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Issue: December 12

Making compost from leaves may need some additional nitrogen

Q. Please give me a formula for making compost with leaves.

Jack C.
Roswell

A. Composting is as much art as science, but knowledge of the science helps the compost artist. Scientific research tells us that the ratio of carbon (carbohydrates) in the leaves relative to the nitrogen (protein) (C/N ratio) should be about 30 parts carbon to 1 part nitrogen for optimal composting. Freshly fallen tree leaves that froze on the tree before falling are very close to that optimum ration and will compost well if they are shredded and kept moist (even in the winter). Older leaves and those that are brown can have a C/N ratio of 80:1. They will need additional nitrogen to speed decomposition. Leaves will decompose naturally without added nitrogen, but compost will form more slowly. The nitrogen can be added in the form of manure or commercial nitrogen fertilizer (nitrate fertilizer is more effective in cold weather than ammonium fertilizer). Carbon and nitrogen are needed by the fungi and bacteria that decompose the leaves to form compost. When the C/N ratio is optimal the microorganisms multiply most rapidly and consume the leaves quickly.

The C/N ratios of various compostable materials are available in NMSU Extension Publication H-110: Backyard Composting (available online at http://aces.nmsu.edu/pubs/_h/H-110.pdf). If you have brown leaves with a C/N ratio of 80:1 (worst case ratio) and you choose to add dairy manure (C/N ration 20:1) to balance the compost pile's C/N ratio, you should add 4 parts manure by volume to 1 part leaves. This will give you a C/N ratio of 32:1 in the compost pile (close enough). The formula to use is parts manure times C/N ratio (4 x 20:1 = 80:4) plus the ratio of one part leaves (80:4 + 80:1 = 160:5 = 32:1).

If you choose horse manure (25:1), you will need to add 7 parts manure to 1 part leaves to get a ratio just under 32:1. However, with poultry manure 15:1) you can add only 3½ parts manure to 1 part leaves to get a ratio of approximately 29:1. Kitchen vegetable wastes may also be used, but their ratio is very variable (11:1 to 20:1) and more difficult to calculate. However, remember getting the ratio exactly right is not critical to creation of compost. If the nitrogen component is too high (low C/N ratio), there may be some ammonia odors noticed near the compost pile, but compost will form. If the carbon component is too high (high C/N ratio), then compost will just form more slowly.

You can shred the leaves (with a shredder or a mower with a bagging attachment) before adding them to the compost to speed the rate of compost formation. Adding water to maintain the moisture level inside the compost pile and turning the compost also help speed composting. During the winter, composting occurs more slowly, but with a proper C/N ratio, moisture (to keep the compost feeling like a rung-out sponge), and periodic turning to allow aeration, compost should form by spring. You can separate any uncomposted material and return it to the compost pile and add the completed compost to your garden.

Thank you for composting.


For more gardening information, visit the NMSU Extension publications World Wide Web site at http://aces.nmsu.edu/pubs/_h.

Send your gardening questions to:

Yard and Garden, Attn: Dr. Curtis Smith
NMSU Agricultural Science Center
1036 Miller Rd. SW
Los Lunas, NM 87031

Curtis W. Smith, Ph.D., is an Extension Horticulture Specialist with New Mexico State University's Cooperative Extension Service. NMSU and the U.S. Department of Agriculture cooperating.