08 Jan2002

Overstory #96 - Sheet Mulch: Greater Plant and Soil Health with Less Work

Written by Craig R. Elevitch and Kim M. Wilkinson.

Introduction

Mulch is a layer of decaying organic matter on the ground. Mulch occurs naturally in forests; it is a nutrient rich, moisture absorbent bed of decaying forest leaves, twigs and branches, teeming with fungal, microbial and insect life. Natural mulch stores the nutrients contained in organic matter and slowly makes these nutrients available to plants. Mulch also protects soil from desiccation by the sun and wind, as well as from the erosive effects of rain and run-off.

Mulch forms a necessary link in nutrient cycling vital for our soils. When mulch is absent for whatever reason, the living soil is robbed of its natural nutrient stores, becomes leached and often desiccates. Natural terrestrial environments without a litter layer are usually deserts. Non-desert plants grown in bare soil require constant fertilization, nutrient additions, and water, not to mention the work required to keep the soil bare.

"Sheet mulch" is a four-layered mulch system for use around crops. The four layers (or "sheets") mimic the litter layer of a forest floor, and optimize the weed control and fertility benefits of mulch. The sheet mulch technique described here is for use with trees or in gardens. The techniques can also be adapted for landscaping and other agricultural uses. Sheet mulch is a simple and underutilized technique protecting soil, reducing weed competition, and restoring fertility.

Benefits of sheet mulch

* Improves nutrient and water retention in the soil
* Encourages favorable soil microbial activity and worms
* Suppresses weed growth and competition around crops
* Reduces labor and maintenance costs as compared to bare soil culture
* Provides crops with organic matter and nutrients
* Improves plants vigor and health, often leading to improved resistance to pests and diseases
* Enhances soil structure

Basic techniques of sheet mulching

Once you get the hang of it, sheet mulching can be used almost anywhere.. It may be used either in establishing a new garden or tree planting, or to enrich existing plantings. Below is described sheet mulching to cover an area such as a garden on a small scale, then, how to sheet mulch around a tree. In both cases, mulch is applied to bare soil or on top of cut weeds. New plantings are planted through the mulch, or a small area is left open to accommodate established plants and trees.

The benefits of mulching justify putting the energy into doing the job right, using ample materials. Collect all of the materials (as outlined below), and complete the mulching process in one session. A reduction in maintenance and increase in plant vigor will more than pay off the initial effort.

Sheet mulch is put down in four layers to mimic natural forest mulch: well decayed compost, weed barrier, partly decayed compost and raw organic matter, as described below.

Steps for applying sheet mulch

Step 1: Prepare site

To prepare the site, knock down tall weeds and woody plants with a scythe, brush cutter, or by trampling the existing vegetation so that it lies flat. A poultry or pig tractor system (see The Overstory #50--Animal Tractors) is an excellent method of site preparation. There is no need to remove vegetation, unless it is woody or bulky. It fact the organic matter left now will decay and add nutrients to the soil. Once vegetation in the area is flattened proceed to lay down the sheet mulch.

Step 2: Add concentrated compost and mineral amendments (Layer #1)

Whether you are mulching bare soil or weeds, "jump start" microbial activity by adding high nutrient material which stimulates soil life. This material also accelerates the decay of weeds and grass under the mulch. Suitable materials are enriched compost, poultry or stock manure, worm castings, feather meal or similar at the rate of about 2.2 kg/m2 (50 lbs/100 ft2). If the soil is overly acid, which is common in disturbed soils or those treated with conventional fertilizers, add lime. A soil analysis will indicate the need for adjustment of pH or mineral amendments. This is the appropriate time to add the recommended doses of minerals such as phosphorous and potassium.

Step 3: Water well

Now, soak the area well with water. This is essential as it starts the natural process of decomposition. Also it is much easier to soak the ground now, before the remaining layers of mulch are applied.

Step 4: Apply a weed barrier (Layer #2)

Most cultivated areas harbor untold numbers of weed seeds. There are also weed seeds blown by wind, animals and people. Soil borne seeds are lying dormant and waiting for sunlight, moisture and space to sprout. Simply pulling or killing growing weeds will not erase the weed problem: more seeds will sprout almost as soon as the soil is exposed to moisture and light. Therefore the next step in mulching is to put down an organic weed barrier. This barrier prevents the germination and eventual emergence of weeds through your mulch.

