Availability to Plants

Copper is found in soil at extremely small concentrations of 5 – 50 ppm (parts per million), and mostly in rock minerals. Copper, as the Cu²⁺ ion,  binds tightly to organic matter, more so than other micronutrient cations such as zinc (Zn²⁺) and manganese (Mn²⁺).

It is these Cu-organic complexes which regulate the mobility and availability of copper to plants via the soil water, and the amount of hydrogen (H⁺) ions present which drive how much Cu²⁺ is absorbed by and desorbed from organic matter. Copper availability generally decreases as the concentration of H⁺ decreases (and pH rises), and is extremely low in highly alkaline calcareous soils.

Copper is extremely immobile in soil as it adheres strongly to organic matter. As organic matter concentrates in the upper soil horizons, copper tends to also concentrate in these same horizons, especially if applied in fertilisers. Copper concentration decreases down a soil profile, unlike the more leachable nutrients such as nitrogen, potassium, sulfur, calcium, magnesium and iron.

Copper in Biochemistry

Plants use very small amounts of copper. Copper makes up from 2 – 20 ppm of dry plant tissue, which is about a tenth of that of manganese and one-twentieth that of iron.

Copper is an essential enzyme cofactor, in that many enzyme reactions will not occur unless it is present. It has roles in photosynthesis and chlorophyll production, cellular respiration, lignin synthesis, and the metabolism of carbohydrates and proteins. Copper adds colour to flowers, and flavour and colour to fruits and vegetables by increasing the sugar content. Copper is also important in seed production.

Copper Deficiency Symptoms

A copper deficiency is more likely to occur on sandy soils with little organic matter, in soils with very high organic matter content, and in high pH soils.

Copper is not mobile in plants, and deficiencies show first in the youngest leaves. The deficiency may vary from species to species, but chlorosis (an insufficient production of chlorophyll) resembling that of iron is common. Small necrotic (dead) patches may form within the chlorotic regions and especially on the leaf margins (edges).

The newest leaves may be very small in size, dull in colour, and wilted. Flowers may be dull in colour too. As with a zinc deficiency, the internodes (lengths between the nodes on a stem) are shorter than normal, and the plant develops a compact appearance.

High amounts of nitrogen, phosphorus, potassium, and zinc can interfere with copper uptake.

Copper Toxicity Symptoms

High amounts of copper are toxic to growth generally. Too much copper can burn root tips, reduce branching, and affect the uptake of iron, zinc and molybdenum. High levels of copper inside a plant can displace iron ions from physiologically important compounds, causing a chlorosis that resembles an iron deficiency but which is really a symptom of copper toxicity. Another sign of toxicity is greener than usual new growth that then shows signs of iron or other micronutrient deficiencies.