Composting Guide (Continued)

What Factors Affect the Composting Process?

Composting speeds the natural decomposition process.  There are many factors that can affect the acceleration of the composting process.  Those include the following:

  • The concentration of carbon and nitrogen in the organic material
  • The volume of the material being composted
  • Aeration of the pile
  • Moisture content within the pile
  • Surface area of the pile and particle size
  • The temperature of the pile

These factors will each be discussed in further detail.

Organic Material Being Composted

The balance between the amount of carbon and nitrogen in the organic material is one of the most important factors in composting.  All organic material contains both carbon and nitrogen, but the ratio differs from one material to another.  Microorganisms used in the composting process need carbon to give them fuel and nitrogen to make protein.

Some organic material like leaves, cornstalks, straw, bark, paper, sawdust, and wood chips are high in carbon.  Think of these carbon-rich organic materials as “browns”.  Other material like manure, hay, vegetable scraps, and grass clippings are high in nitrogen.  Think of these nitrogen-rich organic maters as “greens”.

The ideal carbon to nitrogen ration is 30:1 because it is at that ratio where microbial activity is at its greatest.  If the C:N ratio is too low in the compost pile, the microorganisms may lose nitrogen into the air as ammonia.  If the C:N ratio is too high in the compost pile, the process will be very slow and inefficient.  Several materials with differing C:N ratios can be blended to achieve an ideal C:N ratio.

Leaves, straw, and trimmings from nonwoody plants are appropriate for composting.  Leaves are the most common material in most backyard composting piles.  Any branches, twigs, and tree limbs greater than 1/4th inch in diameter should first be run through a woodchipper before being added to a compost heap.  Vegetable garden and flower bed trimmings are great sources of nitrogen for the compost pile.  Kitchen scraps like coffee grinds, eggshells, fruit, and vegetable peelings, and more can be composted. Blood, bone meal, and manure all make great additions to a compost help.  Grass clippings can be composted, but care should be taken when doing so because grass often has been treated with pesticides and herbicides.   If you do decide to compost grass clippings, you should allow at least a year of composting time to make sure the residue of such chemicals is not a problem with the compost gets used.

Materials That Should Not Be Composted

You should not use human or pet feces in compost piles as they can transmit diseases.  Avoid using wastes like meat, bones, eggs, peanut butter, cheese, etc. in composting piles as they attract rodents.  It is possible to compost weeds, diseased plants, and other pests, but only when mixed into the center of a “fast” compost pile where temperatures reach 120° to 140° F.

Some materials like corn cobs, corn stalks, corn husks, citrus rinds, nut shells, and palm fronds decompose very slowly and should therefore be shredded prior to adding to a compost pile.  This will lessen the time required to decompose them.

You should avoid adding charcoal, coal ashes, and automotive petroleum products to compost heaps.  Charcoal typically does not decay in compost bins.  Coal ashes contain high levels of sulfur and iron which are toxic to plants.  And all automotive petroleum products should be taken to a recycle center.

Compost Pile Additives

There are three main categories of additives used with compost heaps:

  • Inoculants
  • Activators
  • Lime

Inoculants consist of dormant microorganisms.  These are rarely used since leaves, kitchen scraps, soil, and other compost materials already contain enough bacteria for typical composting. Activators contain sugar or a source of nitrogen like ammonium sulfate.  Typically, activators are only used if you need additional nitrogen to accelerate the decomposition if the pile does not contain other low carbon (high nitrogen) materials such as manure.  Lime is added to raise the pH of the pile making it less acidic/more alkaline.

Nitrogen is needed by microbes to grow, so a shortage in nitrogen can slow down the composting process.  Nitrogen fertilizer is often added to composting heaps containing lots of “brown” materials already very low in nitrogen like sawdust or straw which would otherwise decompose very slowly.  When mixed correctly with leaves, grass clippings can provide nitrogen to speed up composting.  You can use blood meal, poultry litter, or manure as organic sources of nitrogen.  Otherwise, fertilizer with 10-30% nitrogen can be used.  Most nitrogen sources are first mixed with water and then sprinkled on the compost pile as it is being built.

