SC Master Gardener Course: Composting

Wendy S. Delmater
By Wendy S. Delmater on Mon, Jan 6, 2014 - 12:54pm

Clemson Master Gardener Training Manual

As many of you know, I've been taking a Master Gardener course online through the Clemson University Cooperative Extension. I'm going to do a series of posts on the various topics studied, to try and share what I've learned.  I will strive to give you useful information for any climate. This post will be about Composting.

Why compost? It improves soil texture, or tilth, which is the ability of soil to provide a good growing medium. Composting causes coarse, sandy soils to retain more water and helps fine, clay soils to have better drainage. It adds available nutrients to the soil to nourish plants.

Why add your organic yard clippings and kitchen waste to a landfill when it can be used to nourish your garden? I, personally, bring any woody items we cannot burn in our woodstove to our local recycling center, but leaves and grass clippings break down fairly rapidly in our compost pile. Here's how the science of it works.

There are two types of composting: using aerobic or anaerobic bacteria. Aerobic is the preferred method, because it does not "smell."  One of the main reasons you turn compost over with a fork, shovel(turning compost should be done about once or twice a month) or--in the case of large composting setups at the municipal level, a heavy equipment loader--is to work oxygen into the soil for aerobic bacteria.

But bacteria are only the smallest of the organisms in a compost pile. There are level-one insects and invertebrates that ear the organic material, and level two invertebrates that eat the level-one organisms. Their waste products and decomposing bodies become part of the compost. Mite, millipedes, centipedes, sow bugs, snails and slugs, spiders, springtails, beetles ants, flies, earthworms and other types of worms can be found in your compost pile.

Then there are three levels of bacterial. The first, active at around 0 to 55 degrees F, are psychrophilic bacteria. The second-level, mesophilic bacteria are active at temperatures of 550 to 100 F. Then a third category of bacteria, thermophilic, are active at 100 to 140 degrees: a temperature that kills most weed seeds and diseases. 

Your compost also has actinomycetes and fungi. When all of the above organisms are done, you will not be able to tell what sort of plants matter went into the nice, brown compost. It will have a lovely earthy smell and crumbly texture.

Factors that will affect you composting are what type of organic materials you add, moisture levels (moist, not wet), the volume, and aeration (turning the pile). Large pieces break down more slowly than small ones;  too much brown matter, woody hard-to-break-down things like dried stalks and wood chips will slow things down. This affects the carbon-to nitrogen ratio, or C:N. Green matter, or things like grass clippings should be mixed in with brown matter, aiming for a C:N of 30:1. Here is a chart of estimated C:Ns of various materials. If your compost gives off a strong ammonia smell, it may need more brown matter (carbon).

Here is what NOT to put in you compost pile: human or pet feces, meat, bones, grease, peanut butter, whole eggs (shells are fine), or dairy products. Human and pet fecal matter can transmit diseases,  and the others not only encourage pests like rats and mice, but are more likely to smell and use anaerobic bacteria.

Your compost should be at least 3-square feet to allow for heat to build up in the center. The maximum dimensions are 5-ft x ft-ft by any length. It will lose volume somewhat as carbon is released as carbon dioxide. Check moisture by squeezing the compost in a gloved hand. If dry, add enough water that squeezing a fist full of compost only yields one or two drops of water. You can use a temperature probe to see how hot the pile is: if it gets over 140 degrees, turn it.

Once your compost in done, let it cure. Let it sit until it cools to air temperature. This is essential or you may harm your plants. If you buy spent mushroom compost, let it sit out-of-doors for six months to leach out potentially harmful levels of salts, and be aware that it may have pesticide residues.

Here is an article on methods of composting. Red wriggler worm composting is another whole topic, but if you're interested, watch this video.

Some folks recommend compost as mulch but in a hot climate the nutrients will leach right out of your compost. And then I find it gets weeds. My personal opinion is that compost should be worked into your garden soil. Mulch with things like bark chips, straw, or pine straw.


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