Municipal Leaf Composting

In today's cities, autumn leaves are a major concern and their collection is a big budget item. Many cities that have suitable space and means, have adopted a program to compost autumn leaves to create leaf mold compost that is an excellent amendment to soil as well as being a product that can be sold to local nurseries, landscape companies, and home gardeners.

A successful municipal composting operation requires careful attention and planning to the site selection, the collection system, the management of the materials, utilization of the product, and community involvement and support. 

Healthy compost provides all the organisms and nutrients required and are in the proper forms for a tree to take up.   Healthy compost holds nutrients in non-leachable forms so they remain in soil until the plant requires the nutrients.

Composts that are stable and possess significant amounts of humic acids have the ability to bind nutrients and heavy metals as moisture passes through the product layer.  The organic nature of compost also improves the cation exchange capacity of the soil, increasing sites in which nutrients can be bound as well as providing a home for microorganisms to proliferate.  Organisms found within compost have the ability to degrade organic contaminates such as hydrocarbons.  Furthermore, as humic materials accumulate in the soil, the productivity of plants increase.

Large Scale Composting
The municipal composting site is frequently a political decision and one that often generates neighborhood opposition.  This opposition comes from a lack of understanding about how the composting process works.  One often used argument deals with leachate from the compost.  However, leaves, grass clippings, and wood chips or wood debris do not create leachate problems.  Animal manure wastes, while providing nutrients to the compost, could cause leachate problems.  Another uninformed complaint deals with odor.  However, since a properly managed leaf composting operation generates only minor odor problems, there is actually little reason for complaint. 

The composting site can be any former landfill, unused park, or vacant land.  The site must have a total acreage that will equal approximately one acre of compost site per square mile of municipality, or 1 acre per 30 miles of streets, whichever is less.  It is more efficient to operate the entire operation at one site, but in larger cities this is not always feasible.  If residents are expected to deliver leaves to the site themselves, the site needs to be accessible to most everyone in the municipality.  If the municipality operates a citywide clean up, then access is less important.  The site should be gently sloping, well drained, and hard.  Surface drainage should not go directly into brooks or catch basins.  The site should be on a constant slope and open so there is room to maneuver equipment as well as store the leaves. 

Newly arriving leaves should be piled in windrows that are as long or as wide as the available site and as high as the municipal equipment (loaders) can reach for maintaining the piles.  The pile should be as wide as it is high and should run up and down the slope so that the pile does not trap rainwater.  The windrows are designed to let air and moisture enter the pile.  One tip many communities take advantage of is to build twin windrows when the leaves are being delivered to the site.  At the first turning of the piles, they are combined into one. Since the most rapid amount of leaf size reduction occurs in the first month, the twin piles can easily be managed as one large pile after that first month.  As a general rule of thumb, 1000 cubic yards of leaves on the street will eventually become about 200 cubic yards of composted leaf mold.

Size reduction can also be accomplished by mechanically shredding the leaves prior to composting.  Shredding will reduce the amount of time necessary to complete the composting process by almost 50%.  Uncomposted shredded leaves can also be used as mulch spread in gardens or around a tree.  However, since the leaves decompose very quickly, they have to be replaced annually.  Large pieces of shredded leaves can also be blown away if the landscaped area is windy.

Ingredients and Procedures:
The following provide all the necessary steps required to insure successful composting.

• Air – Ideally, perforated pipes should be run through the compost pile.  A more practical solution is to loosely stack the leaves in long, narrow rows using a front-end loader.  Oxygen is essential for preventing anaerobic decomposition while promoting aerobic composting and a minor, but not unpleasant odor.

• Turning – Turning the pile mixes the materials, re-aerates the compost, and provides a check on the progress of composting.  The frequency of turning speeds the composting.  The minimum interval is four days; the more practical is one month.  The more you turn the pile, the more the compost tends to become bacterial because any kind of disturbance destroys fungi by breaking up their mycelia. 

• Nutrients – Nitrogen is very important for feeding the composting bacteria.  The best source would be to add manure to the compost during the turning procedure.  If manure is unavailable or not allowed, the next best sources of nitrogen are weeds, grass clippings, aquatic weeds and commercial nitrogen fertilizer.  Except for the fertilizer, all these products will also supply the heat required for composting.

