Is There Such a Thing as Too Much Compost?


    too much compost

    Guest Rant by Amy Campion 

    If we gardeners agree on anything, it’s that compost is wonderful stuff.  We can never have enough of it.  We make it ourselves in heaps and bins and barrels, and we ask for more of it on our birthdays.  Compost makes clay soil loosen up and helps sandy soil hold water; it nourishes our vegetables and feeds all the micro-organisms that keep our soil healthy and alive.  Most of us would plant our veggies in pure compost if we could.

    I’ve recently come to question my own faith in that dogma.

    Steve Solomon has been quietly shattering garden myths for years.  He is best known for his advice on organic vegetable gardening in the Pacific Northwest; his book Growing Vegetables West of the Cascades is in its sixth edition.  His most recent book, The Intelligent Gardener:  Growing Nutrient Dense Food, however, is broader in scope, addressing food-growing in all parts of the temperate world, and exploding some commonly-held Truths along the way.

    One of his beefs with too much organic matter deals with the nutritional imbalances in most compost.  If you use a lot of compost, and that compost is heavy in potassium—which it probably is, especially if straw, hay, stems, or sawdust went into making it—your vegetables will not be as nutritious as they could be.  Farmers—even organic ones—commonly add potassium to their crops to boost yields, but the increased bulk is lacking in nutrients.  Extra potassium causes plants to pack on carbs—not the complex proteins that your body truly hungers for.

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    As for using compost to loosen clay soils, Solomon says this does work, but there is a much easier way.  He says that clay soils are often tight because of an excess of magnesium in relation to the amount of calcium present.  Bring these two into proper balance, and your soil becomes magically springy and clod-free.

    Soils do need some organic matter, for sure, and vegetable gardens a little more than ornamentals, since so much of it is being taken away by the crop.  He suggests keeping organic matter at its natural state plus one percent.  Natural soil organic matter levels in the U.S. range from about 2.5% in the South to 6% in the North.  In most cases, achieving this level would mean adding only a half-inch of compost to a new vegetable bed and a scant quarter-inch addition each year after that.

    Could you bring yourself to add that little?  Would you, if you knew that’s all it needed?

    Amy Campion always uses the correct amount of compost on her garden in Portland, Oregon.  You can find her at whatbloomswhen.  


    1. Sometimes mulches are just put on top of the soil for the purpose of weed control and moisture retention. This is also a convenient use of the trees that are being destroyed because they aren’t native. Chipping them and spreading them on the ground is cheaper than hauling them away.

      The problem with that approach is that most species of native bees nest in the ground. These heavy mulches prevent the native bees from getting access to the soil, so they are unable to nest in your garden. The loss of pollinators in your garden is not benefiting your garden.

      Here’s an article about the heavy mulches being used on public lands in California as a result of the destruction of our non-native trees:

      • Mary, you are so correct. Too much mulch is not a good idea. If you simply take a hike through the woods, you would probably never see piles of wood chips or other mulch materials piled around the trees and shrubs. (And this business of making huge mounds with it is crazy!) Nature just doesn’t do that. However, concerning compost, I didn’t realize the affect it had on the nutritional content of our veggies. It makes perfect sense. I will definitely have to add this book to my reading list since building good soil is a soapbox I stand on…a lot! ~Julie

      • Solomon has some interesting things to say about mulches as well. Used under ornamentals is fine, but used in the vegetable garden, they may not be such a good idea. He believes that they don’t actually conserve much moisture–that plants are what really pull moisture out of the ground, so if your goal is to conserve water in the vegetable garden, then wide plant spacing is the key. Also, there’s the fact that mulched beds warm up slower in spring. And, that mulches can harbor “plague level populations” of insect pests and slugs, which are not killed off in our mild PNW winters (in colder parts of the country they are).

        I was not aware of ground-dwelling bees not being able to nest. That’s a good point.

        • Spacing plants further apart leads to lower water use? Greater spacing equals fewer plants. Fewer plants equals less water demand. No great surprise there.

          I would like to see some actual research on his proclamations. Studies on mulching show mulched retains more moisture than non-mulched regardless of plant transpiration.

