Growings On: Compost -- What to put it on

Roger Gates

Photosynthesis, the remarkable production of plant tissue from carbon dioxide, water and sunlight, is balanced in natural systems by equally remarkable processes of decomposition. Think about what a forest might look like if there were no organisms actively decomposing all the plant material, dead leaves and old trees, for example. Composting harnesses these decay processes in order to allocate decomposed plant material advantageously.

Two particularly beneficial ways to use compost are as a soil amendment and as mulch.

In very general terms, soil is composed of mineral matter, water, air and organic matter. If organic matter is ignored, soil can mistakenly be viewed as a lifeless, sterile growth medium. In contrast, adequate organic matter can support numerous organisms so that soil become a dynamic living system. Scientists describe all the organisms that decompose plant material as the detrital food web. Applying compost to landscape or garden soils can enhance the supply of organic matter and fuel the detrital organisms.

Complete decomposition would result in the same residue as burning. Little more than mineral ash would remain. Those mineral elements do provide useful nutrients for plant growth, but are in very limited supply in compost. Compost is therefore not a substitute for needed fertilizer.

Addition of compost does improve the potential productivity of the soil because of the way it modifies soil physical, chemical and biological properties. Soil structure, sometimes referred to as tilth, influences water infiltration and drainage, water-holding capacity and nutrient availability. Soils with poor structure may be excessively droughty or become waterlogged. Most desirable plant are not tolerant of extremes in moisture conditions and are most productive with more optimum conditions.

Supplying adequate, degradable plant material provides a food source supporting many organisms that effect soil structure. Earthworms, for example, require organic material as a food source. Their activity creates channels in the soil, which improve water infiltration and drainage. Improving soil structure with compost addition also reduces the tendency of a soil to form a crust, enhancing seedling emergence as well as water infiltration.

Ongoing additions of compost will increase organic matter and improve soil structure. University of Georgia Extension suggests the addition of 1 to 2 inches of well-decomposed compost annually. The lower rate should be used for sandy soils. Typically, composts derived from yard waste (grass clippings and/or leaves) have a pH between 7.0 and 8.0. This slightly alkaline composition may be beneficial to generally acid soils in our region. However, acid loving plants, such as azaleas and blueberries may not benefit from that kind of compost addition.

Compost provides an excellent source of mulch which can be used in gardens to suppress weeds, reduce soil erosion, and reduce loss of soil moisture. Mulches will also cool soil temperatures during the summer and warm them in winter. Yard waste compost makes an ideal mulch for annual and perennial gardens. Compost should be applied as a 3- to 6-inch layer around the base of plants. Warm temperature during summer will increase decomposition of the mulch layer so periodic additions will likely be beneficial.

Complete compost decomposition is not required when it will be used as mulch. Partially decomposed compost applied in the fall will warm the soil during winter as it continues to heat during decomposition. If uncomposted or partially composted leaves are used, one tablespoon of a high nitrogen fertilizer should be added to each bushel of mulch to optimize the carbon nitrogen ratio. More information about compost use, such as for potting soil, can be found in UGA Extension Circular 816 “Composting and Mulching.”

If you would like to turn your excess plant waste into useful compost, join Amy Hartline, recycling and education program coordinator for the Dalton-Whitfield Solid Waste Authority, at Lakeshore Community Garden on Saturday, June 29, from 9 to 11 a.m. Amy will discuss and demonstrate proper composting procedures using small “kitchen counter scale” containers up to large, garden size approaches.

Roger Gates is the agricultural and natural resources agent for University of Georgia Extension, Whitfield County. Contact him at

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