Soil health and nutrient management in high tunnels

To determine how much nitrogen to add to your tunnel, calculate your nitrogen credits and subtract them from your total nitrogen needs.

Estimating N credits

Irrigation water:

Water used for irrigation in Minnesota often contains nitrates, which should be included as a credit towards your total plant nitrogen needs. The following equation can be used for estimating nitrate contributions from irrigation water:

N credit in lb./acre per inch of water applied via irrigation water = (Nitrate N (ppm) from your water test) * 0.225 * inches of water applied

• Most crops require about 1 inch of water per week, so if you are growing a 5-week spinach crop, you would multiply by 5 inches.
• Some fruiting crops like tomatoes and cucumbers require closer to 1.5 inches of water per week once flowering begins.
• If irrigating a tomato crop over 20 weeks, you could multiply 1*8 + 1.5* 12 to represent 1 inch for the first 8 weeks or so and 1.5 inches for the next 12 weeks.

Soil nitrate tests:

The amount of nitrogen listed on your soil test in parts per million (ppm) can be converted to pounds of available nitrogen with the following equation:

• Test result ppm * (sample depth in inches / 3)

For example, 20 ppm from a 12-inch depth soil test:

• 20*(12/3)) = 80 lb. N per acre
• If you sampled to only 6 inches and your soil test reads 20 ppm, your equation would be 20*(6/3) = 40 lb. N per acre

Cover crops:

Cover crops in the legume family (beans, peas, clover, vetch) can fix nitrogen from the atmosphere, making it available for future crops. Even non-legume cover crops provide a minor nitrogen credit because they prevent nitrogen from leaching out of the soil.

This video provides step-by-step instructions for calculating a nitrogen credit from your cover crop.

Previous applications: 

If you previously applied composted manure in your high tunnel, some residual nitrogen may be available from the prior year. However, nitrogen is not 100% available in year 1 of a composted manure application.

• For fresh dairy manure, 40-60% of the N is generally considered available in the first year, and 30-40% of the N is available in the second year.
• For composted dairy manure, 5-20% of the N is available in the first year, and 10% will be available in the second year.
• For fresh poultry manure, 50-75% of the N is available in the first year, and 20-25% is available in the second year.
• For composted poultry manure, 30-50% of the N is available in the first year, and 10% is available in the second year. 

Farmer Jill plans an average tomato yield and expects to use around 350 pounds of nitrogen per acre.

Her high tunnel beds are 90 feet long and 3 feet wide (with an extra 1 foot between each bed for a walkway), and she has 270 square feet of growing space per bed.

• 350*270/43560 = 2.17 lb. of nitrogen per bed.

However, Jill also has some nitrogen credits to factor in, which will lower her rate.

Soil organic matter: Jill’s high tunnel has 4% organic matter, which is considered medium. This means she can take a 20 lb./acre credit. 

Irrigation water: Jill’s irrigation water has 15 ppm nitrate. She will irrigate ~ 1 inch per week for the first 7 weeks that her plants are in her high tunnel and 1.5 inches per week for 13 more weeks, for a total of 26.5 inches.

She multiplies the 15 ppm in her water by 26.5 inches of water for the season and multiplies again by 0.225 for a total of 15*26.5*0.225=89.43, which she rounds up to 90 for ease of calculation.

Soil nitrate: Jill collected a soil sample down to 1 foot, and her soil test reads 20 ppm nitrate nitrogen. Therefore, her soil has 80 pounds of nitrogen per acre available.

Cover crops: Jill planted an Austrian winter pea cover crop in September and allowed it to grow until April, so she grew quite a bit of biomass. From this, she expects 40 lb. of nitrogen per acre to become available over the season.

Previous applications: She did not apply any manure products last year, so she is not taking a credit from previous fertilizer applications.

She has the following nitrogen credits:

• 20 lb. from soil organic matter
• 90 lb. from irrigation water
• 80 lb. from soil nitrate
• 40 lb. from her cover crop
• Total: 230 lb. N per acre

She can subtract this from her total N needs (350 lb. per acre), and her new goal is to supply 120 pounds of additional N per acre to her tunnel.

Finally, at this stage, she scales her recommendation down to her high tunnel size. She has 270 square feet of growing space per bed in her high tunnel.

120 lb. of nitrogen per acre * 270 square feet per bed / 43560 sq feet per acre = 0.744 lb. of additional N needed per bed.

