Copper is a micronutrient in plants and an important constituent, in small amounts, of the human diet. It is a naturally occurring element in the soil and it can be found as a metal or in a variety of ores. It is a constituent of many man made alloys and is used in wire and some coins.
Copper plays a range of roles in plants. It facilitates respiration and photosynthesis and is important for plant metabolism. It is a component of a variety of enzymes and plant cell walls so it is important for plant strength. Copper also affects the flavour, sugar content and storage life of fruit.
A variety of factors can affect the availability of copper including:
- Root growth ‑ copper doesn't move through soil easily so anything that reduces root growth also prevents plants from taking up copper.
- pH ‑ copper availability is higher in acidic soils and lower in alkaline soils.
- Organic matter ‑ organic matter reduces the availability of copper (though it is released as organic matter breaks down).
- Moisture ‑ copper is less available in waterlogged soil.
- Zinc ‑ excess zinc reduces copper availability.
- Nitrogen ‑ excess nitrogen can prevent copper from being transported around plants and nitrogen deficiency can reduce the uptake of copper.
- Phosphorus ‑ excess phosphorus can reduce copper uptake.
Ideally, for healthy and productive soil, the concentration of copper should be 2‑50 mg/kg. it would be a good idea to investigate the reasons behind high levels if a copper concentration of greater than 6000 mg/kg is detected in your soil.
Copper deficiency is more likely to be seen in plants grown in sandy, alkaline soils.
Symptoms of Copper Deficiency
Copper deficiency in plants can result in poor growth, delayed flowering and sterility. As a result of the latter, seed saved from copper deficient plants, even plants that recover from a copper deficiency, will often have a poor germination rate. Uniform chlorosis may also occur on new leaves and leaves may curl under and/or wilt. In some cases, interveinal chlorosis may be the initial symptom with chlorosis extending to the veins after prolonged deficiency.
Treating Copper Deficiency
The first step to treating copper deficiency is to check and if necessary adjust the pH of the soil. Copper is most easily absorbed by plants if the soil pH is between 5.5 and 6.5 though adequate uptake should still occur provided the pH is between 4.5 and 7.5 and the soil isn't waterlogged. If the soil is boggy, take steps to improve the drainage of the soil by adding organic matter and aerating the soil with a fork. Alternatively, a quick solution is to transplant the affected plant to a raised bed or other better drained site.
While organic matter can reduce the uptake of copper, it is still important to have enough organic matter in the soil for all its other benefits so don't be tempted to stop adding organic matter to the soil. At the same time, digging too close to plants can disrupt root development (among other things) and this can severely limit a plant's ability to absorb copper and other nutrients so organic matter is best added as mulch rather than being dug into the soil.
If symptoms persist after the above steps, it would be a good idea to have your soil copper, nitrogen, phosphorus and zinc levels tested. If any of these elements are not present at appropriate levels, then you may consider taking more direct action to solve the problem ‑ clicking on the other elements (nitrogen, phosphorus or zinc) will take you to that element's page of this website. To add more copper to your soil, copper sulfate (a foliar spray and/or a soil amendment) is probably the best choice.
If any of your plants are also suffering from fungal infections, a copper based fungicide may be an effective treatment so decide whether you will try such a treatment before amending the soil as copper fungicide treatment will likely increase your soil's copper concentration.
Copper toxicity in plants can inhibit iron uptake and can also stunt growth. Excess soil copper can inhibit seed germination. High soil copper levels can occur as a result of excessive use of copper containing fungicides and industrial activity (such as mining).
When reviewing the results of a soil copper test, be alert for copper levels that are close to the maximum recommended value as well as excesses. High but not excessive levels are significant because if any of your fruit trees, roses or other plants that are susceptible to fungal diseases develop such diseases, you may need to resort to a copper spray (there are some that are suitable for organic gardens if you garden that way). If so, you'll need to be especially careful during application to ensure you don't end up increasing your soil's copper level.
If you find excessive levels of copper in your soil, ensure your soil has enough phosphorus and zinc, that the pH isn't too low (too acidic) and add plenty of organic matter on a regular basis. You'll also need to be careful not to add any additional copper when fertilising or treating diseases. Microorganisms can be affected by high levels of copper so care should be taken to promote their growth (providing organic matter is a good start but you should also reduce digging as this disrupts the growth of microorganisms).