URBANA, Ill. - New gardeners often ask, “What is compost and do I need to buy it?”
When starting a new garden, it is hard to imagine we should initially spend more time, money and energy on compost than on plants, said a University of Illinois Extension horticulture educator.
“It is plants we want,” said Nancy Pollard. “Yet without adequate compost (decomposed organic matter) in the soil, the plants won’t thrive. It is the soil that provides nutrients, and the compost that holds adequate water and provides good drainage to make the difference between just surviving plants and the thriving plants we covet.”
Good soil, that is soil with a good amount of organic matter, will hold lots of water available to plants, yet drain well so roots stay healthy. Soil can hold roughly 1.5 quarts of water per cubic foot of soil for each percent of organic matter. So increasing the organic matter content from 1 to 2 percent would increase the volume of water to 3 quarts per cubic foot of soil, or at 3 percent to 4.5 quarts of water.
“What a difference that extra water makes in times of drought stress,” she said. “It is the difference between needing multiple applications of water per week compared to only needing additional water every week or two. So adding compost in the beginning saves you time and money and gets you much better plants in the long run.”
In a very small garden, work the organic matter in with a spading fork ($50- $100) or for a larger garden, invest in a broad fork (about $200), Pollard recommends. Many gardeners prefer the broad fork to a rototiller, as the broad fork aerates more deeply, preserves soil structure, brings up fewer weed seeds, cuts up far fewer earthworms, requires little maintenance, and is less costly and less jarring.
How much compost should be added?
Pollard recommends that for 100 square feet of soil, add one (or a proportional combination of) the following:
8 cubic feet of peat moss; OR
15 cubic feet of compost; OR
4 bushels (about six 5-gallon buckets) of grass clippings. (Do not use lawn clippings from grass that has been treated with sprays containing fungicides, insecticides, or herbicides.)
When most un-decomposed organic matter is added to the soil, microbes begin decomposing the carbon materials, utilizing soil nitrogen in the process. In order to compensate for this nitrogen loss when using un-composted straw or leaves, you will need to apply supplemental fertilizer containing nitrogen. For 100 square feet of soil, add either one bale of straw or four bushels of leaves, as well as, in either case, 1 pound of nitrogen fertilizer (to help microorganisms break down organic matter).
Adding organic matter in the fall is best and the spring is second best. Any time of year is better than not doing this, Pollard cautioned.
“I did not say add garden soil or potting soil to your garden unless you are making raised beds,” she noted. “If using raised beds, create a soil and compost mix with up to a maximum of 50 percent compost by volume. So you likely don’t need more soil in your garden, you likely need more compost.
“Why are those extra quarts of water made available by organic matter important? Water is critical because plants use water to maintain plant turgor (hold them up). Water aids in cell division and growth; water provides pressure to push roots through soil; water is required in photosynthesis to make carbohydrates; water moves food and minerals around the plant; and water is critical for stabilizing plant temperatures,” Pollard said.
Lots of compost initially and top dressing with compost annually means the plants will likely have adequate water available and won’t be stressed so easily. The plants will flourish, and the gardener will be proud and proclaim they have a green thumb! Interesting that brown matter (compost) is what makes for a green thumb!