To heat, or not to heat?
Why “hot” compost?
To be sure, hot composting is hard work! If you’re motivated, however, hot composting will
reward you with rich compost in less time and can help rid compost of most weed seeds and
fungal plant diseases (IF you choose to add such materials to your compost). To successfully
kill most of the “nasties” in your compost, your pile needs to sustain heat between 130°F and
150°F (but below 170°F) for about week or more. You’ll also need a few extra materials to
ensure proper heating, such as a mega-rich nitrogen source and a thermometer.
Adding additional nitrogen helps your compost to really heat up, but you’ll want to use an
organically-rich nitrogen. Using pure nitrogen, like an ammonium nitrate fertilizer, is too
intense and can pickle your beneficial microbes. Instead, use a plant-based nitrogen source
such as store-bought compost activator, organic fertilizer, or, as a cheaper alternative, plantbased rabbit or poultry feed pellets or plant-based meal (corn, canola, etc). Rabbit pellets can
be found at local pet stores and the others can be found at farm stores. If you have access to
manure, you can use it to provide the necessary nitrogen boost.
Ideally, you want to start your hot heap with weathered materials such as weathered plants,
weeds or leaves that were piled the previous fall and allowed to sit through the winter. Also,
try your hand at hot composting during the summer months when warm temperatures will
aid in decomposition.
Recipe for a hot heap1
1) Choose a location in your yard at least 8’ square and place your weathered leaves or
plants there. Your pile should be at least 12 cubic feet in size (approximately a hiphigh pile of material). Use a hoe to chop the material into smaller bits and sprinkle the
pile as you mix it to ensure even moisture.
2) Mix in some fresh green material equal to about 20% of the heaps mass. Now, move
the pile aside and start to build layers, with your material on the bottom followed by a
heavy dusting of your chosen activator, a sprinkle of water on the activator topped
with about 3 inches of completed compost or soil. Repeat the layering until you’ve
used all your plant material. If using rabbit pellets or poultry pellets (soybean meal),
you’ll need about 20 lbs. If using canola or alfalfa meal (not pelletized) you’ll need
about 10 lbs. Organic fertilizer (a blend of meals) can also be used and you’ll need
about 8 lbs.
3) After three days, thoroughly mix and turn the heap, adding water to moisten any dry
pockets. Expect some heat, but not uniformly throughout the pile.
4) Mix the heap every two or three days for at least two weeks. Move material from the
outside of the heap to the center. After the first week, you should notice the heap
heating in the center. Your thermometer should read above 130°F from day 6 to day
12. DO NOT miss any turnings during this crucial period!
5) After two to three weeks, the heap will start to cool down. Continue to turn if every
few days and add water if necessary. When the heap has stopped heating, cover the
heap with a cloth or tarp and let it cure for at least a month. If your heap is near trees,
place a weed barrier or some burlap under the pile to discourage invading roots.
Recipe for a hot bin2
1) Layer the compost as shown in the diagram below. Use weathered materials for the
organic layer, about 8-10 inches deep, with larger material placed on the bottom of
the pile. If you’re adding grass clippings, be sure to mix them thoroughly with other
plant material to avoid matting. Water this layer until moist, but not soggy.
2) Apply a 1-2 inch layer of manure or a thick coating of fertilizer or animal
pellets/meals as described above.
3) Add a one-inch layer of soil or completed compost.
4) Repeat, layering and watering until your pile is approximately 5 feet high.
5) Keeping your pile in the bin requires less turning. Monitor the compost by using a
long temperature probe and as soon as you see the temperature begin to cool, turn
your compost to bring the outside materials to the center. The composting process is
essentially complete when turning no longer produces an increase in heat.
Additional hot heap recipes to try1
Once you’ve tried your hand at a few hot heaps, you’ll be able to experiment with different
materials to find what best suits your composting needs. All of these require moisture,
mixing and curing per the above instructions and need to be at least 12 cubic feet in size.
* 3 parts shredded leaves (weathered over winter), 1 part dehydrated cow manure (your
rich nitrogen source), 1 part green or partially-rotted garden waste.
* 20 pounds rabbit food (alfalfa pellets), one wheelbarrow load each of shredded dry leaves,
fresh grass clippings and green garden waste
* 3 parts fresh grass clippings (rich enough in nitrogen to not need an additional activator) 1
part kitchen scraps, 1 part moist, shredded paper
Why won’t my heap heat??2
There are several reasons why a carefully built heap may not heat:
* Your pile is too small. Think big! Follow the guidelines above.
* There isn’t enough nitrogen. Add some! Pellets, manure, grass clippings… just be sure to
add these in layers.
* There isn’t enough oxygen. Turn, turn, turn!
* It’s too wet. Turn and add more dry material.
* It’s too dry. Turn and add water while doing so. Search out and moisten dry pockets!
Last but not least
When striving for hot compost, you should avoid adding fresh material. If you strive to be a
constant composter and want to hot compost, you may want to have more than one project
at a time.
Sources:
1. Pleasant, B. and Martin, D. The Complete Compost Gardening Guide. Storey Publishing, 2008. 319 pp.
2. Rosen, Carl et al. Composting and Mulching: A Guide to Managing Organic Yard Wastes.
http://www.extension.umn.edu/distribution/horticulture/DG3296.html#toc
Handout compiled by:
Anne Sawyer, U of MN Master Gardener, Rice County, for “Turn Your Garbage into Gold”, Just Food Co-op
presentation, 19 April, 2011