By Thomas R. Halbach, University of Minnesota Extension

April is a great time of the year to review your composting program. Gardening plans should include both the selection of plants and a soil fertility program that will meet the needs of those plants. Composting should be part of those plans.

Composting is primarily a microbial process that converts plant materials such as grass clippings and leaves to a more useful soil amendment. Gardeners have used compost for centuries to increase soil organic matter, improve soil physical properties and supply some of the essential nutrients for plant growth.

Organic materials need to be placed into a pile 36 to 44 inches tall. This gives the amount of vertical height needed to heat up the piles. Next, composting microorganisms need to be provided with the correct amount of air, water and food to heat up to 131 degrees to 150 degrees. These temperatures reduce weed seed viability and pathogens.

Aerobic microorganisms need oxygen to live. Free air space inside the pile should be between 55 percent and 70 percent when the pile is built and will help assure that oxygen can get into all parts of the pile. Course particle size and small twigs can help increase air space in a compost pile.

The moisture content in the compost pile should be kept between 46 percent and 61 percent. This is “moist” but never “wet.” A “wrung-out sponge” feeling is just about the right amount of water. To maintain this level of moisture may require weekly or more frequent watering of the pile, especially in hot windy weather.

Compost microorganisms thrive with a carbon to nitrogen ratio of 25:1 to 35:1. Whenever possible adjust feed stock mixtures to get into this range. Fall tree leaves have a carbon to nitrogen ratio of about 90:1. To get into the proper carbon to nitrogen ratio you will need to add two cups of urea (46 percent nitrogen); or five cups of 22-01-14 fertilizer or an equivalent amount of nitrogen from other sources for each 100 pounds of leaves in the pile.

Turn the pile two to five times to mix the cooler outside materials into the hotter inside of the pile in order to get faster and more uniform composting. If you start composting now, you should have some very nice compost ready to apply to the garden next October at a rate of one-half inch to 1 inch and worked into the soil 6 inches deep. Using finished compost to grow new plants completes the cycle.

For more information about composting, such as how to purchase or make a structure for backyard composting, visit http://www.extension.umn.edu/gardeninfo/ and click on “soils and composting.” April is a great time to begin your compost program for 2008!

(Thomas R. Halbach is an Extension educator in waste management and water quality.)