Copper and Zinc Accumulation in Manured Soils

Considerable research in the past 5-10 years has focused on how manure applications affect soil nitrogen (N), phosphorus (P), and potassium (K) levels, both from plant nutrition and environmental perspectives. However, manure consists of more than just N, P and K. A recent survey of the trace metal content of dairy manure in New York State showed levels of copper (Cu) and zinc (Zn) that were elevated compared to all other trace metals analyzed (McBride and Spiers, 2001). Similar trends were seen in Vermont by Jokela and others (2005). Both Cu and Zn are added to most dairy rations as part of a mineral mix; however the highest concentrations result when hoof-bath treatment solutions containing copper or zinc sulfate are disposed of in the manure stream. An average corn silage crop will remove about 0.014 lbs of Cu and 0.016 lbs of Zn per ton of silage (35% dry matter). Given current average manure Cu and Zn data for New York, as described by McBride (2001), and a yield of 17 tons of silage per acre, an application of more than 700 gallons of liquid dairy manure per acre will result in more Cu being added than removed with harvest. Similarly, 160 gallons per acre of this manure will be enough to apply the amount of Zn and average corn crop will remove. These calculations show that on our dairy farms even the lowest manure application rate will result in the addition of more Cu and Zn than required by the crop. Copper and Zn are not as mobile as nitrate due to their high affinity for sorption to organic matter so these elements tend to accumulate in the soil. The question remains: is this a problem for our current farming systems? Case study We analyzed soil profiles from a case study farm in Steuben County with over 40 years of dairy and/or poultry manure addition. With this study we wanted to determine the long-term effects of manure application on Cu and Zn accumulation in the plow layer, its distribution over the soil profile and bioavailability in the soil. The soils of the farm are classified as either Wellsboro or Oquaga channery silt loams. We do not have accurate historical manure spreading records or analyses over the past 40 years so we used the total P content of the soils as an estimate of manure history (higher total P concentrations representing longer durations of manure application or higher loading rates). Agronomic soil tests such as Mehlich-3 and Morgan can not reliably predict when copper or zinc toxicity might occur to plants or microbes (interpretations have mostly been limited to identification of deficiency situations), but they can be used to monitor soils for evidence of increased Cu and Zn accumulation over time. As the farm’s historic fertility program includes Mehlich-3 soil tests, we report on trends in Mehlich-3 soil test data but similar trends would be observed if the Cornell Morgan test was used. Cu and Zn accumulation in the plow layer The Cu and Zn content of the 0-7 inch plow layer were both accumulating with manure application (i.e. as total P levels increased). Copper accumulation was higher in the dairy manure amended fields while Zn accumulated more rapidly in the fields that had a history of poultry manure addition. The increased Cu accumulation in dairy manure amended soils could be attributed to CuSO4 used in hoof bath treatments. The higher Zn accumulation in poultry manure