Chuck Bornt, ENYCHP & Steve Reiners, Cornell University                                                                                                          

Those of you that have worked with me long enough know that I have some pretty strong opinions when it comes to certain things and foliar feeding vegetable crops is one of those topics on which I have some opinions. The bottom line is, I can't really find good research information on what to use or what rates etc., but over the years I have learned a couple of things that I would like to share with you.                                                                                                                      

 Let's define the nutrients I'm talking about. Micronutrients are needed by plants in low amounts, from just a few ounces per acre for molybdenum to a few pounds per acre for zinc, manganese, boron, copper and iron.  Compare that to macronutrients like nitrogen, phosphorus and potassium that are needed in amounts ranging from 40 to 150 pounds per acre. Also considered macronutrients are sulfur, calcium and magnesium which may be needed in the 20 to 40 pound range. First, I believe foliar feeding micros is only part of the solution and is meant as a temporary corrective measure! Foliar forms of these micros may be more readily available to plants compared to soil applied forms. However, foliar feeding should be considered only part of the nutrient management plan.  Continue to soil sample and address micronutrient deficiencies through liming/pH corrections when possible. In many cases, these materials can be added to many of the dry or liquid starter fertilizers we use.                                                                  

Determining if you have a micronutrient deficiency is sometimes very difficult as often the symptoms look the same as some environmental issues. The best way to tell if you have a micronutrient issue is to collect a foliar sample and send it to a lab that can run an analysis for you.  I have had good luck with Waters Agricultural Labs in Kentucky: www.watersag.com (also a location in Georgia). Their turnaround time is usually quick and they supply you with recommendations including foliar feeding recommendations. You can also submit a soil sample from the same field to determine if your soil levels are also low.  When looking to take a foliar sample, the recommendation for most crops is the youngest fully expanded leaf.  Collect at least 15—20 leaves from across the planting (composite sample like you do with soil sampling) and put them in a paper bag (do not use plastic bags) and get them in the mail as soon as possible.  I would recommend that you not pull samples on a Thursday or Friday since they could sit in the post office for a day or two before being delivered.  You can find more information at their website on nutrients they analyze for, con       tact information and fees.

I also thought this information from the Michigan State Bulletin was important when treating a micronutrient deficiency:  "For a preventive spray program, spray the crop about four weeks after emergence or transplanting. Because many micronutrients are not readily translocated within the plant, a second spray will be needed two weeks later to cover the new foliage. When a known nutrient deficiency develops, spray the crop with the appropriate nutrient at the recommended rate every 10 days until the deficiency is corrected. Complete coverage of the foliage is important, especially for iron. Adding a wetting agent to the spray solution will improve the coverage and may increase absorption, especially in crops with waxy surfaces, such as cauliflower and onions.

Micronutrients may be mixed with most fungicides and insecticides. However, some combinations are incompatible and may injure crops. When in doubt, spray only a limited acreage until compatibility is established. Any injury will usually appear within 48 hours." I should also add that a minimum of 30 gallons of water per acre should be used.

I am not a believer in delivering the necessary macronutrients such as nitrogen, phosphorous and potassium by foliar means—but with that said, I do think that there are times when plants may respond to these nutrients being applied as a foliar.  Most vegetables require these three nutrients in large quantities (40—150 lbs per acre). Soil biological processes make these nutrients available, and plants have been evolved to take these nutrients up most efficiently through their roots, not their leaves and stems. Here comes the "however" - over the last couple of years I have seen where adding a couple of pounds of these nutrients, especially nitrogen during stressful times does seem to help the plant "weather" the stress and help it recover quicker when the environment turns more favorable. In particular, I have seen where a foliar feeding nitrogen on sweet corn damaged by hail did help the plant recover quicker.  However, the key is making sure you have some foliage left there for the nutrients to be taken in.  Calcium and magnesium sprays can also help feed plants when soil application is not practical.

