Sheet composting offers several of the benefits of composting, while stripping away the labor of having to constantly turn your piles. Although this technique is most often used on large scale gardens and farms, if you have the right equipment (i.e. a rototiller), you can do it on a small scale as well.
As with all composting techniques, sheet composting offers growers a great way to lower their fertilizer costs while also recycling "waste" organic matter. I am always hesitant to use that term "waste" for food scraps and crop residues. In reality these products are just as important as the crops themselves...after all, this year's composted "waste" could very well turn into next years crop.
There are several different methods of sheet composting, however, they all hinge on a couple key factors. First, they all involve adding organic matter (i.e. leaves, grass clippings, weeds, etc.) to your soil. Second, they all require some period of "rest" or fallowing before the next planting. Below are the three different methods of sheet composting:
The Traditional Method
More often than not, when someone refers to sheet composting, they are referring to this technique. The steps involve adding organic matter in layers on top of your soil, and then passing over these materials with a rototiller. The key is to make several passes so the organic matter is thoroughly shredded and readily available to soil microbes.
As I've said before, I am not a big fan of tillage, however, when tending to large areas, the benefits often outweigh the disadvantages.
This technique is best done in the fall as it allows the soil microbes to feast over the winter and replanting to occur in the spring. Depending on the level of microbial activity in your soils, you may have to allow these sheet composted areas to rest for an entire season.
Green manuring refers to the technique of growing an annual crop with the specific intention of turning it back under into the soil. For instance, farmers and well-versed gardeners (like Gardener Ed in our Tribe) will often grow nitrogen-fixing crops such as legumes (clover, soy beans, vetch) and then till them back into the soil once they reach a desired size.
Please note that green manuring deserves a lot more attention than what I am giving it here; entire books have been written on the subject. So please do yourself, and your soils, a favor and investigate it further. Here is a great little guide to start you on your way.
As the name implies, this final technique is better
described as a means of mulching rather than composting.
How do you do it? You can start by covering your soil with a layer of cardboard, then add a layer of well-rotted manure or compost, and top that with a thick layer of organic material such as leaves, straw, or spent hay. Please note, you do not incorporate this material into your garden soil.
Needless to say, sheet mulching is a very slow approach to building soil organic matter. And in order to really make the most of this technique, you must start with a microbe-rich soil. Remember, the microbes are your work force and they're the ones that will eventually break these materials down.
I think they say it best in The Rodale Book of Composting - "many experts agree that you would do well to combine the labor-saving advantages of mulch with a seasonal composting program."
My biggest warning surrounding this composting technique is as follows - Know your compost ingredients! Don't add a bunch of carbon (e.g. leaves) to your soils without also adding sufficient nitrogen (e.g. grass clippings) to compliment it, especially if you want to plant into the soils the following season. Remember, you're essentially composting in place, so all of the same rules for making good compost apply (i.e. proper ingredients in the proper ratios, moisture, and oxygen).
For more information about the hazards of overloading your soils with carbon, please read Lloyd's comments about his mishap with sheet composting his wheat fields.
No matter which sheet composting technique you choose, it is important that you monitor your results. As per usual, I recommend taking an annual soil test as a means of tracking exactly what your soil does and does not need. In the words of the late Carey Reams, "why guess when you can be sure"?
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