Copper has been used for over 100 years as a fungicide in horticulture but what does it do to sensitive beneficial non target organisms like earthworms? The Good Soil project funded by the National Heritage Trust has found that the use of copper based fungicides can have detrimental effects on the earthworm populations of the north coast.
Copper is an essential element and required by all organisms, in fact deficiency results in reduction in biological function. However, elevated concentrations of copper are toxic and when found in soils may result in a range of effects including reduced biological activity and subsequent loss of fertility.
Why is copper such a problem?
Firstly, copper, being a metal, will naturally accumulate in soils binding to the clay and organic material. Secondly, the section of the soil where most biological activity occurs is also the place where most copper will accumulate when applied as fungicide, that is in the surface layers. This is why it is of such concern for the biology of the soil.
What did the Good Soil Project do?
The project looked at earthworm numbers and earthworm avoidance in orchard soils contaminated by copper from fungicide use . Earthworms are often used, as indicator species’, to determine detrimental levels of a variety of chemicals in the environment as they are very sensitive to changes in the soil environment. Worms are mobile and can move away from areas where excess copper (or any other chemicals) will harm them which shows, very clearly, where and when damaging chemicals are located.
As worms move away from areas high in copper the vital functions they perform are removed. Earthworms are vital in maintaining a health soil, through their feeding and burrowing activity. They aid in decomposition and incorporation of organic matter, increase the number of water soluble aggregates, improve water infiltration, aeration, drainage and root penetration, and increase microbial activity. Earthworm casts and burrow walls have higher concentrations of total and plant-available nutrients than surrounding soil and it has been recognised that surface feeding species distribute micro-organisms, spores, pollen and seeds both horizontally and vertically, Earthworms also reduce plant pathogens through digestion of fungal spores. By removing worms from orchards soils, this vital link in the soil biology is removed, and soil degradation will follow.
Orchards with high levels of copper in their soils surveyed in the project were found to have fewer worms in addition to other indicators of poor soil health. Mulch remained on the soil surface longer as there were fewer soil organisms to break it down and there appeared to be very little bioturbation ( mixing) between the soil and mulch above. In fact, earthworms were shown to avoid soil residues of copper between 25-50mg/kg, residue levels which can be attained by only 2 or 3 years of fungicide application.
How do you prevent this happening?
Once there is excess copper in the soil it is impossible to remove as it doesn't break down. The best thing to do is prevent the accumulation in the first place. Using less copper based fungicides and/or using alternatives is a step in the right direction. If soils are high in copper, it may be possible to dilute the effect of the residue by applying organic material and maintaining the soil in a generally healthy state. This will reduce the bioavailability of the residues making them less toxic.NSW Agriculture, through sponsorship from the Rural Industries Research and Development Corporation (RIRDC) Organic Sub-Program, is currently conducting a review of alternatives to copper for disease control in the organics industry. Many of the findings of this review will be applicable to conventional horticultural production.
For more information:
Merrington, G., Rogers, S. L. and Van Zwieten, L. (2002) The potential impact of long-term copper fungicide usage on soil microbial biomass and microbial activity in an avocado orchard. Australian Journal of Soil Research 40(5) 749-759.
Abigail Jenkins, Soil Advisory Officer NSW Agriculture Ph: 02 6626 1357