Soil health and nutrient management on diversified vegetable farms: What we learned from studying soils at 100 Minnesota farms

To understand soils on small-scale diversified vegetable farms, Extension educators visited 100 farms in the spring of 2023 to test fields and high tunnels. Tests included soil nutrients, physical properties, and soil health metrics. Here are some of the key things we learned:

High tunnel soils can become more alkaline over time, limiting nutrient uptake in plants

We started with four hypotheses for why high tunnel soils might become less productive over time—a common complaint among growers. One idea was that salt gradually builds up in tunnels, causing toxicity problems with plants. Another was that nutrients and organic matter might become depleted over time. Both of these hypotheses were wrong. 98% of tunnels and 100% of fields were non-saline, and in general, high tunnels had more organic matter and nutrients than fields (often a lot more).

We did see two other things: the soil pH (measure of acidity) in high tunnels was significantly higher than in fields, and it tended to increase over time. In more than 50% of high tunnels, the pH was high enough to limit nutrient uptake. The other thing we saw was that high tunnel soils were generally very dry. While we didn’t see impacts to soil structure, some growers struggled to get their soil to absorb water once it became dried out. Water would instead pass through quickly, making irrigation a challenge. 

Soils generally have too much phosphorus, which is an environmental concern

The most common fertility input used by 100 farms participants was composted manure. While this can be a great source of fertility, it tends to be high in phosphorus and potassium, but low in nitrogen. If composted manure is applied as the primary nitrogen source, growers can end up with way too much phosphorus and potassium in their soil.

Indeed, 87% of high tunnels and 84% of fields in our study had “very high” levels of phosphorus and did not require any additional phosphorus fertilizer. The average high tunnel had 2-3x the amount considered “very high”, and a few had over 10x this amount.

Excess phosphorus is an environmental concern; phosphorus is a limiting nutrient in freshwater systems, and when phosphorus enters lakes and rivers, it causes algal blooms. When the algae dies, oxygen is depleted and aquatic life is negatively affected. Studies from the Midwest have shown that the amount of soil phosphorus on a soil test is a good predictor of the amount of phosphorus likely to leave a farm, either through runoff and/or leaching. 

Photo: Algal blooms are driven by excess phosphorus reaching freshwater ecosystems. Photo: Minnesota Pollution Control Agency

Tillage impacts soil structure and health

The frequency and intensity of tillage was the major factor influencing aggregate stability and compaction in both high tunnels and fields. More tillage was linked to fewer large soil aggregates and more compaction. Aggregate stability is important for soil health, because it represents the soil’s ability to resist erosion. Compaction prevents plant roots from accessing nutrients and growing. Tillage influenced these soil health metrics more than any other factor.

What can we do with this information?

We developed a few recommendations for growers based on what we learned in this study:

• Work with your local Extension educator to develop nutrient management plans that maximize plant health while avoiding over-fertilizing. Do not add additional phosphorus if your soil test shows high concentrations.
• Consider using more cover crops, particularly legume cover crops like peas, beans, hairy vetch, or clovers. Legumes fix nitrogen from the atmosphere, and a legume cover crop can provide a significant portion of your nitrogen needs without any extra phosphorus and potassium. Using cover crops in the winter in high tunnels can also help to retain soil moisture. Check out our many videos about cover crops for vegetable farmers on our Small Farms YouTube channel.
  
• Take care when thinking about flooding your tunnel. Some farmers do this to flush salts out of the soil, but our results did not indicate salt problems. Depending on your soil and how much nitrate is in your soil, this practice could lead to nitrogen loss.
• Reduce tillage when possible. Check out Extension’s resources about reducing tillage intensity in vegetable crops on our website and consider joining the Climate Resilient Organic Vegetable Production community of practice to learn about reduced tillage alongside fellow Upper Midwest vegetable farmers.
• Acidifying irrigation water may help to resolve pH issues in high tunnels. For conventional growers, there are many resources available online for acidifying irrigation water with sulfuric acid or nitric acid. Organic growers are limited to naturally fermented citric acid. Reach out to your local Extension educator for support if you would like to try this practice.
• Mitigate excess phosphorus on the landscape by working with conservation support programs like NRCS or the Minnesota Ag Water Quality Certification Program to add cover crops, strips of perennial vegetation, and buffers to your farm. These practices help to prevent erosion and surface runoff. 

Learn more about these results

For a more in-depth look at these results, join us for sessions at the following winter conferences: 

• Minnesota Fruit and Vegetable Growers Association Annual Conference
  
• Sustainable Farming Association of Minnesota Annual Conference
• Marbleseed Annual Conference
• We'll also be hosting a series of field days around the state in 2024, to be announced soon
  

Authors: Natalie Hoidal & Shane Bugeja, hoida016@umn.edu & sbugeja@umn.edu