Using Plant Sap Analysis to Develop an Optimal Nutrient Profile for Lowbush Blueberries

Progress report for FNE21-997

Project Type: Farmer
Funds awarded in 2021: $14,985.00
Projected End Date: 02/28/2024
Grant Recipient: The Benson Place
Region: Northeast
State: Massachusetts
Project Leader:
Meredith Wecker
The Benson Place
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Project Information

Project Objectives:

The objectives are:

  1.       To use sap analysis to identify key macro and micro nutrient variations in low bush blueberry plants that display both very high and very low production, disease resistance and fruit quality.
  2.       To determine the extent to which production, disease resistance, and fruit quality in lowbush blueberries can be increased by foliar applying key macro and micro nutrients to different, larger field plots (containing genetically diverse plants displaying a mix of high, low and average production, disease resistance and fruit quality).
  3. To take the first steps in creating an ideal sap analysis profile for lowbush blueberries. My hope, given the limits and scope of this project would be to provide a useful working profile that could inspire and be refined by further research.

If this project is successful, growers will have increased information and, if desired, be able to compare sap analysis results from their fields with an ideal profile that is correlated with high levels of plant nutrition in order to obtain improved fruit quality, increased yield and heightened plant immunity. The industry and consumers can benefit from a more profitable, ecological approach. 


The cultivation of lowbush blueberries is highly specialized. It differs from most agricultural systems in that it involves the management of an existing ecosystem, a lowbush blueberry barren.  Like oak trees in a forest, there is remarkable genetic diversity on a given farm. Unlike most crops, high performing varieties are not selectively bred and planted.  Lowbush blueberries grow naturally in interwoven patches with dense mat-like root systems ranging from 2-20 ft in diameter. Each patch is really one genetically unique plant, called a clone. Clones vary greatly in productivity, insect and disease resistance, and fruit quality; some fail to ripen all their fruit, some have more flavor, others are more susceptible to diseases, etc.  Because of the interwoven mat-like nature of the patches, replacing poor varieties with higher performing ones is impractical; lowbush blueberry farmers, mostly, are stuck with the genetics evolved on their particular farms.

Thus, supplying nutrients with more precision provides a promising path towards minimizing this limitation by optimizing the genetic potential of each plant. The vast array of cellular processes that are catalyzed by enzymes require significant quantities of minerals, especially micronutrients. As a plant’s nutrient status improves, photosynthesis increases, plants produce more sugars and proteins, and complex phytochemicals, such as antioxidants, increase.  Highly mineralized plants have heightened levels of resistance to pests and diseases. For example, many insects thrive on simple sugars and free amino acids, whereas the complex sugars and proteins that are so highly nutritious for humans are difficult for them to digest.  Supplying sufficient quantities of minerals also affects fruit quality: optimizing calcium levels produces a firmer fruit that stores longer, adequate copper insures against splitting, and providing manganese, a mineral needed for hydrolysis, can increase photosynthetic rate resulting in not only increased yield, but also flavor and even ripening. Agriculturalist John Kempf writes, “Nutritional integrity has at least as big, if not a bigger impact on harvest timing and quality than genetics.”

Contrary to the image conjured by branding lowbush blueberries as “wild”, the nearly 200,000 acres of lowbush blueberries under cultivation in the US and Canada are managed primarily with chemical inputs. Little research has been conducted on the crop to determine optimal levels of nutrients, especially micronutrients. Conventional wisdom about aspects of lowbush blueberry nutrition can lead to a vicious cycle of decreased plant health and the subsequent necessity of pesticides and fungicides. For example, sulfur, in conjunction with herbicide, is often promoted as a means to reduce weed pressure by lowering the pH to 4.0, a level many weed species cannot tolerate.  But a 4.0 pH inhibits calcium absorption, and the combination of high sulfur and inadequate calcium results in a softer fruit skin that is more easily penetrated by the ovipositors of lowbush blueberry pests such as spotted-winged drosophila and blueberry maggot fly. Chemicals used to combat pest infestations further weaken the plant and the cycle continues. 

