Q. I’m composting in plastic trash cans with holes. It’s taking a very long time to make compost despite adding carbon to my grass clippings and kitchen waste. I water and turn it every few days. What am I doing wrong?  Compost piles need to be a certain size for them to be efficient enough to generate the heat needed for weed free and pathogen free compost. A. Compost ingredients are divided into two categories; “brown” ingredients which are loaded with carbon, and “green” ingredients which have much more nitrogen in them. Typical “brown” ingredients might be things like shredded paper, cardboard, sawdust and pulverized woodchips. Commercial composts generate high enough temperatures to kill all weed seeds and human and plant pathogens. Compost thermometers like this one are used to measure the compost temperature before turning it. Turning it cools the compost, aerates it and mixes microorganisms on the cooler surface back into the entire mix.             Typical “green” ingredients can be scraps from fruits and vegetables, green parts of plants including leaves and soft stems. Brown and green ingredients must be in proper balance to achieve a ratio of carbon to nitrogen between 20:1 to 40:1. Machines like this compost turner move down a row of compost called a windrow and mixes it by turning it over thus aerating it.             Composting is controlled “rotting” of a mixture of these ingredients. Brown and green ingredients are finely shredded and mixed together, some water is added, a small amount of soil or fresh compost and it is turned, or aerated, when the center of the compost gets hot. If heat isn’t produced by a compost pile, then one of the necessary ingredients is missing or in short supply.             Ideally, microorganisms from soil or fresh compost feast on moist carbon and nitrogen found in the compost ingredients, heat is produced and the entire mixture “rots” in a few months if these “rotting” microorganisms also get air. Air is provided by turning this mixture periodically or injecting air into the pile.             Commercial composters turn large compost piles when temperatures are about 160°F toward the center of the pile. These high temperatures are needed to destroy human and plant pathogens and weed seeds.             Small amounts of compost are more difficult to start than large piles because of our desert environment. In our desert environment, place small composters like trash cans out of the wind and protect them from the sun.             Make sure microorganisms are in the mixture. Add a couple scoops of fresh compost or a pound of garden soil to this mixture. Add extra nitrogen such as high nitrogen fertilizer or blood meal if you think too much “brown” ingredients are in the mixture.