Compost and Mulch
by Andrea Peck

Q. What is the difference between mulch and compost?
Jane Lauridsen, Los Osos

A. There is no question that when creating a sustainable garden, mulch is a necessary tool in the gardener’s arsenal.  Similarly, compost, otherwise called ‘black gold,’ with its ability to improve even the worst soil, is an excellent way to reuse garden waste.  But, while mulch and compost have their own stellar qualities, can they be used interchangeably?

Mulch, in essence, is anything that covers the soil to prevent weed growth and water loss. Mulches can be inorganic or organic.  Black plastic, rock, even reused rubber from tires are used as inorganic mulches.  While inorganic mulches do not amend the soil, they do retain soil moisture as well as block the sun, thereby preventing weeds, slowing down evaporation and protecting soil from erosion.
Organic mulches form a much larger list and extend from leaf litter, grass clippings, straw and wood chips.  Over time these organic mulches break down and enhance the soil. Because they are essentially ‘on top’ of the soil, these mulches will not have much effect on nitrogen levels in the soil during the process of breaking down.

Compost, on the other hand, is an equal amount of carbon (brown materials such as leaf litter) and nitrogen (green materials such as kitchen waste) mixed together and set aside in a pile or contained in an open box to decompose.  After the process of decomposition is finished, it should be dark and crumbly with an earthy smell.
Compost is best worked into the soil, near the plant root zone, at a depth of 6 to 8 inches.  Because it has had a chance to fully decompose, necessary nutrients are immediately available for plant use.  Compost can be used as mulch but not vice-versa.  In order for weeds to be suppressed, compost should be placed around the plants at a depth of four inches or more to block the light that weed seeds need.  But, while weed seeds on the soil before the compost is added will be prevented from growing, any weed seed that lands on top of the compost will find a nice spot to grow.  Nevertheless, if you have enough compost to use as mulch, the quality of your soil will greatly improve.

Got a Gardening Question?
Contact the University of California Cooperative Extension Master Gardeners: at 781-5939 from 1 to 5 p.m. on Monday and Thursday; at 473-7190 from 10 a.m. to noon in Arroyo Grande; and at 434-4105 from 9 a.m. to noon on Wednesday in Templeton. Visit the UCCE Master Gardeners Web site at groups.ucanr.org/slomg/ or e-mail mgsanluisobispo@ucdavis.edu.