Phosphorus

For phosphorus, we recommend the following field recommendations from the Nutrient management for commercial fruit and vegetable crops in Minnesota.

Potassium

The following potassium fertilizer recommendations are on a per-acre basis for high-yielding crops (intensively managed).

The following potassium fertilizer recommendations are on a per-acre basis for low to medium-yielding crops (less intensively managed).

For crops that are harvested once, such as carrots, beets, head lettuce, etc., follow the guidance in the Nutrient Management for Commercial Fruit and Vegetable Crops in Minnesota guide for phosphorus and potassium. These crops do not necessarily have higher yields in high tunnels, but they mature faster.

Composted manure and plant-based composts are commonly used in high tunnels in large volumes. However, to avoid salt build-up, take care when using these products, particularly composted manures.

Tips for adding organic matter with compost

• Many composted manure products are sold pre-packaged with the N-P-K contents listed on the bag. Use these numbers to factor your composted manure into your fertility calculations.
• Incorporate compost and manure into the soil rather than leaving it on the surface. This helps prevent compost from becoming very dry or hydrophobic on the soil surface.
• Use plant-based composts in high tunnels unless your soil is deficient in phosphorus. While plant-based compost is generally very low in nutrients, phosphorus can build up when it is used in large quantities.
• Plant-based composts and manure from neighbors will rarely come with listed N, P, and K contents.

If you are buying compost from a commercial facility, ask whether they can provide an analysis. If an analysis is unavailable, we recommend testing your compost and manure. The University of Minnesota soil testing laboratory can test compost, as can many private laboratories. Test for the following: 

• pH: The pH of compost can be highly variable. The ideal pH range for most plants is 6-7. Because high tunnel soils tend to increase in pH over time due to high pH irrigation water, avoid using composts with a pH above 7.4, at least in large quantities.
• P, K, and Nitrate: Consider the nutrient contributions in your fertility planning. Our webpage about interpreting your compost report provides step-by-step instructions. [Interpreting your compost report]
• Soluble salts: If your soluble salt levels are >2 mmhos/cm, take caution when applying it to your soil.
  • Look at the specific salts in your compost. If a large percentage of the salts are sodium (Na), do not apply the compost to your high tunnel.
  • If they are mostly potassium, calcium, magnesium, ammonium, or nitrate, make sure to add only enough compost to meet your plant nitrogen needs, and work the compost into the soil to prevent surface salt accumulation.
• C/N ratio: An ideal range for the carbon to nitrogen ratio of compost is about 25:1, though if the goal is to provide nitrogen from your compost, a lower ratio would release more nitrogen in the first year it is applied.
  • If your C/N ratio exceeds 25:1, your compost can compete with your crops for nitrogen.

No matter which base fertilizer you use in your tunnel, you will likely need to supplement with a more concentrated source of nitrogen, phosphorus, or potassium to meet your crop needs, particularly nitrogen and potassium.

Conventional nitrogen fertilizers

If you’re using conventional nitrogen sources, incorporate around 1/4 the total amount at planting, and then apply the rest via fertigation over the course of the growing season.

• Common conventional fertilizers to incorporate at planting include urea, monoammonium phosphate, diammonium phosphate, or ammonium nitrate.
• Common water-soluble N sources for fertigation include calcium nitrate, potassium nitrate, and urea-ammonium nitrate. 

Nitrogen fertilizers for organic farms

Supplying adequate nitrogen to high-tunnel crops is a significant challenge for organic farmers. Options for concentrated organic nitrogen include: 

• Feather meal (13-0-0): This product is sometimes sold in a pelleted form that may be easier to apply but does not work well with fertigation or foliar applications.
  • Nitrogen is quickly available.
  • It can lead to salt build-up in the soil if applied in excess of plant needs or if not mixed thoroughly into the soil.
• Bloodmeal (12-0-0): Nitrogen is quickly available.
  • It can be applied as a foliar spray, but it may clog drip lines if applied via fertigation.
  • If applied in excess of plant needs or if not mixed thoroughly into the soil, it can lead to salt build-up in the soil.
• Fish emulsion (5-1-1): Nitrogen is quickly available.
  • It can be applied as a foliar spray, but it may clog drip lines if applied via fertigation.
  • If applied in excess of plant needs or if not mixed thoroughly into the soil, it can lead to salt build-up in the soil.
• Cover crops: Legume cover crops can provide some nitrogen, reducing your total N needs (see below for more guidance about cover crops). 

