Soils 205-90

Lecture 17- Micronutrients

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28

 638-667

 

A. Fe, Mn, Cu, Zn, B, Mo, Cl, Co, Ni

 

B. Deficiencies

occur with specific plant-nutrient combinations

 

C. Deficiency vs Toxicity

Examples of Values for the Northwest

  Plant Tissue Concentration in mg/kg or ppm
Element Deficient Sufficient Toxic
Zn <15 15-150 >250
Mn <25 25-200 >250
Cu <4 5-20 >20
B <20 20-50 >100

Sufficiency zone is relatively small- must manage very carefully to correct for deficiencies and avoid toxic levels

 

 

D. Role in plants

x enzyme systems

- metabolic functions

- "reused" - small amounts required

 

E. Form used

Fe = Fe+2, Fe+3 Mo = MoO4-2
Mn = Mn+2 B = H3BO3
Cu = Cu+2 Cl = Cl-
Zn = Zn+2  
Co = Co+2  
Ni = Ni+2, Ni+3  

 

F. Source

1. inorganic minerals and complexes

2. organic matter component and organic complexes

 

G. Micronutrient Cations

» Fe, Mn, Cu, Zn, Co, Ni

1. More soluble and available at acid pH values

¾(+ OH-)¾®

Fe+3 -------------------------------- Fe(OH)3

¬¾(+ H+)¾

soluble -------------------------------insoluble

available ---------------------------- unavailable

also: Cu+2 ¬¾® Cu(OH)2

and others

2. Insoluble and less available in lime-containing soils

Zn+2 + CaCO3 ¬¾¾¾¾® ZnCO3 + Ca+2

calcareous soil --------> insoluble

3. Oxidation state - particularly, Fe & Mn

(a)

Fe2O3 (Fe+3) ¾(- O2)¾® Fe+2
MnO2 (Mn+4) ¬¾ (+ O2)¾ Mn+2

oxidized form --------------- reduced form

insoluble ------------------- more soluble

(b) high moisture + acid soil

- toxic Mn levels

(c) high O2 (well aerated) + high pH (calcareous)

- low Fe and Mn

(d) deficiency & toxicity varies with plant variety

4. Chelates = organic compounds that form stable complexes

(a) EDTA is a common synthetic chelate

 

 

 

 

(CuEDTA)-2

 

(b) EDTA keeps cations in solution

Fe+3 ¾¾¾¾¾® Fe(OH)3
(FeEDTA)-1 ¬¾¾¾ Fe(OH)3
plant --------------------insoluble
available

(c) Natural occurring organic complexes

  • if soluble - increase nutrient availability
  • if insoluble - decrease nutrient availability

 

H. Micronutrient anions

» Cl, Mo, B

1. Cl-

=> very soluble

=> nonreactive

=> ubiquitous

2. H3BO3 ¬¾¾® H2BO3- (borates)

=> reacts weakly with soil

- fairly mobile (can leach)

=> high rainfall - deficiency

- highly variable

3.

¬¾¾¾¾(+ H+)¾¾¾¾¾

H2MoO4 ¬¾¾® HMoO4- ¬¾¾® MoO4-2

molybdic acid ----------------------- molybdate ion
acid soil ----------------------------- alkaline soil
low availability ---------------------- readily available

¾¾¾( liming the soil )¾¾®