Backyard Composting – The Basics for a Hot Pile

It's a Natural Process

In nature, there is a cycle of life, death, decay, and growth. Plants draw the nutrients they need for life from the soil and air around them. As the plant or some of its parts die, they fall to the ground where they decay, or decompose. Decomposition releases the nutrients back into the soil where they will be available for other plants to use.

We can manipulate the conditions under which this natural process occurs in order to speed up the production of available nutrients by adding humus through compost. Composting is simply the management of the natural decomposition process to make it more efficient.

Since the goal of composting is the decomposition of organic matter, and since all organic matter eventually decomposes, you are virtually guaranteed success! There may be times when compost pile management practices need to be adjusted, but this should not be taken as failure. So enjoy yourself, because it is impossible to fail at composting! The goal is to make the best compost you can out of your available feedstocks, without creating any nuisance conditions.

Regulating Factors

Most of the work of decomposition, in or out of a compost pile, is done by microorganisms (bacteria, actinomycetes, protozoa and fungi). The composter's goal is to create favorable conditions for these microorganisms to grow and do their work. We especially want to encourage the aerobic microorganisms, the ones that need air to survive and grow.

Anaerobic microorganisms (living in the absence of air) will decompose organic matter, but some of their by-products are very undesirable. When anaerobic microbes are at work, they release methane gas that is explosive at certain concentrations and a potent greenhouse gas, nitrogen containing compounds such as ammonia which smell bad, and some sulfur containing compounds that smell like rotten eggs. These anaerobic microorganisms will make your compost pile smell very bad, so encourage aerobic ones instead. The Trouble-Shooting Guide at the end of this section will give you tips to avoid this problem.

The aerobic microorganisms also have by-products, but they are not unpleasant. The bacteria in particular will give off body heat. The bacteria are very small, but there are lots of them (billions in a handful of active compost), so heat will build up in the pile to temperatures above 120 degrees F. At these high temperatures, the thermophilic (heat loving) bacteria will take over and go to work. These are the bacteria we like best, because they work the fastest, and produce enough heat to destroy most weed seeds and pathogens! That is why we want our compost piles to get hot. Microorganisms generate that heat.

Air

Our first regulating factor is air. When building our compost pile, we should use enough bulky material to allow air circulation. Bulky materials in a compost pile are the dry, coarsely chopped ingredients. Items such as stems, ground or shredded wood, pine cones, and large dried leaves can be excellent bulking materials. By letting air circulate through the pile, we encourage the growth of aerobic microorganisms (the good guys), including actinomycetes, which produce the "earthy" aroma that gardeners like so much. Turning the pile over to fluff it up makes it easier for air to circulate through the material too.

Water

Composting microorganisms also need a moist environment. This is our second regulating factor — water. A thin coating of water over each small particle of material in the pile will keep the microorganisms from drying out. The little critters need moisture, but not too much, because the good ones also need to breathe oxygen. Don’t drown them in too much water! Without air, the anaerobic microbes move in and the pile gets pungent. A good rule of thumb is to keep the pile as damp as a wrung out sponge.

An easy way to tell if you have added enough water to a compost pile is to pick up some of the material in your hand and squeeze it. If water runs out, there is too much and you should add some more bulking material. If the material falls apart when you open your hand, you should add a little more water. If the material sticks together for a few seconds, there is probably just enough water.

Carbon and Nitrogen

Easy access to air and water is very important to the microbes doing the work of decomposition, but they also need "food". Like us, they consume carbon to burn for energy. Dried leaves, straw, peanut hulls, paper, wood chips, bark, and other "brown stuff" are excellent sources of carbon.

Our microscopic work horses also need nitrogen to make protein for growth and reproduction. Fresh grass clippings, vegetable scraps from the kitchen, garden plants, fresh leaves, and other "green stuff" are very good sources of nitrogen. There are many sources of nitrogen that are not green, such as carrot peels, apple cores, orange rinds, manure, and coffee grounds. There is one thing that almost all good nitrogen sources have in common and that is that they are all fresh.