Underneath this weed barrier grasses and weeds die and quickly become food for earthworms. The worms turn and aerate the soil.

Of the four sheet mulch layers, the weed barrier has no natural counterpart on the forest floor. In the forest, weeds do not sprout because there is "no room for them," which simply means a lack of space above and below the ground, and a lack of light. By planting an area properly, there will eventually be no room for weeds. The weed barrier is needed only for establishment of the mulch, and disappears with time. If your area is planted appropriately, weeds will not emerge after the decomposition of the weed barrier.

Materials for the weed barrier that work well are: cardboard, 4 - 6 sheets of newspaper, burlap bags, old carpets of natural fiber, worn-out clothing, gypsum board, or any other similar biodegradable materials. Banana or other large leaves also work if laid down in several layers. Overlap the pieces of the material so as to completely cover the ground without any breaks, except where there are plants you want to save. Around these leave a generous opening for air circulation around the root crown. Care in laying down the weed barrier without gaps will save you the headache of emerging weeds later on.

Both water and good air circulation are necessary for healthy soil. Although the weed barrier forms a physical and light barrier, it is essential that is be permeable to water and air. Overlapped pieces of organic material as recommended above let water and air slowly permeate between and through them. If the weed barrier is applied too thickly, the soil can become anaerobic. Also, for the same reasons plastic mulches are not recommended for most situations.

Step 5: The Compost Layer (Layer #3)

This layer is on top of the weed barrier--it must be weed seed free. Well conditioned compost, grass clippings, seaweed or leaves are ideal materials to spread over the weed barrier. Any weed-free material mixture at the right moisture level for a good compost will do. This should form a fairly dense layer about 8 cm (3 inches) thick.

Step 6: The Top Layer (Layer #4)

The top dressing mimics the newly fallen organic matter of the forest. It also must be weed-seed free. Good materials for this layer include leaves, twigs and small branches, hay, straw, fern or palm fronds, coffee chaff, macadamia nut shells, chipped tree prunings, sawdust, bark, coir, bagasse, etc. The top layer will slowly decompose into lower layers, and therefore must be replaced periodically; it represents reserves of compost. This layer should be about 8 - 13 cm (3 - 5 inches) deep. Many materials suitable for the top layer often have a pleasant cosmetic appearance. For this reason, there should be no hesitation in using sheet mulch in all cultivation from landscaping to gardening to permanent orchard crops. In fact, as you use mulch, bare soil will begin to seem ugly and undesirable.

When the soil is amended and sheet mulch applied properly, there will never be a need to turn the soil. Earthworms do the tilling. The only task left is to keep the soil covered by replenishing the mulch.

Sheet mulch around trees

Planting trees with mulch assures optimal conditions for survival and early growth. The method is a specialized version of the steps above. Use locally available materials, and adapt this method to your situation. If you are unsure about the benefits of mulch, apply mulch to some trees and not to others planted at the same time. Usually the difference in growth and vigor is amazing.

1. Prepare the planting area and plant the tree the way you usually do.

2. Amend soil around tree out to a radius of 0.5 - 1 meter (1.5 - 3 feet) with a light layer of nitrogen fertilizer, such as chicken manure, and other mineral amendments if necessary. Water well. (If you are mulching an established tree, be sure to amend out to the edge of the crown of the tree, also called, "the drip line.")

3. Spread a permeable weed barrier around the tree in a ring shape, leaving a gap of 15 cm (6 inches) diameter around the trunk of the tree for air circulation. Make certain there are no gaps in the barrier through which weeds can emerge. If you are using loose materials such as paper that might blow away, water the weed barrier layer now.

4. Spread compost and/or mulch about 15 cm (6 inches) thick over the weed barrier, again making sure it is several centimeters away from the stem of the plant for good air circulation.

5. Leave a generous gap in the mulch around plants to allow for air circulation. Otherwise, the mulch can lead to rot on the plant stem.

Cautions/considerations

In most cases (with exception of the last point below), the benefits of sheet mulching outweigh the costs. However, be sure to watch out for these potential problems and be prepared to handle them if they arise.

* Slugs and snails are particularly fond of mulch. Especially during the dry season, slugs will be attracted to mulch. This can be a threat to certain small tree seedlings and many garden plants.

* Poultry such as chickens, turkeys, Guinea fowl and more love to scratch in mulch. This may be a problem in certain situations, although usually not much more of a problem than without mulch.

