Copper Accumulation

Copper products such as copper sulfate are considered “Allowed with Restrictions” in organic farming. The annotation on the National List states that they “must be used in a manner that minimizes accumulation of copper in the soil.” How can they be applied so that copper won’t accumulate?

As an elemental contaminant, copper is prone to build up in the soil after repeated use, resulting in phytoxicity and damage to soil organisms, especially in orchards and vineyards. Plants and soil organisms differ widely in their tolerance to copper. Depending on soil conditions (such as pH), it has been estimated that 10-200 milligrams of copper per kilogram of soil is safe for 95% of organisms. Note that the range is wide, and that some organisms may not be safe at those concentrations. 

The intent within the organic standards was to have loading rates low enough that copper would not accumulate to levels with adverse effects. Since plants use copper, if the loading rate doesn’t exceed what plants can take up each season, then accumulation can be minimized. Other amendments such as compost, lime, and organic matter from crops need to have sufficiently low levels of copper, as well as the capacity to buffer, sequester and dilute the copper so that the annual loading of copper is at or below the background levels in the soil and phytoxicity does not result. 

One approach crop producers could use is to take a baseline analysis and include in the farm plan periodic tests. If the level of copper shows increases, then the rates of application can be reduced and other methods used. Copper use should be discontinued in cases where tests show a long-term trend of accumulation, or when baseline levels are high.

This article first appeared in the summer 2005 edition of the OMRI Materials Review newsletter, and was revised in July 2022 by Senior Technical Coordinator Peter Bungum.