Plant Sap Analysis (PSA) is a promising new tool for measuring plant nutrient status in that it seems to more accurately reflect nutrient uptake and availability than previously available methods.  Performing plant sap analysis throughout the growing season, and comparing results to an ideal profile for that crop allows growers to create a finely tuned fertility program that addresses deficiencies and excesses in order to optimize productivity, fruit quality and immunity. Currently, this technology has not yet been adopted by lowbush growers and too few samples taken, so an ideal profile for lowbush blueberries is not yet determined; lowbush blueberry results are currently compared to profiles for highbush blueberries. Although there are many similarities between these two Vaccinium species, the nutrient content of the fruit and sap differs significantly. 

This project would be a strong first step towards ascertaining this crop’s “ideal” and “tolerable” ranges of key nutrients using plant sap analysis with the long-term aim of creating of a benchmark that growers throughout the US and Canada can work from to increase yield and address components of fruit quality that affect profitability, such as uniform ripening, fruit size, skin firmness (related to storage life and insect resistance), and flavor. Establishing a profile that correlates with heightened plant immunity also has the potential to significantly reduce the use of pesticides and fungicides in lowbush blueberry farming.


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  • Lily Calderwood
  • Nathan Harman - Technical Advisor


Materials and methods:
  1. Perform PSA to identify nutrient variations in patches with very high and very low productivity, disease resistance and fruit quality.
    1. Disease resistance:  Mummy berry is the prominent disease on our farm, and the one we will study. While scouting the whole farm in the mid-bloom, 10 clones each, with very high and very low rates of infection will be identified. Two leaf samples from each group will be taken, with leaves pooled between the 10 clones.* Patches with increased susceptibility due to low or wet ground won’t be sampled. An infection rate value (1-5) will be recorded for each clone. 1= very low, 5= very high
    2.     Productivity: 
      1. Number of blossoms as a gauge of productivity controls for pollination variability. After field scouting the entire farm in the early-mid bloom, 10 clones each, with high and low numbers of blossoms will be selected and sampled, pooling leaves from clones. The number of blossoms on each stem will be counted and recorded on 10 stems for each clone.
      2. Prior to harvest, 10 clones with the highest density of berries and 10 clones with the lowest density of berries will be selected by eye. 2 samples from each will be collected. Total weight of all berries in a 3X 3 area within the clone will be recorded. 
    3. Fruit quality: Factors measured will be Brix, fruit firmness, consistency of ripeness, and fruit size. After scouting in the middle of the harvest season, 20 clones each of high and low fruit quality will be selected.  The fruit quality of those clones will be assessed using the fruit quality evaluation point system described at the end of the Materials and Methods section**. From this assessment, 2 samples from the 10 clones with the highest fruit quality and 2 samples from the clones with the lowest fruit quality will be taken.
    4. Bloom and early harvest sap results will be reviewed and discussed with my technical advisor and key nutrient differences identified. Based on this data, two nutrients with the most variability between high and low performing plants (Key nutrient A and Key nutrient B) will be selected for further study.
  2. Applying micro and macro nutrients to test plots:
    1. Plots will be staked out four fields with relatively uniform sun exposure and slope as well as very low weed pressure. The plots will be separated by a 15 foot buffer to avoid spray drift. There will be 4 treatment plots of 1/4 acre each. The four plots will be replicated in 4 different fields for a total of 16 treatment plots.

      Treatment 1: Control- no treatment

      Treatment 2: Base Foliar Mix (BFM)

      Treatment 3: BFM+ higher levels of Key Nutrient A

      Treatment 4: BFM+ higher levels of Key Nutrient B

      The Base Foliar Mix will be designed to replicate the average sap analysis profile of the plants with very high mummy berry immunity, yield, and fruit quality. This mix may be adjusted throughout the study in response to the ongoing sap tests, but the same mix will be used on each of the three treatments each week. The BFM will also include Key Nutrients A and B.

