High Tunnel Soil Fertility Management: What You Need to Know

Are you looking to enhance your knowledge of soil fertility management in high tunnel farming? Look no further! Let’s explore the crucial aspects of High Tunnel Soil Fertility Management that can help you grow abundant and healthy crops.

Spring has arrived, and with it comes the excitement of planting cool and warm-season crops. If you plan to grow crops in a high tunnel, submitting soil samples to your nearest laboratory is essential. If you haven’t done so already, it’s a good idea to do it now. Additionally, you should analyze your irrigation water, as this will provide you with valuable information that could help you manage your high tunnel fertility program.

Growing in a high tunnel is different from growing crops outdoors, as there is no natural rainfall inside the tunnel. Therefore, it is crucial to understand the water and soil test results as they provide essential information for the fertility program. High tunnels have unique challenges, such as temperature extremes, salt accumulation in the soil, nutrient deficiencies due to faster crop growth, and faster release of nutrients from organic materials.

To address these issues, growers can plan to manage their soil fertility and moisture issues timely. You can add water-soluble fertilizer to your irrigation system throughout the growing season or add compost and other soil amendments before planting and during the growing season. This article will focus on water quality and how to interpret your soil test report.

Importance of Assessing Water Quality

High alkalinity irrigation water in greenhouse bench crop production can cause a rise in soil or media pH over time, leading to zinc, manganese, and iron deficiencies in specific crops. High tunnel growers also experience this problem. To avoid this, it is recommended that the total alkalinity and pH of your irrigation water should be between 0 to 100 ppm CaCO₃ and 5.5 to 6.5, respectively. Ideally, it should be about 80 ppm CaCO₃ with a pH of 5.8. If the alkalinity levels exceed 150 ppm CaCO₃, it can lead to an increased incidence of clogged drippers. It is important to note that water pH and alkalinity are not strictly related, so it is essential to test for both.

There are two options to deal with high alkalinity and pH. One is the acidification of irrigation water while you irrigate, which involves adding an acid like citric, sulfuric, phosphoric, or nitric to reduce the irrigation water’s alkalinity. This is perhaps the fastest and most cost-effective method to manage the soil pH in the root zone. Rates can be calculated using the ALKCALC resource. The other more costly option is to perform reverse osmosis, which will remove all minerals and reduce the water’s buffer capacity to almost zero. Another option would be to acidify the soil with elemental sulfur (Figure 1), which is a much slower process. Elemental sulfur is inexpensive and has the highest acidification power (0.32 lb needed to neutralize 1.0 lb of CaCO₃) of common amendments used to lower the soil pH. Alternatively, ammonium-based fertilizers can be used, but a higher application rate is needed to decrease soil pH. This may require application rates that are higher than the nitrogen needs of the crop, and in sandy soil, nitrogen might be lost due to leaching. The use of ammonium-based fertilizers is therefore only recommended when you need a minor pH reduction (up to 0.3 pH units) (Gatiboni et al. 2020).

Ensuring that iron, manganese, and sulfate levels are within acceptable ranges is also essential. Iron above 5 ppm could be toxic to the plant, causing iron precipitates to form at the emitter and plugging your irrigation system. Similarly, manganese levels above 1.5 ppm and sulfate levels above 240 ppm could cause emitter blockage. Keeping track of the sodium and chloride levels in the water is also essential. High levels (>50 ppm Na and >70 ppm Cl) can increase soil salinity, especially in the top 2-4 inches. They must be managed carefully, mainly because the high tunnel cover prevents natural rainfall from washing or leaching excess soluble salts from the soil. The elevated soil temperatures inside the tunnel increase soil microorganism activity, releasing nutrients from organic materials such as manures and composts into the root zone faster.

Understanding your Soil Test Report

The optimum pH varies by crop, but it is generally accepted that the ideal pH range for organic and mineral soils is 5.3 to 5.8 and 6.0 to 7.0, respectively. Nitrogen, phosphorus, potassium, calcium, magnesium, boron, and molybdenum are most available in mineral soils when the pH is between 6.0 and 7.0. Zinc, manganese, iron, and copper are the most available at a soil pH below 6.5. Therefore, maintaining mineral soil pH between 6.0 and 6.5 is desirable. The available aluminum increases significantly as the mineral soil pH decreases below 5.5. This can further contribute to soil acidification and aluminum toxicity, which inhibits root growth. The target pH range for organic soils is between 5.3 to 5.8. The lower pH range is acceptable because organic soils have very low aluminum levels (Warncke et al., 2004).

