Compost activators: worth it or not? What they really do and when to use them

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Powders, granules, liquids, “starter” packets: garden center shelves are packed with compost activators that promise faster, odor-free compost. We even make our own bokashi activator. That's all the more reason to be straight with you: a well-managed compost pile usually doesn't need an activator.

That doesn't mean these products are never worthwhile. Here's what an activator really does, when you don't need one, when it can give things a boost, and how to do without one using free alternatives.

What an activator really does

The word “activator” actually covers two families of products that work in completely different ways:

• Nitrogen sources (manure, horn meal, poultry droppings, nettle tea, some commercial products): they feed the microorganisms already present. They're useful when the compost contains too much carbon-rich brown material and is short on “greens.”
• Microorganism inputs (starter cultures, bacteria, effective microorganisms): they aim to “seed” the compost with extra microbes.

But compost microorganisms are already everywhere: on peels and leaves, in the soil, and in mature compost. So there's no need to add an inoculant or activator to the pile: soil and finished compost already contain the microbes that will break down plant matter.

When you don't need one: a well-balanced compost pile

A compost pile that gets a balanced mix of green and brown materials, enough air, and the right moisture needs nothing else. Let's be frank: composting usually goes very well without these products, as long as you balance what goes in and keep it aerated.

The few studies available point the same way. In 2010, a team of researchers working with household waste compared a control compost with composts seeded with effective microorganisms (EM) or a fungus, Trichoderma: they found no significant difference in the end result. Another study from 2015, which tested two commercial EM products, concluded that they shortened composting time only marginally.

Above all, no activator can make up for compost that is too compacted, waterlogged, or dried out. If yours isn't breaking down or smells bad, the cause is almost always the mix, air, or water: start with our guide Smelly or slow compost: troubleshooting, and review the balance described in Green and brown materials.

Key takeaways

An activator is never a requirement for success. The right mix, air, and moisture do most of the work; at best, an activator gives a little boost.

When a boost can help

There are a few situations where an extra input makes sense, as long as you don't expect miracles:

• Lots of brown material all at once, for example a big load of fallen leaves or shredded branches in the fall: a nitrogen input gets decomposition going again. This is where an activator is most useful.
• Starting a new composter: a few shovelfuls of garden soil or mature compost provide a varied population of microorganisms right away. A commercial product isn't necessary, but it does no harm.
• Winter: compost slows down in the cold, and no activator really changes that. What helps is insulating the pile, protecting it from rain, and keeping a good mix.
• Odors: some gardeners use effective microorganisms to reduce bad smells, especially from food scraps. But a smell is first and foremost a sign of too little air or too much green material, and that's what you should fix first.

Natural activators, free or nearly free

Before buying anything, take a look at what you already have on hand:

• Mature compost or garden soil: one or two shovelfuls in the heart of the pile make the best “starter” there is.
• Manure (horse, poultry, rabbit): rich in nitrogen and microorganisms.
• Fresh nettles or comfrey, placed in the center of the pile, or nettle tea, a very good decomposition activator.
• Roasted horn meal or poultry droppings, sandwiched between layers of leaves.
• Fresh grass clippings, in thin, well-mixed layers: they heat up quickly and supply plenty of nitrogen. We cover them in our guide to green yard waste.

For hot composting, where the rise in temperature depends heavily on the available nitrogen, these inputs placed in the heart of the pile are especially useful: see our guide Hot composting (Berkeley method).

Willy's tip

Always set aside a bucket of mature compost when you harvest. When you restart an empty bin, just spread it over the first layer: your new scraps will be colonized by the microorganisms from the old compost.

Effective microorganisms (EM) and bokashi

Effective microorganisms, or EM, are a mixture of microorganisms produced by fermentation, mainly lactic acid bacteria, often combined with yeasts and photosynthetic bacteria depending on the label. For conventional composting, independent evaluations are still scarce, and the trials mentioned above showed no clear benefit. So we present them as an optional aid, not a guarantee.

Bokashi is a different story. Here, the activator is part of the method: it supplies the lactic acid bacteria that start fermentation in the sealed bucket. Without it, scraps are likely to rot instead of fermenting. Our Bokashi EM compost activator, a bran fermented with EM, is made first and foremost for this purpose. It also comes in handy in a conventional composter, as we explain below.

If you'd like to try EM on your compost, EM-A Microferm activated solution is ready to use: the manufacturer recommends diluting it in water and spraying it between layers. EM-1, the mother solution, is mainly used to make your own activated solution or fermented bran. Detailed instructions for both are in our guide to effective microorganisms, which also reviews what the science says.

Our bokashi activator in the composter: when and how

We make our activator on the farm primarily for bokashi, but it also has a place in a backyard composter. It doesn't replace a good mix, air, and moisture, but it can be genuinely helpful:

• To fight bad smells: sprinkled over food scraps, it adds lactic acid bacteria that make them ferment rather than rot, which cuts down on strong odors. This is especially useful in summer, or when you add a lot of wet peels at once.
• To start a new composter: a handful over the first additions, along with a shovelful of garden soil or mature compost.
• To revive stalled compost, once moisture and aeration have been corrected.

How much to use: a handful, or about two tablespoons, spread over each addition of food scraps, then covered with a layer of brown material. There's no need to put any on dead leaves or shredded branches. If a smell persists despite everything, go back to the causes covered above: too little air or too much green material.

Willy's tip

Keep a jar of activator next to your kitchen compost pail: add a pinch each time you fill it, and the scraps will arrive at the composter already fermenting, with no strong smell when you open it.

Making your own activator

As we've seen, the simplest options cost nothing: mature compost, garden soil, nettles, or manure. If you practice bokashi, you can also make your own fermented bran. Our kit for making your own Bokashi & Compost activator brings together wheat bran, EM-1, and molasses. Here's how our recipe works:

1. Warm 17 fl oz (500 ml) of non-chlorinated water to 77–86 °F (25–30 °C). Let tap water stand uncovered for 24 hours so the chlorine evaporates.
2. Add 1 fl oz (30 ml) of EM-1, stir, then add 1 fl oz (30 ml) of molasses.
3. Pour it over 2.2 lb (1 kg) of wheat bran and knead: the bran should be damp but not soaking wet.
4. Pack it into a black bag tied with a knot, place it in an airtight container, and keep it somewhere warm and dark.
5. Wait three weeks without opening it. A fermented smell and a light white film are signs of success.

Dried and stored away from light, it keeps for about a year. All the steps are detailed in our bokashi guide.

The bottom line

A well-balanced, aerated, and properly moist compost pile usually doesn't need an activator: the microorganisms are already there. A nitrogen input can help when there's too much brown material, and a few shovelfuls of mature compost are enough to get started. Effective microorganisms remain an optional aid for conventional compost, with results that vary from study to study; there, our bokashi activator is mainly helpful against food scrap odors and for starting or reviving a compost pile. For bokashi, on the other hand, it's essential. Before you buy anything, always start by checking the mix, air, and water.

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Other topics in the guide

Vermicomposting30 fact sheets →Bokashi24 fact sheets →EM microorganisms26 fact sheets →