Dear Tom –
We use a granulated poultry litter product that is OMRI (Organic Materials Review Institute) listed for organic production. I read that those products may have high levels of metals that can accumulate over time and damage our soil. Should we be concerned? Thanks.
— Karla in Metaphor, NC
Dear Carla –
A specific answer to your question depends on your soil type, pH, cropping system, soil supplements, and pest management strategies. To provide a general answer I will use some information on copper in tomato production. Copper is used in poultry feed so it ends up in poultry litter-based soil supplements. It is also present in several materials such as copper sulfate used to manage late blight and other tomato diseases.
Copper is an important micronutrient that tomatoes need to thrive, but high levels can damage plants. High copper levels can also raise the crop copper content to a point where it is toxic to humans and livestock that eat the crop. Copper is more stable in the soil that some nutrients but copper levels vary as it is applied in the form of soil supplements and sprays and as it leaves when rainfall and irrigation leach it from the soil. Copper is also carried off in crops and perhaps plant debris. Soil type and pH influence this dynamic system. Acid conditions help metals dissolve and move through the soil and groundwater. Organic soils tend to hold metals more effectively than sandy soils.
According to the trade group the Copper Development Association Inc. “The inclusion of up to as much as 0.1% copper sulfate in the diet of bacon and pork pigs and broiler chickens stimulates appetite and produces increased growth rate with a marked improvement in feed conversion” in pigs and broiler chickens. http://www.copper.org/resources/properties/compounds/copper_sulfate02.html ) One supplement vendor (http://harmonyorganics.net/catalog.html#General%20Purpose%20Fertilizers) implies that supplements made from egg laying chickens are less likely to have high metal levels than broilers and that organic chicken operations are less likely to have metals in their poultry litter than conventional farms. Symphony ™ is a layer litter-based supplement which they offer as an alternative to Harmony™. Both Harmony and Symphony are reportedly produced on the same farm – Harmony with conventional litter and Symphony with organic layer litter. Symphony is not OMRI listed and they offer no copper content information. Harmony is approved by OMRI and the copper content is listed. Symphony is about a dollar more per bag from one source. It probably contains fewer metals in general and if the layers are organic, the feed that they consume is required to be non-GMO. Some organic growers may think Symphony is worth an extra dollar to support an organic operation.
Here are some figures that may help us unravel the issue of copper in tomato production and soils.
- A typical copper level in soils = 20 ppm (parts per million (approximately mg/kg)) (Brady – The Nature and Properties of Soil)
- Minimum soil copper for tomato health = 2-3 ppm (Knott’s Vegetable Growers Handbook)
- Maximum copper for plant health = 40 ppm (sandy soil), = 60 ppm (loam soil), = 100 ppm (clay soil) (Harrison et al 1999).
- Harmony™ copper content = 40 ppm (Seven Springs Farm)
- Recommended vegetable application rate for Harmony = 1525 lb/A = 0.00062 ppm if soil wt = 85lb/ft3
- Copper increase per year with Harmony (two applications) per acre = 0.00000005 ppm
- Years to reach 60 ppm (upper limit in soil) with Harmony as the only copper source = 1,200,000,000 yrs
- Copper applied in one spray of copper hydroxide fungicide = 1.06 lb/A (Champ WG™ label) 50% Cu
- Increase in copper per spray = 0.13 ppm at label rate
- Increase in copper with weekly sprays over an eight week season = 1.04 ppm
- Years to reach loam soil limit (60 ppm) with annual sprays = 58 years
- Copper sprays allowed in Netherlands and Denmark = 0 lb per acre (copper is banned)
- Copper limit in other EU countries = 5.4 lb/A/yr
(Note; some vineyards in Europe have used copper for 100 years or more)
- Human health limit for copper in food = 10 ppm http://www.ncbi.nlm.nih.gov/pubmed/12403270)
- Typical copper leaf concentration in tomatoes = 4-6 ppm http://wholefoodcatalog.info/nutrient/copper/vegetables/16/
- Copper removed in tomato crop at 20,000 lb fruit/A per acre = 0.1 lb per acre per year
- Copper added with Harmony at two applications per year = 0.122 lb per acre per year
- Soil concentration to produce plants with copper that exceeds human health limits =
- Pak choi = 430 ppm (http://www.ncbi.nlm.nih.gov/pubmed/12403270 )
- Celery = 313 ppm
- Chinese cabbage = 339 ppm (no figures presented for tomatoes)
Please understand that these are farmer calculations and not those of an agronomist or toxicologist and are at best a general guide to practitioners. It appears that copper buildup in soils is a bigger issue with copper fungicides than with poultry litter-based soil supplements by large factor (millions). Decades will be required to reach plant toxicity even with repeated annual sprays of copper. In places like Europe where copper has been used for decades, it becomes an important issue for soil health. Looking toward many more generations of American farmers it probably should be a concern in the US as well. Since copper is not very mobile in good agricultural soils of an expected pH, growers probably should exercise care with any copper spays. Sprays can be more limited under cover since the copper is less likely to wash off. Plastic mulch may protect soil to some degree by intercepting overspray. Removing copper-coated crops from production areas may also help.
Copper is an important tool in late blight management as indicated by the photo above of fungicide trials to control late blight in potatoes. The green blocks were treated with copper products. The dead plants were not (Photo credit: Alex Stone, Oregon State University.)
I hope this information is helpful. Comments are welcome. Thanks for your question.
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Tom Elmore is co-owner and operator of Thatchmore Farm in Leicester NC. He has grown certified organic fruits and vegetables for 25 years and serves on the Boards of the NC Greenhouse Vegetable Growers Association and the Organic Growers School.