      In pursuit of determining optimal nutrient status, it will be necessary to determine the levels at which those nutrients become unneeded or even detrimental. This will clarify the target to approach, but not surpass it. This will be done incrementally; the technical advisor will slightly raise the application rates, looking for changes in plant health on the sap analysis, while I monitor any visual changes in the field. If improvements are observed, the application rate will be slightly increased until no further benefit or a decrease in plant health is observed. Because it is easier to produce undesirable effects when using high rates of individual nutrients when other nutrients are imbalanced and in order to replicate the approach a farmer using this method would actually use, the BFM will be used with each treatment plot.

    2. Initial PSA will be performed prior to any foliar applications, to test any pre-existing differences between plots and initial foliar recommendations made.
    3. Weekly foliar sprays will be applied using a backpack sprayer using rates recommended by the technical advisor specific to each treatment plot.
    4. Sap analysis will be performed biweekly on each treatment.  After the results of each sap analysis, a consultation with the technical advisor will result in adjustments to the materials applied per trial.
  3. Measuring productivity, fruit quality and disease resistance:
    1. Disease resistance will be measured by estimating the percentage of blossoms infected with mummy berry during the mid-late bloom. A rating 1-5 will be given to each plot with 1 being the least infected and 5 the most infected.
    2. Fruit quality (brix, evenness of ripeness, fruit size and berry firmness) will be evaluated using the fruit quality evaluation system**. Sample locations in each plot will be selected at random for each quality being measured and measurements recorded for 16 stems in each plot.
    3. Productivity will be measured in two ways:
      1. Production potential will be measured in order to control for variations in pollination. Total blossoms will be counted and recorded on 16 stems in each plot.
      2. Yield will be determined by harvesting and weighing the berries from each plot before the sorting process. Raking will be done in the mid-season, ideally on the same day, but on no more than 3 consecutive days.
  4. Data will be analyzed with the help of Nathan Harman and Lily Calderwood using JMP software to perform an ANOVA analysis.
  5. The beginnings of an optimal lowbush blueberry sap nutrient profile using the collected data will be created with the help of Nathan Harman and Advancing Eco Ag. This profile will be made available to growers, labs and university extensions- for use and further refinement.

*Because lowbush blueberry leaves are so small, many leaves of both old and new are required for each sap analysis sample. A sample taken from a given clone might require most of its leaves; therefore, a sizable area is needed to collect leaves without significantly damaging the plants. Although sap analysis of each separate clone would provide a larger data set, pooling the leaves from a few clones with similar characteristics will prevent damage and provide insights into nutrient variations between clones.

**The Fruit Evaluation System will measure brix, fruit size, even ripening, and fruit firmness and assign values to each sample using a point system:

  • Brix will be measured using a digital refractometer. Berries on sample stems will be crushed using a hand held press and the brix recorded.  Points for each sample will be calculated by multiplying the brix reading x 5.
  • Fruit size will be measured by selecting 100 ripe berries in each sample location and recording their weight. Point score will be 100*(W-1.2)/1.2 where W is the weight of the one hundred berries in ounces (1.2 is our average hundred berry weight).
  • Even ripening will be measured by counting the total number of berries and the number of ripe berries in a sample. The point score will be the percentage of ripe berries.
  • Fruit firmness will be measured using a penetrometer for soft fruit with a range of 0.4- 11 lbs. The point score will be the pounds multiplied by 10. 
Research results and discussion:

We have been working on data analysis and plan to get our final report done by the end of March. So far, we have analyzed fruit quality data for the second year treatment plots and found statistically significant differences in Brix between treatment plots:


                                          diff                   lwr                     upr                       p  adj

Control-Base                -0.8265625     -1.60269881.     -0.05042619    0.0318394

Mn-Base                       -0.6562500     -1.43238631       0.11988631       0.1297022

Zn-Base                        0.0921875      -0.68394881      0.86832381       0.9899232

Mn-Control               0.1703125           -0.60582381       0.94644881     0.9416310

Zn-Control                   0.9187500.       0.14261369       1.69488631     0.0129616

Zn-Mn                       0.7484375          -0.02769881         1.52457381    0.0632718

Participation Summary

Education & Outreach Activities and Participation Summary

10 Consultations

Participation Summary:

2 Farmers participated
1 Number of agricultural educator or service providers reached through education and outreach activities
Education/outreach description:

I have collected all the data for the project and will be making a final report and slide show presentation.

