To Dig or Not to Dig
|When I first began gardening, the gold standard for preparing new perennial beds was double digging. In The Garden Primer (Workman, 1988) – my gardening bible at the time – Barbara Damrosch described the process this way:|
…I remove the soil from a section of the bed to the depth of a spade and set it aside. Then I loosen the soil under it with a digging fork, working organic matter, lime and whatever else the soil needs into the subsoil. I replace the topsoil layer, but only after I have added soil amendments to it as well. (p. 35)
For decades, I followed this practice, using double digging to amend my poor sandy soil and make it a welcoming home for plants.
More recently, however, double digging has come into disfavor. This topic came up early in my Master Gardener course. One classmate reported seeing bumper stickers in her affluent coastal town that read “Friends don’t let friends double dig.” The University of Maine scientist who taught our class on soils seemed mystified by this controversy. The university’s agricultural scientists warn against damaging soil structure through “recreational tilling,” but they consider tilling in soil amendments an important part of taking care of the soil.
To understand my dilemma, you need to understand that my property sits on a big glacial sand deposit, and that my soil is what soil scientists call “loamy sand.” The diagram above shows different types of soil textures, depending on their mix of sand, clay and silt. As you can see, “loamy sand” is just one step up from pure beach sand (in the lower left corner of the triangle); it is a mixture of 70-85% sand with small amounts of clay and silt and has almost no organic matter. Not only are there relatively few plants that will grow happily in this loamy sand, but it is also very poor at retaining either moisture or organic matter. So if I want to garden here, the question is not whether to add organic matter to the soil, but how.
To decide whether or not to double dig my new flower beds, I needed to consider (1) alternative methods for adding organic matter to the soil and (2) the reasons why double digging has fallen into disfavor. Let’s start with the alternatives. One is to add organic matter on top of the existing soil and let it work down into the soil over time. This is the method I use to recharge organic matter in existing flower beds. But I didn’t consider this a workable alternative for my new garden areas; I’m in my late sixties and can’t afford to let years go by while I wait for organic matter to build up in the soil. Another alternative is the “lasagna” method, which involves covering the existing ground with cardboard or wet newspaper and then piling several inches of compost on top. The idea is that the cardboard or newspaper kills any sod or weeds while the organic matter improves the soil. I used a variation on this method to create the raised bed in my back garden, but I don’t want my front garden to consist of raised beds and it seemed as though I would have to pile the organic matter up quite high in order to put plants in right away (rather than waiting for the weeds to die, the cardboard or newspaper to decompose, and the organic matter to work its way down into the soil). A third alternative is tilling rather than digging, spreading several inches of organic matter on top of the soil and then turning it under gently with a garden fork.
To decide between tilling and double digging, I needed to look at the arguments against double digging. The main ones are (a) that soil plays an important role in carbon sequestration so that digging releases carbon into the atmosphere and (b) that digging destroys soil structure. I quickly decided that carbon sequestration wasn’t really an issue in my case; it is the organic matter in soil that provides carbon, and the big problem with my soil is that it lacks organic matter. Soil structure, however, is a trickier issue. Soil structure refers to the way that particles of soil are arranged, particularly the ways that bacteria, fungi, and organisms like worms bind particles together into clumps or soil aggregates. In my earlier double-digging days, I would carefully break up clumps into friable crumbs of soil. Now I know better; soil aggregates are an important part of healthy soil structure, and disturbing the soil inevitably damages soil aggregates. Because my soil had already been disturbed by the construction process, however, there weren’t many soil aggregates to damage.
I decided that double digging is a reasonable way to add organic matter to this backfilled soil. Sure enough, as I have worked with the soil, I have encountered very few clumps. I dig out soil to the depth of my spade and pile it into my wheelbarrow, removing weeds and their associated roots as I go. I also remove any rocks or large stones, but I try to leave any clumps of soil undisturbed. Occasionally, I encounter a lump that may be a soil aggregate or may be a soil-covered rock. In the past, I would have squeezed it to find out, breaking apart the clumps of soil. Now, I tap these lumps gently against the side of my wheelbarrow. If they are rocks, they make a metallic “thunk” sound, but soil aggregates make a soft thud. When I put the soil back into the ground, compost and cow manure have been mixed in with the sand and loamy clumps.
As I worked to amend the soil on my back slope, I noticed a big difference in soil structure when I reached an area that had not been disturbed by the construction. Here my digging did disturb soil aggregates, and a different method for adding organic matter probably would have been preferable. This means that I will need to rethink how I add organic matter to my soil as I begin (next year) to work on sections of the front garden where the poor sandy soil was not disturbed during the construction of my house addition.