Sustainable Living Through Organic Gardening

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Making Compost

May 7th began Compost Awareness Week. By making compost, we create the conditions for micro-organisms (MO) to digest organic matter (OM) thus producing the miracle plant food, humus. Humus is a product of death and decay which makes nutrients available for renewed plant life. When we make compost we are using our knowledge of these relationships. Thus, compost makers are in touch with the natural cycles of the Earth.

Humus is the most reliable plant food. From its surface, plant roots pull off whatever combinations of nutrients they require. Compost heaps produce humus and thus keep soil at maximum health with a minimum expense. So it is essential for organic growers to make compost.

There are many different approaches to making compost. Frequently, gardeners make piles. In the static pile, materials are added as they become available. In the thermal pile, the ingredients are assembled and then the pile is built. Both types of compost can be built in a variety of boxes, bins or in open piles.

Thermal piles have the advantage of heating up and destroying weed seeds.

There are several keys to composting. One of the most important is a smorgasbord of inputs. At a minimum, three ingredients of different textures are used to aid aeration. They are placed in layers 1-2 inches thick. Another key is to include a 1/2 inch layer of top soil to inoculate the pile with micro-organisms. It is key to aim for a carbon/nitrogen ration of 25-35 parts carbon to 1 part nitrogen. Another key is to keep it moist, about like a squeezed out sponge. The pile must have air, to prevent it from going anaerobic, a few twigs will help. Pay attention to the pile’s temperature, 140 degrees F. for 3-4 days will kill weed seeds, but if the temperature gets near 165 degrees F the pile should be turned to cool it down and prevent it going anaerobic.

Let’s determine the C/N ratio for the example pile. Using materials found on most city lots, grass clippings 19/1; vegetable food scraps 15/1 and leaves say 60/1. 19 + 15 + 60 = 94/3 would give a C/N ratio of 31/1, within the target ratio. We are also adding soil between the layers, which would also reduce the C/N ratio a bit.

Making compost is like making bread or wine so practice makes perfect. If you were successful in making a thermal pile it should be turned when it starts to cool and cured for 2-3 months. Static piles will need to cure for 6 months or longer. Compost is ready to use when it is dark and rich looking and smells good. A maximum dressing of 1/2 inch per crop should added to the garden.

John Jeavon’s book, How to Grow more Vegetables, has a very good chapter on Compost.

Following are some sample C:N ratios of organic matter:

Target C/N Ratio
25 – 35 / 1