Composting Additives

Bernalillo County Extension Master Composters

apples iin compost
Photo courtesy of Markus Spiske

January 2021

Some composters choose to use additives with the goal to enhance the decomposition process or boost the quality of the finished compost. Additives may or may not aid or hinder the composting process. Their use is an individual composter’s choice. Informed thoughtful use is advised.

Composting additives may be called activators, accelerators, or inoculants. In some instances, a product may include all actions or just one. Sometimes intentions and functions may overlap in the same product.

These additives may be liquids, powders or granules. Some are homemade. Some are manufactured. Further research may be necessary for any particular product.

Activators stimulate the decomposition process. Accelerators speed the process once it has started. The word inoculant is sometimes used in a haphazard fashion by some composters to refer to any product added to a composting setup. For our discussion, an inoculant is an additive which confers a microbial load to the composting process. The theory is that by adding additional microorganisms to the setup they will enhance decomposition. That is, more microbes may initiate and accelerate the decomposition process.

Activators in the form of microbial inoculants, in general, are not required for the decomposition process to begin or to proceed. Microbes, both active and dormant, are already present on and in any composting feedstock. Given appropriate air and moisture, greens and browns in the setup should provide adequate nutrients for microbial proliferation. A simple guideline is to combine two parts browns (by weight) with one part greens. Attention to an appropriate “diet” for microorganisms is a required step in getting microbial activity started and sustaining that activity. See Nitrogen Sources below, as well as our Desert Composting Recommendations.

The Bokashi closed container method does require inoculation with EM-1 organisms which regulate the anaerobic fermentation process in a closed container. This is often a dry bran fiber which has been inoculated with the effective microorganisms. See Bucket Composting with Bokashi and EM-1 Microbial Inoculant Gardening.

Types of Additives to Consider

Nitrogen Sources: Activator + Accelerator

Nitrogen from natural sources is the microbial nutrition source which provides protein for reproduction and cell wall formation. Nitrogen is the “spark” which fires up microbial proliferation when combined with browns (carbon). Nitrogen is a composting accelerator.

Some natural nitrogen sources:

  • Fruit and vegetable scraps and pulp, coffee and tea grounds
  • Green yard clippings, freshly cut grasses, green leaves, green weeds without seeds
  • Vegetarian animal manures and urine, human urine
  • Alfalfa hay, meal, and pellets
  • Hair, feathers, fur, and blood meal
  • Wine, cider, and beer making leftovers
  • Fish emulsion and other fish products
  • Compost and worm castings

Soil: Microbial and Mineral Inoculant

Well-informed gardeners know that an amended fertile soil has its own microbial ecosystem, so adding small amounts of soil is considered a useful inoculant for the composting process. Soil particles also carry minerals necessary for microbial vitality. By adding your local soil, you are adding indigenous microorganisms. There is no specificity regarding the amount to be added to any setup. Further research may be necessary. Adding too much soil may make the system heavy and cause compaction of the moist organics, which leads to an anaerobic situation. If not corrected, this leads to odors. A suggestion: sprinkle-spread a cup of fertile soil on top of every 5 inches of organic material as you build up.

Compost and Worm Castings: Microbial Inoculant + Nutrient Additive

Compost itself and worm castings contain millions of microorganisms which, when added, may enhance the decomposition process. A suggestion: sprinkle-spread a cup of compost or worm castings on top of every 5 inches of organic material as you build up.

Compost Tea: Microbial Inoculant + Nutrient Additive

A simple compost tea brew may be sprinkled as layers of organics are added to a composting setup. Compost tea should be aerated by frequent stirring. The brew should not be allowed to become anaerobic, which will be odiferous and possibly contain pathogenic microbes and organic acids.

Worm Castings Tea: Microbial Inoculant + Nutrient Additive

Worm castings may be used to make an aerobic tea. Sprinkle finished tea on feedstock layers as they are built up.

Actively Aerated Compost Tea: Microbial Inoculant + Nutrient Additive

This is prepared with an active aeration system run by a powerful air pump. Often a sweetener such as unsulfured molasses is added to feed the microorganisms, thus improving microbial proliferation. Sprinkle finished tea on feedstock as you build.

Manure Tea: Inoculant + Nutrient Additive

Current science practice advises to avoid anaerobic manure brews. These are odiferous, produce organic acids, and may grow microbial pathogens. Any tea should be aerated by frequent, regular stirring with a paddle or an air pump. Aerobically prepared manure tea may be sprinkled on feedstock as layers are built.

Herbal Teas: Biodynamic preparation: Mineral + Nutrient Additive

Teas may be made from herbaceous plants such as nettles, comfrey, chamomile, dandelion, nettle valerian, horsetail, etc.. The residual nutrients from the plants are in the tea. Herbal teas may be sprinkled on feedstock layers as they are built. Again, all teas should be prepared aerobically.

Manufactured Compost Maker Additives: Inoculant + Activator + Accelerator

These products may combine a nitrogen source, dormant microorganisms, a carbohydrate nutrient source and possibly enzymes in the form of granules, powders or liquids. They may be useful when little organic greens (nitrogen) are available. Those made from natural rather than man-made materials support the microbial process. Synthetic sources of nitrogen such as ammonium sulfate do not provide protein which is necessary for microbial cell wall formation and reproduction. Natural nitrogen sources are better at this task. Read the label ingredients and then follow the directions. See Nitrogen Sources above.

Charcoal/Biochar: - Microbial Inoculant

Charcoal broken to walnut size nuggets may be added to a composting setup. Charcoal is very porous, providing spaces for microbial colonization over time. Once the charcoal is biologically activated with microbes it is called biochar. Biochar may be added to a subsequent setup as an inoculant or left in the finished compost

Rock Powders: Mineral Additive + Microbial Nutrients

Rock powders provide minerals for microbial nutrition. Minerals are already present in organics added to a composting setup. A consideration would be whether the soil which you intend to amend needs more of a certain mineral or minerals? Close observation of your plants and a soil test would help answer that question. (See Appropriate Analyses of New Mexico Soils.) Whether to add more minerals to a composting setup and how much to add is an individual's choice and is worthy of further research.

Do you have questions?

If you do, see Ask A Master.