Just a Spoonful of Micro's Keeps Vegetables Movin' Along

All vegetables need micronutrients but they all don't have the same requirements. The six micronutrients required by vegetables are boron, copper, iron, manganese, molybdenum and zinc. They are called micronutrients because they are needed in lesser amounts than nitrogen, phosphorus or potassium.

Soil Affects Availability

Micronutrient availability for plant uptake depends on many factors. Soil and environmental conditions can cause temporary or chronic micronutrient deficiencies. Table1 describes soil management and characteristics, and other factors affecting micronutrient availability. Managing these factors will to improve availability.

 

Table 1. Soil management and characteristics, and other factors adversely affecting micronutrient
Nutrient Soil characteristics limiting availability Soil management and other factors
limiting availability
Boron, B
  • dry soil
  • pH extremes (acidic or alkaline)
  • low organic matter
  • exposed or eroded subsoil
  • coarse sandy soils
  • some crops, i.e. cole crops (broccoli, cauliflower), have high boron requirement and are sensitive to deficiencies
  • excessive rainfall may leach on sandy soils
 
Copper, Cu
  • sandy soils
  • increasing soil pH
  • organic soils ( >20% organic matter)
  • availability may also be low in soils with very little organic matter
  • excessive levels of aluminum, phosphorus and iron may reduce plant absorption of copper
Iron, Fe
  • calcareous soils
  • poorly drained soils
  • soil pH > 6.8
  • low soil organic matter
  • combination of high pH, high lime, wet cold soils and high bicarbonate levels (lime-induced chlorosis)
 
  • excessive levels of molybdenum, copper, manganese, or phosphorus
  • plant genetics
     
Manganese, Mn
  • soil pH > 6.7, decreases availability; most available at pH 5.0-6.5
  • high organic matter decreases the availability of manganese; this may be a concern on muck soils
  • high organic matter mineral soils
  • excessive water
  • poor aeration
 
Molybdenum, Mb
  • as soil pH rises, availability increases
 
  • low soil moisture restricts movement to plant roots, reduced mass flow and diffusion
 
Zinc, Zn
  • high pH soils
  • soils with marginal zinc levels with excessive phosphorus applications
  • organic soils
  • eroded or leveled soils where subsoil is exposed
 

 

Soil and Tissue Testing

A soil analysis reports the soil's pH. Knowing and managing your soil's pH is crucial to managing micronutrients. Soil tests give micronutrient levels in part per million (ppm). Unlike testing for macronutrients such as phosphorus and potassium, micronutrient soil tests can be challenging. For some micronutrients, accurate measurement and relating estimated levels back to plant response or requirements is difficult.

In Ontario accredited soil tests are only available for manganese and zinc. Using the same soil laboratory for micronutrient testing will help in the results' consistency. Overtime, this can assist you in evaluating your micronutrient fertilizer program. Table 2 outlines some pertinent aspects of soil micronutrient testing.

In most cases, tissue testing should be used along with soil testing. All micronutrients can be evaluated using tissue testing. Sample timing, plant development stage and the plant part being sampled are crucial to tissue analyses interpretation.

Table 2. Micronutrient Soil Tests

Nutrient

Soil testing

Boron
  • No accredited Ontario test
  • Soil boron levels are often less than 1 ppm making it difficult to get an accurate measure

Copper

Iron
  • No accredited Ontario test
  • No soil test available that correlates well with plant uptake or fertilizer response

 

Manganese
  • Accredited Ontario soil test: Manganese (Mn) Index relates phosphoric acid extraction plus the soil's pH to determine availability

Molybdenum
  • No accredited Ontario test
  • No soil test available that correlates well with plant uptake or fertilizer response

 

Zinc
  • Accredited Ontario soil test: Zinc (Zn) Index uses DTPA extractable zinc and the soil's pH to determine availability

 

Correcting Micronutrient Deficiencies

If needed, micronutrients can be applied as a soil or foliar application. Soil and environmental factors can limit a soil applied micronutrient fertilizer's effectiveness. These factors need to be managed, if and when they are required. If soil characteristics, such as pH, limit micronutrient availability, foliar applications are recommended.

Crop Response to Micronutrients

Crop response to nitrogen, phosphorus and potassium applications is usually predictable. But crop response to micronutrient applications is quite complex and not as predictable. Micronutrient requirements and response to fertilizer applications varies between crops. Generally, crops respond more significantly if the soil is deficient in the micronutrient being applied or affects its availability.

Table 3 describes crop response to micronutrients. Highly responsive crops (H) often respond well to the micronutrient fertilizer if the soil micronutrient concentration is low. Medium responsive crops (M) are less likely to respond. Low responsive crops (L) do not usually respond even at the lowest micronutrient levels. For example: asparagus will probably not respond to zinc applications, even if it is predicted to be deficient in the soil. However, on a manganese deficient soil, beans will probably respond to manganese applications.

Table 3. Crop response to micronutrient fertilizers

Crop

Boron

Copper

Manganese

Molybdenum

Zinc

Asparagus

L

L

L

L

L

Beans

L

L

H

M

H

Broccoli

H

M

M

H

--*

Cabbage

M

M

M

M

L

Carrots

M

M

M

L

L

Cauliflower

H

M

M

H

--

Celery

H

M

M

L

--

Cucumber

L

M

H

--

--

Lettuce

M

H

H

H

M

Pea

L

L

H

M

L

Peppers

L

L

M

M

--

Potato

L

L

M

L

M

Radish

M

M

H

M

M

Red beets

H

H

H

H

M

Spinach

M

H

H

H

--

Sugar beet

M

M

H

M

M

Sweet corn

M

M

M

L

H

Tomato

M

H

M

M

M

Source: OMAFRA Publication 611 Soil Fertility Handbook, 2006. p. 72
* -- = no information available.


Micronutrients are crucial to your crop's development, yield and quality. Testing your soil every 2-3 years and knowing its pH will assist in managing soil micronutrients. Use the Ontario accredited soil test to manage zinc and manganese. If you do soil test for the other micronutrients (boron, iron, copper and molybdenum), be aware of the soil test limitations. For consistency use the same soil testing laboratory and the same extraction. Also, be aware that crop response may not always agree with estimated soil test levels.

For more information on vegetable crops, micronutrients, soil fertility and Ontario accredited soil tests, visit www.ontario.ca/crops or purchase a copy of the following publications available from your regional OMAFRA office or by contacting ServiceOntario: www.ontario.ca/publications or 1-800-668-9938:

  • Publication 363, Vegetable Production Recommendations, 2010
  • Publication 611, Soil Fertility Handbook



For more information:
Toll Free: 1-877-424-1300
Local: (519) 826-4047
E-mail: ag.info.omafra@ontario.ca
Author: Christoph Kessel - Horticulture Crop Nutrition/OMAFRA
Creation Date: 1 April 2010
Last Reviewed: 1 April 2010