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Composting 101 Lisa and Kathleen Putnam

Updated: Nov 14, 2019

As discussed in our previous posts, keeping your soil alive and nutrient-rich by using crop rotation and cover crops is paramount for a healthy garden. To further keep your soil vibrant, you need to replace what you are harvesting by adding compost several times during the year. If we remove 100 pounds of tomatoes, we need to replace that biomass with 100 pounds of a combination of compost, water and sun.

Making your own compost is easy and rewarding. There are various methods and techniques of composting:

  • Hot, or active, composting

  • Passive composting

  • Digging a hole

  • tumblers

The most important aspect of a successful vegetable garden is healthy soil. As our previous post emphasized, keep your soil alive and fertile by keeping it covered with mulch (living mulch, like plants, or ground mulch, like straw), always have plants growing in the soil to feed the Soil Food Web, grow a cover crop every second or third crop and add one to two inches of high-quality compost in between crops.

What is compost? Compost is the key ingredient in organic gardening and helps to keep your soil alive with microbial life. It is essentially organic matter that has been decomposed and recycled as a fertilizer and soil amendment.

Compost is composed of four ingredients: carbon (browns), nitrogen (greens), water and air. The recipe for a perfect compost is 3 to 4 parts carbon, 1 part nitrogen, water to moisten, and air (which you add by turning the entire mixture). Typical browns include shredded newspaper or cardboard, dead leaves, wood shavings, straw and other carbon-rich organic matter. Typical greens are grass clippings, kitchen waste, green leaves, weeds (without seed heads) and farm animal waste. Materials you do not want to add to your compost pile include meat products, bones, dairy, oils/fats, or domestic animal (cat/dog) waste. Egg shells are okay.

Whether you are performing active or passive composting, the process for the decomposition of organic materials is ultimately the same. Different microorganisms have distinct roles in the composting process. In the first phase, bacteria actively break down nitrogen; in fact, bacteria make up about 80-90% of the microorganisms in a compost pile, numbering in the billions in a single gram of compost. For the first 24 hours, the compost pile temperature will rise from mesophilic, or moderate temperature, to thermophilic, or high temperature. This thermophilic phase lasts about two to three weeks. The bacteria will produce heat, which will bring your pile temperature up to 140-160 degrees very rapidly. When your compost pile reaches 140 to 150 degrees (you can measure the temperature with a compost thermometer), you will need to add air and water. To add air, you turn your pile by using a garden fork to move the pile (the top becomes the bottom). While you are turning it, have a hose handy to add enough water so when you squeeze it, a drop or two of water comes out. You want the pile to be moist, but not soaking wet. You will need to turn your pile and add water about four or five times during the thermophilic phase.   The next phase is the cooling, or maturation, phase, which can last from several months to several years. This is when the fungi move in to break down the carbon material. During this phase, you do not need to turn your pile or add water. Your pile just sits there in ambient temperature and decomposes on its own. Your compost is ready to use when you cannot recognize any of the primary ingredients (apple core, etc.). The compost keeps getting better and more stable as it decomposes. It eventually turns into humus, which is a highly stable organic matter.

Hot (or active) composting

When building a pile for hot composting, you will want to make a pile that is at least three feet cubed. This is the minimum amount of biomass the bacteria need to begin breaking down the nitrogen. The process of hot composting is described above. Once you start your hot compost pile, you have officially started the thermophilic phase and you cannot add new material. If you have additional material, you will need to keep your scraps for your next pile.

Passive composting

Digging a hole One passive method is simply to bury your kitchen scraps. The hole should be at least 8 to 10 inches deep to avoid any rodent activity. You simply dig new holes, fill them with scraps, cover them up with soil and then move to the next spot. This is an easy way to add organic matter and nutrients to your soil. We would do this for perennials, but not in a vegetable bed.

Tumbler A tumbler is another handy way to deal with the incoming stream of yard waste and kitchen scraps. It is best to buy a tumbler on the larger side. This assures that it is easy to turn. Our experience is that tumblers make sticky and clumpy compost, but if you add more finely chopped-up carbon material, that should help.

Composting tips

  • There is no need to buy any specific products for your compost pile other than what we have already described. The microbes are plentiful and will do the necessary work for you. 

  • The smaller the pieces of raw material are that you add to your compost pile, the faster the decomposition will occur.

  • During the entire process, the compost pile should smell fresh, somewhat like a forest floor. If it does not, usually one of two things have gone off track. Either too much nitrogen is present, which means you need to add more carbon. Or it has become too wet and you should turn the pile to try and dry it out or add more dry carbon.

  • An easy way to promote composting in your home is to place a receptacle on your counter. After preparing meals at home, put the scraps in the container and take it out to your compost pile.

How to use your compost when it is ready We like to put a two- to three-inch layer of compost on top of our vegetable beds. We do not roto-till it in as tilling disrupts the microbial community in the soil. We simply place it on top, and cover it with straw; however, you can work it into the top couple of inches of soil, without major disturbance to the life in the soil. After you place your compost, make sure you protect it by adding a layer of mulch (straw or grass clipping) over the top. Sun, wind and rain can be hard on the microbes in the soil and compost. Composting is a win-win for everyone. It helps give plants that nutrient-rich, living humus — one of the healthiest ingredients around. It also keeps table scraps out of the garbage, easing the strain on our landfills. The Environmental Protection Agency (EPA) estimates that 30 million tons of food waste ends up in landfills each year, causing more than 3.5 billion tons of greenhouse gases. Happy composting!

Lisa and Kathleen Putnam

Lisa has a BS in Agricultural Economics from UC Davis. She also studied nutrition science there. She currently operates a small sustainable organic farm in Woodside and is a lifetime gardener, a UC Master Gardener (1999) and Master Composter (2010). Lisa’s passions are composting and the Soil Food Web. She teaches both summer and winter vegetable gardening at Lyngso, Common Ground and several local garden clubs.

Kathleen is a professional organic vegetable gardener and an ISA Certified Arborist serving the Mid-Peninsula region. She has a degree in Environmental Horticulture from City College of San Francisco and is a UC Master Gardener. She teaches classes about vegetable gardening and fruit tree pruning throughout the Bay Area, at Common Ground, Lyngso, San Francisco Community Gardens, Los Altos Garden Club, Portola Valley Garden Club, San Mateo Master Gardeners and the San Francisco Professional Gardeners Association. 

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