Hero Image

Marin IJ Articles

The full scoop about soil amendments

  • Nanette Londeree
  • Fall is for planting. Whether you're ready to set out cool season vegetables, add a specimen tree or divide your perennials, you can get a jump start on next season's growing by planting now. It's also a great time to invigorate beds for spring planting.

    If your garden is blessed with oodles of chocolate-colored, crumbly, vibrant soil just waiting to be used, you needn't read any further. But if, like many Marin gardeners, you've got soil that looks like brick in the summer and glue in the winter, you may want to transform it into plant-friendly soil by amending it — adding materials to improve it.

    But what do you add? A visit to your local nursery can put you on overload with the dizzying array of products, so it's helpful to understand what you want from a material before buying it.

    First some terminology: an amendment is any material mixed into the soil that indirectly aids plant growth by improving the condition of the soil, like its structure or texture, water retention or microbial activity.

    The terms soil conditioner and amendment are often used interchangeably, both serving to improve the chemical, physical or biological properties of soil. Mulches are organic or inorganic materials placed on the soil surface to help prevent weed growth, conserve moisture and add organic matter to the soil as they break down. A number of materials used as soil amendments can act as a fertilizer by providing nutrients to the soil, or be applied to the soil surface as mulch.

    What do you want from the amendment? Are you looking to loosen up heavy clay soil? Improve drainage? Lower soil pH?

    It's a good idea to do a soil test before making any significant changes; simple test kits for soil pH and major nutrients are available at local nurseries. Then consider how long the material will last in the soil, whether it retains water and/or improves permeability (the rate at which water moves through the soil), or if it may present any problems from excess salts, weed seeds or disease-producing microbes (like fresh manures). Finally, think about the practical aspects of the material — its availability, cost and ease of handling.

    Many amendments are used to change soil texture — the way a soil feels. Our ubiquitous clay soil has tiny particles that feel sticky when wet, retain moisture and drain slowly; once dried it can be hard to penetrate the surface.

    "The soil experienced gardener's crave is deep and easy to work," says Elizabeth P. Stell, author of "Secrets to Great Soil."

    The fastest and easiest way to improve heavy soils is by adding lots of organic matter — compost, earthworm castings, humus or tree bark are good sources.

    "Organic matter is the magical stuff that transforms a pile of sand — or a lump of potter's clay — into fertile soil," Stell says.

    Common inorganic amendments include gypsum — it helps increase permeability, improve aeration and drainage. Perlite is a light material with an extremely large surface area that holds moisture and nutrients and vermiculite is similar; it's a fluffy material that absorbs many times its weight in water.

    The pH of the soil (how acid or basic it is) affects availability of nutrients to plants, especially iron, and the activity of soil microorganisms. Most plants do best in soil with a pH range of 5.5 to 7.5. To raise soil pH, incorporate materials with calcium and magnesium carbonates — dolomitic or calcitic lime, into the soil. If you're after lowering pH, consider peat moss, coffee grounds, elemental sulfur or ammonium sulfate.

    Many amendments are readily available at little to no cost — you can make your own compost and earthworm castings or collect coffee grounds from the myriad coffeehouses that give spent grounds away for free. Other materials may cost you, especially if you purchase in small quantities. Some local landscape supply places allow you to "do it yourself" and fill large bags of amendments making it much more economical. A lot of soil to amend? You can purchase it by the truckload.

    How much of the amendment you need to add will vary depending on the condition of the soil you're working with, what you're adding and what you want it to do to the soil. Don't overdo it — too much can burn tender roots and leaves, tie-up nitrogen so it's not available to the plant, or retain too much water.

    To learn more about the right path to manage soils for restoration and production, attend a UC Marin Master Gardener talk on Nov. 1 at Marin Art & Garden Center in Ross (www.marinmg.org). For more information on soil amendments, go to ucanr.org/sites/sacmg/Soil_Amendments/.

    So, give your soil a boost now by incorporating some amendments into it; your plants will reward your autumn efforts with exuberance next spring.