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Is There Such a Thing as Too Much Compost?

Is There Such a Thing as Too Much Compost?
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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.

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.  

Posted by

Amy Campion

on July 22, 2014 at 8:05 am, in the category Eat This, Guest Rants.

6 Comments

    • Mary McAllister
    • 22nd May 2015

    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.

    • admin
    • 23rd January 2016

    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

    • Amy Campion
    • 1st February 2016

    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).

    • skr
    • 30th April 2016

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

    • skr
    • 28th June 2016

    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.

    • Vincent Vizachero
    • 29th September 2016

    Simply planting more densely will adequately prevent erosion will still allowing plenty of soil access to ground-dwelling bees.

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