The Basic Compost Recipe:

Air + Water + Brown Stuff (carbon) + Green Stuff (nitrogen) = Compost

Now that we know the ingredients in the compost recipe, we need to determine the proper amounts of each. As discussed above, bulky material is needed to create spaces for air. This is usually the "brown stuff". We also know that water is important, but too much water can cause problems (remember the "damp as a wrung out sponge" test). Now, what is the best mixture of carbon and nitrogen for our microbial composters?

The ideal ratio for the carbon to nitrogen mix is 30:1 - thirty parts (by weight) of carbon for each part of nitrogen. All organic materials contain both carbon and nitrogen, but not in the same ratio. Almost everything has more carbon than nitrogen in it, even the “green stuff”. Grass clipping have a carbon to nitrogen ratio of about 19:1, or less than the ideal amount of carbon. The average ratio of carbon to nitrogen in dried leaves is 60:1, twice as much carbon as we're shooting for. So, in order to achieve the ideal carbon to nitrogen ratio (C:N), we usually need to mix different materials, or feedstocks. Below is a chart of various materials and their C:N ratios.

Nitrogen
Food Scraps 15:1
Grass Clippings 19:1
Coffee Grounds 20:1
Horse Manure 25:1
Carbon Sources
Dried Leaves 60:1
Pine Needles 90:1
Newspaper 125:1
Sawdust 625:1
 

It is possible to achieve a 30:1 ratio by mathematically calculating the exact amounts of the various feed stocks using their C:N ratio averages. But, since we will mostly be dealing with material commonly found in our kitchens and backyards, there is an easier way to estimate the proper mix.

To achieve a C:N ratio close to 30:1, mix dry leaves and green grass clipping ½ and ½ by weight or about 3 volumes of leaves to one volume of grass.

If you've ever bagged your yard trimmings, you've probably noticed that grass clippings are a lot heavier than dried leaves, maybe two to four times heavier. This half greens and half browns by weight is a pretty good rule of thumb for most compostable materials in and around your house. Do a little experimenting to find out what works best for you.

Compost Pile Volume

The volume of your pile does have an effect upon the composting process. A pile that is too large becomes difficult to handle and has a greater chance of producing pockets of anaerobic activity deep inside it. On the other hand, a pile that is too small will not support enough microbes to generate the heat needed for rapid decomposition and to kill weed seeds. Let's say our compost pile is contained in a square compost bin. The outside four to six inches act as insulation, allowing the middle of the pile to heat up to the desired temperatures for the thermophilic (heat loving) bacteria to take over. As the supplies of air, water, carbon and nitrogen become depleted, the process slows down until the compost pile cools to the ambient temperature.

The ideal size of a compost pile is around one cubic yard or 27 cubic feet (3 ft X 3 ft X 3 ft, for example). Anything less than one cubic yard is too small to house a large enough population of bacteria to produce the desired heat. A pile over two cubic yards not only becomes hard to turn and handle, but can also generate anaerobic conditions if it's not well-aerated.

Compost Bins

Compost bins are for the benefit of the human; the microbes will do their thing with or without one. A bin can help, though, to contain the materials and maintain the optimum volume. It can also discourage unwanted guests such as varmints and neighborhood pets. A lid may be placed on top to further insure that the composting materials will not be disturbed.

Compost bins can be made of just about any material that is not harmful to living things. There are commercially produced plastic, wire, metal, and wooden bins. Some people make their own using concrete blocks, wire, bricks, or wood. Wood pallets are a good size to build a bin out of if you can get some. A key element in construction is to always make sure there are plenty of openings for aeration. Also, the bin should not have a bottom.

Building the Pile

After gathering all of the materials together, you are ready to start building the compost pile. To make a hot pile, 8-10 standard yard waste trash bags of material will be required in the proper brown to green ratio. For example, use 3-4 bags of dried brown material to each bag of fresh vegetative material like green grass or vegetable scraps. If using vegetable scraps, you might need to increase the brown material in the ratio to keep the mix from being too wet..