Table 1. Nutrient sufficiency ranges for vegetables, potatoes and corn*

 

ELEMENT

VEGETABLES

Most recently mature leaf

POTATOES

Petioles most recently mature leaf sampled

at midseason

CORN

Ear leaf sample at initial silk

Percent (%)

NITROGEN

2.5 - 4

2.5-4

2.76-3.5

PHOSPHORUS

0.25 - 0.8

0.18-0.22

0.25-0.5

POTASSIUM

2 - 9

6-9

1.7-2.5

CALCIUM

0.35 -2

.36-.5

0.2-1

MAGNESIUM

0.25 - 1

0.17-0.22

0.15-0.6

SULFUR

0.16 - 0.5

0.21-0.5

0.15-0.5

Parts per million (ppm)

MANGANESE

30 - 200

30-200

20-150

IRON

50 - 250

30-300

20-250

BORON

30 - 60

15+40

4-25

COPPER

8 - 20

7-30

6-20

ZINC

30 - 100

30-100

20-70

MOLYDENUM

0.5 - 5

0.5-4

0.1-2

*Vitosh, M.L., D.D. Warncke, and R.E Lucas. 1994. Secondary and Micronutrients for Vegetables and Field Crops. https ://ww w. ms u. ed u/ ~war ncke// E 0486. pd f

See Table 3 for rate recommendations. This foliar application followed by either a sidedress application or injection via a drip system for crops on plastic would be a way to promote nutrient uptake and keep that plant moving along. I think and hope that most crop advisors and salesman would think along these same lines and tell you that you need to make sure you're doing your best to provide the crop with the nutrient needs through amending your soils and not through relying on foliar applications. With that said, if you have questions about foliar nutrients, sampling or other fertilizer questions

Table 2. Relative response of selected crops to micronutrient fertilizers*

Crop

Mn

B

Cu

Zn

Mo

Fe

Asparagus

L

L

L

L

L

M

Broccoli

M

H

M

--

H

H

Cabbage

M

M

M

L

M

M

Carrot

M

M

M

L

L

--

Cauliflower

M

H

M

--

H

H

Celery

M

H

M

--

L

--

Cucumber

H

L

M

--

--

--

Lettuce

H

M

H

M

H

--

Onion

H

L

H

H

H

--

Parsnip

M

M

M

--

L

--

Pea

H

L

L

L

M

--

Pepper

M

L

L

--

M

--

Potato

H

L

L

M

L

--

Radish

H

M

M

M

M

--

Snap beans

H

L

L

H

M

H

Spinach

H

M

H

H

H

H

Sweet corn

H

M

M

H

L

M

Table beet

H

H

H

M

H

H

Tomato

M

M

H

M

M

H

Turnip

M

H

M

--

M

--

 

Highly (H) responsive crops will often respond to micronutrient fertilizer additions if the micronutrient concentration in the soil is low. Medium (M) responsive crops are less likely to respond and the low (L) responsive crops do not usually respond.

 

Table 3. Suggested rates and sources of secondary and micro- nutrients for foliar application*

 

Element

Pounds of element/ acre

 

Suggested Source

Calcium (Ca)

1-2

Calcium chloride or calcium nitrate

Magnesium (Mg)

1-2

Magnesium sulfate (Epsom salts)

Manganese (Mn)

1-2

Manganese sulfate or finely ground manganese oxide

Copper (Cu)

0.5-1

Copper sulfate or copper oxide

Zinc (Zn)

0.3-0.7

Zinc sulfate

Boron (B)

0.1-0.3

Soluble borate

Molybdenum (Mo)

0.06

Sodium molybdate

Iron (Fe)1

1-2

Ferrous sulfate

1Iron is not usually deficient in New York vegetable soils

*Vitosh, M.L., D.D. Warncke, and R.E Lucas. 1994. Secondary and Micronutrients for Vegeta- bles and Field Crops. https://www.msu.edu/~warncke//E0486.pdf