• Bacteria – Commercial bacterial compost starters are available; however, the occasional mixing of compost with previously composted soil should provide sufficient quantities of bacteria seed.  If the same site is used year after year, the bacteria can be obtained by scrapping up the top inch or two of soil when turning the pile over for the first and second time.  After the composting action has begun, additional bacteria do not need to be added.

• Heat – The optimum temperature is 140°F (60°C).  This is no trouble to obtain in summer but composting over winter requires special insulation with hay or uncomposted leaves, which will also protect the pile from winter rains.  When temperature has cooled to 100°F (38°C), the compost action is finished.  Weeds, green vegetation, and manure speed up the heating and composting action.  The top 2 feet of surface area are unlikely to decompose as readily as the interior of the pile.  Therefore, when turning the pile, exteriors should be moved to the center of the new pile and the centers moved out to cure at the edge.

• Moisture – Rainfall is generally sufficient, but a sprinkler may be necessary to supplement natural rainfall and to insure the moisture content remains at a wet but not dripping condition.

Turning the pile on a rainy day allows moisture to be mixed throughout the entire pile as the compost is being turned.  It also provides an opportunity to utilize equipment that would not normally be working on outdoor projects in the rain.  If the piles are near residential areas, rainy day turning takes advantage of the fact that residents' windows are likely to be closed, minimizing the chance for odor release and reduce the chance of complaints from neighbors. 

As the composting action proceeds, the pH value fluctuates from acidic in the beginning to neutral at completion regardless of the product being composted.  When the compost cycle is completed, the row may be screened to remove any uncomposted leaves, sticks, etc.  The material is then piled to cure.  Curing allows the compost to stabilize so nutrients are released when added to the soil instead of consumed by bacteria and continued decomposition.

Small Scale Composting
There are many small scale programs and methods of composting available for homeowners and small companies.  For example, a 14-day compost is good for small amounts of residential yard waste material.  This is accomplished with a rotating barrel that contains fresh green grasses or leaf clippings and is rotated every two or three days.  By contrast, static composting, which is simply making a pile of leaves and letting them decompose without any management, requires 2 to 3 years and anaerobic decomposition that produces odor and alcohol problems.  There are many other methods in between these two extremes.

Residential Composting
Many communities actively encourage residents to develop backyard composting areas to eliminate leaves and grass from ever getting into the municipal waste stream.  The making of composted leaf mold is actually a simple process, but a thorough knowledge of the process is required in order to insure success.

Many residents have compost piles in their yards.  Compost piles can be filled with any biodegradable, non-animal waste product.  Municipalities encourage this effort because it saves the cost of removal while enhancing the local environment, home garden, and soil quality. 

Compost bins are best made with wood slats or wire sides and in three sections so the compost can be moved from one bin to the next as the compost decomposes.  Wood posts and beam designs are also quite popular as are plastic containers that are designed to be compositors.  This concept is very popular with small landscape companies who have a small tractor and a front end loader to move the compost piles from one wood section to the next, right down a line to a pile of cured compost at the end.

Another popular residential approach is the trench method.  Begin by digging a long open trench in the garden.  The compost products are placed in the trench and then covered with soil from a second trench right beside the first.  Dig the trenches a shovel blade deep.  If trimmings from trees and shrubs are included, go deeper so the branches and twigs are completely buried.  In the second year, consider going at right angles from the last trenching in the garden. 

In an average suburban garden, during the course of the summer enough compost material will be generated to cover an area that is 8 feet wide and 25 feet long.  In the fall, repeat the process again with leaves.  This process takes a lot of effort but it works.  In a 15-year period, up to 1 foot of rich, black, well-drained loam can be produced.

Compost develops good soil structure by binding pieces of soil together and by building airways and passageways through the soil.  Good movement of air and water are vital to the health of plants and the soil/food web itself.  While it seems contradictory, good soil structure allows water to drain from excessively wet soil and helps soil to hold water when soils start to dry out.