If you are composting yard trimmings, you should generally not need to add any lime.  The early stages of decomposition yield organic acids which lower the pH of the pile.  Composting microorganisms and invertebrates perform best when the pH is between 4.2 and 7.2.  Adding lime to composts can be detrimental since it converts ammonium nitrogen to ammonia gas causing much needed nitrogen to be leaked into the atmosphere.  The addition of lime is typically only required if you are composting large quantities of vegetable or fruit waste, sawdust, pine needles, or pine bark.

Volume of Composting Material

Compost piles should not be very large.  The objectives are for the pile to be small enough to hold in the heat generated by the decomposers yet large enough get air into the center of the pile.  Typically, composting piles are at least 1 cubic yard in size (3’ x 3’ x 3’).  They can be as large as 5’ x 5’ x any length.

Aeration of the Composting Pile

As mentioned at the start of the chapter, microorganisms involved in aerobic composting, which is what we have been covering thus far, require oxygen to efficiently decompose organic waste.  While decomposition without oxygen can be achieved via anaerobic composting, such decomposition processes are very slow, emit foul odors, and often create chemical compounds that are toxic to plants.  Because of the foul odors emitted by anaerobic composting, aerobic composting is recommended for residential settings.

Oxygen can be introduced to the pile in several ways.  Simply mixing or turning the pile a couple times per month will typically provide the necessary oxygen to the decomposers thus speeding up the decomposition process.  Raising the pile off the ground will allow airflow from both the bottom and top of the pile, speeding decomposition.  Another alternative is to place PVC pipes through the compost bin with predrilled holes in the PVC.  Fresh air can flow through the ends of the PVC pipes, through the holes in the pipe, and into the center of the pile.

Moisture Content of the Compost Heap

Compost piles require moisture for microbial activity crucial to the decomposition to occur efficiently.  Those living in areas with little rainfall will be required to water the compost heap periodically to maintain an efficient decomposition rate.  Enough water should be added to moisten the pile, but you should avoid overwatering as doing so can replace the air in the pile with water, suffocating the microorganisms.  This will result in anaerobic conditions which will slow down the decomposition process and product foul odors.  Pile that are too wet should be turned frequently to increase air.  Piles that are too dry should be turned and sprinkled with water.

Particle Size of Composting Materials

Small particles have more surface area than do large particles.  The more surface area microorganisms have available to work on, the faster the decomposition process.  For this reason, grinding composting material into smaller particles will accelerate decomposition.  A shredder or woodchipper can be used to do reduce particles sizes of larger materials.  Leaves can be reduced in size simply by mulching them with a lawnmower before raking them.  If the mower has a bag attachment, you can also collect the mulched leaves as you mow them. 

Temperature of a Compost Pile

Heat is generated during the decomposition process because of all the physical and chemical reactions taking place within the pile.  The temperature of the composting heap is important to the biological activity.  Not enough heat can result is slow decomposition.  Too much heat can kill microbes and invertebrates crucial to the process.

There are two main types of microbes that contribute to decomposition:

  • Mesophilic organisms that live and function most efficiently at 70° to 100° F, and
  • Thermophilic organisms that live and function most efficiently at 113° to 155°

While higher temperatures kill disease organisms and weed seeds, moderate temperatures promote the growth and activity of the most efficient decomposers, mesophilic bacteria.

Pile temperatures between 90° and 140° F are ideal for rapid decomposition.  However, if temperatures exceed 140° F, many of the microbes and invertebrates will die or become less active.   When this happens, the pile should be turned to increase airflow and cool it down. 

If a pile never reaches 120° F, you may be required to add nitrogen and/or water. Cold weather can also prevent the pile from heating.

Should your pile develop a strong odor of ammonia, it contains too much nitrogen.  This can typically be corrected by adding more “brown” ingredients which are high in carbon.