Micronutrients

Chapter 15

Co, Cu, Fe, Mn, Ni, Zn (cations)

B, Cl, Mo (anions)

Concentration in plant < 1/10 theconcentration of macronutrients

B and Mo exist as oxyanions.

Deficiency and Toxicity

Deficiency, sufficiency and toxicityranges

Causes of deficiencies

High yieldsHigh analysis fertilizersSoil conditions

Fe deficiency peachMn deficiency cotton

High yields = fast depletion.High analysis = little input.

By high analysis what is meantis few impurities, including micronutrients. Older forms(low analysis) contained whoknows what but some micros.

Two soil conditions affectingmicronutrient availability arepH and Eh (redox potential).

Causes of toxicities

Sufficiency range is narrow

Incidental applicationsLike Cu and Zn in sludge

Soil conditions

Mn toxicity cotton

You ought to be concerned aboutwhat you add to your soil –youmight be adding something youdon’t want there.

There are regulations governingthe composition of land-farmedwaste, including contents of thewaste and build-up of metals inthe soil. But nothing is perfect.

pH and Eh can cause highconcentrations as well as low.Let’s look at these factors.

Availability of Cations

pH

Fe3+ + 3OH- Fe(OH)3 ↓

Will over-liming induce deficiencies?Again, good to appreciate chemical equilibrium. Here, increasing [OH-], i.e.,decreasing [H+] and increasing pH leads to precipitation of the metal micronutrients, as shown for Fe, decreasing their solution concentrationand availability for plant uptake. The answer, obviously, is yes.

Redox conditions (aeration)

Reduced forms are more soluble

Fe2+ > Fe3+ and Mn2+ > Mn4+

Toxicity is possible if pH lowCurious thing but true that the chemically reduced form (e.g., Fe2+) ismore soluble at any pH than the chemically oxidized form. Thus, couplewet (anoxic, anaerobic) conditions conducive to microbially-mediatedreduction with low pH and you might get toxically high concentrations.On the other hand, there is a tendency for wet soils to approach neutralpH. Whether conditions are right for toxicity depends on initial pH andconcentration of the metal.

Availability of Anions

Cl- generally plentiful

B deficiency common

H3BO3 + H2O H+ + B(OH)4-

Borate anion bound to soil colloids

Availability greater at (low / high) pH?As per the equilibrium, as pH increases, there is more B(OH)4

-, so availability decreases.

Exception to generality that availability greater low pH

Mo

Liming increases availability?

Yes. Liming increases pH and decreases the adsorption of the Mooxyanion. So, we’ve a contradictory situation with availability and pH.This, too, is part of the reason why a compromise in soil pH is bestfor most plants, i.e., 5.5 or 6.0 to 7.0.

Chelates

Organometallic complexes with 2 ormore sites capable of bonding to metal

Synthetic and occur in humus

Increase solubility micronutrients

Cu2+ + EDTA4- CuEDTA2- Cu2+ +EDTA4- CuEDTA2-

The below equilibrium is shifted strongly to right. When thetotal concentration of Cu in solution produced by adding Xamount of Cu as a salt to the soil is compare to the totalconcentration of Cu in solution when X is added as the EDTAcomplex, there is more in solution with the complex.

Free Cu2+ is subject toadsorption and precipita-tion whereas the complexisn’t.

The downside is that EDTA (example chelate)can form complexes with other metals, like Ca2+

which is vastly more abundant in the soil than Cu2+.Fortunately, the Cu complex is strongly favored, off-setting this problem.

Applied as foliar spray or banded

Chelates used with Cu, Mn, Zn and Fe

Very stable complexes exist with these metals so that competition withCa, etc. is not a serious limitation to the use of chelated complexes tosupply the micronutrients.

Micronutrient Management

pH 6 to 7 optimal

Moisture / aeration affects Fe and Mn

Application methods to avoid fixation

Application methods

Banded more efficient than broadcast

Foliar sprays avoid fixation

Incorporation into fritted glass gives slow release as glass weathers

Yes, so long as youdon’t burn the foliagethis works becausethe dose is small.

Been around fora while.

Same principle as with P.