Building healthy soils?

I love living in Seattle…but I’m getting increasingly impatient with the City’s “Building Healthy Soils” propaganda.  For years I’ve questioned their recommendation to perpetually amend landscape soil with organic material to no avail.  Let’s see what you all think of their “fact sheet” (which you can read here in its entirety).

“The best way to improve the soil is to add plenty of compost or other organic matter throughout the entire planting area before planting. Thoroughly mixing these materials deep into the soil helps provide water, air and nutrients to plant roots.”

Hmm.  No mention of how to determine IF your soil needs improving; without a soil test, you have no idea what your baseline organic matter level is.

But perhaps this recommendation is only for vegetable gardens and annual beds?  Nope.  In the next paragraph, we’re told to “Mix in organic material before planting lawns, perennials, trees and shrubs.”  We’re given helpful how-to instructions:  “Use a shovel or digging fork to mix amendments into the top 6 to 12 inches of soil. It is important to amend the entire planting bed — not just small holes for each plant. When planting individual trees and shrubs in lawns or existing beds, amend an area at least 3 feet wide, or 3 to 5 times as wide as root balls over 12 inches in diameter. Rototill large areas where digging is impractical.”

Now we’ve got a serious problem.  This practice is recommended for existing beds.  Not only will extensive digging or rototilling destroy any soil structure you might have, it will also take out the roots of any desirable plants in the vicinity).

But let’s continue to ignore reality and go on to the annual recommendations for adding compost to soils.
“Clay soils: 16 cu. feet (.6 cu. yard) = 2 inch layer of compost for new gardens. Use 1 inch per year in established gardens.”
“Sandy soils: 24 cu. feet (.9 cu. yard) = 3 inch layer of compost for new gardens. Use 1 – 2 inches per year in established gardens.”

Is the compost used as a mulch in these existing gardens?  No – the guidelines are prefaced with this instruction:  “Gardens: mix compost to 10- to 12-inch depth.”  (Can’t say this does much for promoting root growth either.)

This document shows a breathtaking lack of understanding of how landscapes function, especially over the long term.  It takes an agricultural practice (annual organic amendment of crop fields) and misapplies it to permanent landscapes.  It is devoid of the research which continues to show that improper soil amendment can cause serious problems such as soil subsidence, perched water tables, and nutrient overloads.  This last point is especially important to anyone living near aquatic ecosystems, since excess nutrients always end up in the water.

Before you plant this year, find out what your soil needs before amending it.  And remember that mulching is the natural (and sustainable) way to add organic matter to the soil.

Published by

Linda Chalker-Scott

Dr. Linda Chalker-Scott has a Ph.D. in Horticulture from Oregon State University and is an ISA certified arborist and an ASCA consulting arborist. She is WSU’s Extension Urban Horticulturist and a Professor in the Department of Horticulture, and holds two affiliate associate professor positions at University of Washington. She conducts research in applied plant and soil sciences, publishing the results in scientific articles and university Extension fact sheets. Linda also is the award-winning author of five books: the horticultural myth-busting The Informed Gardener (2008) and The Informed Gardener Blooms Again (2010) from the University of Washington Press and Sustainable Landscapes and Gardens: Good Science – Practical Application (2009) from GFG Publishing, Inc., and How Plants Work: The Science Behind the Amazing Things Plants Do from Timber Press (2015). Her latest effort is an update of Art Kruckeberg’s Gardening with Native Plants of the Pacific Northwest from UW Press (2019). In 2018 Linda was featured in a video series – The Science of Gardening – produced by The Great Courses. She also is one of the Garden Professors – a group of academic colleagues who educate and entertain through their blog and Facebook pages. Linda’s contribution to gardeners was recognized in 2017 by the Association for Garden Communicators as the first recipient of their Cynthia Westcott Scientific Writing Award. "The Garden Professors" Facebook page - "The Garden Professors" Facebook group - Books:

21 thoughts on “Building healthy soils?”

  1. In my experience, the soil in Seattle is glacial clay or sand, and needs organic matter. I’ve never had a garden here where the soil wasn’t low on organic matter, so I won’t argue with them on that. Obviously, that may not be true in other areas, but they’re not addressing other areas, just Seattle.

    The rest is utter nonsense.

  2. Deirdre, in the dozens of soil tests we’ve run in Seattle from restoration sites to urban landscapes, we’ve had only a few below 5% OM (5-10% is ideal and sustainable in our region). More OM than that can’t be replaced naturally and isn’t sustainable over the long term. But in new developments, where topsoil has been scraped away, you are absolutely right that OM will be low. It just underscores the importance of testing soils first!

  3. you mention adding mulch — i mulched around my vegetable garden last summer, and am going to reuse the spot for vegetable gardening this year. should i turn the soil to mix the mulch in, or just leave it all as is?

  4. Dan, I’d just leave it as is and add more to the top if it’s gotten too thin. Mixing could bring weed seeds to the surface – the last thing you want!

