Trenching

Trenching loosens your soil and adds cured compost to a depth of three feet. Although this method is the most intense of all of those described here, it’s the most efficient way to loosen one’s soil and add the needed organic matter to this depth and was commonly done by gardeners just a few generations ago.

If there’s any genius to this method, it’s the way it gains access to each new foot or so of the deepest soil—to loosen it and add cured compost—by “leapfrogging” trenches down the garden, simultaneously loosening the upper layers of soil naturally in the process.

The Method

First, loosen the soil in the area you intend as a bed about a foot deep with a digging fork and clear it of weeds. If sod is present, remove it or smother it far enough in advance that it sufficiently degrades into the soil. If you have enough lead time, you have more options for the latter. For more information, see our post on dealing with sod.

Second, amended the entire bed area with a two-to-four-inch layer of cured compost, applied right on top. Don’t worry about mixing it into the soil. It’ll get mixed in naturally during the trenching process.

Third, starting on one end of the bed, dig a trench approximately a foot-and-a-half wide and two-feet deep, and place the removed soil in a pile just out of the way at the other end of the bed, where it will be added back later in the process. Note that the trenches pictured here are all one-foot deep instead of two.

A wheelbarrow and tarp tend to make this and a couple of the latter steps easier. The wheelbarrow makes walking the soil to the other end easier. Buckets, garden tubs, old galvanized washbasins, or the like work fine; they just take a bit more hefting. The tarp makes moving the last several shovel-fulls of soil a little easier when you add this pile back to the bed toward the end of the process since it allows you to simply drag them to the bed and dump them on as soon as they’re light enough. Using a tarp also makes the final clean-up easier. Once you drag the last bit of soil to the bed and dump it on, you’re done with cleanup. There’s no final soil to be raked out of the grass or that has filled the spaces between your mulch, essentially ruining it.

Fourth, add a two-inch layer of compost to the soil exposed underneath this trench and mix it in with a digging fork, loosening the soil an additional foot (typically the length of the tines on a good digging fork) in the process. Be sure the compost and existing soil are mixed well and the soil is thoroughly loosened while you have the chance. Gaining access to this depth of soil to loosen it and mix it with compost is the crux of this method, so don’t scrimp on loosening and compost once you’ve done the hard work of digging down to this level. Again, note that the trenches in all of the photos presented here are only a foot deep instead of two.

Fifth, dig a second trench two-feet deep immediately adjacent to the existing trench. This time, however, instead of removing its soil to the other side of the bed, simply shovel it directly into the first trench, filling it with freshly loosened soil and naturally mixing in the cured compost placed on top in step two in the process. Break up and loosen any clumps that need extra help.

Just like the first trench, once the second trench is dug to a depth of two feet, add two inches of cured compost to the bottom of it and mix it in with a digging fork to a depth of another foot, thoroughly loosening the soil to three-feet deep in the process. Once this is thoroughly loosened, use the soil from your third trench to fill your second, and add compost to and loosen the lower soil. Continue moving trenches over down the bed like this, adding compost and loosening the soil to three-feet deep as you go, until you reach the final trench. At the far end of the bed, use the soil set aside from the initial trench to fill the final trench, and rake the bed into an even, gently sloping shape.

Because it mixed the top two feet of soil, and hence the varying aerobic and anaerobic microbes present at different levels of unloosened soil, trenching could only be done once the soil had been improved enough over several years of what was called “bastard trenching.”