- 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)
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.
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
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.
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
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.
|Crop||Stage of growth||Plant part sampled||Approx. number of plants or leaves to sample|
|Apples||July 15-Aug. 15||Leaf from middle of current terminal shoot||60|
|Asparagus||Mature term (Aug.)||Fern from 17-35 in. up||20|
|Beans, snap||Initial flowering||Young mature trifoliate||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||Mid-growth||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|
|Lettuce||Heads, half-size||Wrapper leaf||20|
|Onions||Mid-growth||Top, no white portions||25|
|Peas||First bloom||Recently mature leaflet||50|
|Peppers||Early fruiting||Young mature leaf||20|
|Potatoes (leaf)||40-50 days after ermergence||Fourth tip from leaf||20|
|Potatoes (petiole)||40-50 days after ermergence||Petiole from fourth leaf from tip||40|
|Pumpkin/squash||Early fruiting||Young mature leaf||15|
|Radishes||Mid-growth to harvest||Young mature leaf||40|
|Raspberries||First week in Aug.||Leaf 18 in. from tip||50|
|Spinach||30-50 days old||Young mature leaf||35|
|Strawberries||Mid-Aug.||Young mature leaf||20|
|Sweet corn||Tasseling to silk||Ear leaf||10|
|Tomatoes||First mature fruit||Young mature leaf||20|
|Watermelons||Mid-growth||Young mature leaf||15|
From Cornell University
How to 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 access the form here.
- 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:
- Check for the sample cost (costs are listed on the form)
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.
See page 34 of the "Nutrient Management Guide for Commercial Fruit and Vegetable Crops in Minnesota" for information on interpreting foliar test results and determining fertilizer needs of fruit crops.
Additional resources are available:
- Strawberries: The Cornell University Organic Production and IPM Guide for Strawberries provides strawberry foliar nutrient recommendations.
- Apples: Eric Hanson at Michigan State University wrote a guide to apple nutritionthat includes recommendations for interpreting foliar test results.
- Grapes: 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.
Nutrient sufficiency in grape petioles sampled at full bloom (mid to late June) and at early veraison (mid July to mid Aug.)
|Nutrient||Full bloom||Early veraison|
Pages 35-36 in the "Nutrient Management Guide for Commercial Fruit and Vegetable Crops in Minnesota" provide tables showing ideal nutrient concentrations for various crops. Again, for many growers this is above and beyond standard recommendations.
This approach can be very helpful to farmers who want to, for example, maximize tomato production in the last few weeks of the season. For tomatoes, peppers and cucurbits, the guide provides ideal nitrogen concentrations for multiple growth stages from flowering to early fruit set to late fruit set.
Reviewed in 2020