Micronutrient Chemistry
and Management
 Introduction
 Role of Micronutrients
 Cycling of Micronutrients
 Sources of Micronutrients
 Deficiency versus Toxicity
 Management of Micronutrients
Introduction

Of the 18 elements known to be essential for
plant growth, nine are required in such small
quantities that they are called micronutrients

Iron (Fe), manganese (Mn), zinc (Zn), nickel
(Ni), copper (Cu), boron (B), molybdenum
(Mo), cobalt (Co), and chlorine (Cl).

Animals including humans also require most
of these elements in their diets.
Introduction

The term micronutrient or trace elements do not imply
that these elements are somehow less important than
the macronutrients

In fact, the effects of micronutrients deficiency can be
very severe in terms of stunted growth, low yields,
etc.

When micronutrients are needed, small quantities
produce dramatic results

Most micronutrients come from soil parent materials.
All micronutrients are found in varying quantities in
igneous rocks.
Interest in Micronutrient Chemistry
1. Intensive plant production practices have increased
2.
3.
4.
5.
crop yields, resulting in more removal of micronutrients
from soil.
Use of high analysis fertilizers has reduced use of
impure salts and organic manures which formally
supplied some micronutrients.
Better analytical methods of plant nutrition are helping
to diagnose micronutrient deficiencies that may have
been unnoticed.
In some cases, food grown on soils with low levels of
trace elements provide insufficient human dietary levels
of certain elements that the crops show no signs of.
Problems of toxicity due to oversupply of elements.
Role of Micronutrients

Micronutrients play very complex roles in plant nutrition
–
–
E.g., Participate in the functioning of several enzyme systems
But their specific functions in plant & microbial growth
processes varies. (See Table)






Cu, Fe, Mo, act as electron carriers in enzyme systems
Zn, Mn act as bridges between enzymes and their substrates
Mo, Mn, are needed for N transformations
Ni is a component of urease
Zn is needed in protein synthesis
See Table for details
Element
Function
Deficiency Symptoms
Fe
Component or co-factor for many enzymes.
Essential for chlorophyll formation.
Interveinal chlorosis of younger
leaves.
Mn
Activator of many enzymes; essential for
photosynthesis, N metabolism, and N assimilation.
Interveinal chlorosis of younger
leaves.
Zn
Present in several enzymes.
Promotes growth hormones.
Promotes seed maturation and production.
Small chlorotic leaves. Scallop
leaf edges.
Ni
Essential for enzymes.
Needed for grain filling, seed viability.
Needed for Fe absorption, and urea metabolism
Non-viable seeds, toxic levels of
urea in legume leaves
Cu
Present in several enzymes.
Important in photosynthesis, protein and
carbohydrate metabolism, and probably N fixation;
Stimulates lignification of all plant cell walls.
Pale yellow-bleached appearance
of leaves, lack of firmness in
leaves & stems. Die back of tips
of stems.
B
Activates certain enzymes.
Facilitates sugar translocation.
Essential for cell division and development.
Chlorosis of younger leaves.
Internal browning of stems and
fruit. Many flowers abort.
Mo
Essential for N transformations (fixation and N
assimilation).
Activator of enzyme in nitrate reduction.
Uniform chlorosis of whole plant.
Extreme curling of leaves giving
the whip-tail effect.
Co
Essential for nitrogen fixation; found in vitamin B12
Not known
Influences photosynthesis and root growth
Not known
-
Cl
Deficiency & Toxicity of Micronutrients




At low levels of a nutrient, deficiency and reduced plant
growth may occur (deficiency range).
As the level of nutrient is increased, plants respond by
taking more of the nutrients and increasing their growth.
If the level of nutrients is sufficient to meet plant needs
(sufficiency range), raising the level further will have
little effect in plant growth.
At some level of availability, the plant will take up too
much of the nutrient for its own good (toxicity range)
causing adverse physiological reactions to occur (see
graph)
Deficiency Versus Toxicity
Micronutrients Cycling in Soils
Plant and
animal
residues/wastes
Minerals
and
precipita
tes
(oxides,
silicates,
sulfides,
etc)
Ions in
soil
solution
Adsorbed
on clay and
humus
colloids
Chelates
in soil
solution
Drainage
water
Soil
organisms
Soil organic
matter
Source of Micronutrients


Parent materials influence the micronutrient
content of soils
Deficiencies and toxicities of micronutrients may
be related to:
–
–
–

The total contents of the elements in the parent rocks
The chemical forms of the elements in the rock
Solubility and availability of the elements
Forms of Micronutrients
–
–
Inorganic forms –silicates, oxides, sulfides, etc
Organic forms –complexes of humus and the elements
Conditions under which
micronutrients my be limiting:
1.
2.
3.
4.
5.
6.
Highly leached acid sandy soils
Organic soils
Soils that have been intensively cropped and
fertilized with macronutrients
Soils very high in pH (except for Mo)
Eroded soils
Parent materials
Conditions under which
micronutrients may be toxic:
1.
2.
3.
4.
soils low in pH (except for Mo)
soils with freshly exposed 1 minerals
containing these or other heavy metals
mine tailings (because of a & b)
Other inorganic reactions
Soil Management and
Micronutrient Needs
Soil Management and
Micronutrient Needs

Always keep in mind the conditions under
which micronutrient deficiencies and toxicities
are likely to occur
–
–
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–
Changes in soil acidity
Soil moisture
Fertilizer applications
Plant selection and breeding