The Pros and Cons of Cultivating Soil

In response to my recent post on Winter Mulches for Vegetable Gardens, reader Monica Reinagel suggested I follow up with more information on the pros and cons of cultivating vegetable garden soil. "A couple of times a year (spring and autumn), I turn over my entire garden to a depth of about a spade and chop it up – to kill weeds (or turn under cover crops) and loosen up the soil. But I've read that it's best to do as little cultivating as possible so as not to disturb the top layer of soil. The turning and chopping takes a lot of effort. Should I stop?"

To answer this question, I looked into the effects of cultivation on soil nutrition, earthworms, beneficial fungi that live in the soil, and management of the soil's weed seed bank.

Shall we begin with some basic soil biology? The aeration caused by cultivation causes nitrogen to be released, especially in fertile, organically-enriched soil. This is fine when you're starting a new crop, but it makes less sense when winter is around the bend. For this reason, there are big differences between cultivating soil in spring, when planting gets under way, and digging about in the fall. In fact, nutrient losses from fall tillage and subsequent winter erosion make so little sense that Purdue University researchers have used the phrase "recreational tillage" for fall cultivation that makes soil look well groomed, but negatively affects its overall quality for growing plants.

In any season, earthworms can be the innocent casualties of unnecessary soil cultivation. Underground networks of earthworm burrows comprise a natural drainage system beneath many gardens, which is partially destroyed each time you cultivate. When the entire garden is dug and turned at once, earthworms may be forced to leave the garden in search of better habitat.

Beneficial fungi that assist plant roots in taking up nutrients and water also are set back by soil cultivation, but they do not perish. But if you're going for truly superior vegetable garden soil, why interrupt the process? Austrian research indicates that building excellent organic soil with a full cadre of beneficial microorganisms can take as long as 15 years in a cold winter climate, so you don't want to cause unnecessary stops and starts by digging in when you could cover crop or mulch instead.

Gentle Soil Cultivation Guidelines

And so, in answer to Monica's excellent question, I propose the following guidelines for cultivating your vegetable garden soil:

• Be as gentle as possible when cleaning up after spent crops, disturbing only the top few inches of soil. Save deep cultivation for when you are preparing soil for new plantings, and need to mix in compost and organic fertilisers.
• Do use your spade to turn under cover crops, or to chop through the roots of spent plants to help them decompose faster. But when want to loosen and aerate your soil, a digging fork or a special tool called a broad fork (shown on the right) can do the job with minimal disturbance to the soil's secret world. A digging fork or broad fork is especially useful in spring and fall, when earthworms are most active. These are the worst times to use a rotary tiller.
• Cultivate beds or rows individually, so that parts of your garden remain undisturbed. Earthworms, ground beetles, and many other beneficial life forms prefer stability to change.
• Avoid cultivating in the fall if you can mulch instead. Pull out spent plants and compost them, and then scuff up the surface with a hoe to kill weeds. Mulch through winter, and save actual cultivation for spring.
• Never cultivate when you know an abundance of weed seeds are present. Sweeping them up or sprouting them out makes more sense than mixing them in. Should an area of your garden receive a heavy rain of weed seeds, use the false seedbed technique to reduce the number of weed seeds present. This involves cultivating only the top inch of soil in spring, allowing the weed seeds to germinate, and then repeating the drill. The spot can then be planted with a fast-growing summer crop that quickly forms a dense, light-blocking canopy such as cucumbers or bush beans, and the rehabilitation is complete.

By Barbara Pleasant