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Interbay Mulch

11 years ago

There has been much posted on the forums about a soil building method known as Interbay Mulch. It is a method developed by a site coordinator at the Interbay P-Patch in Seattle and has recently been featured in an article in Organic Gardening. The P-Patches are a large network of very well run Community Gardens. It is catching on quickly in other P-Patches but the folks at Interbay used the system for a full season before it started being adopted elsewhere.

The Interbay Mulch is the use of a layer of burlap placed over the top of various organic material that you pile up on top of soil. Organic matter decomposes much faster on top of the soil than it does if tilled in as long as it is covered and kept moist and dark. The byproducts of this process enrich and feed the soil under it in some very interesting ways.

A few years ago a four day course on soil ecology was held at Oregon State University. This course was attended by one of the Interbay site coordinators and the idea of Interbay Mulch occured to him while driving back to Seattle.

The OSU microarthropod expert, Dr. Andy Moldenke, said in his presentation that 90% of all soil organisms live in the top inch of the soil. Dr. Elaine Ingham of SoilFoodweb then said in her presentation that plant health and productivity depends on that biological activity in the soil. These two bits of information inspired the concept of using a burlap "cover" to create a deep,dark,and damp "litter layer".

The Interbay Mulch attempts to expand the conditions in that biologically active one inch litter layer to 6, 12, or even 18 inches. The idea is to recreate and magnify the natural conditions that produce rich, fertile soil. The use of damp burlap (coffee sacks are readily available in the Seattle area but purchased burlap can be substituted) proved to be the ideal material for enhancing the biological activity associated with the humus production.

Depending on your soil needs, and the amount of organic material you have access to, you would build a mulch 6" to 18" deep.

Mix the materials well and wet them down. Cover over this with the burlap. The burlap should also be damp. You could soak them in a large barrel (they will be HEAVY) or water over the "hump" with a sprinkler until everything is nice and damp. Monitor the pile for moisture content and water if any materials are dry. Decomposition comes to a halt when materials dry out. The mulch will probably be reduced to 30% it's original height when it is finished and will have turned into a rich, dark and most importantly biologicaly active humus.

The breakdown/decomposition time will vary depending on your zone and the time of year you build your mulch. Expect 4-6 months on average before you can move the burlap aside and plant.

This has been compared to a long-known method of soil building known as "sheet composting" and, more recently, "Lasagna" gardening. While there are similarities in these methods and some common benefits, the Interbay approach differs in that it is proven to be a faster and more effective means of producing a large volume of rich humus as well as a rich diversity of bacteria, fungi, protozoa, beneficial nematodes, microarthropods, and worms.

The Interbay method actively seeks to create conditions that culture the maximum populations of beneficial soil microorganisms thereby achieving the maximum benefit from the inputs.

Interbay Mulched soil, according to the lab that tested it, is "uniquely active".

Living soil is successfull soil! And best of all, it's free!

What is the purpose of the burlap?

Here is a list of benefits attributed to the burlap cover:

  1. Maintains the optimum dark, moist conditions prefered by most decomposer organisms.
  2. Encourages colonization by, and reproduction of, soil organisms as well as the microorganisms associated with compost and those introduced with new materials. Using the burlapped system, the biodiversity and abundance of life in finished humus is extremely high according to the analyses done.
  3. Protects from driving rain and minor freezes. Burlap, if dried out, will absorb a lot of rain before letting it gently pass into the mulch.
  4. Burlap covers the mulch but, also acts as host to a multitude of fungi and other organisms, performs an important habitat role.
  5. Protects worms, etc from birds. Birds consume a huge quantity of worms. In an Interbay Mulch worms reproduce and populate without predation.
  6. Burlap covers the mulch but is permeable enough for required oxygen to reach all parts of the system.

The materials used to create an effective pile, or "Humps" as they are called, can be gleaned from a number of sources. Almost any organic material can be utilized and it has been shown that the wider the variety of inputs you can use the richer the resulting humus will be. People who have done this have been very creative in finding sources of suitable materials.

A few of these include:

  • Yard waste and grass clippings
  • Shreaded newspapers
  • Spent grain from breweries
  • Coffee grounds from Starbucks
  • Vegatable waste/trimmings from greengrocers
  • Used floral arangements
  • Pulp left over from Juice Bars
  • Shredded fall leaves
  • Composted manure

The ingredients you use to build the "Humps" can be fine tuned to produce specific conditions and soil chemistry depending on what type of crop you wish to grow in each bed. i.e. using some pine needles to prepare a bed for more acid loving plants.

To make the whole idea fun and interesting the latest version of the recipe was modelled after a local juice bar menu.


Enough burlap bags to cover the area you are developing. (2 layers work better than one).

  • Basic Recipe (Just a guideline. Mulchers are encouraged to be creative):
    50% leaves 50% fresh garden debris (put any seed heads in compost bin)

  • Supercharged:
    Add one wheelbarrow of young or unfinished compost per 100 sqare feet of area.

  • Supercharged plus:
    Add one bucket of leaf mold per 100 sq. ft of area.

  • Nitrogen Boost:
    Add fresh grass clippings, carrot pulp, espresso grounds, wine pressings, seaweed, spent grain, composted manure.

  • Mineral Boost:
    Add fresh seaweed, eelgrass, glacial till, granite dust.

  • Calcium Boost:
    Add egg shells, ground oyster shell.

  • Carbon Boost:
    More leaves, rotted sawdust, rotted straw, dryer lint, rotted burlap

  • Energy Boost:
    Add "green" materials (low C:N) such as kitchen waste over the winter to increase worm density.

It has been noted that, over time, Interbay Mulched soil can generate up to 750 lbs. of available nitrogen per acre per quarter from the nitrogen cycling activity of the micro and macroorganisms living in it.


One user points out that in some areas this process may attract scroungers. Skunks, raccoons and such looking to make a meal of the worms and other critters working to break things down under your "Humps'. This can be annoying as well as messy. You could place a layer of chicken wire over the burlap and pin it to the ground but critters can be determined if they sense an easy meal is to be had.

If you need to build humus and tilth, and aren't plagued with four legged scroungers, it has much to offer as it utilizes materials that might otherwise wind up in the waste stream and ultimately landfills.