Skip to main content

Test, don’t guess: Is foliar nutritional analysis right for your fruit crops this growing season?

Madeline Wimmer, UMN Extension Educator- Fruit Production & Soon Li Teh, Extension Specialist and Assistant Professor in Grape Breeding and Enology
Image: a plant nutritional pyramid showing from bottom to top those nutrients supplied by air and water: Carbon (C), Hydrogen (H), and Oxygen (O); primary macronutrients: Nitrogen (N), Phosphorous (P), and Potassium (K); secondary macronutrients: Calcium (Ca), Magnesium (Mg), and Sulfur (S); micronutrients: Molybdenum (Mo), Boron (B), Nickel (Ni), Copper (Cu), Zinc (Zn), Iron (Fe), Chlorine (Cl), and Manganese (Mn). 
Graphic created by Madeline Wimmer, 2023.
Plant nutritional analysis—also known as foliar/leaf/petiole testing—is the practice of analyzing nutrition within a plant to determine adequate, excess, or deficient nutrient levels. Oftentimes, this involves collecting a plant’s leaf, leaf with petiole, or only the petiole at a specific growth stage. Plant nutritional analysis is a critical step in determining any potential nutrient deficiencies, but how do you determine when this step is right for the fruit crops you grow? Continue reading to learn more about when and how to collect samples for plant nutritional testing, and how to interpret the results you’ll receive from the analysis.

Soil testing vs plant nutritional testing

The University of Minnesota recommends completing a soil test for any planting site before establishing perennial fruit crops and then testing regularly, about once every 3-5 years, or when an issue is suspected.
In contrast, plant nutritional analysis can indicate which nutrients are actually present within a plant at a given time during the growing season. Just as the human diet does not always directly reflect the nutrients our bodies can absorb, a plant nutrient deficiency may not necessarily reflect a soil nutrient deficiency. Sometimes it can be because of an imbalance of soil nutrients, improper soil pH, or issues with the plant’s roots impacting nutrient uptake. Collectively, these factors challenge the conventional advice that adding more nutrients is the only solution to amending plant deficiencies.
Another major difference between soil and plant nutritional analysis is that the soil testing does not typically include soil nitrogen levels due to logistical issues, while plant nutritional tests can indicate plant nitrogen levels. Because these tests are complementary, plant nutritional testing cannot be a substitute, but should be used in conjunction with soil testing.
Image: A grape leaf showing labeled parts in white: the leaf petiole and the leaf blade. Different crops require either leaf blade or petiole testing and in the case of grapes, petioles are used for analysis. Image sourced from UMN Extension.

When and how to test

When to test

Because nutrient levels fluctuate throughout the season, depending on physiological demands like vegetative growth, fruit set, and ripening, the time in which the test is done matters. Some resources will list a calendar time of year for testing, but growers should consider the typical growth stage during that time and aim to adjust the testing time accordingly during years when plant growth is ahead or behind what is typically expected. Some crops, like grapes, have multiple windows when testing can be done, while others have one clear window.

Table 1: Recommendations for when and how to collect samples for foliar analysis based on fruit crop.
Information from this table was extracted from UMN Nutrient management for commercial fruits and vegetables guide, which was originally adapted from the Plant Analysis Handbook.

Images: Infographic displaying visual guidelines for grape petiole sampling at bloom, opposite from basal clusters, (left) and during veraison, in which the petioles from leaves 5,6, or 7 from the growing tip can be collected for analysis (right). Graphics created by Madeline Wimmer in 2017 for the Wisconsin Fruit article, “Petiole sampling for determining fertilization needs for cold climate grapes.”

How to test

While testing instructions are specific to each type of fruit crop listed below, general instructions still apply:
  1. Collect samples of the proper tissue at the suggested time of year.
  2. When testing plants that are NOT symptomatic, avoid sampling plants that are atypical in your vineyard/orchard/fruit stand, such as plants on border rows, subject to major pest or disease damage, or overly vigorous.
  3. Keep symptomatic and “healthy” or non-symptomatic samples separate.
  4. Take only one sample per shoot unless instructed otherwise.
  5. Dusty or dirty samples can be lightly rinsed with water, but not soaked for longer periods.
  6. Place samples in a clean, labeled paper bag to either dry at room temperature or be sent to a lab immediately for analysis. Plastic bags should only be used for already dried samples.
  7. Samples can be submitted to theMN Soil Testing laboratory, where growers can fill out this Diagnostic Plant Analysis Test form and either drop samples off directly or mail them into the lab with a fee appropriate for the requested test(s). Growers will notice on the form that there are options to test single nutrients (e.g., only nitrogen or sulfur) as well as options to test multiple nutrients.
Note: Other labs also offer plant nutritional analysis throughout Minnesota. To locate a list of labs that offer this service, visit the Minnesota Department of Agriculture website.