  5. After 4 years of annual applications of a 2″ compost/4″ organic mulch sandwich I have 3″ of rich humusy garden soil atop nutritious red Virginia clay. The clay is lightening and the worms are burrowing. However, I get more long lasting results (4-5 years) with a one-time winter cover crop of alfalfa.

  6. Irene, you’re a woman after my own heart! I’ve done the same on our landscape (also a heavy clay, since we used to be wetlands before Seattle lowered the level of Lake Washington). I’ve been putting arborist chips down for about 10 years, and we’re also developing a nice organic layer. I agree – let the worms do the work!

  7. We have an area that we are mowing to keep down the blackberries and bindweed until we are ready to do something with it. I over seeded with white clover in hopes of increasing the fertility in the mean time (I have a mulching mower) in the mean time. This will work, won’t it? As much as I love arborists’ chips, blackberries come right up through it.

  8. Deirdre, we’ve done the same thing…the trick is to mow the blackberry flat and immediately cover it with at least 1 foot of chips. Less than this will allow the blackberries to find their way through. A foot of chips keeps all but the most aggressive canes from coming through, and they can be easily pulled (and then paint the stem with Roundup). You can look at my web page under “Case Studies” and see what we did with the Heron’s Glen project. You do have to watch the edges of your area, as they can be invaded by weeds from the outside.

    1. I read your case studies on invasive species. I see your point on Glyphosate but what if invasive species are near a vegetable garden? I am concerned that the chemical will stay in our soil and food?? I know in the agricultural business they will apply a lot over their crops but I really want my crops free of possibly toxic chemicals. My other question is I recently removed a tree stump using a muttock and there is a big hole in my yard. When you can’t use the same soil in our yard ( because it’s quite a big hole and to get the amount of the soil to fill in, I would need to disturb the soil in other areas of my yard), what type of soil can I purchase from a landscape supply store? My soil is clay loam, do I need to find similar type of soil? Once I fill in the hole, I plan to mulch with woodchip to prepare for future planting. Thank you!

      1. Hi Ayumi –
        If glyphosate is properly applied – for instance, dabbed on to the cut end of a stem – it is not going anywwhere. It’s not going to sit in the soil, and it’s certainly not going to be taken up by plants! They would die if that was possible. Much of what you read about glyphosate online is fear-mongering and you should ignore it unless the source is an expert on the topic, meaning they have significant educational and practical experience in the fields of plant and soil sciences.
        In terms of filling the hole – you can’t buy anything that is going to work. See if you can find some “free dirt” in your general area where people might be getting rid of topsoil. Then as you say mulch, wait, and plant!

  9. In nature, leaves fall and become the ‘nutrition’ for next season as they break down. If people are cleaning up their garden beds too well, they are not replacing that nutrition. Doesn’t it become a matter of amending versus fertilizing? By amending annually, one is replacing the nutrition. I never dig in compost or mulch.

  10. “Amending” as used in this pamphlet means working into the soil. “Topdressing” is laying down mulch. I’m all for the latter!

  11. Great post and discussion. Love it! LSC, would you post a soil test as a Friday puzzler to give us some real-life scenarios?

  12. Various reasons, including the size of the area, the massive roots of the blackberries (they were 8″ tall when we moved in), and the pretty bulbs that come up in the spring, make arborists’ chips impractical. So, what do you think of my clover idea?

  13. Great idea, Daniel! I’ve got some I could scan in (and I’m sure the other GPs do as well). Deirdre, I see your problem now. I would suggest consistently cutting the blackberry canes and painting them with Roundup. Otherwise, they will subsume your clover and everything else!

  14. “I would suggest consistently cutting the blackberry canes and painting them with Roundup. Otherwise, they will subsume your clover and everything else!”

    That’s where the regular mowing comes in. I’m hoping the roots will starve and give up before I do.

  15. After reading your columns, I realized I should be using arborist wood chips for mulching for landscaping and around my fruit trees (leaving a doughnut). How should I be treating my p-patch and raised beds? In the past I’ve been using a bag of compost and some organic fertilzer, but have continued to battle lots of weeds. Would mulching with wood chips work? Or would the wood chips inhibit seed germination? I was unable to plant a cover crop this year because of moving, and I’m worried about battling weeds in my new poorly maintained p-patch.

  16. Amy, I’ve been hearing from more and more vegetable gardeners that using wood chip mulches does not have a negative impact on their desired plants. If it were my own garden, I’d probably put down a layer of compost, then top that off with wood chip mulches. That way, you’ve got an organic buffer between your vegetables and the nitrogen-poor wood chips.

  17. I was just delivered 2 yds of arborist chips that contain alot of cedar, unlike the sample I saw. My plan is to use it on perennial beds, not particularly trees or shrubs, will it be too acidic?

  18. Paula, those chips will be just perfect. Mulches do not change the acidity of an entire soil profile. It’s possible that the interface between the soil and mulch could be affected, but this isn’t where your plant roots are.

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