MicronutrientsSection N

Soil Fertility and Plant Nutrition

Micronutrients in Plants• Transition metal micronutrients:

Fe, Mn, Zn, Cu, Mo, Ni - are important as enzyme cofactors, metal components of enzymes, and in electron transfer reactions. Immobile in plants.

• Non-metal micronutrientsB - important for cell division,

meristematic tissue. Immobile in most plants.

Cl - important for osmotic regulation. Mobile in plants.

Micronutrient Mobility

• In plants:– With the exception of Cl, all

micronutrients are immobile in plants. However, remember about B?

– Significance?• In soils:

– All except Cl are immobile in most soils.

Deficiencies

Mn - pecan

Mn - lemon

Manganese• Manganese activates many enzymes

including metabolism, energy transport and fatty acid synthesis.

Deficiencies

Zn - corn

Zn - citrus

Zn - pecan

Zinc• Zinc as manganese activates many

enzymes. It is also essential for synthesis of auxin, a key hormone controlling cell growth.

Iron• Iron plays a function in

photosynthesis.

Deficiencies

Fe - beans

Fe - peanuts

Deficiencies

Mo - cauliflower

Boron• The function of boron in plants is not

well understood. It is involved in carbohydrate transport, and cell membrane and cell wall development

Deficiencies

B - alfalfaB-canola

B - cauliflower

Micronutrients in Soils• Transition metals

– occur mostly in primary and secondary minerals. Fe is the most abundant. Abundance of Fe increases as soils weather. Zn deficiency is the most widespread of micronutrient deficiencies.

• B– Occurs in SOM (50%) and in minerals such

as borates. Is most common in arid soils.• Cl

– Most abundant in arid soils and near coasts

Incidence of Zn Deficiency

http://www.iza.com

Chemical Availability

• Available Nutrient forms Cl Cl-

B H3BO3

Mo MoO42-

Fe Fe2+, Fe3+

Mn Mn2+, Mn3+

Zn Zn2+

Cu Cu2+, Cu+

Availability• Most important factors influencing

availability of micronutrients:Soil texture (clay content)Degree of soil weatheringpH - most important Redox potential (aerobic vs. anaerobic). Fe, Mn, Cu much more available under anaerobic conditions.

pH

Relative N

utrient Availability

pH and Nutrient Availability

Fe, Mn, Cu, ZnReason: Solubilityof minerals

MoReason: AnionAdsorption

ClReason: Usually leached from acidsoils

BReason: leachedfrom acid soils.Chemically unavailablein alkaline soils

5.0 7.0

Effects of Redox on Availability

• Availability of Fe, Mn, Cu are directly affected by redox– Under anaerobic conditions, Fe, Mn, and Cu

become more availability– Can become toxic, particularly Mn

Mn toxicity

Deficiencies of Micronutrients• Conditions most likely to result in

deficiencies of:– Fe, Mn, Cu, Zn, Ni:

• Weathered, sandy soils.• Alkaline, aerobic soils. Cu deficiency likely in

organic soil

– Mo• Weathered, acid soils and sandy soils

– B• Weathered, acid soils and sandy soils

– Cl• Weathered, acid soils far from seacoasts

Differences Among Plants

• Some plants are adapted to conditions of low micronutrient availability and rarely suffer deficiency:– Ability to change the rhizosphere pH and

redox to increase nutrient availability.

– e.g. some roots secrete enzymes called phytosiderophores to increase Fe availability.

Fertilizing with Micronutrients

• In alkaline soils, Fe, Mn, Cu, and Zn may react readily with soil constituents, rendering them insoluble and unavailable.

– Therefore, use either chelated nutrients for soil application, or foliar applications of salt solutions.

– B - use borate materials. Over-fertilization with B can cause toxicities

– Cl - fertilizers are rarely used

Chelated (EDDHA)-Fe

Fe-EDTA

FeN

NO

O CH2

CH2

CO CH2

CO CH2

OCO

CH2

OCO

CH2

Chelated IronFe -EDTA

• Applying Fe fertilizer to an alkaline soil:Fe3+ + 3 OH- FeOOH + H2O

Applying Fe chelate to an alkaline soil:

Fe-Ch + 3 OH- FeOOH + Ch3- + H2O

Chelate Mode of Action

(available)

(available)

(unavailable)

(unavailable)

Micronutrients• Threshold between deficiency and

toxicity narrow.• Efficiency of utilization for metal

micronutrients a challenge due to soil reaction.

• Often used where not needed.• Periodically needed and not used.• Fine tuning required in most areas

substantial research required in a few.

Summary - Micronutrients • Most important factors influencing

micronutrient availability in soils:– pH, redox, weathering, texture

• Most micronutrients are immobile in soils and plants– Diagnosis, treatment

• Metal micronutrients are normally applied as chelates or byfoliar application.