Micronutrients (Fe, Mn, Zn, Cu, B, Cl, and Mo) December 2005 Jeff Skousen Professor of Soil Science Land Reclamation Specialist WVU

Why Important NOW?! - Crop Yields- more micros removed from soil, and some soils cannot release fast enough - Fertilizer Purity- impurities containing micros are removed.

Fe in Soils Fe in soils 0.7 to 55% Most iron in primary minerals and clays Low solubility of Fe compounds in soils Adsorbed iron and total Fe has little influence on soil solution or plant available iron Solubility of Fe minerals is very low (10 -6 to M)

Fe in Soil Solution Fe 2+ and Fe 3+ Fe 3+ dominant in well drained soils Solubility pH dependent Fe (OH) 3 + 3H + = Fe H 2 O For every unit decrease in pH, Fe 3+ concentration increases 1,000 fold

Iron Chelates Iron complexation by natural or synthetic compounds may increase dissolved Fe concentrations and plant availability Chelates are soluble organic compounds that transport Fe from soil minerals to plant roots Natural organic acids such as citric acid and oxalic acids have chelating properties

Fe in Plants Absorbed as Fe 2+ or Fe 3+ Important in oxidation reduction reactions in plant cells Used in chlorophyll synthesis Immobile in soils and plants Deficiency symptoms in younger leaves Sufficiency range is 50 to 250 ppm Fe ppm or more (low pH)

Iron deficiency

Factors Affecting Fe Availability Soil pH (deficiency in calcareous soils) Poor soil aeration and formation of carbonates in wet cool soils Organic matter: Low chelates Interactions with Cu and Mn Excess Cu, Fe, Mn, P, and Zn reduce Fe uptake

Iron deficiencies are more likely to occur in: a) high pH b) high organic matter soils c) calcareous soils d) a and b e) a and c

Zinc Soil solution concentration governed by pH Transported to roots as chelate Absorbed as Zn 2+ by plants Immobile in soils and plants

Zn Deficiency Symptoms White or yellow veins on leaves Short internodes, rosette appearance of the leaves Early loss of foliage Malformation of fruit Zn deficiency in sandy soils, calcareous soils, soils high in P, eroded soils

Zinc deficiency

Factors Affecting Zn Availability pH Zn incorporation on iron oxide surfaces Zn incorporation into the dolomite crystals Immobilization by high molecular weight organic compounds Flooding incorporates Zn into iron compounds and increases pH

Zinc availability in soils increases with: a) decreasing soil pH b) increasing CaCO3 content c) liming d) flooding d) all of the above

Copper and Manganese Immobile in soils and plants Cu is very strongly complexed by soil organic matter and specifically adsorbed by layer silicates Solubility of Cu and Mn increases 100 folds for each unit decrease in pH

Copper Sources Swine manure is high in Cu Copper sulfate, copper acetate, and copper ammonium phosphate are common sources of Cu Soil applications at rates of 1 to 20 lb/acre Foliar applications for emergency

Copper deficiency

Mn in Soils Mn 2+ in reduced soils and Mn 4+ in oxidized soils Mn becomes available by reduction and chelation Mn is immobile in soils and plants

Micronutrient Anions Boron absorption as H 3 BO 3 Molybdenum is absorbed as MoO 4 2- Chlorine absorbed as Cl -

Molybdenum in Soils Total Mo in soils range from 0.2 to 5 ppm Soil solution concentrations < 4ppb Solubility increases 10 fold for each unit increase in pH P enhances Mo uptake and sulfate reduces Mo uptake Both Cu and Mn reduce Mo uptake

Boron Mobile in soils, immobile in plants Adsorbed on iron oxides and calcium carbonates Complexed with organic matter Boron becomes less available above pH 6.3 to 6.5 Liming strongly acid soils may cause temporary B deficiency

MIRACLE FERTILIZERS! Usually contain micronutrients, and That’s what makes them magical!

Conclusions All micronutrients are immobile in plants A very narrow range between deficiency and toxicity

Conclusions In general, the metals (Fe, Cu, Mn, and Zn) are more available at lower pH. The anions (B, Mo, Cl) are available at higher pH. The keys to proper micronutrient availability are: moderate pH maintaining Organic Matter content

Which micronutrients are taken up by plants as anions? a) Fe, Cu, Mn, Zn b) B, Mo, Cl

Which statement regarding Mo is false: a) Mo in soils is adsorbed by plants as MoO 4 2- b) Excessive Mo is toxic to grazing animals c) Mo is more available at low pH

Which of the following lists the universally accepted essential micronutrients? a) Fe, Mn, Cu, Zn, Cd, B, Mo b) Fe, Mn, Al, Si, Se, Cl, Mo c) Fe, Mn, Cu, Zn, B, Cl, Mo d) Fe, Mn, Cu, B, Cl, Se, As

What are important factors to ensure that micronutrient deficiencies are unlikely to occur? a) maintaining moderate pH b) having adequate levels of OM c) fertilizing with NPK d) using manures e) growing rice

How do chelates change micronutrient availability? a) they release micros through OM decomposition because they are soluble organic molecules b) they decrease them by adsorbing micros c) they increase them because they raise pH e) no effect, but they run fast

Which percentage of students is most likely to miss a micronutrient question on the test? a) 28% b) 50% c) 94% d) 100%