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In permaculture, sheet mulching is an agricultural no-dig gardening technique that attempts to mimic the natural soil-building process in forests. When deployed properly and in combination with other permaculture principles, it can generate healthy, productive, and low maintenance ecosystems.
Sheet mulching, also known as composting in place, mimics nature by breaking down organic material from the topmost layers down. The simplest form of sheet mulching consists of applying a bottom layer of decomposable material, such as cardboard or newspapers, to the ground to kill existing vegetation and suppress weeds. Then, a top layer of organic mulch is applied. More elaborate sheet mulching involves more layers. Sheet mulching is used to transform a variety of surfaces into a fertile soil that can be planted. Sheet mulching can be applied to a lawn, a dirt lot full of perennial weeds, an area with poor soil, or even pavement or a rooftop.
Typical layers of natural soil.
A model for sheet mulching consists of the following steps:
The area of interest is flattened by trimming down existing plant species such as grasses.
The soil is analyzed and its pH is adjusted (if needed).
The soil is moisturized (if needed) to facilitate the activity of decomposers.
The soil is then covered with a thin layer of slowly decomposing material (known as the weed barrier), typically cardboard. This suppresses the weeds by blocking sunlight, adds nutrients to the soil as weed matter quickly decays beneath the barrier, and increases the mechanical stability of the growing medium.
A layer (around 10 cm thick) of weed-free soil, rich in nutrients is added, in an attempt to mimic the surface soil, or A horizon.
A layer (at most 15 cm thick) of weed-free, woody and leafy matter is added in an attempt to mimic the forest floor, or O horizon. Theoretically, the soil is now ready to receive the desirable plant seeds or transplants.
Variations and considerations
Often the barrier is applied a few months before planting to ensure the penetration of roots of newly planted seeds.
Very thick barriers can cause anaerobic conditions.
Some permaculturists incorporate composting in steps 5 and/or 6.
Sheets of newspaper and clothing can be used instead of cardboard.
Before step 4, an initial layer (2-3 kg/m2) of matter rich in nutrients (such as compost or manure) may be added to bolster decomposition.
Some varieties of grasses and weeds may be beneficial in a number of ways. Such plants can be controlled and used rather than eradicated. See also: mulch, list of beneficial weeds.
One variation of mulching, called hugelkultur, involves using buried logs and branches as the first layer of the bed.
Avoidance of potentially dangerous pesticides, especially herbicides.
Reduction of overall maintenance labor and costs.
Most of the materials required to sheet mulch can be collected at no cost, and materials can be substituted for those readily available in certain areas. For instance, suburban areas may have a plentiful supply of leaves, and farming communities may have spoiled hay and manure.