Foliar testing for fruit and vegetable crops

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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Fruit crops

The best time to take fruit foliar samples depends on the crop. It is critically important to take the sample at the appropriate time to get accurate and meaningful results.

Be sure you are collecting the right leaf tissue; for example, grape growers submit petioles, not leaf blades. The position of the grape leaves to sample depends on the time in the season.

Use the foliar testing guidelines table below.

Vegetable crops

The exact timing for foliar sampling varies by crop. Generally, sample when plants are entering the reproductive phase (flowering and starting to fruit). However, you may decide to sample multiple times from high-value tomatoes (especially indeterminate), peppers and cucurbits to maintain optimal fertility. See the foliar testing guidelines table below.

Always sample from fully expanded, mature leaves. For potatoes, sample petioles and not leaf blades. Do not sample newly emerged or senescing (declining) leaves.

A sample is made up of many leaves (see the foliar testing guidelines table for the number of leaves required for each crop). 

For tomatoes, submit either the full leaves or just the petioles. Leaves can be used for diagnostic purposes, but petioles are used to predict fertilizer needs. If unsure, consult with your testing laboratory before submitting the sample.

Penn State University has further recommendations for tissue sampling in tomatoes. Keep in mind that for tomatoes, a leaf is the entire leaf structure coming off of the main stem or leader and has multiple blades.

Foliar testing guidelines for fruit and vegetable crops

Get a representative sample by collecting leaves from randomly selected plants throughout the field or area of interest. Include plants of varying levels of health and vigor rather than selecting the healthiest plants.

To diagnose a problem such as discolored leaves or poor fruit set, collect a sample from the area of the field exhibiting the problem and a second sample from a healthy area of the field to compare nutrient levels and determine if it is a nutrient-related issue.

Foliar test recommendations

Crop	Growth Stage	Plant Part Sampled	Approx. Number of Leaves Needed
Apple	July 15-Aug 15	Leaf from middle of current, terminal shoot	60
Asparagus	Mature fern	Fern from 17-35 inches up	20
Beans, snap	Initial flowering	Young mature leaf	50
Beets, table	Mature	Young mature leaf	20
Blueberries	First week of harvest	Young mature leaf	50
Broccoli	Heading	Young mature leaf	15
Brussels sprouts	Maturity	Young mature leaf	15
Cabbage	Heads, half grown	Young wrapper leaf	15
Cantaloupe	Early fruiting	Fifth leaf from tip	25
Carrots	Midgrowth	Young mature leaf	25
Cauliflower	Buttoning	Young mature leaf	15
Celery	Half-grown	Young mature leaf	20
Cucumbers	Early fruiting	Fifth leaf from tip	20
Eggplant	Early fruiting	Young mature leaf	15
Garlic	Bulbing	Young mature leaf	25
Grapes	Full bloom	Petiole from leaf opposite basal fruit cluster	75
Grapes	Veraison	Petiole from newest fully formed leaf	75
Lettuce	Heads, half size	Wrapper leaf	20
Onions	Midgrowth	Top, no white portions	25
Peas	First bloom	Recently mature leaflet	50
Peppers	Early fruiting	Young mature leaf	20
Potatoes	40-50 days after emergence	Petiole from fourth leaf from tip	40
Pumpkin/Squash	Early fruiting	Young mature leaf	15
Radishes	Midgrowth to harvest	Young mature leaf	40
Raspberries	First week in August	Leaf 18 inches from primocane tip	50
Spinach	30-50 days old	Young mature leaf	35
Strawberries	Mid-August	Young mature leaf	20
Sweet corn	Tasseling to silk	Ear leaf	10
Tomatoes	First mature fruit	Young mature leaf	20
Watermelons	Midgrowth	Young mature leaf	15

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.

Sources

• Nutrient Management Guide for Commercial Fruit and Vegetable Crops in Minnesota
• Organic Production and IPM Guide for Strawberries
• Apple Nutrition
• Soiltest.cfans.umn.edu
• Berry Leaf Tissue Testing (video)
• Taking a Foliar Sample: Vineyards and Orchards (video) 
• Tissue and soil nutrient testing for cold climate grapes

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