Sheet mulching is a fancy phrase for building a massive compost pile across the surface of a field or garden. Rather than pile up your manure, leaves, crop residue, and other organic materials in a squat pile, you spread it all out where the compost is needed, skipping the effort to build the pile, turn it, water it, and otherwise coerce it to break down into rich, brown earth—a process that goes on for months.
But sheet mulching does even more than that.
Beneath the layers of compost materials, sheet mulches typically include a layer of cardboard to keep grass and weeds from growing through—a great way to smother unwanted vegetation or convert a sod lawn into a garden. Before long, both weeds and cardboard decompose and feed the soil with organic matter, while you pat yourself on the back for finding such a clever way to recycle and relieve yourself of the constant chore of weeding.
Sheet mulching also traps moisture in the soil. Every farmer and gardener knows that mulching is a must to cut down on irrigation, but the cardboard used in sheet mulching is much more effective at trapping moisture than typical wood chips or straw. When I used the sheet mulching technique in the parched landscape of California, I found the moisture in the soil lasted at least five to 10 times longer than a thick layer of mulch on its own.
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