University of Vermont Extension 
Department of Plant and Soil Science

Summer News Article

Dr. Leonard Perry, Extension Professor
University of Vermont
A compost pile only makes desirable compost for the garden if conditions are proper.  There are certain signs to watch for that your compost bin may need some help.

If your compost has a rotten smell, this may mean your compost is too wet or too compacted.  In either case, sufficient air isn’t getting to the microorganisms that are what make materials decompose into the final compost.  To add more air, turn the pile with a garden fork or similar tool.  You can add a dry, porous material such as sawdust or straw if the pile seems too wet.  Another option is to break a larger pile into smaller ones.

On the other hand, if you smell ammonia, this indicates there is too much nitrogen and not enough carbon.  These same microorganisms use carbon for food, and nitrogen to make proteins.  Without these, or with the improper balance, the microorganisms wont do their job effectively.  So if you smell ammonia, add more high carbon material such as straw and less high nitrogen materials such as grass clippings and vegetable scraps.

You should aim for about 30 parts carbon to one part nitrogen, by weight, although this doesn’t have to be exact.  Coarse, woody material such as twigs, leaves and paper are usually high in carbon.  Moist, dense material such as manures are high in nitrogen.  A rule of thumb some use is that brown indicates carbon, and green indicates nitrogen.  Lush, green grass clippings are a great source of nitrogen, even greater if you fertilize your lawn.  However, in general, it is best to mow regularly leaving clippings on the lawn. 

A problem I have in our colder northern climate is the compost pile not heating up properly.  Composting microorganisms do their job in the range of 95 to 160 degrees F.  Too low and they work slowly if at all.  Ideal in the interior of compost piles is about 120 to 130 degrees F.  Temperatures can be measured with compost thermometers—basically a dial on a long rod—obtained at complete garden supply stores or online. 

If over weeks or months your compost just isn’t progressing, or the season is cool, consider if your pile is too small.  Large piles hold heat in the interior better.  Not enough moisture, poor air circulation, and lack of nitrogen also are reasons the compost pile might not be heating up properly.  In addition to tips already mentioned, try insulating the pile with straw to hold in heat more effectively.

Another reason compost might be progressing slowly, if at all, is that the acidity is too acid or alkaline.  These same microorganisms prefer a neutral to slightly acid environment.  Many materials you add to compost are acidic, hence the reason a sprinkling of lime often is recommended.  Too much lime, or too many wood ashes which serve the same purpose, and the pile will be too alkaline (high pH).  You can check this with inexpensive soil test kits from garden stores.  Add more materials if the pH is too high.

Got pests?  Raccoons, chipmunks, and even rats are attracted to meat scraps or fatty good wastes in the pile.  Don’t add these types of waste.  Also, don’t add weeds from your garden, if they have gone to seed, nor diseased plant parts.  These will cause future garden problems.

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