Monday, May 16, 2011

To Dig or Not to Dig: Are 'No-Dig' Planting Methods for Real?

photo by Jim Richardson, National Geographic

One recent garden trend that is spreading with inexorable speed is the “no-dig” or “no-till” method of planting.  The basic idea is that plants are installed directly into the ground without tilling or turning over the soil.  While this method is centuries old, it challenges conventional gardening practices of tilling and breaking in the soil before one plants.

I’ve been aware of this method for a while, but have been surprised by how quickly it has become dogma, particularly within sustainable landscape circles.  When teaching a class on soil preparation, I mentioned tilling and watched as many of the students recoiled in protest.  “Isn’t tilling bad?” one student immediately asked.  I was taken aback.  ‘No-dig’ is not just an idea, but a doctrine, a creed, a badge of one’s eco-credentials. Proponents spread the message with revolutionary fervor. 

So is it time to put your tiller on Craigslist?  Let me weigh in on this complex issue and hopefully provide some clarity.  The gardening world has more than its fair share of old wives tales and superstitions.  This is particularly true with anything regarding soil.  We understand so little about what goes on in the soil, yet we dig, till, fertilize, and amend it with reckless zeal.  When it comes to soil cultivation, what’s true?

Here’s the bottom line: ‘no-dig’ is great, but not when the soil is severely compacted. 

After going through quite a bit of research, the evidence certainly favors the ‘no-dig’ approach.  Part of me really wanted to find flaws with this method; after all, breaking the soil before planting just feels so natural, so downright human.  Egyptian paintings 1200 years bc show people plowing fields.  But the evidence generally supports the wisdom of not digging.  Why? 

Soil is the weathered mantle of the earth.  It is not really even a solid, but a mix of mineral solids (45%), organic matter (5%), and water and air (25%). In its undisturbed state, soil is generally layered with the top few inches containing organic matter, the next few inches containing topsoil (soil that is mixed with organic matter), below that subsoil (generally denser and less organic), below that decomposed rock, and below that rock.  This basic structure is vitally important to plants, as this layering creates large pores (macropores) through which air and water move.  In addition, soil’s layered horizons create a living network of bacteria, fungus, molds, and other critters that symbiotically support plant growth.  Plant roots move through soil pores, following the air and water.  Plants grow their roots by connecting their root hairs onto mycorrhizal webs in the soil.  These fungi supply plants with nutrients and water while in return, they obtain sugars from the plant through photosynthesis.  At least 95% of plants are known to use mycorrhizal webs.

Mycorrhizae attach to roots
Tilling and digging disrupt this vital soil network.  When you sink a shovel in the ground and flip the soil over, you break the mycorrhizal network.  When you till, you may be breaking the soil into fine enough particles that it compacts even more, obliterating many of the necessary pores in the soil.  In fact, studies have shown that plants grown without tilling initially outperform plants grown in tilled soil.  At a larger scale, no-tillage agriculture has been proven to reduce erosion, increase crop yields, and decrease greenhouse gasses.

So should you get rid of that tiller?  Not just yet.  While the ‘no-dig’ method is generally a good way to plant, it does not work well in heavily compacted soils.  Compacted soils are common in human-disturbed landscapes.  Soils can get compacted enough that air and water no longer move through it.  Soils compacted to this level are deadly to plants.  If you plan to plant in heavily compacted soils, some method of decompaction—subsoiling, tilling, or aeration—is absolutely necessary.  Think throwing organic matter on top is good enough?  No way.  The problem with severe compaction is that it never goes away.  If air and water can’t move through it, it will not cure itself.

How can you tell if your soil is compacted enough to justify decompaction?  One of the simplest tools is a hand-held penetrometer.  This device has a rod that gets shoved into the ground and measures the resistance in pounds per square inch.  A small dial on the top will let you know what psi the soil is.  Generally anything above 250 psi should be decompacted.  Penetrometers cost a few hundred bucks, so they’re definitely worthwhile for a large site, or if you evaluate a lot of sites (all you landscape architects out there—get one for your firm).  But penetrometer are crude tools and vary somewhat depending on the velocity you shove it in the ground.  Other ways of measuring soil compaction is through Proctor test.  Generally anything above 85% proctor will inhibit root growth. 

Photo by William Cullina shows the effects of organic matter on post construction site.  The soil on the left received no organic matter. 
The soil in the middle had organic matter added only to the top few inches.  The soil on the right had organic matter incorporated
throughout.
Don’t want to bother with complicated compaction equipment?  The simplest way to test for compaction is to shove a pointed rod or stick into the ground.  If it can easily penetrate the ground to a depth of 8-12”, your soil is probably fine.  If it can’t, you may need to consider some method of decompaction.