Phosphorus sources

Most high tunnels in Minnesota have adequate phosphorus, and if growers are relying on balanced inputs that contain N, P, and K, additional phosphorus does not need to be supplied.

However, in cases where phosphorus needs to be added in a targeted manner, apply your phosphorus fertilizer at planting.

• Common conventional options include monoammonium phosphate and diammonium phosphate.
• Organic options include rock phosphate or compost and manure-based products.

Be aware of the waiting period for harvest after raw manure application:

• 90 days for produce that does not touch the ground, such as tomatoes, peppers, sweet corn, sweet peas, and others.
• 120 days for vegetables that are in direct contact with the soil, such as carrots, potatoes, and zucchini.

Potassium sources

The need for potassium increases dramatically once fruiting begins. Typically, in a high tunnel, about 1/3 of the potassium should be applied at planting, and the rest is applied via fertigation.

Potassium nitrate and potassium chloride are water-soluble and work well with fertigation systems. However, these products are usually not allowed in organic systems.

If you are using potassium sulfate or potassium magnesium sulfate (more common in organic systems), you can apply most of the potassium upfront and then side-dress the rest midway through the season.

When secondary and micronutrients are required, they should generally be incorporated before planting.

Calcium and magnesium 

• Calcium and magnesium are usually adequate if soil pH is maintained in the optimum range of 6-7.
• Dolomitic lime supplies both Ca and Mg.
• If lime is not required (i.e. if the soil pH is above 6), gypsum (Ca-sulfate) and Ca-nitrate can be used to supply Ca.
• Epsom salts (Mg sulfate) and potassium magnesium sulfate will supply Mg without changing pH

Sulfur

Various sulfate compounds, such as gypsum, Epsom salts, potassium sulfate, and potassium magnesium sulfate, are sources of secondary nutrient sulfur.

Other micronutrients

Minnesota soils generally contain adequate amounts of micronutrients, but deficiencies may occur under certain soil conditions and on crops with a high demand for specific nutrients. For information on fertilizer sources of micronutrients, refer to the Nutrient Management for Commercial Fruit & Vegetable Crops in Minnesota guide.

Compost will help supply micronutrients, and soluble micronutrient fertilizers can be applied through fertigation. Foliar sprays may be useful in alleviating micronutrient deficiencies that occur during the growing season.

Fertigation is the process of injecting one or more agricultural plant nutrients into irrigation water for application to the plant-soil root zone to meet a portion of a crop’s fertilizer needs.

Fertigation is common in high tunnels with drip irrigation systems, as fertigation allows you to fine-tune your nutrient applications and easily add more fertility throughout the growing season.

This is especially important when using black plastic mulch with heavy-feeding crops; fertigation allows you to supplement fertility when the soil is covered with mulch.

There are very high-tech options for fertigation as well as very simple systems.

Learn more about designing a fertigation system and the different types of injectors.

Choosing the right products for fertigation

Nitrogen and potassium fertilizers are the most common nutrients applied by fertigation to vegetable crops. Some formulations of phosphorus and micro-nutrients can also be used if compatible with the irrigation water (pH should be less than 6.5).

• Because of precipitation problems, special precautions must be made not to mix P fertilizers with calcium nitrate and iron.
• To avoid precipitation problems, two stock tanks should be used, one for calcium nitrate and iron chelate and the other for the remaining fertilizers.
• Another way of avoiding precipitation problems is to apply and incorporate all the P before planting, based on a soil test as discussed above.

Fertigation timing

Fertigation is typically practiced during several irrigation events over the course of the growing season to “spoon-feed” 1/2 to 3/4 of the plants’ total nitrogen needs for the season. Fertigation can even be used to supply 100% of a crop’s nitrogen needs.

In high tunnel plastic-mulched rows, the frequency of fertigation can vary from once a month to once a week to each day there is an irrigation event, depending on the plant’s stage of growth. In general, the frequency of application is not as important as the total rate applied.

The table below can be used as an example of how to apply N and K to tomatoes using fertigation. This schedule is based on the case study above (“Jill’s tomatoes”) and assumes a 230 lb./acre nitrogen credit based on soil nitrate before planting, irrigation water, and cover crops.

