A 3-D view of tilling in Russia c1915

My post on lasagna gardening, which linked to a brief article by horticulture professor Linda Chalker-Scott seems to have opened a can of worms, so to speak. Two issues came up in the comments on my post: the wisdom of using cardboard in a lasagna mulch and the pros and cons of double digging/tilling. Let’s address them in separate blog posts, beginning here with double digging/tilling.

There are some very persuasive arguments in favor of a no-till, leave the soil alone approach. Chalker-Scott in the comments section of her post on lasagna gardening says,

. . . double digging (the equivalent of tilling in agriculture) disrupts natural soil building. No-till agriculture is increasingly preferred as being more protective of the soil ecosystem. I think the same philosophy could be applied to home gardens as well. You’re right, you can boost production with a more aggressive approach to soil amendment – a similar argument is often made in conventional agriculture (compared to organic agriculture) to till, use excessive fertilizers, pesticides, etc. I guess it depends on how you regard the soil – as a medium for growing vegetables or as an ecosystem (and I’m not being judgmental). It’s a philosophical choice.

No-till agriculture advocates argue that tilling oxidizes organic matter leading to a loss in soil fertility and the creation of carbon dioxide which, in turn, leads to global warming. A case can also be made that tilling creates a soil “crust” that interferes with water penetration. And tilling disrupts mycelial networks and other soil organisms that, research has shown, form important symbiotic relationships with plant roots.

But what about heavily compacted soils? How do you turn a lawn or driveway into a garden? It’s in these cases that I, in the past, have used double digging.

Double digging proponents would argue that the practice should be distinguished from tilling in that, unlike tilling, you don’t invert the soil structure as much when you double dig. Double digging keeps the same soil profile while loosening heavy compaction. Double-digging advocates distance themselves from the use of roto-tilling machines which invert deeper layers of soil with surface layers that contain more organic matter.

But there are alternatives to double digging and tilling that will break up compacted soils. Scott Kleinrock at the Huntington Ranch turned a former construction parking lot into a productive edible landscape without double digging or tilling. Kleinrock used what I’d call a kind of toolkit of de-compaction strategies:

The application of a thick mulch (Chalker-Scott suggests a minimum of 12 inches). It’s surprising how many earthworms start doing the tilling for you with a thick mulch layer.

Planting soil busting cover crops with thick tap roots like Daikon radish

The use of a broadfork or deep spader

Peaceful Valley’s “Deep Spader”

A broadfork in action

Broadforks and deep spaders get air and water into compacted soils without the damage tilling can do. Unfortunately broadforks and deep spaders are very expensive (around $200) and heavy. The deep spader Kleinrock used came from Peaceful Valley Farm and Garden Supply. I’ve tried it and it works nicely, though it’s still hard labor. If you knew how to weld you could probably make a home brew deep spader or broadfork. I’ve also successfully used a regular, inexpensive garden spading fork in moderately compacted soils.

This is clearly a topic on which reasonable people can disagree, but the no-till folks seem to have the upper hand in terms of the science. As with all gardening problems, though, context is king. Environmental factors and economic issues (those expensive broadforks) intersect in our urban gardens in complex ways. You have to make up your own mind. I’d say if you’re going to double-dig do it only as a last resort and after considering all the alternatives. While, under some circumstances, I might double-dig I would never till with a roto-tiller or invert soil structure with a shovel. But after seeing the dramatic improvement in soil at the Huntington Ranch in less than a year, I’m more inclined to try de-compaction alternatives. You could also just build raised beds and import better soil (though that strategy gets expensive).

I’ve created a poll on the right side of this blog on which you can cast a vote on tilling vs. not-tilling vs. double digging. And consider leaving a comment–I’m interested in what readers think about this complex issue.