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Marin IJ Articles

What’s in your compost?

  • Fay Mark
  • One of the best ways to improve your home garden soil and support plant health is to apply organic matter that has been recycled and decomposed as a soil amendment, i.e. compost. Composting is a natural process that provides beneficial microbes and is a key ingredient in organic gardening and farming. It can be used in your container garden, vegetable and flower gardens, with shrubs and trees, and on established lawns.

    If you make your own compost, you can control what goes in it and can keep the harmful stuff out. Table scraps, fruit and vegetable scraps, eggshells, leaves and grass clippings that are not exposed to herbicides or pesticides are a few of the many materials that you can add to your compost pile or worm bin. Among things to avoid are meat, bones or fish scraps (they will attract pests), perennial weeds (they can be spread with the compost) or diseased plants.

    But what if you do not have the space and time to make your own compost? You can purchase compost, but be aware of what you are buying and how your garden plants may react to it.

    Plants in your garden that are particularly sensitive and vulnerable to potential pesticides and herbicides in compost are the nitrogen-fixing, legume family of plants such as peas, beans, lentils and clover, and the nightshade or solanaceous family of plants such as tomatoes and potatoes; the sunflower composites family of plants such as sunflowers, petunias, daisies, lettuces and asters; and the cucumber/cucurbit family of plants such as cucumbers, squashes, pumpkins and watermelons.

    Because only certain highly regulated herbicides survive commercial composting, and treated materials are not legally in the compost stream, odds of getting contaminated purchased compost are low.

    That said, here’s how you can test your compost if you suspect something isn’t right. A simple pot bioassay involves growing beans or peas, which are sensitive to the presence of these toxins. To test for herbicide residue, mix equal parts of the compost and potting mix and fill a few small pots with it. Plant a few pea or bean seeds or a tomato transplant in each pot. Let the plants grow for two to three weeks. Signs and symptoms of damaged plants include stunted growth where the main growth tip stops growing and the lateral buds begin to grow, cupping of leaves, failure of secondary leaves to grow after the seed leaves emerge, and in legumes, compound leaves stay.

    If you purchase compost, studying labels will help inform your purchase decision. Look for organic compost that has been tested or approved for organic use. The US Composting Council offers a seal of testing assurance designed to provide compost testing, labeling and information disclosure to consumers. Also look for certification by the Organic Materials Review Institute (OMRI). OMRI does not test compost products, however it does approve products for organic use. The OMRI seal assures the suitability of products for certified organic production, handling and processing under specific conditions.

    Organic compost can be a wonderful soil amendment for your garden, as long as you know what is and isn’t in it. You can find more information about how to create one of nature’s best soil amendments at the UC Marin Master Gardeners’ website at marinmg.ucanr.edu. Look for the section titled “great gardening information” where you will find the link to “gardening resources” for more information about “soil and compost.”