the mid-7 range, limits the plants from accessing key nutrients. An additional challenge in organic tunnel systems, is that nitrogen levels often start too high in the spring because we tend to frontload our fertility at pre-planting. This leads to too much early vegetative growth and a slower start of fruit production, with foliar tests showing that the plants start to run out of nitrogen for all those lush leaves mid-season, which can cause unwanted surges in later fruit growth.

New tunnels provide a few years grace period where you can float by on the native soil fertility before the cumulative effect of adding too much of the wrong sorts of fertilizer, or watering with highly alkaline water catches up with you. This leads to all sorts of imbalances starting to showing up. We saw this happen in our own tunnel with just over three years of production, with two light applications of composted animal manure. Outside the tunnels, we’ve worked hard to slowly bring our phosphorus levels out of the basement, while in the tunnels our soil phosphorus literally surged from 2 to 140 lb/acre!

There are some low-tech options to deal with these issues. Some growers utilize moveable tunnels, rotate the crops through the tunnels, or pull plastic from their tunnels over the winter. However, moveable tunnels don’t work for all sites or systems, and taking a season without plastic or your most profitable crop in your tunnel isn’t economically viable for all operators.

The goal of Judson’s work, aided by all the participating farmers (THANK YOU ALL!), was to narrow down what issues most concerned long-term tunnel growers. Through trials, observation, and researcher-farmer discussions, the growers’ experience was put into best management practices that both maintain soil health and are realistic options given the time and financial stresses on growers. For farmers new to long-term soil management in high tunnels, this project’s research will help you proactively manage soils to avoid problems, even as it provides guidelines for longer-term tunnel operators to rectify soil issues.

If you are a new grower (or just new to tunnels), NOFA-NY has growing resources focused on providing high tunnel resources and highlighting some of the results from this project. You can find help for getting started with season extension and tunnels here. Cornell Vegetable Program’s full Best Management Practices document can be found here,  and more reports from Judson’s work can be found at here. Some of the highlights of the best practices are shared below.

We hope this helps you as you gear up for a new high tunnel season or take your first high tunnel plunge! Before you think about implementing these practices, remember to check out the full document over at the Cornell Vegetable Program, as well as NOFA-NY’s high tunnel resources. Happy growing!