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Donald H. Steinegger, Extension Horticulturist
Susan D. Schoneweis, Extension Coordinator

This Guide discusses the advantages of compost, the compost heap, ingredients, uses, and instructions for making compost.

Compost, a mixture of partially decomposed plant material and other wastes, is used in the garden to amend soil and fertilize plants.

Advantages

The chief advantage of compost is its ability to improve soil structure. Cod garden soil is loose, has a high water-holding capacity and adequate drainage. Addingcompost to heavy clay soil improves drainage by improving soil structure. Compost also absorbs water and improves the water-holding capacity of sandy soils. If one intends to conserve moisture or develop a xeriscape landscape requiring little water-it is essential to improve the water holding capacity of the soil.

In addition to improving the structure of "problem" soils, decomposing compost also slowly releases plant nutrients. Unless applied in very large amounts, compost will not provide all the nitrogen that highly productive crops require. Organic gardeners can supplement generous compost applications with manure to produce good yields without the addition of other fertilizers. Making and using compost also allows the gardener to recycle garden wastes and reduce the trash disposal burdens of municipalities.

Making Compost

The decay which creates compost is the same process which naturally occurs at the soil surface in a field or on the forest floor. However, certain composting practices encourage this decomposition to occur more quickly.

Breakdown is hastened by gathering compost ingredients together in a heap. A heap allows for the large buildup of the organisms which cause decomposition. The organisms' growth is encouraged in a number of ways. Materials are shredded to provide a large surface area for decomposition. The organisms also require the proper balance of different types and sizes of ingredients in the heap. Lastly, the growth is encouraged by providing oxygen and sufficient moisture.

The Compost Heap

Piling plant materials together in a heap encourages the growth of microorganisms which decompose the residues. A compost heap of the proper size and ingredients can heat up to 150 F. The heap should be at least 4 feet wide across the bottom and 3 to 4 feet high. Smaller compost heaps may dry out too fast and slow decay. If a heap is very large, air may not get to the center, giving the compost a bad odor.

Heaps can be freestanding or made in more permanent compost "bins". Various materials can be used to make compost bins, including: concrete blocks, scrap lumber, lath fencing and wire mesh. Bins should be partially open on the sides to allow air to enter the heap. They should be constructed to allow the compost to be aerated by turning and to be easily removed. Locate compost bins or heaps in the shade to prevent the compost from drying out too fast.

Compost Ingredients

Compost can be made from any organic refuse such as leaves, grass clippings, weeds, kitchen scraps, or animal wastes. Some organic materials however, should not be added to compost heaps. To be safe, do not use grass clippings from the first two cuttings after applying lawn herbicides.

Do not add cat manure to compost heaps because it may contain parasites which can infect humans.

Do not add bones, meat scraps or greasy substances to compost; they may attract rodents, and the fats in them will slow decomposition.

Do not add diseased plants or weeds that have formed seeds to the compost heap since disease organisms or weed seeds may not be destroyed during decomposition.

Substances sold as "bacterial activators" are not necessary for successful composting.

Coarse materials like branches or cornstalks should be shredded into smaller pieces to decompose faster. Strive for a mixture of differently-sized ingredients as you construct the compost heap. This ensures plenty of air in the heap since the microorganisms require oxygen to decompose the ingredients. A heap made of only one ingredient like grass clippings or leaves will tend to mat down. Decomposition will occur slowly or the decomposition may be undesirable and produce ammonia or sulphur-like odors. These odors indicate that the compost heap requires more air inside it.

Decomposition is also encouraged by using the right balance of types of plant refuse. Two nutrients, carbon and nitrogen, are particularly important in building a compost heap. Green materials like fresh grass clippings, weeds, and kitchen scraps are high in water and relatively high in nitrogen, while dry materials like leaves and straw are high in carbon. The balance between the amount of carbon and the amount of nitrogen in a material is called the carbon-nitrogen ratio (C/N ratio). See Table 1 for the C/N ratio of some materials commonly used in compost heaps. Materials with high C/N ratios are high in carbon; low C/N ratios indicate high nitrogen content.

Heaps constructed only of materials with low C/N ratios may decompose too quickly, and much of the nitrogen will be lost as gas (ammonia). Heaps made of materials with high C/N ratios like straw or pine needles will decompose very slowly. The ideal C/N balance for effective composting is around 30. For example, combine similar amounts of leaves (50/1) with manure (20/1) to obtain a ratio of 35/1.

To ensure a proper balance between high nitrogen and high carbon materials, compost heaps are often constructed in layers. Layers are a convenient way of measuring how much of each ingredient goes into the heap. The layers disappear as the compost is mixed during decomposition.

Making a Compost Heap

A general guideline is to use 1/3 green material and 2/3 dry material. Form a heap by alternating 4- to 6-inch layers of dry carbon materials with 2- to 3-inch layers of green nitrogen material or animal manure. (If you are low on nitrogen materials, sprinkle 1/2 cup of 5-10-10 or 10-10-10 over the dry materials.)

Add a shovelful of garden soil between each layer to introduce the necessary microorganisms to the heap. If the materials in the heap are not moist, sprinkle each layer with water as you build the heap.

Cover the finished heap with a plastic tarp to keep it from drying out and to prevent rain from washing away nutrients.

The microorganisms need water and oxygen to do their work. Check the heap as it decomposes to make sure it is moist enough. A compost heap should feel as moist as a squeezed-out sponge. If necessary, add water by sprinkling the heap as you turn it. Overwatering the heap will reduce the oxygen the microorganisms require.

It is important that air be added to the heap by regularly turning and mixing the materials. The first turning should be three or four days after the heap is formed, and subsequent turnings once a week. Use a shovel or pitchfork to fluff and mix the ingredients. Large heaps can be turned by removing the top layer and using it as the base for a new heap. As ingredients are moved from one heap to the new one, air is incorporated.

Failure of the heap to heat up in a few days indicates the decay process has not started. Add more nitrogen materials and be sure that the heap is sufficiently moist. Compost that smells strongly of ammonia or is slimy requires more frequent turnings and the addition of high carbon materials. How soon compost is "finished" will depend on the size and balance of the ingredients used, the air temperature, and how fast decomposition was encouraged by keeping the heap moist and turning it regularly.

Using Compost

Finished compost is dark brown and crumbly and smells like potting soil. Small pieces of the leaves or other ingredients may be visible. If the compost contains many materials which are not broken down, it is only partly decomposed. Adding partly decomposed compost to the soil can reduce the amount of nitrogen available to plants. The microorganisms will continue to decompose but will use soil nitrogen for their own growth, restricting the nitrogen's availability to plants growing nearby.

Allow partly decomposed compost to break down further before using it around growing plants. Or add extra nitrogen (such as in manure or commercial fertilizer) to ensure that growing plants will not suffer from a nitrogen deficiency.

Finished compost has many uses. It can be spread over and incorporated into garden soil in the spring or fall. Generally, a layer of compost I to 3 inches deep is applied. Compost can be used as an ingredient in potting mixes. For a fine texture, sift compost through a 1/4-inch wire mesh screen. Mix with equal parts of soil and sand, or scratch lightly into the soil surface of potted plants.

Top dress growing plants like vegetables, flowers or shrubs with compost by spreading a 1- to 2-inch layer around the plants. half-decomposed compost can be applied more thicKly as a mulch around trees or shrubs. It will conserve soil moisture and slowly release nutrients to plants.

Incorporate compost when establishing a new lawn or when rejuvenating patches of turf by reseeding. Mix 1 inch of compost into the top 2 to 3 inches of soil. Compost should not be applied to established turf because its high organic matter content can encourage thatch formation.

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