      • While the thick mulch layer is a problem for native bees, it is fantastic for other parts of the ecosystem. It greatly reduces erosion and sedimentation that can severely damage waterways. There is always going to be pros and cons to any action.

        • Plus, I don’t want bees to nest in my garden. Elsewhere, yes. We have a brush pile out back that they are more than welcome to nest in. My garden, however, gives bees too great of a chance to sting somebody. They can nest somewhere else.

    2. People go overboard with compost all the time thinking it can’t hurt. But you can pollute the watershed with nutrient overload from too much OM. I have seen far too many soil tests cone back with sky high K or P because of excessive compost additions.

      With regards to Mary’s mulch comment, yes you don’t see piles of wood chips in nature. Well, you can sort of see that around fallen trees when the dry rot causes them to break apart. And though not wood chips, the duff layer under a native California oak grove can easily be a foot thick of leaves and fallen branches. You also don’t see houses sitting on scraped subsoil in nature either. Deep mulch layers are a good way to kick off the recreation of a topsoil layer on heavily disturbed soils. Gardeners don’t need to recreate every habitat for every creature that could possibly utilize the landscape.

    3. About keeping ground uncovered in order to provide space for bee nests, you may end up nuking the nests if they’re close to where you’re gardening and lead to bee stings. I had a nest in a small border in front of my house once where I was unable to garden anywhere near it without getting stung, Susan

    4. Well, this isn’t a problem for me yet; I’m turning my desert-scraped suburban yard into garden, and only 2/3 finished. But when i do finish in a couple of years, what do I do about the garden trimmings and kitchen waste? Shouldn’t I be minimizing the garbage sent to the landfill, and recycling organic material as much as possible?

    5. I think the over-application of compost is a problem for SOME ocd, type A gardeners, but my experience in a design practice and handling several independent maintenance crews in the LA area is that applying compost is something done once in a blue moon. I have a compost layer as mulch when installing a garden, and request yearly applications, but this almost never happens. For the passionate home gardener, yes, too much “love” can be, well … too much. But is over-composting REALLY a problem? I really want to read this book, as I think all of us who grow food want that food to be healthy and nutritious, but how much of a differential in nutrition are we talking about? Thanks for posting, very interesting!

      • I do apply compost at least once a year on my veggie beds. After all, I have a compost pile, so I might as well use it. However, I probably only put about as much as the author is recommending. If I get around to my highly overgrown ornamentals, I put some compost on them once a year as mulch, and right before I put a new planting in.

        As for mulch, I really should buy some for the flower gardens, but I just can’t ever convince myself to shell out the cash.

    6. I have never really thought about this. I guess compost is something I always took for granted. I always figured that if only good stuff went in, then only good stuff would come out.
      As a carpenter, I always had a surplus of sawdust around, but was very careful about the type of sawdust to put in my compost based on the fact that several wood varieties (some mahogany and some cedars in particular) have an irritating effect on skin and airways. seemed not good to add that to my vegetable compost.
      Time to rethink the old compost pile again….J

    7. Last fall I finally got my act together and spread compost on an ornimental bed. I now have a gorgeous rampant pumpkin plant running amok giving the garden a tropical feel. And crab grass. I obviously am not a compost expert and don’t get it hot enough. I can deal with the extra weeding and love the pumpkins. But, some Asian Solomon seal is getting weird looking leaves and dying. The European ginger is dying off. Even th sweet clethra is getting some funky leaves. Did I introduce a nasty fungus with the compost was my first thought. My second thought was the hard winter we had, first one we have had in years. Now I’m wonder was it too much compost.

      • Joe Lamp’l’s blog on Growing a Greener World has a post called “Killer Compost” that you may find helpful.
        If your pumpkin is thriving though, it seems it wouldn’t be the compost.

      • We are retaining about half our backyard for play, the rest of our lot will be garden. I have two compost streams: one goes into the compost pile. But garden trimmings that could present a problem when added to the garden (such as crabgrass root fragments or certain annual seeds) I throw in the back yard and mow it once a week. Few of these sprout anything that can withstand weekly mowing. (Crab grass that survives this can be easily seen and uprooted.)

        Suburban compost piles rarely get hot enough to kill pathogens or seeds. A few problem plants go in the trash; these are uncommon – perhaps one fistful weekly.


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