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 - 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 - alfalfa B-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

Chemical Availability Available Nutrient forms ClCl - BH 3 BO 3 MoMoO 4 2- FeFe 2+, Fe 3+ MnMn 2+, Mn 3+ ZnZn 2+ CuCu 2+, Cu +

Availability Most important factors influencing availability of micronutrients: Soil texture (clay content) Degree of soil weathering pH - 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–B –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

Applying Fe fertilizer to an alkaline soil: Fe OH - FeOOH + H 2 O Applying Fe chelate to an alkaline soil: Fe-Ch + 3 OH - FeOOH + Ch 3- + H 2 O Chelate Mode of Action (available) (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 by foliar application.