University of Vermont
Department of Plant and Soil Science

gmg logo   Summer News Articleline

COMPOST HAPPENS, OR DOES IT?

Dr. Leonard Perry, Horticulture Professor Emeritus
University of Vermont

A compost pile only makes desirable compost for the garden if conditions are proper.  If you’ve begun composting in a bin or pile, and it isn’t turning into that rich dark organic matter that you see in photos and buy in bagged compost, the process 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 every few days.  You can add a dry, porous material such as sawdust or straw if the pile seems too wet.  Another option is to break the pile into smaller ones.  If you do this, though, it may not get hot enough for these microorganisms. 

If, on the other hand, your compost is too dry it won’t decompose.  If you’re in a dry area, or drought without much natural rain, moisten layers as you add materials to your pile, and re-wet them as needed.  They should be as moist as a wrung-out sponge.

If you smell ammonia, this indicates that 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 won’t 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.

You should aim for about 30 parts carbon to one part nitrogen, by weight, although this doesn’t have to be exact.  A rule of thumb that some use is to add two to three parts (by volume) of brown materials (carbon containing) to one part green (nitrogen containing).  If your compost is decomposing slowly, perhaps you have too much brown material and need to add more green.

Fallen leaves, straw, sawdust (not from pressure treated wood), paper (shredded paper decomposes more quickly), cardboard and woody material such as twigs (again, best shredded) are high in carbon.  Moist, dense material such as manures, coffee grounds, vegetable and fruit trimmings, and green gardening trimmings are high in 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 and to leave the clippings on the lawn to decompose there. 

Even with the right ratio of brown and green materials, your compost may proceed slowly or not at all if there are no microorganisms.  This is the reason many add layers of soil in between layers of green and brown materials.  You might aim for about five to eight inches deep of the brown, two to three inches deep of the green, and then a layer of soil or composted manure one to two inches deep.  Repeat these layers until your pile is high enough or bin is full.

A problem that many, including me, 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 a temperature and they work slowly, if at all.  Ideal temperature 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 (pH) is too acid, or too 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 (to raise the pH).  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 food 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.

Follow all these tips for an “active” pile, and you should end up with good compost, eventually.  Be patient, as in cooler climates and with small piles or bins as in most home gardens, compost may take up to a year to be ready.  But your soil will be better for it in the end, and you’ll be recycling all these great organic materials into your garden rather than into a landfill.

In fact, going to the landfill will soon no longer be legal in Vermont for food scraps.  Vermont, in 2014, passed a Universal Recycling Law—the first such in the country--- banning leaf and yard debris by 2016 and organic food scraps from landfills by 2020.  These can be disposed of properly at recycling centers.

Short on time or don’t care to worry with all these steps for active composting?  Then you can just add wastes to a bin and let nature take its course with “passive” composting.  It takes longer to make compost than actively managing the pile, but is easier and better than not composting at all.

Return to Perry's Perennial Pages: Green Mountain Gardener Articles-- your reliable source of gardening information for over 50 years.