leaf mold/compost/soil - Ask Extension
This time of year there are lots of articles stating that "leaving the leaves" creates new soil. However, it is my understanding that soil ...
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leaf mold/compost/soil #855171
Asked November 13, 2023, 8:52 AM EST
This time of year there are lots of articles stating that "leaving the leaves" creates new soil. However, it is my understanding that soil comes from decomposed inorganic matter (rocks) and that compost and leaf mold are organic soil amendments. However, after many internet searches, I can't really find definitive info on the difference between leaf mold, composted leaves and soil and how they should (and maybe shouldn't) be used. Can leaving lots of leaves in perennial garden beds cause problems from too much organic matter? Do those leaves really decompose in just a year, or do they blow away over time? Can the decomposed material really make it into my dry hand packed clay by itself to improve the soil structure, or does it need to be manually worked in. And finally, I have asian jumping worms in my garden, how does that change the equation?
Montgomery County Maryland
Expert Response
You are correct that soil is derived from the weathering of rocks. It's referred to as "mineral soil" and contains sand, silt, and clay particles. Organic matter makes up a small, but very important part of soils. (Soil as a whole contains both this mineral component along with organic matter, water, and air in the pore spaces between particles.) Organic matter, including compost, is added to soil to improve its structure (especially given the typical compacted state of soils altered by human activities like construction), to increase biological activity (beneficial microbes), and provide nutrients. You can learn more in our soil and compost resource pages.
"Leaf mold" describes tree leaves undergoing decomposition, usually at an advanced stage. For most people, it can be synonymous with "cold composting" -- that is, composting at its own pace, rather than accelerated by manipulating a pile to stay hotter, where well-oxygenated and moist conditions promote the proliferation of microbes that break down the material.
Spreading a 4-inch layer of whole or shredded leaves over garden beds will provide essentially the same benefits as mulch made from bark or wood chips -- it helps protect soil from erosion, insulates perennial plant crowns and roots from winter injury, and suppresses winter annual weed seed germination (chickweed, etc.). Yes, intact/unshredded leaves may blow around a bit, and any whole or shredded leaves that do not completely decompose by spring can still be used as mulch.
Where possible, it's best to incorporate compost into the top 4-8 inches of soil, assuming doing so will not disturb existing plant roots. Still, compost spread on the ground in perennial beds will slowly filter into and improve the soil, partly due to ground heaving and cracking during winter and mostly due to the activity of a plethora of soil-dwelling organisms. It's very common to have too little soil organic matter, and rarer to have too much soil organic matter.
Jumping worms live close to the soil surface and can speed-up the conversion of this valuable leaf layer containing different stages of decomposition (often referred to as "duff" in forest habitats) into compost, to the detriment of certain wildflower species that rely on duff to insulate seeds and as a resource to root into. Soil loosened by jumping worm invasions may also be more vulnerable to erosion. Fortunately, in gardens at least, jumping worms don't seem to be causing catastrophic impacts, at least so far. In our opinion, the benefits of retaining leaf "litter" for wildlife (overwintering shelter for insects and other animals being an important benefit) outweigh the potentially desirable habitat it creates for jumping worms.
Miri
"Leaf mold" describes tree leaves undergoing decomposition, usually at an advanced stage. For most people, it can be synonymous with "cold composting" -- that is, composting at its own pace, rather than accelerated by manipulating a pile to stay hotter, where well-oxygenated and moist conditions promote the proliferation of microbes that break down the material.
Spreading a 4-inch layer of whole or shredded leaves over garden beds will provide essentially the same benefits as mulch made from bark or wood chips -- it helps protect soil from erosion, insulates perennial plant crowns and roots from winter injury, and suppresses winter annual weed seed germination (chickweed, etc.). Yes, intact/unshredded leaves may blow around a bit, and any whole or shredded leaves that do not completely decompose by spring can still be used as mulch.
Where possible, it's best to incorporate compost into the top 4-8 inches of soil, assuming doing so will not disturb existing plant roots. Still, compost spread on the ground in perennial beds will slowly filter into and improve the soil, partly due to ground heaving and cracking during winter and mostly due to the activity of a plethora of soil-dwelling organisms. It's very common to have too little soil organic matter, and rarer to have too much soil organic matter.
Jumping worms live close to the soil surface and can speed-up the conversion of this valuable leaf layer containing different stages of decomposition (often referred to as "duff" in forest habitats) into compost, to the detriment of certain wildflower species that rely on duff to insulate seeds and as a resource to root into. Soil loosened by jumping worm invasions may also be more vulnerable to erosion. Fortunately, in gardens at least, jumping worms don't seem to be causing catastrophic impacts, at least so far. In our opinion, the benefits of retaining leaf "litter" for wildlife (overwintering shelter for insects and other animals being an important benefit) outweigh the potentially desirable habitat it creates for jumping worms.
Miri