* Rodents can find a cozy home in mulch. Certain rodents such as rats and voles can readily debark certain trees.

* Pigs love good, moist soils which harbor worms, and will grub out sheet mulch if they have access to it. Do not use sheet mulch if pigs have access to the area; they will be attracted to it and will destroy both your work and your plantings.

The ongoing process

Once a mulch is established, the soil is covered and weed-free. With time, the mulch materials will decay and begin disappearing into the soil from the bottom up. The rate decay will be fast during the hot and wet season, and slow during dry or cool periods. As in a natural forest, the mulch must be replaced or soon the mulch will disappear and the soil will once again see the sun and the weeds will take over. As long as organic materials are added to the surface of the mulch, there is no need to lay down the weed barrier layer after the first time. From time to time mineral amendments of phosphorous and calcium can be added to the mulch. However, for the most part maintaining sheet mulch requires only replenishing the top layer (Layer #4) with materials such as leaves, trimmings, husks, etc.

It is best to design a garden or orchard mulch system to produce its own top layer mulch in sufficient amounts. To make mulching as efficient and easy as possible, use mulch materials which are readily available. With good planning, mulching of gardens and orchards can become a regular part of maintenance—just mulch with handy materials such as grass clippings, plant prunings (roughly chopped), animal bedding, etc. Eventually, other tasks such as watering, fertilization and weeding will be reduced. The overall maintenance burden in mulched conditions, when efficiently using materials on hand, can be less than in conventional systems (which may use frequent tillage or applications of herbicide).

There are many ways to produce sufficient mulch at your site. Grass clippings, for example, represent nutrient rich mulch material. Deep rooted, vigorous growing plants that readily regrow after hard pruning can be cut 3-6 times per year to provide mulch. For example, several nitrogen fixing trees will produce copious amounts of green matter. Each should be evaluated for its appropriateness for a specific site before planting. Many other plants produce large amounts of organic matter including various trees and bunch grasses (e.g. vetiver or lemon grass). Also, many water plants such as water hyacinth and kelp are good mulch materials. Because fast growing, vigorous plants are desirable for mulch production, when selecting plants for this purpose extreme caution should be taken to avoid introducing weedy species.

Sheet mulching as described here seeks to recreate the organic mulch layer of the forest with a minimum of effort from people. Properly planned, a garden or orchard system will produce its own raw mulch in sufficient amounts and people are involved only in putting this material back onto the ground where it belongs.

References

Curry, M. 1996. Sheet Mulch Now! In: The Permaculture Activist, issue No. 34-A, August 1996.

Hemenway, T. 2001. Gaia's Garden: a guide to home-scale permaculture. Chelsea Green Publishing Company, White River Junction, Vermont, USA.

Mollison, B. 1990. Permaculture: A Practical Guide for a Sustainable Future, Island Press, Washington, DC.

Stout, R. 1998. Gardening without Work: For the Aging, the Busy, and the Indolent. Lyons Press.

Original source

Elevitch, C.R. and K.M. Wilkinson. 1998. Sheet Mulching: Greater Plant and Soil Health for Less Work. Permanent Agriculture Resources, Holualoa, HI.

About the Authors

Craig R. Elevitch is an Agroforestry Specialist with both public and private sector experience in tropical agroforest and forest management. He has a Master's degree in Electrical Engineering (Dynamical Systems) from Cornell University. He has conducted numerous agroforestry research projects and species trials in cooperation with organizations including the USDA Sustainable Agriculture Research and Extension, Cornell University, and the University of Hawaii. Craig is founding editor of The Overstory and co-author of Agroforestry Guides for Pacific Islands. Contact him at: P.O. Box 428, Holualoa, Hawaii 96725 USA; E-mail: craig@agroforestry.net.

Kim M. Wilkinson is a reforestation professional and a freelance writer. She has B.A. degrees in Anthropology and Ecology from Emory University, and is a Certified Permaculture Consultant and Master Gardener. She lives on and manages an organic agroforestry project on the Island of Hawaii. Through her nursery, Future Forests, she grows many thousands of tree seedlings annually for people planting agroforests and reforestation projects. She is co-author of Agroforestry Guides for Pacific Islands, as well as editor for The Overstory. Contact her at: P.O. Box 575, Holualoa, Hawaii 96725 USA; E-mail: kim@agroforestry.net.