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Foliar nutrient testing

Quick facts

  • Foliar (leaf) testing helps determine fertilizer needs for your crop and soil.
  • Knowing what nutrients your crop needs allows for better fertilization and increases production potential for high-value fruits and vegetables.
  • Each crop has its own unique needs for how to submit a foliar nutrient test.

In order to check whether a crop is taking up sufficient nutrients from the soil, growers should collect samples of the plant leaves, also known as a foliar sample, and submit them to the UMN Soil Testing Laboratory for a foliar nutrient test.

Why do a foliar nutrient test?

There are 14 plant nutrients from the soil that are considered essential for the growth of all plants. Testing plant leaves for their nutrient content indicates whether the plants have optimal levels of these nutrients, or if fertilizer is needed. 

These nutrients are divided into six macronutrients and eight micronutrients :

  • Macronutrients: Nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg) and sulfur (S)
  • Micronutrients: Iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), boron (B), molybdenum (Mo), chlorine (Cl) and nickel (Ni)

Fruit crops

Person placing apple tree leaves in paper bag for testing

When done properly, foliar nutrient testing is the most reliable way to determine current fertilizer needs for fruit crops. While soil tests determine the nutrient, pH and organic matter contents 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. Soil scientists recommend that fruit growers conduct foliar testing every year.

Annual vegetables

For growers who already have a strong nutrient management program, but would like to go the extra mile for high-value crops such as high-tunnel tomatoes, foliar testing can provide valuable information to refine a fertigation strategy.

While a basic soil test at the beginning of the year is an effective tool to determine the nutrient needs of that field before planting, soil tests are not very reliable for predicting nitrogen content of the soil or determining how much nitrogen fertilizer to add. Nitrogen application rates are based on soil organic matter percentage since higher organic matter soils are better able to retain nitrogen.

A foliar test provides further information about how well the plant is taking up nitrogen from the soil and can help identify deficiencies in micronutrients if symptoms on the leaves suggest a nutrient problem.

Collecting samples and timing for a foliar test

Fruit crops

Always 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.
  • See the table below for when to sample for each major fruit crop.

Vegetable crops

The exact right timing to sample varies from crop to crop, but generally, the best time for a foliar test is right as plants are entering the reproductive phase (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.
  • For tomatoes (especially indeterminate varieties), peppers, or cucurbits, you may decide to sample multiple times to maintain adequate fertility.

Which leaves to sample

Full grapevine leaves

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, sample petioles instead of leaf blades for the most accurate results. Refer to the videos below for a guide on collecting fruit crop samples and the table below for vegetable crops.

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 healthy and unhealthy plants.


How to get a foliar nutrient test

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

  1. Go to soiltest.cfans.umn.edu.
  2. On the homepage, scroll down to see a list of downloadable forms. Click on "Diagnostic Plant" to download the file. You can also access the form here.
  3. Fill in the form accordingly.

The Multi-element spectroscopy and nitrogen test will be sufficient in most situations. This test analyzes an array of macro- and micronutrients important for plant and crop development.

Mail in the following:

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

Ship to:

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 for interpreting results of a foliar nutrient test.


Annie Klodd and Natalie Hoidal, Extension educators for fruit and vegetable production

Reviewed in 2020

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