These fertigation rates would result in the total application of 114 lb./acre of N and 350 lb./acre of K2O. 

In most high tunnels, multiple crops are grown with varying fertilizer requirements. To meet the demands of various crops in one house, two approaches can be taken:

1) fertigate the crops at different times to allow for varying rates to be applied or 

2) fertigate to meet the demands of the crops requiring the lowest amounts of nutrients and make up the difference with pre-plant fertilizer.

Tissue analysis can determine if nutrients are limiting or at excessive levels. Particularly in organic systems where nutrient management is less precise, tissue testing can give growers a sense of whether their plants are getting what they need mid-season.

Learn more about tissue testing for fruit and vegetable growers.

In a 2023 study of 100 farms in Minnesota comparing high tunnel and field soils at each farm, Extension researchers found that soil pH tends to increase over time in high tunnels. In 50% of high tunnels, the pH was high enough to limit nutrient uptake. The pH was highest in tunnels with high soil calcium concentrations and irrigation water with high alkalinity.

In the same study, just under half of the high tunnels had some salt accumulation. 

• Slightly saline soils (2-4 mmhos/cm) can stress very sensitive crops like carrots, onions, strawberries, and raspberries.
• Moderately saline soils (4-8 mmhos/cm) can stress a wider variety of crops, including cabbage, celery, lettuce, peppers, and sweet potatoes.
• Salts were correlated with excessive inputs.

There are several common strategies for getting salt and pH back to acceptable levels. 

Manage inputs carefully

The high tunnels in this study with high salts also had high soil nitrate levels. Over-applying fertilizers can lead to pH and salinity issues, so careful nutrient management is a critical preventative strategy for managing soil problems.

Remove plastic

Remove the plastic every few years during the winter to help flush excess salts out of the soil and potentially reduce soil pH. Some growers use farm equipment to push snow into tunnels during the winter for a similar effect. 

Flood your high tunnel

But take care. Some farmers do this to flush salts out of the soil, but depending on your soil and how much nitrate is in it, this practice could lead to nitrogen leaching into groundwater. Additionally, if you flood your tunnel with water with a high alkalinity level, this practice is unlikely to have a positive effect.

Acidify irrigation water

This 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. Keep in mind that this practice may actually lead to more problems, as the acids used to bring the pH down can add additional salts to the soil. 

In a 2023 study of 100 vegetable farms in Minnesota comparing high tunnel and field soils at each farm, our UMN Extension team identified a few factors that meaningfully affected soil health metrics, like aggregate stability, organic matter, and bulk density (a measure of compaction). 

Reduced tillage was associated with better soil health. Farms that tilled more intensively had lower organic matter and lower aggregate stability.

Organic certification was correlated with higher organic matter and less compaction.

The addition of plant-based composts was correlated with higher organic matter and better aggregate stability. Both fresh and composted manure were not correlated with the same benefits.

Farmer experience: The more experienced farmers in the study actually had worse soil health than the beginning farmers in terms of aggregate stability, organic matter, and compaction. We attributed this to more intensive use of tillage as farmers gain more experience and scale up.

Reducing tillage is one key soil health strategy for high tunnels and fields alike. For ideas and alternatives to tillage, see Reducing tillage intensity in vegetable crops.

Cover crops

One approach to mitigating soil health challenges in the high tunnel is to use cover crops over the winter or during any period when a crop is not being grown. Cover crops can supply organic matter, and the increased presence of living roots in the soil benefits soil structure.

Establishing and maintaining a cover crop stand will require you to irrigate outside of the usual crop rows and cropping times; however, this additional moisture may also benefit soil conditions.

Many of the high tunnels we visited in the 100 Farms project were extremely dry and lacked structure. Water moved so quickly through the soil that it was hard to maintain soil moisture.

Legume cover crops can supply nitrogen without increasing phosphorus levels, which is particularly important for growers using organic practices. Legume crops “fix” nitrogen from the atmosphere, which is gradually released into the soil as residues break down after the cover crop is terminated.

Based on preliminary research, we recommend the following legume cover crops for high tunnels:

• Fall planting: Austrian winter pea, hairy vetch
• Winter planting (after a winter spinach crop and before your spring/summer crop): field pea, crimson clover
• Summer planting: cowpea, crimson clover