Gardening part 2. Organic is not always “organic”

Once you’re sure the garden where you’ll be planting produce is as free of toxic chemicals as possible (see prior post), the next step is to make sure products you use in the garden are also free of added pesticides or other potentially harmful materials.

First, I need to clarify what the term “organic” means when referring to soils, composts, and fertilizers. We chemists use “organic” to refer to carbon-based molecules like carbohydrates and proteins. In layman’s terms, that means that the material was likely derived from plants and animals instead of “inorganic” substances such as minerals like salt and granite. However, the term “organic” is also used to refer to food that is grown without prohibited substances like pesticides and synthetic, fossil fuel-based fertilizers.

Those two uses of the word “organic” are not the same thing and can be confusing when you’re trying to buy soil or compost that’s free of pesticides. Manufacturers of garden products use the term “organic” the same way chemists do, in that the products are derived from plant and animal matter, but not necessarily free of pesticides or other possibly toxic components. (Note that there will likely be trace levels of pesticides in all soils and composts sold, even if they weren’t added intentionally, since those chemicals are used widely around the world.)

In addition to pesticides and synthetic fertilizers, biosolids made from sewage sludge may be present in store-bought soils, fertilizers, and composts. According to several sources,1-4 they don’t have to be listed on the label. Biosolids are frequently applied to agricultural fields in addition to being sold to consumers for their gardens. Although these materials are rich in nutrients, they can be contaminated by PFAS, heavy metals, pharmaceuticals, as well as other toxic chemicals when industries also use the municipal waste disposal system.5-6 Some of these chemicals have been measured in edible tissues of plants grown in biosolids-treated soils,7 as well as earthworms living in the treated soils.8

Animal manures, commonly sold as fertilizers for use in gardens, can also contain heavy metals that have been accumulated by the animals, and food grown in soils amended with the manure can take up these toxic elements. In one study, chicken manure was found to contain higher levels than that from cattle.9

Using mulch to inhibit the growth of weeds is a great way to avoid using herbicides in your garden, as well as reduce water evaporation. However, there are a couple of things to watch out for to minimize introducing toxic chemicals to your garden. Avoid plastics, which can leach chemicals like plasticizers. Also, it’s best not to use newspapers in garden beds where you grow food, due to the presence of toxic chemicals in some inks.10

What can you do to avoid these sources of toxic chemicals in your garden?

Using straw or other plant matter to mulch garden beds can help replace problematic products. Another excellent alternative to store bought products is to compost your own food scraps, dry leaves, and woody material from your yard. You might also try a worm bin or indoor composting system. If that’s not practical for you, there are resources listed below that can help you avoid products that might contain toxic chemicals.

There are third party organizations that provide certification for garden products like soil and compost to ensure that they are free of pesticides and synthetic ingredients:

  1. California Department of Food and Agriculture (CDFA)
  2. Washington State Department of Agriculture (WSDA)
  3. Organic Materials Review Institute (OMRI)
  4. Look for USDA “Registered Organic Input Material” on the label.

These organizations offer partial lists of products to avoid that are known to contain biosolids:

Finally, if you can’t find products using the resources listed above, contact the company making the product you’re interested in to ask whether they’re pesticide-free and exclude biosolids from their mix.


  1. Modern Farmer,
  2. Vinje, E. April 11, 2019. Is your compost made of sewage sludge?
  3. Sludge News by the Resource Institute for Low Entropy Systems. List of branded products containing sewage sludge:
  4. Sierra Club. (downloaded 8 May, 2023):
  5. Gray, J. L. 2017. Commonly used chemicals transported to agricultural fields through municipal biosolids application. USGS Environmental Health Program.
  6. Perkins, T. 2020. Questions remain about using treated sewage on farms. Civil Eats.
  7. Wu, C. et al. 2010. Uptake of pharmaceutical and personal care products by soybean plants from soils applied with biosolids and irrigated with contaminated water. Environ. Sci. Technol. 44:6157-61.
  8. Focazio, M. and D. Kolpin. 2014. Biosolids, animal manure, and earthworms: Is there a connection? USGS Environmental Health Program.
  9. Zhen, H. et al. 2020. Long-term effects of intensive application of manure on heavy metal pollution risk in protected-field vegetable production. Environ. Pollut. 263:114552.
  10. Jadhav, S. et al. 2021. Health risks of newspaper ink when used as food packaging material. Lett. Appl. NanoBioScience 10:2614-23.

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