Some composters promote the importance of special inocula (bacterial activators), supposedly containing several pure strains of laboratory organisms or other biological factors essential to decompose organic matter and fix nitrogen. They call them "enzymes," "hormones," "preserved living organisms," "activated factors," "biocatalyst," etc. In fact, several commercial composting processes are built around the use of some special inoculum, often known only to its discoverer and proponent, who claim it to be fundamental to the successful operation of the process. The need for inocula has always been debatable, and most composting studies have strongly indicated that they are unnecessary.

Bacteria are always present in every bit of organic matter, whether manure, vegetable waste, leaves, and can be eliminated only by drastic sterilization methods. So it makes sense that inocula is not necessary for the composting process. In any case, the number of bacteria is rarely a limiting factor in composting because, provided the environmental factors are appropriate, the indigenous bacteria, which are much better adapted when forms attenuated under laboratory conditions, multiply rapidly. Thus the rate of composting is governed simply by the environmental conditions.

The vast number of enzymes involved in decomposition, as well as the difficulty and expense involved in isolating and synthesizing them would make composting with enzymes highly impractical even if satisfactory preparations were available. The addition of enzymes to raw compostable materials is unnecessary because bacteria synthesize efficiently and rapidly all the enzymes required. The term "hormones" is popularly used to designate the growth factors and vitamins needed by bacteria or other organisms. Organic matter usually contains all the growth factors needed for normal growth. Also, growth factors and vitamins can be produced by microorganisms and will undoubtedly be produced in sufficient quantities in a mixed microbial population to meet normal requirements.

The terms "biocatalyst" and "activated factors" are applied to various biological materials that are supposed to activate and accelerate decomposition and stabilization of organic material. Experimental investigations with sludge digestion and activated-sludge treatment of sewage indicated that biocatalyst did not affect either of these processes. In some cases the "activator" usually supplied some material that was lacking in the compost. For example, straw or paper, which does not contain the necessary biological nutrients, is not decomposed readily alone, but if nitrogen and phosphorus are added, the straw and paper will serve as the source of carbon for decomposition.

Using horse manure, compost material, normal soils, and special commercially prepared bacterial cultures in the composting of mixed garbage and refuse was investigated. Similar materials were composted with and without these different inocula, and although rich in bacteria, none of the inocula accelerated the composting process or improved the final product. There was no significant difference in the temperature curves or in the chemical analyses of the material at different intervals during the composting period. Inocula failed to alter the composting cycle because the indigenous microbial population was adequate.

There have been some interesting recent research studies suggesting that special preparations made from specific plant and other substances used in producing compost on Biodynamic farms can make a difference in the composting process. In these studies, Biodynamic treated composts maintained an average 3.4 degree C higher temperature throughout the 8 week active composting period, and reached maturity faster than the control compost.

On the whole, though, when the environment is appropriate, varied indigenous (originating in a particular region) biological population will multiply rapidly and composting happens. Microbial inoculation would be useful only if the biological population in any emerging environment were unable to develop sufficiently rapidly, or take full advantage of the capacity of the environment to support the increase in numbers. In such a case, a time lag would occur which could be overcome by supplementing the initial indigenous population to the refuse. However, no such time lag has been observed in these experiments or in composting the usual materials that contain a large indigenous bacterial population.

Some groups of organisms in the mixed microbial population apparently remain inactive until the environment is satisfactory for their growth, emerging to perform a role in the succession of steps in the stabilization process. Since the process is dynamic and any individual group of organisms can survive a wide environmental range, one population may begin to emerge when another is still flourishing and yet another is disappearing. Hence, when any group of bacteria is capable of multiplying at a rate equal to that of its developing environment, any addition of similar organisms as an inoculum would be superfluous.

The best inoculum a home composter could use would be a shovelful of their own already finished compost, which would contain the microorganisms present for their specific feedstock.

Successful compost operations that don’t use special inocula in the Netherlands, New Zealand, South Africa, India, China, the USA, and a great many other places, provide convincing evidence that inocula and other additions are not essential in composting.