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Master gardener in training: the finer points of composting

May 5, 2010 |  7:11 am

In the UC Extension Master Gardener class I'm taking, we’ve heard that 50% of landfill-bound garbage is organic and that 70% of that can be composted at home. The payoffs can be huge, not only for the plants but for our water bills. With compost in the soil (and mulch on top), I should be able to cut my watering by one-half to two-thirds. If there’s one message that is repeated by speakers in class or at landscapes we visit, it’s that composting and mulching are the essentials of successful gardening.

Next week, I'll talk about mulching. This is composting week. I have my fertilizer factory going in my worm bin, but the first harvest is still more than a month away. In the meantime, my seedlings are getting impatient to get into the ground, so it’s time to check my composts: a tumbler, a Smith & Hawken half-cubic-yard BioStack box and a ragged heap of branches, vines and sticks that haven’t been cut up enough to go into either container.

MG_lasagna_compost162 Composting may be referenced on 4,500-year-old Mesopotamian clay tablets and may hardly seem like a new idea to many gardeners, but there are tricks to doing it right. You'll find at least a dozen different techniques, from cold “lasagna” sheet mulching to hot bin composting.

At its most basic, composting is the reduction of complex things into simpler components that then can be deployed as soil nutrition. Lots of life contributes to this process — worms, beetles, ants, cutworms and, overwhelmingly, microbes.

There are four phases of composting, the most dramatic being the third. That's when the temperature rises above 104 degrees and the thermophiles, the heat-loving microbes, take over. Some think thermophiles were the first forms of life on Earth 4 billion years ago, inhabiting hot, dark, oxygen-free places such as hot springs and volcanoes. When I pay attention to my composts --  especially the bin -- I can get my very own miniature Eyjafjallajokull going in a few hours, depending on what I add. (That hasn’t stopped the LAPD helicopters from circling overhead, however.)

Compost can be used not only as an additive to loosen up compacted soil but as a mulch or a potting soil mix (combined with 50% worm castings and 20% vermiculite). Partially decomposed compost added to your soil, however, will suck away any nitrogen before it can get to your plants. To prevent that from happening, I was taught these guidelines:

Material put into your compost bin or pile should be smaller than an inch. You will hear many compost recipes, but the simplest is one cubic yard of material that is 50% green and 50% brown. Watch your pile and adjust accordingly. If it’s too wet, add brown. If it's too dry, add green and water. Be careful not to let the pile get soggy. It should be the moisture of a wrung sponge. It’s ready to use when it resembles soil, which can be anywhere from two months to never, depending on how attentive you are. The cheapest ingredient you can add is heat from the sun.

What constitutes green material? Think any fruit or vegetable scraps, grass clippings, horse manure, crushed eggshells.

Brown material includes dry leaves, non-glossy junk mail, newspapers, cardboard and wood chips.

I have been warned repeatedly against adding meats, bones, grease, dairy, pet food, pet feces, litter, chemically-treated plants, charcoal ash and melon seeds. Some compost purists avoid foliage from nightshade plants, including tomatoes and potatoes.

-- Jeff Spurrier

Photo credit: Ann Summa

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