Someone wrote to us the other day asking a question about cold composting. Apparently he had heard that "cold composting", where food scraps are put directly into the soil to decompose, robs the soil of certain nutrients and wanted to know if this was true or not. The short answer is yes, but not cold composting per se. Read more to find out why, but first let’s take a quick look at what cold composting is.
Cold composting is the easy way to compost. It requires minimal labor and is suitable for most hobby gardeners. In fact you might already compost this way without even realizing there’s a name for it. If you have a compost pile or bin where you throw kitchen scraps, grass clippings (tsk, tsk, remember they’re good for the grass!), weeds, and leaves to decompose naturally or have tried sheet composting then you’re already cold composting. Once the material is composted or broken down by microbes it becomes humus or the type of organic matter that is a plant’s best friend.
Now back to the answer. Cold composting itself does not deplete the soil of nutrients; however, when kitchen waste, straw, leaves, wood chips (but not mulch), and the rest are placed directly on soil as “compost”, then yes, an important nutrient, namely nitrogen is depleted in soil. The reason for this, is that microbes use nitrogen in the decomposition process thereby depleting the soil of this important nutrient. In summary, if you’re using kitchen scraps and large chunks of organic materials it’s probably better to do your composting in a bin and use the humus to enrich the soil than it is to use the raw material. On the other hand if it's grass clippings and weeds, you can always try lasagna gardening.
Recommended reading:
Let it Rot!: The Gardener's Guide to Composting
Teaming with Microbes: A Gardener's Guide to the Soil Food Web
Basic Composting: All the Skills and Tools You Need to Get Started