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Extension > Garden > Yard and Garden > Soils > Composting and mulching guide > Preparing and maintaining the compost pile

Preparing and maintaining the compost pile

This information is provided for those who want to make compost quickly. For those who are more patient, simply piling the yard waste with few or no inputs or turning will also work, but the process will take much longer (more than one year).

Layer the compost pile as you add materials to facilitate decomposition by ensuring proper mixing. An example of the layering process is illustrated in Figure 4. Each pile ideally should be about five feet high. Put down organic wastes such as leaves, grass, and plant trimmings in a layer eight to ten inches deep. Coarser materials like twigs, stalks, and chipped branches will decompose faster if placed in the bottom layer. Water this layer until moist, but not soggy. A nitrogen source should be placed on top of this layer. Use one to two inches of livestock manure, or a nitrogen fertilizer such as ammonium nitrate or ammonium sulfate, at a rate of one third of a cup for every 25 square feet of surface area. If these nitrogen sources are not available, 1/3 cup of 27-3-3 lawn fertilizer per 25 square feet of surface area will also work. Do not use fertilizer that contains herbicide or pesticide. Other organic sources of nitrogen that can be used are green grass clippings, lake plants, cottonseed meal, or blood meal. Grass clippings tend to mat and should either be mixed well with other materials, such as wood chips or leaves, or placed in layers only one to two inches thick.

Figure 4

You can apply about a one-inch layer of soil or completed compost on top of the fertilizer layer. One purpose of adding soil is to ensure that the pile is inoculated with decomposing microbes. The use of soil in a compost pile should be considered optional. In most cases, organic yard wastes such as grass clippings or leaves contain enough microorganisms on the surface to effect decomposition. Studies have shown that there is no advantage in purchasing a compost starter or inoculum. Microbes multiply as rapidly from the soil and/or added organic wastes as from the purchased inoculum. The microbes already in the soil and on organic materials are just as efficient in decomposing the waste as those provided by the commercial inoculum. Adding soil, however, will help reduce leaching of mineral nutrients such as potassium released during decomposition. Repeat the sequence of adding organic waste, fertilizer, and soil (optional) until the pile is completed, remembering to water each section to the 50% to 55% moisture level.

The carbon-to-nitrogen (C/N) ratio determines how long decomposition will take. For rapid composting, the initial C/N ratio should be in the range of 25/1 to 30/1, or simply 25 to 30. If the initial C/N ratio is above 35, the process will be considerably slower. The C/N ratio of common organic yard wastes is shown in Table 2. Materials can be blended and mixed to achieve an initial C/N ratio of 25 or 30. Over time, the C/N ratio will generally decrease.

Table 2. Approximate concentration of nitrogen and carbon-to-nitrogen (C/N)*ratio of various materials used in municipal and backyard composts5

Material Nitrogen % dry weight C/N ratio wt/wt
Alfalfa 2.5% 12 to 19:1
Grass clippings 2.2% 15 to 25:1
Fruit wastes 1.5% 25 to 45:1
Sugarbeet 0.7% 30 to 40:1
Leaves 0.7% 40 to 80:1
Sawdust 0.2% 100-750:1
Wood 0.09% 200 to 1300:1
Paper 0.12% 200 to 800:1
Table scraps 3.0% 11 to 15:1
Livestock manure 2.0% 10 to 30:1
* See text for further explanation. **C/N ratio will increase as the proportion of bedding increases.

To prevent odors and hasten decomposition, the pile should be turned occasionally. Turning also exposes seeds, insect larvae, and pathogens to lethal temperatures inside the pile. Turning may be done by inverting segments of the compost, or by shifting the pile into another bin. The compost pile should be kept moist but not waterlogged. Odors may occur if you add excessive amounts of wet plant materials like fruits or grass clippings, or from over watering--both will cause anaerobic conditions. A properly mixed and adequately turned compost heap will not have objectionable odors. An actively decomposing pile will reach temperatures of 130-150 degrees F in the middle. Low-cost temperature probes are available to help monitor temperature in the pile. Reasons for the pile not heating up may be: too small a pile, not enough nitrogen, lack of oxygen, lack of free air space, too much moisture, or not enough moisture. The pile should be turned when the temperature in the center begins to cool. This will introduce undecomposed edge material into the center and subsequently regenerate heating with a "new" food source for the microbial community. The composting process is essentially complete when mixing no longer produces an increase in heat in the pile.

Small amounts of fresh materials may be added provided that the pile is occasionally turned. Vegetable wastes should be buried inside the pile to avoid attracting rodents. If enough material is available, it is best to make a new pile instead of combining with old compost.

Generally, a well-managed compost pile with shredded materials under warm conditions will be ready in about two to four months. A pile of unshredded material, left unattended may take years to decompose. Outdoor piles prepared in the late fall, in Minnesota, will not be ready for use the following spring. When the compost is finished, the pile will be about half its original size and have an earthy smell to it.

Questions sometimes arise about spontaneous combustion in compost piles. Spontaneous combustion is the occurrence of fire without the application of an external heat source and can be caused by chemical, biological, or physical processes. Organic material can ignite spontaneously due to biological activity at moisture contents between 26-46% moisture if the temperature exceeds 200 degrees F. These high temperatures only occur with restricted air flow and piles exceeding a height of seven feet. Spontaneous combustion happens to stored hay or silage and only in rare cases to compost. No documented cases of spontaneous combustion have been reported for compost piles smaller than seven feet. Most reported fires occurring in compost piles are the result of external sources such as matches or the addition of hot ashes. In short, a well maintained compost pile with temperatures less than 150 degrees F will not spontaneously combust. If a compost pile gets too hot--more than 160 degrees F--you can cool it down by 1) reducing the size of the pile; 2) adding water to 55% moisture; or 3) mixing in coarse, bulky material such as wood chips. Compost piles work best at temperatures between 130-150 degrees F.

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