Cooperative Extension Publications

By Lily Calderwood, Ph.D., wild blueberry specialist and associate professor of horticulture, University of Maine Cooperative Extension; Leala Machesney, wild blueberry research and Extension professional, University of Maine Cooperative Extension

For information about UMaine Extension programs and resources, visit extension.umaine.edu.
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Introduction

Soil and leaf testing are essential management tools for successful wild blueberry production. Together, these tests provide a clear picture of both the growing environment and the plant’s actual nutrient status. Soil testing determines the soil pH, CEC (cation exchange capacity), and organic matter content. Soil pH and CEC directly influence the availability of nutrients to wild blueberry plants. Leaf (foliar) samples determine which nutrients are adequate and deficient in the plant at the time samples were taken and guide decisions on the type and rate of fertilizer to apply. Regular testing allows growers to apply only the nutrients required, reducing unnecessary fertilizer costs and labor while improving crop performance.

In addition to nutrient management, knowing your soil pH is essential for proper weed management. Applying sulfur to fields reduces grass and broadleaf weed species’ ability to grow, increasing resource availability for wild blueberry plants. When used together, soil and leaf testing support informed decision-making and long-term field productivity. Routine sampling should be conducted in the nonfruiting, or prune, year, and combined with visual field monitoring. A healthy wild blueberry stand has a high density of stems and green leaves. Discolored foliage, weak growth, or poor stem density may indicate nutrient imbalances or other underlying issues, such as insect pests or disease, that require attention.

Understanding Nutrient Deficiency Symptoms

Although leaf discoloration or distorted growth can result from nutrient deficiencies, this is not always the case. Testing will confirm whether nutrient amendments are truly necessary. Other conditions may mimic deficiency symptoms, including:

• Insufficient or uneven soil moisture
• Poor drainage restricting root growth
• Soil compaction or erosion
• Insect or disease damage
• Fertilizer burn
• Herbicide injury
• Periods of cool growing weather

Before applying fertilizer, growers should evaluate whether environmental or management factors may be contributing to poor plant performance.

Leaf (Foliar) Testing

Leaf sampling is the most accurate way to determine the nutrients wild blueberry plants are absorbing and using. Established sufficiency ranges for Maine wild blueberry were originally developed by Trevett (1972) and later updated by Santiago (2011). These standards are used by the university laboratory to evaluate plant nutrient status and provide fertilizer recommendations.

Foliar testing helps determine precise fertilizer needs, allowing growers to apply appropriate rates of ammonium sulfate, diammonium phosphate (DAP), or other nutrients only when necessary. This targeted approach reduces costs and improves nutrient efficiency.

Sampling should be limited to low sweet blueberry (Vaccinium angustifolium). Sour top blueberry (Vaccinium myrtilloides) should not be included.

When to Sample

Leaf samples should be collected in the nonfruiting year when new stems in the field reach 90%–100% tip-dieback. Historically, tip-dieback has occurred around the 4th of July in Maine, but today this stage often occurs in late June. Tip-dieback occurs when newly emerging stems stop elongating. The uppermost leaf of plants at this stage may curl, dimple, die and turn brown, or drop off entirely. It is also possible to observe tip-dieback as a point of small, hairlike brown shoot tissue. This stage indicates the shift in plant growth from vegetative shoot elongation and leaf development to reproductive bud development. Foliar sampling during tip-dieback is recommended because nutrients in the plants are most stable during this time of year.

How to Take a Foliar Sample

1. Each sample should represent one field unless you are intentionally comparing different areas within a field.
2. Collect one sample per field (one sample per bag).
3. Walk the field in a zigzag pattern.
4. Sample from 15–30 plants across the field.
5. From each plant, collect middle-aged leaves (not the top, youngest leaves or bottom, oldest leaves) from three stems.
   • Alternatively, cut approximately 90 stems at ground level and strip the leaves.

