Cu

 COPPER

Copper concentration in earth’s crust ranges from 55 to 70 ppm. Sulfides are the predominant minerals of copper in the earth’s crust. Copper deficiency is often the first nutritional disorder to appear in plants grown on newly reclaimed acid Histosols (organic soils) and this condition is referred to as ‘reclamation disease’.

Sources of copper

·         Igneous rock 10-100 ppm of Cu.

·         Sedimentary rock 4-45 ppm

·         Primary minerals contains

·         Chalcopyrite

·         Chalcocite

·         Bornite

Sedimentary minerals

         Oxides, Carbonates, Silicates, Sulphates and Chlorides.

Forms of copper

1. In the soil solution: Ionic and complexed Cu in the soil solution range from10-8 to 10-6 (0.6 to 63 ppb). At soil pH below 6.9, it exists as Cu2+ and above7.0 as Cu (OH)2 and CU (OH)+

Occluded and co-precipitated Cu: Cu is buried in various mineral structures or substituted for other ions or as an impurity in carbonate minerals or trapped in oxides of Fe and Al.

3. Adsorbed Cu

Cu2+ is adsorbed to layers silicate clays, organic matter and oxides of either iron or Al.

4. Clay minerals and oxides

Cu is held tightly with Cu – O – Al or Cu – O – Fe surface bonds.

5. Organic matter

In many soils one fifth to one half of Cu occurs in organically bound form. Organic acid molecules solubilise Cu and make it available to plants. Humic and fulvic acids contain multiple binding sites for Cu, with COO- playing a prominent role.

Critical limits of copper in plants

         Deficiency < 4 ppm

         Adequacy 5 – 10 ppm

         Toxicity > 720 ppm

Factors affecting availability and movement of copper:

i) Texture: Cu+2 in soil solution is usually lower in excessively leached podzolics and and calcareous sands

ii) Soil pH

Mobility of Cu+2 in soil solution increases with increasing pH due to

a) Increased generation of pH dependent sites on colloids

b) Reduced competition with H+,

c) Conversion of highly soluble Cu2+ to less soluble Cu (OH)+ and Cu (OH)2.

iii) Interrelationship with other elements

Excessive use of NPK fertilizers aggravate Cu deficiency. Acid forming fertilizer increase Cu deficiency due to increased abundance of Al3+ in soil. High concentration of P, Fe, Zn and Al depress absorption of Cu.

iv) Incorporation of crop residue: Incorporation of crop residues reduce Cu mobility due to

i) Chemical reactions of Cu2+ and organic compound and other substances originating from decomposing organic residues.

ii) Competition for available Cu2+ by microbes

iii) Inhibition of root development and ability to absorb Cu2+.

v) Plant factors

Crops highly responsive to Cu are carrot, lettuce, spinach, table beets, citrus and onions. Crops tolerant to low Cu are beans, peas, potatoes, lotus species.