Compost Mulch
Another method which many experts feel is the absolute best way to deal with waste and compostable products is to use it as a mulch.  Lush meadows and healthy forests require soil started from mulch, also known as that fresh layer of organic matter (dead grass and leaves) on the surface.  Under that fresh layer of mulch is just-started compost (decaying mulch), followed by finished compost a few inches below the surface.  Below all the compost comes the healthy parent soil with compost mixed throughout.  This soil is ideal for healthy trees.  In one gram of healthy soil there are as many as 3 billion organisms plus a whole range of interactions between all these organisms.

A host of organisms, including bacteria, worms, and fungi, break down the organic matter in mulch and compost and begin to change it into a form that will enhance tree growth by the humidification process and produce rich soil.  Research shows that this process utilizes nitrogen from air above, not the soil below.

Organic Matter Content
High-quality organic soil amendments usually contain at least 20% – 40% organic matter.  Concentrations as low as 25% are often adequate for mulches.  Soil typically makes up the remainder of the compost dry matter.  A moderate amount of inorganic content is desirable as foundation material for compost blankets, filter berms and similar installations.

Particle Size Distribution
Particle size is determined by passing the compost through a set of sieves and determining respective weight fractions retained on each sieve size.  Different distributions serve different purposes.  For example, at least 90% of a compost to be used as a turf or landscape soil amendment should pass through a 5/8-inch screen.  Conversely, composts with larger particles serve as excellent mulches.

Compost Types
Composts can be dominated by either bacteria or by fungi.  Bacteria dominated compost is best applied to herbaceous plants.  For the bacteria to dominate, it should be made from green materials such as 25% high nitrogen ingredients, 45% green plants, and 30% woody material.  High nitrogen materials include manure, grass clippings, and legumes such as alfalfa, peas, clover, and bean plant residues.  Green material includes any green plant debris, kitchen scraps, and coffee grounds, which all contain sugars and proteins that bacteria love.  Woody material includes wood chips, sawdust, and paper products.  The more frequently you turn the pile, the more the compost tends to become bacterial.

Fungi dominated compost is good for mulching and for woody plant growth.  Fungal compost consists of approximately 25% animal manure, 50% green plant material, and 25% shredded wood plant material.  Any kind of disturbance to the compost pile destroys fungi by breaking up the mycelia.  Fungal compost is especially useful for suppressing disease and introducing fungi for root development immediately after tree planting.  Fungal composts can not be turned so it is best they be applied to the surface as a mulch after one turning.

Benefits of Compost
Compost improves low-quality soil by adding: stable organic matter, beneficial soil microbes that out-compete soil diseases for nutrients, water-holding capacity, and nutrient-holding capacity.

Compost serves as:

• a soil amendment,

• a turf top-dressing,

• a mulch,

• an erosion-control agent,

• a water-quality enhancer

Compost improves poor soil by:

• promoting plant establishment,

• improving moisture retention,

• costing less than adding topsoil,

• improving structure, infiltration, and drainage capacity.

Uses of composted leaf mold are many:

• It can be mixed as 1/3 compost, 1/3 sand, and 1/3 soil to create topsoil.

• It can be substituted for peat anytime peat is required in construction or for greenhouse potting soil.

• It makes an excellent top dressing for turf areas and mulch in a garden, eliminating the need for fertilizer.

• It is an excellent cover for construction restoration.

• Many communities with successful composting operations no longer purchase topsoil.  Other communities sell the material to topsoil contractors, greenhouses, nurseries, garden shops, and residents; or they use the material as incentives for sales of other surplus products such as woodchips and lumber from forestry operations.

• Some communities have a Give-Away Day while other communities sell it for $25 – $45 per yard, screened and delivered.

Compost may be added to existing soil for optimizing tree growth.  In certain situations, such as growing trees in a large container, there is a need to amend the planting pit soil. When compost is applied as a soil amendment, it should be evenly and deeply mixed into the soil. The final amended soil should contain at least 20% to as much as 40% leaf mold compost.  Compost amendment is not recommended for sites where trees will be planted in the sidewalk, boulevard, or tree lawn because tree roots often fail to venture out into poorer quality urban soil resulting in root-bound conditions and a shorten tree life span.

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