Micronutrients Section N Soil Fertility and Plant Nutrition

Micronutrients Section 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 micronutrients B - 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 - lemon Mn - pecan

Manganese • Manganese activates many enzymes including metabolism, energy transport and fatty acid synthesis.

Deficiencies Zn - pecan Zn - corn Zn - citrus

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 - cauliflower B - alfalfa B-canola

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 B Mo Fe Mn Zn Cu Cl. H 3 BO 3 Mo. O 42 Fe 2+, Fe 3+ Mn 2+, Mn 3+ Zn 2+ Cu 2+, Cu+

Availability • Most important factors influencing availability of micronutrients: Soil texture (clay content) Degree of soil weathering p. H - most important Redox potential (aerobic vs. anaerobic). Fe, Mn, Cu much more available under anaerobic conditions.

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 p. H 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

Chelate Mode of Action • Applying Fe fertilizer to an alkaline soil: Fe 3+ + 3 OH(available) Fe. OOH + H 2 O (unavailable) Applying Fe chelate to an alkaline soil: Fe-Ch + 3 OH(available) Fe. OOH + Ch 3 - + H 2 O (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: – p. H, redox, weathering, texture • Most micronutrients are immobile in soils and plants – Diagnosis, treatment • Metal micronutrients are normally applied as chelates or by foliar application.