General questions on plant foliar analysis and collecting samples

What is a young, mature leaf?
As leaves develop, they grow to a particular size and shape at maturity. The main shoot growing point (i.e., apical meristem) is found at the tip/end of a shoot. To find the youngest mature leaf, begin at the tip where new leaves are growing and continue going down until you find the most recently matured leaf.

Why do some foliar tests ask for a leaf blade or petiole sample only?
Petioles and leaf blades have different nutrient levels and tissue densities. Choosing one or the other can help with accurate interpretation of the test results.

How often should I do a foliar nutrient analysis test?
At UMN Extension, we recommend doing preventative foliar nutrient testing once every 2-3 years and as needed when deficiency symptoms are present.

How many samples do I need to collect to get a representative analysis of my crop?
The number of samples required will vary for each crop. Refer to table 1 for references related to apples, blueberries, grapes, june-bearing strawberries, and raspberries. If the fruit crop you are planning to test is not on this list, reach out to the lab that will do the analysis to find out what is required.

Can I combine samples from different areas of my field, or should each area be tested separately?
Samples should be representative of the crop you are testing. This means that testing combining samples from land areas with different soil types or management practices could be an inaccurate representation. When there are visual symptoms of a nutrient deficiency, samples should be limited to plants exhibiting these symptoms and not combined with healthy-looking plants; however, growers can submit two separate samples to compare healthy and symptomatic plants.

How do environmental factors, like recent rain or drought, affect the sampling process?
Both drought and rain can impact how well nutrients are transported throughout the plant and taken up from the soil. Additionally, excess rain can leach nutrients from the soil and again, change what is available for plant uptake. This doesn’t mean growers cannot take samples during these periods, but should be aware that it may not be accurate to interpret the results as “normal.”

What are the common signs of nutrient deficiencies that I should look for before deciding to collect samples?
Each nutrient deficiency shows up in different ways. For example, if you are noticing slow growth and chlorotic leaves (i.e., leaves are pale in color), your plants may be experiencing a nitrogen deficiency. Iron deficiencies often tend to show up as an extremely pale-yellow color, and something like boron or zinc deficiency can lead to poor pollination. There are resources out there to help identify nutritional deficiencies for each fruit crop; however, completing plant nutritional analysis before deficiencies show up is the optimal way to diagnose and mitigate nutritional deficiencies in the first plant.

UMN Extension video demonstrating apple and grape petiole sampling

This video (below) shows how to take a foliar sample (tissue sample) in a vineyard and an apple orchard, and shows how to submit the sample to a plant tissue testing lab.
Taking a foliar sample: orchards and vineyards

Interpreting plant nutritional analysis results

Soil testing and foliar testing often work together to paint a picture of the problem. The truth is that not all plant nutrient deficiencies result from soil nutrient deficiencies. Sometimes plants are unable to adequately take up nutrients due to soil environmental conditions (e.g., too cold, too wet, too dry, incorrect pH), nutrient competition (e.g., potassium competing with magnesium), root diseases, pest infestation, or a virus infection. Inadequate nutrient uptake is most clearly noticed when comparing a foliar test showing deficiencies with its soil test.
The following chart is a guide for understanding crop-specific foliar nutrient levels based on the recommended testing time. Notice how grapes have different “healthy” nutrient ranges depending on if the petioles were sampled at bloom or veraison, this is why the plant growth stage matters— for example, a normal nitrogen level range at veraison .80-1.20% nitrogen would be a borderline deficiency if indicated at bloom.

Table 2: Healthy/normal, crop-specific foliar nutrient levels based on the recommended testing time.
Note: label ppm vs % for nutrients…. N-S are % and the rest are ppm.
*— symbolizes a lack of data for these nutrient ranges.
This table originates from the UMN Nutrient management for commercial fruits and vegetables guide, which was originally adapted from the Plant Analysis Handbook.

Additional resources:

Vine Nutrition (Northern Grapes Project ppt by Carl Rosen and Paul Domoto)
Tissue and soil testing for cold climate grapes (UMN Extension webpage)
Foliar testing for fruit and vegetable crops (UMN Extension webpage)
Diagnostic plant analysis request form for UMN Soils Lab

Demchak, K. (2020) “Tissue nutrient analysis for berry crops: Getting the most for your money” PennState Extension.
Rosen, C. and Eliason, R. (2005) “Nutrient management for commercial fruit and vegetable crops in Minnesota” University of Minnesota Extension.

Print Friendly and PDF