Learning Outcomes

2 Farmers reported changes in knowledge, attitudes, skills and/or awareness as a result of their participation
Key areas in which farmers reported changes in knowledge, attitude, skills and/or awareness:

For myself and my husband, we have learned a great deal already. First, the sap analysis results are showing which nutrients are elevated in plants with high levels of productivity and berry quality. This is very exciting because we are getting target ranges for key nutrients that have previously been lacking for lowbush blueberries. We are also beginning to get an idea of which nutrients are best absorbed through foliar sprays and which are not. In addition, it has been interesting to see how the sap from the plants in the control group changed over the course of the season. This show us what nutrients the plants are naturally bio-accumulating from the soil without any inputs.

Project Outcomes

Project outcomes:

During the second year of the project, we learned several things:

  • That most macronutrients only slightly changed in the treatment groups versus the control, even with copious foliar sprays. This was especially true of calcium, phosphorous, and potassium and less true of nitrogen and magnesium.

  • That the blueberry sap showed large changes in the concentrations of micronutrients zinc, copper, boron, cobalt, manganese, and iron, even when only small amounts were added on a regular basis.

  • In our zinc trial both macro and micronutrients . This is significant because previously, there was uncertainty about whether manganese, especially in the plant sap, rose as more zinc was applied. Manganese in the zinc trial rose above the levels of the manganese trial. One question that arose for me is to what extent the blueberry plants distribute nutrients in the field through their root systems.

  • We have not statistically analyzed the data yet to see if these differences in nutrient status of the sap resulted in significant differences in berry quality or yield, but at first glance, there did not seem to be significant differences in berry quality and yield between treatments and even the control. One possible reason for this may have been that the plants were all under stress from a severe drought. For fertilizers to have significant benefit, the plant must have adequate moisture. Our fields are currently unirrigated.

  • The largest differences in berry quality and yield were observed between patches of clones (a patch of genetically identical plants) no matter if a plant was in one treatment group or another. I’m curious if this would also be the case if we were using granular macronutrient fertilizers alongside micronutrient sprays.

Assessment of Project Approach and Areas of Further Study:

So far, the main thing I would do differently if I was designing the project over again, and if the budget allowed, would be to separate out the different aspects of fruit quality and analyze them separately instead of lumping them together. If possible, I would also spend a full year analyzing the different untreated plants in the field before choosing which two nutrients to focus on for plots C and D.  At the completion of the research I am excited to try using some granular amendments to supply certain macro nutrients to see if I can better approach the target values shown in the best quality and highest yielding plants.


I hold my opinion from last year regarding the assessment of my project approach. I would also recognize one of the key design challenges: I had to choose a large enough plot size to harvest enough leaves every two weeks for the sap analysis without depleting the plants of their photosynthetic capacity. However, the larger plot sizes (¼ acre) meant that it was difficult to come close to plot uniformity regarding weed pressure, amount of bed rock in each plot, drainage, etc. I’m not exactly sure how I would solve this challenge in the future. But now that we have better target ranges for nutrient levels in the sap, perhaps we could sample less frequently and, therefore, have smaller plot sizes.

For future research:

  • I am interested in learning more about how bioinoculants such as mycorrhyzal fungi and leaf-surface beneficial bacteria would influence nutrient uptake as measured by the sap.

  • I am interested n sampling in other wild blueberry fields in the country to see if the plants with high yields in the fields have a similar nutrient profile to the ones in our fields.

  • I’m interested in studying Velpar, a herbicide commonly used in lowbush blueberry cultivation. Not much research has been done it. I’d like to see if and how it affects the nutrients in the plant sap as well as the soil biology.

  1. I’m interested in the effects that a thick pine mulch would have on weed pressure in lowbush blueberry fields and what affect, if any, this would have on nutrients in the plant sap.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.