The capacity of soil to hold exchangeable cations is measured and reported as the cation exchange capacity (CEC) of the soil. This value is a good indicator of soil fertility. Good soil has a CEC between 5 and 35 meq/100g soil. Generally, sandy soils have a low CEC, and soils with a high CEC are likelier to have a high clay or organic matter content.

Two tests are performed to determine the phosphorus level: P1 (weak Bray) and P2 (strong Bray). The P1 test indicates the phosphorus that is readily available to plants, and 20 to 50 ppm is an adequate level. The P2 test confirms the level of available phosphorus and part of the active reserve in the soil, and 40 to 60 ppm is a desirable level.

Potassium should be between 150 and 300 ppm, calcium should be between 1000 and 2500 ppm, and magnesium should be greater than 50 ppm.

Soluble salt (a measure of soil salinity) results are presented as the soil’s electrical conductivity (EC) and measured in mmho/cm (mmho/cm = dS/m = mS/cm). An EC below 1.0 (below 640 ppm salt) is considered good, and an EC above 2.5 (above 1600 ppm salt) is unsuitable for crops.

Percent base (cation) saturation indicates the proportion of the CEC occupied by cations such as Ca²⁺, Mg²⁺, and K⁺. Optimum ranges for Ca²⁺, Mg²⁺, and K⁺ are 40-80%, 10-40%, and 1-5%, respectively.

Micronutrient ranges for vegetable crops are between 1 to 3 ppm Zn, 1 to 5 ppm Mn, 11 to 16 ppm Fe, 0.5 to 1.5 ppm Cu, 0.7 to 1.0 ppm B, and 0.11 to 0.20 ppm Mo.

Let’s Take a Closer Look at Compost

Applying compost to the soil is common in many farming systems. Composting involves the controlled biological decomposition of organic materials into nutrient-rich soil amendments or mulches. These materials (feedstocks) may be rich in nitrogen, such as manures and legume plant residue, or rich in carbon sources, such as leaves or straw. An ideal carbon-to-nitrogen ratio for composting is 30:1. Lower ratios can result in excess nitrogen loss as ammonia gas and carbon loss. Nitrogen is insufficient for microbial decomposition at higher C:N ratios, which slows the composting process.

Not all composts are created equally

It is essential to know what is in your compost by analyzing it or requesting that information from suppliers. Test results will help guide how much and when to apply compost to build soil health. Organic amendments like compost do not supply plants with readily available nutrients (apart from inorganic N, which is immediately available) because they are released slowly through microbial decomposition. It may take several years to break down and release nutrients for plant uptake. The decomposition rate is affected by environmental conditions such as soil moisture and temperature. It is important to consider differences in compost decomposition rates for soils in high tunnels compared to open fields. To maximize plant nutrient uptake, apply compost in the Spring. As soil temperatures increase, mineralization will release nutrients during the growing season. Summer compost applications can be beneficial for hay and pasture areas. However, fall application typically increases nutrient loss unless soil temperatures are low enough to immobilize soil nutrients until the following Spring.

What happens if I overapply compost?

When significant, affordable quantities are available, it may be tempting to overapply composts in depleted, compacted, or nutrient-deficient soils. However, over-applying compost, especially manure-based compost, can result in nitrate leaching into groundwater, excessive phosphorus soil concentrations, and high soil salinity. Typically, soils have more phosphorus than crops need when composts have been over-applied for several years, but nitrogen may be lacking. The high phosphorus concentration can reduce the crop’s micronutrient uptake ability (particularly iron and zinc) and reduce yield. Manure-based composts typically have a pH greater than 7, so monitoring pH in heavily composted soils is essential because elevated pH can reduce crop yields.

