Impact of Copper Sulfate on Field Crops

Mike Rankin
Crops and Soils Agent
University of Wisconsin - Extension

        During the past year, there's been several articles in the popular farm press addressing the potential problem of increased soil copper (Cu) levels from the land spreading of spent copper sulfate (CuSO₄) foot bath solutions with liquid manure.  This occurs, of course, because the used CuSO₄ solution is often pumped into manure lagoons, which significantly raises the Cu concentration.

Some things to know about copper

        Copper is an essential nutrient for plant growth.  Like most micronutrients, it is needed in small amounts by the plant (less than 1 lb/acre/year is removed from harvested plant biomass).  Soil minerals and organic matter hold copper very tightly so there is little risk of leaching.  However, once Cu levels become high, they will remain that way for a long time.  Several factors reduce the availability of Cu in soil.  These include high organic matter, high soil pH, and fine textured (clay) soils.  In Wisconsin, plant Cu toxicity situations have been extremely rare.  Toxic levels in the plant cause reduced vigor, poor root development, and leaf chlorosis.

        There have been no reported cases of Cu toxicity in dairy cattle due to high forage Cu levels.  Sheep, on the other hand, are very sensitive to Cu levels (over 20 ppm in the ration and figure on having lamb chops for supper). 

The Miner Institute situation

        One of the first people to identify the potential problem of land spreading spent CuSO₄ solutions was Ev Thomas of the Miner Institute in Chazy, New York.  In 1998, the farm was using about 250 lbs. of CuSO₄ per week for their 160 cow herd.  This amount of CuSO₄ was being spread on 470 acres.  After a couple of years, Thomas noticed an upward spike in manure and forage Cu concentrations.  In 2001, the Miner Institute reduced from 5 to 3 the number of days per week that CuSO₄ was used and reduced the solution concentration.  They also began using tetracycline as an alternative treatment in alternating weeks.  These changes reduced the total amount of Cu being applied on the land from 3300 to 1000 lbs annually.

How much Cu is being applied on WI fields?

        Copper sulfate is 25 percent elemental Cu.  To determine the amount of Cu being applied per year, and more importantly per acre, first determine the amount of CuSO₄ being used per week.  Multiply this number by 52 weeks, then by 0.25 to determine lbs. of Cu applied per year.  Finally, divide by crop acres receiving manure per year to obtain pounds of Cu applied per acre.  For example, if a farm operator is using 250 lbs of CuSO₄ per week and spreads manure on about 400 acres annually, the field-loading rate would be:

(250 lbs./wk) x (52 wks) x (0.25) = 3250 lb Cu/yr

(3250 lb Cu/ yr) divided by (400 acres) = 8.1 lb Cu/ac/yr

It seems that many farms using CuSO₄ solutions fall into the range of 7 to 8 lbs of Cu applied per acre per year.  Remember that some fields may not get manure applied every year.  The following table can be used to quickly estimate the amount of Cu from CuSO₄ applied per acre per year.

        Liquid dairy manure with spent CuSO₄ solution added could easily have a Cu concentration of over 500 ppm (dry matter basis).  To put this in some perspective, liquid swine manure and biosolids (sewage sludge) also may contain similarly high levels of Cu.  Thus, it is certainly not unprecedented to be applying over 5 lbs. of Cu per acre per year to crop land.  Such a practice, in and of itself, will not cause harm to plants or livestock.  In fact, Wisconsin DNR loading limit regulation for biosolids is 66 lb/ac annually (1350 lb. lifetime) on materials ranging from 1500 – 4300 ppm Cu. The difference with biosolids compared to dairy manure is that biosolids are not typically applied repeatedly to the same fields over a period of years.

Recommendations

        Given current knowledge, here are some recommendations relative to CuSO₄ use and land application:

• Because it is difficult to reduce high soil Cu levels once developed, use only enough CuSO₄ to maintain good herd hoof health.  Consider using and rotating effective alternative products with CuSO₄ if feasible.

• Here is just one more reason not to over apply manure or wastewater on cropland.  If a toxicity problem to either crops or livestock is to occur, repeated over application will probably be the cause.

• Be extra cautious of Cu loading on sandy soils.

• On farms where significant amounts of CuSO₄ are being used, periodically test manure and forages for Cu content.

• BOTH herd health and plant health can still be obtained with judicious use of CuSO₄.  There won't be a toxicity issue unless poor management makes it one.