The bottom line is that ‘no-dig’ and ‘no-till’ methods are great for relatively healthy soils.  But if you have a site in an urban area, or one that is under construction, expect compacted soils.  The best way to address deep compaction is to use some kind of subsoiling equipment.  Tillers breaks up soil into fine particles that ultimately compact more densely.  Subsoilers rip the earth--like a knife going through butter--in vertical lines that preserve the soil structure, while at the same time allowing air and water to pass through hardpan.  Subsoiling equipment is typically attached to a tractor, but in smaller sites any kind of trencher (like an irrigation trencher) will have the same effect.  Subsoil in lines 18-24" apart in both directions.  When subsoiling, check for underground utililties prior to ripping the soil.

If you plan to address compaction by adding organic matter, one must add enough organic matter (or topsoil, sand, peat moss) to make a difference.  Don’t just sprinkle some compost in the planting hole—this does almost nothing.  Organic amendments must be mixed to a depth of 18 inches minimally and enough added to reach 25 percent by volume in sandy loam or 50 percent by volume in clay in order to make a positive change in bulk density and macroporosity.  Decompacting soils is expensive and labor intensive, but worth it in the long run.

But if you have relatively healthy garden soil free of compaction, try a ‘no-dig’ approach.  It’s actually much easier, and you will be surprised by the results.

29 comments:

  1. Fascinating! Cultivating soil has always been a rather spiritual exercise for me, although perhaps it's just the act of gardening outdoors that gives me bliss. I've been aware of no-till or zero-tillage practices gaining research and momentum in agriculture, but did not know it had trickled down into gardens. Makes sense though. Subsoiling sounds wickedly complicated, by the way.

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  2. This is the first time I've heard of the no-dig planting method. Except in the valley, one has to go into semi-arid areas far to our east to find soils *as good* as the post-construction site area on the 3rd image!

    In our SW Desert's aridisols (even San Diego and parts of So Cal), planting is done more with a pick and auger. Tilling would not work in most cases, so perhaps we already practice "no-dig"!

    Where we have caliche (most of Abq, most other SW cities, etc), I have had more than one extension agent and civil engineer advise the use of a D9 ripper to break up all that, to gain access to less calcereous subsoils. A caliche layer in Abq can range from several inches to over 3'! Yikes, talk about subsoiling! Not sure even that method worked on one project in Hobbs NM. For growing any trees in such areas, subsoiling seems necessary, though many smaller, desert-native plants can handle some degree of caliche, maturing nicely.

    Thanks for the information and insight as always, and I may have to get a penetrometer that you mention.

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  3. Different soils, different pH's, different slopes, different exposures, varying precipitation patterns, different plants with different preferences.....wow. They've all been studied individually; but how do they fit together?

    It's such an art applying Science to Gardening.

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  4. Hi Thomas,
    Great post! I am a firm believer in not disturbing the soil unless absolutely necessary.

    Purposely causing compaction with machinery is one of the biggest mistakes made by landscape installers. I think if we had less emphasis on hardscapes and more emphasis on plants, we could eliminate a lot of unnecessary compaction.
    Heather

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  5. David,

    I'm fascinated by 'soil preparation' in a desert. The rules must be completely different. Yeah, I've had a hard time getting contractors to actually subsoil. It's pretty labor intensive. But I had one project where we did it on part of the property and not on the other. The results (particularly with the trees) was dramatic.

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  6. Heather,

    When you planted your garden, did you do any soil cultivation, or just plant in-situ? I'd be curious of your experience?

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  7. Where the loamy-sand sandpile where I garden has beds that are finally full of humus and look like chocolate cake when one digs in, I just slip plants into the ground. Where it is still packed sand, I till.

    A few farmers around here (southwest Georgia in the Coastal Plain) use no-till planting but it requires special planting equipment. When they are preparing to plant peanuts, as now, there is much cultivation starting with bottom plows.

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  8. I studied the soil type, geology, and local plant communities and made plant choices based on those factors. We did not till anywhere, added mulch to smother grass, then worked on building healthy layers of organic matter with leaf litter and plant debris to encourage more soil fungi, bacteria and microorganisms.
    Heather

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  9. Thank you, Thomas, for this illuminating post. From the perspective of the flower gardener, I would like to add that tilling the soil also awakens dormant weeds that the gardener was certain had been long conquered.

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  10. We have clay soil compacted by builders, who mixed concrete all over the place. No tilling. I am amazed that the moles manage to tunnel along, in summer, thru concrete/clay - while I waited for autumn rain. I'm learning to plant more of what does grow. And keep adding straw mulch and shredded prunings.

