BY DOUG GERAETS
In my first installment of Musings on The Garden I wrote about not using traditional composting techniques but instead a method of trench composting. Composting in trenches is little more than digging a large trench 20-24 inches deep (mine are usually 4-6 feet by 4-6 feet) and burying house scraps, leaves, plant materials, shredded newspaper, and cardboard covering with about 12 inches of soil and then letting earth worms and soil organisms do their work to decompose the material. This process of composting materials underground is reportedly more anaerobic than aerobic thus not “heating up” as traditional composting does and takes a minimum of 2-3 months in warm months.
I have used a slightly modified version of trench composting. After I’ve dug my trench usually 24-30 inches deep I add compostable materials and soil in 2-3 inch layers in the lower 12-16 inches of the trench. Some gardeners have referred to this as lasagna trench composting and I think the name fits. I try and put back soil in the exact order that it has been dug out of the ground; in other words, deeper more clay-like soil gets layered back first with the high-quality loam soil going back last so that the best soil stays on top of the garden.
What about in the winter when the frozen ground prevents digging? You can either save all your kitchen scraps, leaves, and other materials in a large covered bucket or trash can until spring thaw or, like me, dig a trench and cover it throughout the winter. Then whenever you have a bucket of scraps it gets emptied into the trench and remains there (mostly frozen) until spring thaw when you can begin the process of burying it.
This has worked quite well for me over the past 10-plus years and I’ve not had noticeable problems with pests (usually squirrels, raccoons, or opossums) disturbing the covered materials.
If you can mix the compostable material with soil as you add it back to the trench that is ideal but I must admit I don’t always do that (I just layer it with soil). In addition, I try and add moisture before covering any added material as moisture is needed to promote the composting process. I either water with a garden hose or wait until a good rain so that I know the material is moist as I am covering it with soil.
Two concerns frequently raised as one uses the trench composting method: 1) if you compost diseased garden vegetation is there risk of passing this to future plantings and 2) if you add weed seeds from weeds cleaned up from the yard & garden are you spreading these and making your weed control harder? I avoid adding diseased vegetation to the compost trench if at all possible but sometimes it is difficult to separate from non-diseased vegetation. So I practice a 4-year crop rotation in the garden and bury any possible disease-carrying vegetation at least 4 years away from future plantings of the same family of vegetables (tomatoes are the best example).
As for weed seeds I’m covering composted plant material with 10-12 inches of soil so my expectation is that seeds that deep can’t germinate. Here is advice from Minnesota Extension service site: “Burying infected plant debris below ground reduces spread of the pathogen to other crops and allows the naturally occurring composting soil microorganisms to begin to break down the infected plant debris. This needs to be done in combination with rotation to crops that are not susceptible to the disease (typically a different plant family) for 3 to 4 years. Rotation allows time for pathogens in the infected crop residue to die off.”1
Johnson County Master Gardeners of Iowa 