Ensuring Quality Foliar Samples

Do not include soil particles or other plant material in the sample and avoid collecting leaves from areas with severe insect damage or disease. If leaves are dusty or show visible fertilizer or pesticide residue, gently rinse them with water, shake off excess moisture, and allow the surface to dry on paper towels before placing them in the sample bag. Samples should be placed in a brown paper bag and stored in a dry location until they are mailed or delivered. To prevent nutrient loss and ensure accurate results, send samples to the lab as soon as possible after collection. Submitting leaves only, without stems, may reduce the cost of analysis. Collecting samples with nitrile gloves will ensure particulates from your hands do not transfer to samples and impact your results.

Soil Testing

Soil testing for wild blueberry production focuses primarily on soil pH. The ideal soil pH for wild blueberries is approximately 4.3. At this level, nutrients remain available to wild blueberry plants, and competition from grass and broadleaf weeds is reduced.

If soil pH rises above 4.5, sulfur should be applied to lower it. Generally, 100 pounds of applied sulfur pellets per acre will reduce the pH of the soil by 0.1 units 2–3 years after application.

Soil nutrient tests are generally not used to determine fertilizer recommendations for wild blueberry. Soil samples may be collected at any time, but it may be convenient to collect soil samples when collecting foliar samples for testing at the tip-dieback stage.

How to Take a Soil Sample

1. Each soil sample should represent one field unless you are comparing specific areas within a field.
2. Obtain a soil sample box and form from an Extension office or the Soil Testing Lab at UMaine.
3. Collect one sample per field (one sample per box).
4. Walk in a zigzag pattern across the field.
5. Take at least 10 subsamples using a trowel or soil probe.
6. Sample only the top 3–4 inches of soil.
7. Include the organic matter “pad” layer (dark brown upper layer of soil).
8. Do not include plant material such as leaves, stems, or roots.
9. Mix subsamples thoroughly and place the mixed sample in the soil sample box.
10. Use code 400 for wild blueberry on the submission form.
11. Transport in a dry location.

Submitting Samples

Mail or deliver soil and foliar samples to:

Soil Testing Service
University of Maine
5722 Deering Hall
Orono, ME 04469

Prelabeled tissue bags, soil sample boxes, and forms are available from the Wild Blueberry office at the University of Maine or at your local county Extension office. For questions, please call 207.581.2917.

Understanding Your Test Results

The University of Maine laboratory will analyze foliar nutrient levels and compare them to established sufficiency ranges. Results are mailed or emailed directly to the grower along with fertilizer or sulfur recommendations when needed.

If nutrient levels fall below satisfactory ranges, a specific amendment may be recommended. When a field does not respond to the recommended rates of fertilizer or sulfur, we suggest investigating the soil type, organic matter content, and CEC (cation exchange capacity). In clay soils or soils with high organic matter content, CEC will be high, and may require more fertilizer and sulfur to see a change. In sandy soils or soils with low organic matter content, CEC will be low. Therefore, fertilizer and sulfur will be more likely to leach down through the soil profile, out of plant roots’ reach. Soils with high CEC can hold onto more fertilizer and sulfur, while low CEC soils require more frequent applications of smaller amounts.

Interested in your own field’s response to these materials? You can do your own experiment by leaving at least three unfertilized strips (minimum 100 feet long and 10 feet wide) within the field. Mark these strips clearly. Comparing treated and untreated areas helps determine the effectiveness of an application.

After applying fertilizer or sulfur, leaf sampling should be repeated at the same time of year that the original samples were taken to confirm that nutrient levels or soil pH have reached target ranges. For more information on interpreting foliar test results, refer to Bulletin #2383 Fertilizing Wild Blueberry Fields.

References

Santiago, J. P. “Determining Optimum Macro- and Micronutrient Leaf Concentrations for Lowbush Blueberry (Vaccinium angustifolium Ait.) Using the Boundary Line Approach and Quantile Regression.” Master’s thesis, University of Maine, Orono, 2011. 

Trevett, M. F. (1972). Nutrition and growth of the lowbush blueberry. Maine Agricultural Experiment Station Bulletin, University of Maine.

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Information in this publication is provided purely for educational purposes. No responsibility is assumed for any problems associated with the use of products or services mentioned. No endorsement of products or companies is intended, nor is criticism of unnamed products or companies implied.

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