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  11. Interesting post, Thomas. Isn't there also some recent research that suggests, when planting trees, you should never disturb the soil UNDER the roots? As I remember, it's OK to loosen up the stuff around the roots and add compost or whatever, but the tree needs to sit on reasonably compacted, undisturbed soil or it is likely to shift and sink and suffer.

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  12. Jill,

    You're correct about not digging under trees. The danger of loosened soil is that the tree will settle and then be planted too low. Having the root crown at or just above grade is the ideal scenario for trees.

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  13. Thomas, thanks for this detailed and precise post on dealing with compacted soil. Though I in no way disagree with anything you've written, my own experience indicates that it IS possible to make a garden in unimproved, compacted soil. I think, by any measure, my soil would be called compacted, heavy clay, though the compaction seems to be a natural, not a man-made condition (the garden was formerly woodland). There is very little organic matter, and in very wet periods, some areas even turn anaerobic. But I've managed through trial and error to find plants that thrive. Perhaps the soil isn't as compacted as I think, but I doubt I could insert a stiff rod more than six or eight inches, unless it's very wet. I suppose I just want to make a case for the lazy, money-strapped gardener. I certainly won't be growing tomatoes, and I admit the conditions impose severe constraints on plant selection, but I manage to have an interesting garden regardless of the poor soil conditions. (Anyone want to lend me a penetrometer?)

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  14. My question: I'm about to dig up the grass in the center of a new raised bed (just 6" tall), and add soil after I remove the grass. Maybe I just add the soil on top of the grass and plant? What do you suggest?

    (I can't tell you how happy I'd be not to have to dig up that grass.)

    Thanks Thomas! Great post. News to me.

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  15. James,

    It's great to know a garden as marvelous as yours could be done without heavy cultivation. Perhaps the soil was not as heavily compacted as it appeared, or perhaps the vigorous plants you like can thrive despite the compaction. When I've used a penetrometer to test compaction, I'm always surprised to see how compacted the soil must be before it reaches the unacceptable levels. Maybe the lesson is that what we think of as compacted (clay-not fluffy topsoil) is mostly acceptable to plants, while we should reserve the heavy subsoiling equipment for construction sites, former parking lots, road beds, etc.

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  16. Margaret,

    That should work, although if you're smothering lawn, be patient. I started smothering a large area of lawn back in February, and it's still not fully decomposed. I'm not sure I'm going to wait any longer. Hopefully, the nitrogen loss from the decomposition process won't damage the new plants too bad.

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  17. It's a small space---6.5 long by 3.5 wide---a cypress "box" placed over the lawn. More space for veggies since we've lost sun to growing trees. I guess, as you say, to smother...and will the veggie roots easily work their way through the ground? Should I dig up the earth a little just to work it some before dumping in the soil?

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  18. Grass… how about quack grass? I rescued a garden last year that had been ignored for 5 years. The quack grass had choked the whole 100x12 space. It took all summer to gently lift the earth to loosen it then chase yards of the twiney root system through endangered old perennials. The garden recovered and is looking like it will prosper from the big reduction of the weed. The whole thing about gardens is balance and while we all would love it if we did not need to put shovel or fork or tiller to the soil there are instances when there is no other way to rescue or begin a healthy garden. Unless you like the way quack grass looks choking columbine and astilbe. It's all personal in the end.

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  19. Hi Thomas,

    A most interesting post.

    Your description of subsoiling sounds much like the old rationale for double-digging--which I've done before, most recently when putting in rhubarb starts three years ago (the composted cow manure goes in two feet down). This year the rhubarb is very well grown and ready to harvest. And no tilling in that area, ever--just topdressing of compost/composted manure in fall.

    What about areas where you grow annual vegetables?

    @Margaret,

    In the Midwest, smothering works best if you smother during, say, August and then plant the next spring. I've got an area that I smothered in late fall, and it's going to stay that way for a bit longer before I plant.

    Adrian

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  20. Hi Adrian,

    Thanks for that tip. I am wondering about the annual vegetables too.

    Tomatoes will go in my raised box. Our garden where we've planted for years is now almost all shade. So we are following the sun so to speak.

    We'll use the good soil from the veg garden to fill the raised bed. But...do I till first....maybe so, from what you posted. Hmmmm...this is the question for Thomas.......

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  21. Thomas, in your research did you find out how long it took for the mycorrhizal network to restore itself, assuming there was minimal compaction after the tilling?

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  22. Diane,

    I had a project where Quackgrass went rampant. I feel your pain. Tilling, though, I would suspect would make it worse. Those seeds spread like crazy.