Start with an 8 — 10 inch layer of brown material in the bottom of the bin. Stir in water to make sure it is dampened. Then throw in the green stuff and mix it together. Continue to mix in greens and browns in this manner until the bin is full. Add water as needed.

After a few days, the volume will shrink, allowing you to add another layer. When the pile no longer shrinks at the same rate, it's time to completely turn the material over, adding whatever ingredient might be missing.

Maintenance

The three most commonly asked questions about maintenance of a compost bin are:

  1. Is it okay to add more material?
  2. How often do I turn the pile?
  3. Do I water it?

More material can be added to the pile at any time. If you are adding fruit or vegetable kitchen scraps, they should be buried deep in the pile (at least ten inches) so you don’t attract insects or animals. If you are adding more grass clippings and leaves, add them to the bin as if you were building another layer.

Some people follow a regular schedule of turning the compost pile every few days, some every few months, and some never turn the pile! It's all good; however, you can tell when the pile is ready to be turned by taking its temperature , either by digging into the pile and feeling the heat with your hand, or using a composting thermometer made for this purpose. Composting occurs most efficiently when the pile's temperature rises to between 120 and 160 degrees Fahrenheit and stays there until most of the material has decomposed. Composting can be successful at much lower temperatures-it just takes longer and you might not kill all the weed seeds that might be in the mix.

When one of the ingredients in the compost recipe becomes depleted, the microbial activity begins to slow down and the pile cools off. Now it's time to turn the pile and mix in more ingredients, whichever seems to be in short supply. The ingredients most often needed are water and/or nitrogen.

Water is needed when you build the pile and when you turn it. Watering the top of the pile has little effect on the interior conditions because most if not all of the water runs off. The material that acts as insulation to keep heat in the pile also acts to keep external factors out. Watering when you turn is the most efficient way to get water throughout the pile.

Backyard Composting - Trouble Shooting Guide

ProblemPossible CauseSolution
Rotten Odor Excess Moisture


Compaction
Turn pile. Add dry, porous material such as dried leaves, straw, or wood chips.

Turn pile to fluff material and/or add bulking material such as wood chips.
Ammonia Odor Too Much Nitrogen
(too much green stuff)
Add high carbon material
(brown stuff)
Low Pile Temperature Pile Too Small

Insufficient Moisture

Lack of Nitrogen
Add more material

Add water while turning

Add high nitrogen material (green stuff)
Pests
  • Raccoons
  • Rats
  • Other Animals
  • Insects
Presence of Food – Meat or Animal Products, Dairy Products, or Fats and Oils Remove any meat, dairy, or oil products. These can be buried in the ground at least ten inches deep.
Fire Ants   Pour boiling water on them and mix up the pile to get it hot.

When Is Compost Ready?

Using compost before it is ready can damage plants. Undecayed "brown" materials in the soil can temporarily reduce plant-available nitrogen. Undecayed "green" materials can harbor pests and diseases. Immature compost can also introduce weed seeds and root-damaging organic acids. Compost is ready when:

  • it smells earthy – not sour, putrid, or like ammonia;
  • it no longer heats up after it is turned or wetted; and
  • it has a crumbly texture and it looks like dark soil.

How to Harvest Compost

Compost can be shoveled out of a pile or bin and used just as it is, especially for mulch. Remove undecayed objects by sifting them through a screen. If you are using compost in preparing soil for planting or sodding, sift it through a 1-inch mesh screen. Compost used in potting mixes or as top-dressing on lawns is commonly sifted through a 3/8-inch or 1/2-inch mesh screen.

Make a simple screen by mounting hardware cloth or other durable wire mesh in a sturdy wooden frame that will fit neatly onto the wheelbarrow or other container into which you will screen the compost. You can also use a 3' x 3' piece of wire mesh with a 2" x 2" board fastened to opposite edges. Spread compost onto the mesh frame in a thin layer and shake it; or hold the 2 x 2s (one in each hand) and roll the compost over the mesh to sift it. You can work the finer material through the screen with a paddle if it is clumpy. Add the "oversized" material that remains on top of the screen back into the pile, or into a new pile to help it start composting faster.

Additional Resources