micronutrients section n soil fertility and plant nutrition
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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.