    Adrian--I've never heard of double-digging. Sounds intriguing. Is that a typical technique for vegetables?

    Both Adrian and Margaret: to be honest, I've only dabbled in annual vegetables. I would not claim any expertise there. Though the research in favor of 'no-tillage' farming seems pretty substantial. There are lots of websites dedicated to 'no-dig' vegetable gardens. Perhaps they could provide more experience than I can. Just remember, anything that's in the process of decomposing (such as smothered lawn) pulls nitrogen out of the soil as it breaks down. So if you don't wait for it to be fully decomposed, you may want to give it some extra organic nitrogen fertilizer. If your plants are yellowing, that's probably why.

    Great point about the timing of smothering. I wish I had started it last fall. I did it in late January, and it's still not there yet. I may plant in it this weekend--eek--hopefully my impatience won't result in too much damage to the new plants.

    Vicky, unfortunately, I'm not exactly sure how quickly they can rebound. Charles Dowding, a British gardener, has a great book on the 'no-dig' technique called Organic Gardening: The Natural No Dig Way. He might have more information. As an avid compost tea brewer, I do know that you can brew many of the bacteria and fungus present in soil in a matter of days, and then water them into the soil. But I'm not sure how long it would take naturally for them to come back. With some help, I'm pretty confident they establish quickly. By themselves, maybe longer.

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  23. Thanks Thomas! Using your key words, I found a great resource for my raised bed, no-dig veggie garden. Apparently you layer cardboard first, then wet newspaper, then a layer of hay, the thin layer of manure...then cover with more hay. Very interesting process.

    Still thinking about how I'll approach this.

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  24. I file this under something that gives me anxiety (am I killing my mycorrhizae?), but I just don't have time to lay down cardboard and wait for a bare patch to appear. Plus, there are a lot of weeds in my sod--I'd hate to plant and then find the whole bed infiltrated with weeds and grass. Luckily I garden in raised beds so it is not too tough to dig up a 3x6 spot. I hope the Espoma fertilizer I use, with mycorrhizae, helps in recovery.
    margaret, also google "lasagna gardening"

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  25. Double digging is an old English method in which you dig a deep trench down into the sub soil, add lots and lots of organic material to the soil you've dug out and then replace the mix back in the trench. The resulting soft, fluffy soil starts out higher than the ground level and then slowly sinks back in.

    It is a lot of work, but the results are excellent for perennial beds. I used to do it more, but then the beds were in, and they've maintained pretty good fertility. Also I have pretty good soil, so it's not so necessary. But it might be good in more mediocre situations. Maybe it would work for an initial prep of a vegetable area. I used it on the rhubarb because it will be in its location for a very long time.

    Adrian

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  26. Thanks, Adrian. That's interesting. I have a troublesome spot in my garden next to a driveway--that might be the perfect solution. I'll give it a try.

    Thomas

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  27. One more thing about double digging: I wouldn't suggest it for native plant gardens, because the gardener is using plants adapted to the site, and there are plants adapted to different soil conditions.

    By a driveway, though--good idea. I think it's sort of like making a raised bed, only in the ground.

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  28. I can certainly appreciate this post, as it's something that's been on my mind. I've had to do a major overhaul on the soil in my veggie plots, but that being said, I only till when absolutely necessary. The pentrometer gadget that you mentioned sounds interesting, but I think that any less-sophisticated version would be at odds with our rocky soil here in New England. Glad I found your blog!

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  29. Thank you for this post, I have been a little anxious about double digging my new home's landscape, which I am doing-- for not only is the soil already compacted and greatly lacking organic material, but I have had to have heavy equipment in to pull tree stumps and level the grade.

    Some areas near the house are so compacted and rocky I have to dig with a pick and knife. My compromise is that after removing the topsoil--which is only a few inches in that area, I remove the next 8 inches or so in a separate pile, then dig deep cracks in the soil, slip in my garden fork and push as deep as possible, rock it back and forth to create a larger space without disturbing the layers and pour compost into the crack. Then I mix much compost with the middle layer and return it, finally adding compost to the topsoil layer and returning it to the top surface. A slow and frustrating process.

    I am more worried about bringing pathogenic subsoil creatures up to the growing surface than breaking the beneficial fungus web for 2 reasons: first there doesn't seem to be any as weed and grass roots are stunted and short, and second I have read that they can be restored fairly quickly in loosened & amended soil. Preparing the soil is my priority and I am delaying my gratification of planting.

    Now I am going to think about renting a trencher to speed up preparing my half acre. I am just wondering how much subsoil it brings to the surface.

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