Elemental Contaminants in Fertilizers and Soil Amendments Used in Organic Production
Baker, B. and Tracy, D. 
Key words: fertilizers, soil amendments, elemental contaminants.
Abstract
Elemental contaminants in fertilizers pose a threat to human health and the environment. Organic agriculture can take measures to protect the public and the environment from the long-term effects of these contaminants, also known as heavy metals. Arsenic (As), cadmium (Cd), and lead (Pb) were identified as the top priority metals that need attention. Fertilizers and soil amendments used in organic production were randomly selected, and the laboratory results for the levels of As, Cd, and Pb compared against six different standards based on different models. Organic farmers are advised to avoid using fertilizers that may degrade the average levels found in soils in the United States. Standard-setting bodies are advised to prohibit the use of fertilizers and soil amendments that have As, Cd, and Pb that will result in the accumulation of those elements in the soil when applied at average loading rates on an annual basis.
Introduction
Heavy metals occur naturally in soils. They can also be accumulated through conventional agricultural practices and are found in a variety of industrial by-products, some of which are combined with fertilizers and soil conditioners. The elemental contaminants known as heavy metals pose a threat to food safety and can harm the environment, whether they come from synthetic or natural sources. The Organic Materials Review Institute (OMRI) has identified and characterized the concerns raised by the application of heavy metals found in the fertilizers and soil amendments permitted for use in organic production. The purpose of this study was to determine the levels of heavy metals in a representative sample of organic fertilizers, and to predict how many fertilizers and soil conditioners would meet various standards that limit heavy metal contamination. 
Materials and methods 
OMRI staff used the data submitted by our listed suppliers for product review. Staff drew the files of 50 products at random that had been submitted at some point. To be considered valid, analytical results needed to be clearly linked to and identified with the submitted product. The product needed to comply with organic standards. The sample needed to have been taken within the previous five years. Of the 50 selected, 32 products had valid results of analyses on file for As, Cd, and Pb using EPA’s Strong Acid Digest/ Inductively Coupled Plasma (ICP) methods. 
As, Cd, and Pb were selected as the highest priority elemental contaminants based on a consensus of experts consulted. Other contaminants might also be of concern, but were beyond the scope of this study. 
The analytical results were then compared with six different scenarios:
1) OMRI’s Maximum Concentration of Contaminants Contained in Synthetic Micronutrient Products (OMRI, 2002). 
2) The Association of American Plant Food Control Officials’ (AAPFCO) Statement of Uniform Interpretation and Policy (SUIP) (AAPFCO, 2006).  AAPFCO standards base heavy metal limits on phosphorus or micronutrient content. Limits for compost and manure based products were set based on phosphate content.
3) The US Environment Protection Agency’s (EPA) standards set for metals resulting from the land application of sludge, also known as the 503s (US EPA, 1994). 
4) The Washington State Department of Agriculture’s limits established for metals loading in fertilizers expressed in lb/acre/yr (WSDA, 2006). For the WSDA limits, products were viewed based on the loading rates on their label. For compost, an annual  loading rate of 2 tons/acre (approximately 4.5 metric tons/ha) on an ‘as-is’ basis was used. If dry weight was reported, then an annual loading rate of 1 ton/acre (2.24 metric tons per hectare) was used. 
5) The No Net Degradation (NND) limits are based on the US Geological Survey and other researchers’ estimates of average background levels of heavy metals in soils in North America. (Lepp, 1981; Shaklette and Boerngen, 1984; Gustavsson et al., 2001). 
6) The European Union (EU) limits for metals when sewage sludge is used in agriculture.  (EU, 1986). The Annex IB Lower Limit Value was used.
Table 1 lists limits of the different scenarios examined. Data from the sampled products were entered into an Excel spreadsheet and analyzed using the different model standards. 
Table 1: Limits of the Different Scenarios (ppm)
Scenario	Arsenic (As)	Cadmium (Cd)	Lead (Pb)	
OMRI 1	10	20	90	
AAPFCO2	13	10	61	
503s3	41	39	300	
WSDA4	149	40	290	
NND5	7.2	0.4	19	
EU6	--	20	750	

