|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.
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.