Foliar testing for fruit and vegetable crops

Sampling apple leaves in June, 2019. Photo: Gail Hudson.

Authors: Annie Klodd and Natalie Hoidal, Extension Educators for Fruit and Vegetable Production

Why do a foliar test?

In fruit crops: There are 14 essential plant nutrients derived from the soil that are considered essential for the growth of all plants, and foliar tests tell us whether the plants have optimal levels of these nutrients or if fertilizer is needed.  These nutrients are divided into six macro-nutrients: nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S), and eight micronutrients: iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), boron (B), molybdenum (Mo), chlorine (Cl) and nickel (Ni).  

When done properly, foliar testing is the most reliable way to determine fertility needs for fruit crops. While soil tests determine the nutrient contents, pH and organic matter of the soil, there are many factors that influence the ability of the plants to take up these nutrients. Foliar tests reveal the actual nutrient status of the plant, and determine what fertilizer to apply to optimize yield and plant growth. For that reason, foliar testing every year is generally recommended.

In annual vegetables, a basic soil test at the beginning of the year is your best tool for nutrient management planning. However, soil tests are not very reliable for nitrogen needs. Nitrogen application rates are based on organic matter %, since higher organic matter soils are better able to retain N. For most growers, this estimate is just fine. For growers who already have a strong nutrient management program, but would like to go the extra mile for high value crops (e.g. high tunnel tomatoes), foliar testing can provide you with valuable information to refine your fertigation strategy. Additionally, foliar testing can help to identify micronutrient deficiencies if you're seeing symptoms that lead you to suspect a problem.

When is the best time for a foliar test?

In fruit crops: Always be sure to sample fully expanded leaves rather than newly emerged or declining leaves. For apples, the best time to do a foliar nutrient test is in June, once the fruitlets are developing and the branches are fully leafed out. For grapes, there are two times for foliar testing: During bloom, and during veraison. We are currently at bloom for most grape varieties in Minnesota, so now is the time to do a foliar test. See the table below for when to sample for each major fruit crop.

This video shows how to take foliar samples for apples and grapes.
This video shows how to take foliar samples for blueberries, raspberries and strawberries.

In vegetable crops: The exact right timing can vary from crop to crop in vegetables, but generally the best time for a foliar test is right as plants are entering the reproductive phase (i.e. flowering and starting to fruit). See the table below from the Nutrient Management Guide for Commercial Fruit and Vegetable Crops in Minnesota for suggested timings in each crop. For most crops, one foliar test is sufficient. However, for tomatoes (especially indeterminate), peppers, or cucurbits, you may decide to sample multiple times to maintain adequate fertility. 

Which leaves to sample: Your sample should include fully expanded leaves, collected randomly throughout the field or area of interest. The specific leaves to collect depend on the crop. For some crops, such as grapes, we sample petioles instead of leaf blades for the most accurate results. Refer to this video again for information about which leaves to collect for berry crops, this video for grapes and apples, and the table above for vegetable crops.

Fully expanded grapevine leaves, showing the leaf blade and petioles (red or green stems leading to the leaf blade). Photo: Gail Hudson

If one section of the field is showing problems, collect a sample from that area, and then another sample from a healthy area of the field. This will help diagnose the problem, by comparing nutrient levels between the healthy and unhealthy plants.

Where can I get a foliar nutrient test?

The University of Minnesota offers foliar testing through the Soil Testing Laboratory.

Go to soiltest.cfans.umn.edu. On the homepage, scroll down to see a list of downloadable forms. Click on "Diagnostic Plant" to download the file. You can also click on the form below. Fill in the form accordingly. The "Multi-element spectroscopy & nitrogen" test will be sufficient in most situations. This test analyzes an array of macro- and micronutrients important for plant and crop development.

http://soiltest.cfans.umn.edu/sites/soiltest.dl.umn.edu/files/media/diagnostic_plant_19-20.pdf

Mail in the following together:

• Sample
• Form
• Check for the sample cost (costs are listed on the form)

Shipping address:

Soil Testing and Research Analytical Laboratories

135 Crops Research Building

1902 DUDLEY AVE

UNIVERSITY OF MINNESOTA

ST PAUL MN 55108

How to interpret results:

For both fruit and vegetable crops, the Nutrient Management Guide for Commercial Fruit and Vegetable Crops in Minnesota from University of Minnesota is a great resource.

Fruit: The Nutrient Management Guide for Commercial Fruit and Vegetable Crops in Minnesota from University of Minnesota is a great resource for interpreting foliar test results and determining fertilizer needs of fruit crops (see page 34). Additional resources are available as well. Below are listed additional resources for grapes, strawberries, and apples:

• Grapes - Dr. Carl Rosen, Professor of soil science at University of Minnesota, has written nutrient sufficiency ranges for grapes based on foliar test results (see table below). Growers should compare foliar test results to this table to determine if nutrients should be applied. We are in the process of publishing the full guide on the UMN Extension website.
• Strawberries - The Cornell University Organic Production and IPM Guide for Strawberries provides strawberry foliar nutrient recommendations.
• Apples - Dr. Eric Hanson at Michigan State University wrote a good guide to apple nutrition, including recommendations for interpreting foliar test results. Read it here.

Source: Dr. Carl Rosen, University of Minnesota

Vegetables: The very end of the Nutrient Management Guide for Commercial Fruit and Vegetable Crops in Minnesota provides some excellent tables showing ideal nutrient concentrations for various crops. Again, for many growers this is above and beyond. However, this approach can be very helpful to farmers who want to, for example, maximize tomato production in the last couple weeks of the season. For tomatoes, peppers, and cucurbits, the guide provides ideal N concentrations for multiple growth stages from flowering to early fruit set to late fruit set.