1 OMRI 2002. Table 3.
2Association of American Plant Food Control Officials SUIP 25, Column 2, 
3US EPA, 1994. Table 3 of 40 CFR 503.13, 
4Washington State Department of Agriculture, 2006. WAC 16-200-695 and RCW 15.54.800. 
5Shaklette, et al., 1984; Lepp, 1981; and Gustavsson et al., 2001. 
6European Union Council Directive 86/278 
Results
The results are summarized in Table 2. The highest As levels were found in dehydrated poultry litter and rock phosphate. Conventional broiler production in the US uses As as a parasiticide. It is a known impurity in many rock phosphate deposits. Rock phosphate also had the highest level of Cd, exceeding the limit for all but one of the standards. In the case of rock phosphate, the source of contamination is believed to be natural impurities. The highest Pb level was 66.6 ppm, found in compost made from municipal green waste. Demolition material, such as lumber coated with lead paint, is a common contaminant at such facilities. 
Table 2: Summary of Results of the Random Selection of Fertilizers and Soil Conditioners Used in Organic Production
Parameter	Arsenic (As)	Cadmium (Cd)	Lead (Pb)	
Average (ppm)1	9.6	8.5	12.8	
# (%) Positive2	19 (58%)	18 (55%)	20 (61%)	
Maximum (ppm)	53.6	96.6	66.6	
# >OMRI 2002 (%)	4 (12%)	2 (6%)	0 (0%)	
# >AAPFCO (%)	2 (6%)	0 (0%)	1 (3%)	
# >503s (%)	0 (0%)	1 (3%)	0 (0%)	
#>WSDA (%)	0 (0%)	1 (3%)	0 (0%)	
#>NND (%)	5 (15%)	11 (33%)	4 (12%)	
#>EU (%)	--	2 (6%)	0 (0%)	

1Average of the positive samples.
2Number of samples exceeding the limit of detection reported by the laboratories.

Some of the fertilizers in the sample exceeded average soil background levels for all three of the contaminants. Because Cd has a relatively low average background level, the percentage of fertilizers that exceed background level was the greatest, accounting for about a third of the randomly selected products. 
Discussion
When the various scenarios were applied to the selected fertilizers, The No Net Degradation standard was the most precautionary, as expected. However, it would also be the most restrictive in terms of what fertilizers and soil conditioners could be used. Of the remaining standards, none was consistently more protective. The risk assessment models did not have a consistent estimate of the risks posed by the different elemental contaminants. The loading rate model was the most permissive with Pb, but was stricter with Cd and As. 
Most of the experts noted that soil contamination is a function of both the loading rate and the background level. Thresholds for each of the priority contaminants should be established at two levels: the lower threshold based on a no net degradation policy that requires monitoring for increases in soil levels over time, and the higher threshold based on a loading rate that predicts levels of contamination in soil monitored on an annual basis and expected to increase to a level toxic to plants.  
Conclusions 
As, Cd, and Pb were found in most fertilizers and soil conditioners that were randomly selected. The levels found were within the thresholds of most of the regulatory limits and would not be expected to result in soil degradation. Poultry litter, rock phosphate, and compost made from municipal green waste were found to be the most likely kinds of fertilizers and soil conditioners to exceed regulatory limits and threaten soil degradation. We believe that organic farmers should be made aware of the long-term consequences of applying soil amendments that are contaminated with heavy metals and take precautions to not cause long-term degradation.

Acknowledgments
The authors acknowledge Eric Sideman of the Maine Organic Farmers and Gardeners Association, Will Brinton of Woods End Laboratories, Zea Sonnabend of the Ecological Farming Association, Rose Koenig of the University of Florida,  and Joseph Heckman of Rutgers University.
References 
Association of American Plant Food Control Officials (2006): Official Publication. West Lafayette, IN: AAPFCO.
European Union (2003). On the protection of the environment, and in particular of the soil, when sewage sludge is used in agriculture. Council Directive 86/278 as amended through May 6, 2003.
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Shaklette, H. and Boerngen. 1984.  Element Concentrations in Soils and Other Surficial Materials of the Conterminous United States.  Washington: USGS.
Tracy, D. and Baker, B. 2005. Heavy Metals in Fertilizers Used in Organic Production. http://www.omri.org/AdvisoryCouncil/Metals_in_Fertilizers-b6-2005-02-14.pdf
US Department of Agriculture. Agricultural Marketing Service. 2000. National Organic Program Standards. 7 CFR 205. http://www.ams.usda.gov/nop/NOP/standards.html. Last accessed October 24, 2007.
US Environmental Protection Agency (1994): A Plain English Guide to Part 503 rule. http://www.epa.gov/owm/mtb/biosolids/503pe/. Last accessed October 24, 2007.
Washington State Department of Agriculture. (2006) Metals in Fertilizers. http://www.agr.wa.gov/PestFert/Fertilizers/Metals.htm Last accessed May 2, 2006.