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Foliar testing for fruit and vegetable crops

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.
  • Always sample the right leaves at the right time(s) in the growing season according to established recommendations.

There are 14 essential plant nutrients derived from the soil that are considered essential for the growth of all plants. Foliar tests tell us whether the plants are taking up optimal levels of these nutrients, or if changes to the fertilizer program are 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)

The purpose of foliar testing

Fruit crops

When done properly, foliar testing is the most reliable way to determine fertility needs for fruit crops in established plantings. Soil tests determine the nutrient contents, pH and organic matter of the soil, but there are many factors that influence the ability of the plants to take up these nutrients. In other words, just because the soil is high in nutrient levels, that does not necessarily mean that the plant is getting enough nutrients.  Soil testing is important before planting and can be used in conjunction with tissue testing.

Foliar tests reveal the actual nutrient status of the plant and determine what nutrients may be needed to optimize yield and plant growth. For that reason, foliar testing every year is generally recommended.

Annual vegetables

A basic soil test at the beginning of the year is the best tool for nutrient management planning. 

Foliar tests are most relevant for growers who have already developed nutrient management plans based on soil tests, and who wish to go the extra mile for high-value crops (e.g. high tunnel tomatoes). Foliar testing provides valuable information to refine a fertigation strategy. In addition, potato growers in Minnesota routinely do foliar testing every year.

Additionally, foliar testing helps diagnose problems like foliar discoloration or slow plant growth, which may be related to micronutrient deficiencies.

When and how to take foliar samples

This video shows how to take foliar samples for apples and grapes (06:57).

 

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Ordering a foliar nutrient test

The University of Minnesota Soil Testing Laboratory offers foliar testing for a small fee. A number of private and state laboratories also offer foliar testing.

To use the Soil Testing Laboratory, fill out this form and send in your sample as directed.

There are multiple tests to choose from. Contact the Soil Testing Laboratory for advice on which test to use for your crop. The "Multi-element spectroscopy & nitrogen" test is sufficient in most situations. It analyzes an array of macro- and micronutrients important for plant and crop development.

When mailing or hand-delivering your sample, include:

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

Interpreting foliar test results

For both fruit and vegetable crops, use the Nutrient Management Guide for Commercial Fruit and Vegetable Crops in Minnesota from the University of Minnesota to interpret foliar test results and determine fertilizer needs of fruit crops (page 34).

UMN Extension has created a separate webpage detailing foliar and soil nutrient recommendations for grapevines.

It is not always possible to determine the precise amount of each nutrient to add based on a foliar test. However, comparing the values on your test report with the optimum ranges will indicate if your plants are relatively high or low in each nutrient, and indicate which nutrients, if any, to add.

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

Reviewed in 2021

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