Considerations for the Nutrient Management Plan

Nitrogen is crucial for promoting plant growth. However, excessive nitrogen fertilization can lead to overgrowth and deficiencies in warm and high-light conditions, particularly in fruiting vegetable crops. The type of nitrogen used is also significant. Utilizing an ammonium or urea nitrogen source will decrease the soil’s pH in the root zone. The transformation of these nitrogen forms into nitrate nitrogen is temperature-dependent, happening faster in higher soil temperatures. Excess ammonium nitrogen can lead to calcium, magnesium, and potassium deficiency. Since ammonium is a smaller molecule, plants can absorb it more quickly. Conversely, nitrate nitrogen absorption can cause a slight increase in pH in the root zone.

Phosphorus is most available to plants at a pH between 6.2 and 7.2. Therefore, it is essential to manage soil pH to ensure maximum phosphorus availability. Apart from soil amendments, you can adjust the pH of your irrigation water and nitrogen form to help regulate soil pH in the root zone. Potassium is vital for plant growth and high-quality crop production. Higher soil moisture content means more potassium is available in the soil solution, enhancing its availability for root uptake. However, be cautious. Excessive soil moisture reduces root respiration, limits root activity, and therefore decreases potassium uptake. The uptake of both potassium and phosphorus increases with an increase in soil temperature. The ideal soil temperature for uptake is between 60 and 80°F.

Conversion Factors

Expressing fertilizer recommendations in pounds per acre of P₂O₅ and K₂O is common because fertilizer grades on the product label are given as percent N-P₂O₅-K₂O. To convert the P value to P₂O₅, multiply it by a factor of 2.3. On the other hand, to convert the  P₂O₅ value to P, you need to multiply it by a factor of 0.43478. Similarly, to convert the K value to K₂O, you need to multiply it by a factor of 1.2; to convert the K₂O value to K, you need to multiply it by a factor of 0.83333. If you need to know how much P or K is available in one acre one foot deep, you can multiply the ppm value in the soil test report by a factor of 3.6.

Final Thoughts

Developing a fertility plan

• To develop a nutrient management plan, you need to know what your crop’s nutrient needs are.
• Analyze your soil and water and then, looking at your crop needs, determine what nutrients are needed.
• Ask the laboratory to include recommendations in your soil test report or use tools such as the SWCD Nutrient Management Tool to determine application rates.
• Monitor plant nutrient levels timely.
• Check soil fertility levels annually. The best practice is to collect samples at the same time every year. Soil mineral content can then be tracked over time.

What can you do if the soil salinity in your high tunnel is increasing?

• When the crop is not present, consider applying a lot of water. Irrigate 6 inches of water to flush out 50% of salts from the top 12 inches of soil or apply 12 inches to leach about 80% of the salts (Western Fertilizer Handbook, 10th Ed. 2022. Western Plant Health Association).
• When plants are present, irrigating slightly more than what the plant needs will push the excess salts to the outer limits of the wetting zone of the dripper/root zone.
• Use low salt irrigation water.
• Use fertilizers with a low salt index.
• Limit the use of organic nutrient sources containing animal manure.
• If the soil is poorly drained due to compaction or a hardpan, till the soil to break all restrictive layers. You should consider installing a subsurface (tile) drain in cases where high clay soils or a high water table is present.
• If you are about to replace your high tunnel plastic in the fall, leave the tunnel uncovered. Fall, winter, and early spring precipitation will help leach excess salts.

Too much P in your high tunnel soil?

• Is your soil P (Bray 1) higher than 100 ppm? Then, there is no need to apply additional P₂O₅.
• Hold off on applying additional compost.
• Avoid applying soil amendments that contain P. Draw down on existing P that is available for uptake.

Is your soil pH increasing?

• Avoid applying additional compost
• Do not apply soil amendments containing only nitrate nitrogen. Urea or ammonium nitrogen sources will help to slow down or decrease the soil pH in the root zone.
• Is the water source used for irrigation alkaline or hard? High levels of magnesium and calcium in the water will increase soil magnesium and calcium over time and, therefore, pH. Excess magnesium and calcium compete with potassium uptake and could reduce tomato fruit quality. Amend the irrigation source water with acid, reducing the alkalinity and pH of the water. Monitor the irrigation water alkalinity and mineral content regularly, especially during the summer when water quality can fluctuate depending on the amount of rainwater entering the aquifer or pond you are pulling water from.
• Apply soil amendments such as elemental sulfur to reduce soil pH. This is a long-term management strategy – a reduction in soil pH could be expected 5-6 months after application. Keep on monitoring soil pH.

