Getting the best carbon/nitrogen ratio is an autumn project.
I love composting. Growing, harvesting and eating the bounty of my garden is honestly a secondary benefit of my primary goal; building luscious, thick, rich loamy soil. It’s literally a slow motion manifestation of earth magic. Transforming kitchen scraps, yard waste, and even junk mail into a nutrient dense material teeming with microbes makes me feel like a humus Houdini.
When you make your own compost, you have the ability to amend your soil and provide lasting fertility without purchasing outside inputs. Lugging heavy sacks of materials home from the garden center can be a thing of the past. I have yet to find a commercial bagged compost that looks as good under the microscope as even a poorly managed back yard pile.
Often, however, most gardeners don’t have as much as much compost as they need, especially in the spring. This is because most of us compost with a static compost pile. In a static compost pile, materials are added slowly, generally on a daily basis. The pile slowly grows and breaks down; maybe it gets turned, maybe it doesn’t. The pile is occasionally watered, often forgotten. This is the simplest, but far and away the least effective way of composting. By slowly adding material over time, it often becomes difficult to discern when the compost is “finished.” It often doesn’t heat up enough, because there is insufficient thermal mass for thermophyllic bacteria to generate the heat to sterilize weed seeds and pathogens. It can take months, or even a year or more if the pile is left dry, to fully compost. However, since most of us only generate a small amount of kitchen scraps every day, this is the option we are left with.
But if we leverage the season, this time of year we have an abundance of compostable materials at our dirty little fingertips! Fall is the absolute best time to build a pile, as the sheer volume of materials almost become a liability, with hefty piles of yard waste accumulating when we tidy the yard before winter. Frost-killed tomatoes and summer crops pulled from garden beds accumulate, and if you have chickens I’m willing to wager you’re long overdue to clean out the coop. When we have enough materials to build a compost heap all at once, we can now get into an active compost scenario.
In an active compost strategy, piles heat up quickly, and sustain a high enough temperature to sterilize weed seeds, eliminate diseaseand deter rodents and other pests. Rather than months, we can have finished compost in weeks. In fact, I can turn watermelon rinds, coffee grounds, cardboard, kitchen scraps and any other compostable materials into finished compost in as little as 15 days! Not only can it be done, it’s the standard. In fact, to make compost to USDA Organic standards, the pile must maintain a temperature of 131 degrees for 15 days, being turned at least five times, and the process is complete. The piles can then be left to age, a process which continues to increase the quality of the material.
Building an active compost is simple, and by following a few basic rules you’ll be well positioned with a plethora of compost for spring. This technique is for building a compost pile all at once, and it is important that once it is built, new materials are not added to the pile.
The active compost process
A pile must be at least a 4’ x 4’ x 4’ mass to generate and retain sufficient heat. This size is an ideal, and piles shouldn’t be made much larger, as going too big will compress the pile, limiting oxygen availability.
The materials in the pile should have the moisture of a wrung out sponge. Moisture should be added as the pile is built, and materials like straw, hay or wood chips should be soaked overnight to absorb adequate moisture. If a pile is too dry, microbial activity is halted (remember dehydration of fruits, meats, and vegetables is a useful preservation method). If the pile is too wet, it will go anaerobic, as the water inhibits proper oxygenation.
Our good composting microbes are all aerobic, meaning they thrive in the presence of oxygen. Oxygen is added to the pile every time we turn it, resulting in an increase in temperature, similar to blowing on the coals in a fire.
To build a proper pile, we must pay attention to our carbon to nitrogen ratio, or C:N. C materials, or “browns,” are plant materials that have had all of the nitrogen volatilized out of them; think cardboard, fallen leaves, wood chips, etc. If you seal it in a baggy and come back a week later and it smells fine, it’s a carbon. N materials still have the nitrogen; think kitchen waste, coffee grounds, fresh cut grass. If you seal it in a baggie and come back a few days later and it’s nasty, it’s an N. For general purposes, we aim for a C:N ration of 30:1. C:N charts are simple to find with a quick Google search. This requires some simple averaging math, so ask your local 6th grader if you’re stumped on this one.
The key to this process is the unit of measure. One trick is to use a five gallon bucket and measure the volumes of the materials, not the weight. For example, five five-gallon buckets of a 40:1 material combined with five five-gallon buckets of a 20:1 material would result in our ideal 30:1 ratio. A 4’ x 4’ x 4’ pile requires approximately 110 five gallon buckets worth of measured materials. Mix all ingredients thoroughly as you build the pile.
Temperature matters. We want to maintain a minimum of 131F, and a maximum of 160F. Temperatures lower than 131 will not sterilize weed seeds and pathogens; higher than 160, we begin to lose nutrients to volatilization. To monitor your piles’ temperature use a compost thermometer designed to probe into the center of the pile. With the proper C:N ratio, our pile will heat to 131 degrees within 36 hours, and often in as few as eight hours. If it doesn’t get that hot within 48 hours, then there are not enough N materials in the pile, and you’ll need to add more and remix your pile. Monitor the pile daily, and once the temperature drops below 131, turn the pile. This will result in the pile heating up again. Simply keep monitoring the temperature and turn it every time it drops below that 131 threshold. Once the pile stops heating up, it can be left to age, and voila! You have finished compost, fast.
James Loomis is the Green Team farm manager for Wasatch Community Gardens.