Compost your poo: Waste not, not waste

October 21, 2014

There is an energy cost to everything we do, and so far our activities aren’t self-sustaining. Seven billion of us draw down non-renewable resources, overfill pollution sinks, and use renewable resources more quickly than they can regenerate.

Composting is one way to balance the books, turning kitchen and garden scraps into humus, a brown or black substance that contributes to healthy soil.

Humus feeds microbes in the soil and holds 900 percent of its weight in water—versus sand’s two percent and clay’s 20 percent—giving humus-rich gardens and farms an advantage during droughts.

Each of us wastes about 800 or 900 pounds of one potential humus ingredient each year, and we waste 3-4,000 gallons of drinking water to do it: Human feces contain an estimated $18.67 billion worth of agricultural nutrients annually in the US alone.

Humus is superior to synthetic fertilizers because it stays put and releases its nutrients slowly. Agricultural runoff contributes to “dead zones” in the oceans, which can no longer support fish because agricultural nitrogen displaces oxygen.
From 1950 and 1990, global consumption of artificial fertilizers rose from 14 million tons per year to 140 million tons. This wasn’t an entirely bad thing; it saved lives, but at a cost.

Last summer, Toledo residents spent several days without drinking or bathing water. Runoff had caused a bloom of cyanobacteria—blue-green algae that excreted a toxin, contaminating city water.


In The Humanure Handbook, Joseph Jenkins describes a method for turning shit into gold. You build a box with a toilet seat and place a five-gallon plastic bucket inside. Whenever you make a deposit, cover it with sawdust.

The pile should be 30 parts carbon (sawdust, rice hulls, or other plant matter) to one part nitrogen. Jenkins provides tables for calculating the carbon/nitrogen ratio of different materials. It’s better to add too much carbon because adding too little lets nitrogen escape as smelly ammonia.

Because it gets hot, humanure can digest meat, fat, bones, and pet waste that usually can’t be composted. It also kills seeds in garden scraps, reducing weeds.

Composting humanure happens in four stages. The first features bacteria that prefer temperatures between 68º and 113º F, including bacteria that live in our gut.

They begin to change the appearance of the organic material and raise the pile’s temperature. At 111º, heat-loving bacteria take over and raise the temperature even higher, killing most pathogens.

Temperatures can approach 200º, but this isn’t desirable. There have been fires in very large compost piles that were too dry, flaring at 194º.

High temperatures also kill organisms necessary at later stages. Temperatures between 122º and 130º are adequate to kill pathogens and reduce all but woody materials and very coarse kitchen or garden scraps.

After a few weeks, the heat-loving bacteria have consumed all their food, and the temperature drops. This lets in bacteria that prefer cooler temperatures, plus fungi, actinomycetes, and tiny animals. At this stage, wood and larger things become humus.
The fourth, “curing,” stage is a long period when microorganisms are killing the last of the pathogens. Testing can determine when the humanure has safely turned to humus. People who aren’t confident that this process is safe may prefer to use the humus in orchards or vineyards.

Another process for turning feces into soil uses the larvae of the black soldier fly, which is native to the Southern United States and is not a pest.

Black soldier fly larvae are used to control pollution and odor in factory hog farms and large poultry operations. Conveyor belts beneath slatted floors collect manure and deliver it to the larvae, which reduce volume and pathogen levels. The remaining, odorless material is used as a soil amendment, and the pupae are gathered to feed fish in ponds and tanks.

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