One of the most pervasive images in garden magazines and advertisements is of gardeners working their gardens with tillers of one kind or another. However, even though tilling seems, at first, to be a worthy and useful activity, it isn't always.
Tilling brings oxygen down into the soil, which sounds good, but that
increased oxygen can stimulate the growth of soil micro-organisms that
then get busy with their main job of breaking down organic matter, which
releases nutrients. If crops are planted to absorb and use those
nutrients, then that release of nutrients from tilling can be good. Here we are, though, with winter still at hand and no (or few) crops available to use those nutrients.
Releasing them now would result in waste.
However, a gardener planning to plant potatoes in mid-March might want to till in a
cover crop now, to get the decomposition on that foliage started.
This topic is on my mind because I actually turned one garden bed this past weekend, when we had some beautiful, warm, spring-like weather. It was a bed that had a cover crop on it (more chickweed and deadnettle than winter peas, but it held the soil well), and I want to plant some lettuces into that bed.
This was a case of turning under a cover crop to get a bed ready for planting. (I don't actually "till," since I don't own a tiller, but I do an equivalent activity with my grub hoe and spading fork.)
In my own garden, I try to not turn the soil in each of my garden beds
more than once per year. The big turning is usually in spring, and when
it's time to pull out an old crop and plant a new one, I tend to just
use a tined cultivator to loosen the top few inches for planting any
One reason for minimal tilling is that breaking up the soil also breaks up the underground communities of bacteria, fungi, and other little life-forms that help keep crops healthy. Here in Cobb County, I talk with plenty of gardeners who think that the soil isn't in good shape if they haven't pulverized it to a fine powder, when the truth is that their powdered soil has had the life pretty much beat out of it.
Potentially good reasons to till or turn the soil include mixing in amendments and turning under a cover crop. Some gardeners use a tiller to weed the paths between rows in row-cropped gardens, and some till to break a crusted soil for planting seeds. Those are probably ok reasons, as long as the gardener understands the risks.
Reasons to avoid frequent tilling include maintaining the biological community and avoiding release of nutrients at the wrong time, plus that pulverizing the soil can - paradoxically - lead to compaction of the soil as the powdered bits settle, plus that using an actual tiller for tilling can cause the garden to develop a "hard pan" below the tilled layer of soil. Also, tilling when the soil is too wet, which is a danger for all of us impatient gardeners, can cause the soil to form rock-hard clods that are difficult to break up later in the season.
As always with gardening, there is a lot to think about!