Applying granular fertilizer?

• Some growers might apply granular fertilizer prior to planting their crops. This is a great option. Exact amounts of N, P, and K are applied according to your soil test results.
• Do not leave the fertilizer on the soil surface. Incorporate it well into the soil.
• Granular fertilizer is only available when it dissolves. Make sure that the band of fertilizer is in the wetting zone of your irrigation system and that the soil moisture is at field capacity.
• Is the applied fertilizer immediately available, or is it released over a period of time? Some fertilizer sources have a release period of up to 12 weeks. Make sure that soil fertility levels are sufficient for the stage of crop development. Crop nutrient status could be monitored with tissue tests. To make up for the lack of specific nutrients, liquid soil fertility amendments can easily be applied through the irrigation system post-planting.

Have you started to grow your spring tomato crop?

• Now is the time to ensure that you have sufficient potassium available for uptake. Potassium is especially important for producing high-quality fruit.
• If the soil test potassium is higher than 200 ppm, there is no need to apply additional potassium. The soil potassium target range is 150 to 300 ppm. However, potassium can leach from sandy soils. If you irrigate until the water is below the root zone, then potassium might have moved past the root zone and will not be available for uptake. Watering time and volume need to be considered. Also, consider irrigating clean water first and then, towards the end of the irrigation cycle, add the nutrients and flush the irrigation lines with fresh water. Always aim to deliver the water and nutrients to the volume of soil that is currently occupied by the plant’s roots.
• Do not apply nitrogen in excess of what the crop needs. Excess nitrogen will encourage excessive vegetative growth and delay fruit set.

Use guides such as the Midwest Vegetable Guide, the Indiana High Tunnel Handbook, and the SWCD Nutrient Management Tool to help you plan and manage your fertility program.

Related Articles

Soil and Water Data is Critical for High Tunnel Growers. Issue 716. https://vegcropshotline.org/article/soil-and-water-data-is-critical-for-high-tunnel-growers/

Are You Thinking of Applying Compost to Your Soils? Issue 715. https://vegcropshotline.org/article/are-you-thinking-of-applying-compost-to-your-soils/

Water Affects Efficacy of Soil-Incorporated Fertilizers and Amendments. Issue 712. https://vegcropshotline.org/article/water-affects-efficacy-of-soil-incorporated-fertilizers-and-amendments/

Reducing Blossom End Rot and Yellow Shoulder/Internal White Tissue in Tomato. Issue 688. https://vegcropshotline.org/article/reducing-blossom-end-rot-and-yellow-shoulder-internal-white-tissue-in-tomato/

Resources

Gatiboni, L. D. Hardy, D. Osmond and J. Havlin. 2020. Calculating the Rate of Acidifiers to Lower the pH of North Carolina Soils. https://content.ces.ncsu.edu/calculating-the-rate-of-acidifiers-to-lower-the-ph-of-north-carolina-soils

Kaiser, D.E. and C.J. Rosen. 2018. Potassium for crop production. UMN Extension. https://extension.umn.edu/phosphorus-and-potassium/potassium-crop-production

Mikelbart M.V., S. Hawkins and J. Camberato. 2012. Commercial Greenhouse and Nursery Production. https://www.extension.purdue.edu/extmedia/ho/ho-241-w.pdf

Rosen, C.J. and R. Eliason. 2005. Nutrient Management for Commercial Fruit and Vegetable Crops in Minnesota. University of Minnesota Extension Service. https://conservancy.umn.edu/handle/11299/197955

Sideman, B. 2018. High Tunnel Soil Management Update. UMass Extension. Vegetable Notes, Vol. 30 No. 2. https://ag.umass.edu/sites/ag.umass.edu/files/newsletters/february_15_2018_vegetable_notes.pdf

Sideman, B. 2017. Growing Vegetables: Managing Blossom End-Rot. UNH Cooperative Extension. https://extension.unh.edu/resource/growing-vegetables-managing-blossom-end-rot-fact-sheet-0

Warncke, D., J. Dahl and B. Zandstra. 2004. Nutrient recommendation for Vegetable Crops in Michigan. Michigan State University, Extension Bulletin E2934. https://www.canr.msu.edu/fertrec/uploads/E-2934-MSU-Nutrient-recomdns-veg-crops.pdf