Speeding up your compost with EM and fermenting grass clippings under a tarp

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In the yard, effective microorganisms are mainly used for two things: supporting your compost, and fermenting large quantities of green material, starting with grass clippings. These are two very different uses. The first happens with air, the second without it, and the techniques are not at all the same.

Do you really need an activator to make good compost? We gave an honest answer in our guide Compost activator: useful or not? Here, we get down to the practical side: how to use EM if you choose to, how much to use, and how to turn your grass clippings into fermented organic matter.

Compost and fermentation: two different approaches

Traditional compost is an aerobic process: bacteria, fungi, and then worms and other small creatures break down the material in the presence of air. The pile heats up, settles, and within a few months becomes stable humus.

Fermentation, as in bokashi or silage, works the other way, without air. Lactic acid bacteria convert sugars into acids, the environment becomes acidic, and the material is preserved almost intact. The result is a pre-compost, which will finish breaking down in the soil or in a compost bin.

EM are first and foremost ferments, rich in lactic acid bacteria. In a well-aerated compost pile, they don't change the nature of the process: they simply add to the microorganisms already present. That's why independent composting trials show a modest gain at best. One trial carried out in a tropical climate on rice straw and goat manure recorded a slightly faster rise in temperature and more advanced decomposition after 90 days with EM, but it had no control receiving molasses alone, so it's hard to say how much of the effect comes from the microorganisms. Independent assessments consider the effect of EM in composting plausible, provided that moisture, air, and temperature are well managed.

Key takeaways

Good compost comes from the right mix, air, and moisture; EM can support it. Good fermentation comes from keeping air out by packing the material down and covering it airtight; here, EM play a central role.

Spraying EM in your compost bin

How much to use

The manufacturer of EM-A Microferm Active Solution recommends about 0.8 quart of product per cubic yard of material (1 liter per m³), diluted in twice its volume of non-chlorinated water and applied layer by layer. Other instructions go up to 1.6 quarts per cubic yard (2 liters per m³). For a 106-gallon (400-liter) backyard compost bin, that comes to about 17 fl oz (0.5 liter) of product over the whole filling: each time you add material, a couple of tablespoons, 0.7–1 fl oz (20–30 ml), diluted in 17 fl oz (0.5 liter) of water is all you need.

How to do it

1. Add your material, mixing greens and browns as usual. Balance is still the key: see our guide Greens and browns.
2. Spray the diluted solution over the new layer, or pour it on with a watering can.
3. Keep aerating and checking the moisture: EM don't replace these steps.

You can also sprinkle a handful of Bokashi EM Compost Activator over your food scraps, especially when starting a new bin. And if you have some slightly old homemade EM-A left over, the compost pile is where it will be most useful: our guide Storing your EM explains how to tell whether it's still good.

If you empty a bucket of kitchen bokashi into your compost bin, spread it out and mix it with browns rather than dumping it in as one clump. The method is explained in detail in our guide Bokashi and garden compost.

Fermenting your grass clippings under a tarp

In spring, grass clippings pile up in such quantities that a compost bin can't take them all. Dumped in a thick layer, they mat down and rot. Fermentation offers another option: a kind of "garden bokashi," similar to the silage farmers make.

The method

1. Let the grass dry off for a few hours after mowing. If it's too wet, it ferments poorly, releasing liquid and a rancid butter smell. If it stays very damp, mix in a little drier material, such as dead leaves or chopped straw.
2. Moisten it with EM: a 5% solution, or 6.4 fl oz of EM-A per gallon of non-chlorinated water (50 ml per liter), sprayed on as you mix, so the grass is damp but not soaked. For a large volume, the manufacturer recommends about 1.6 quarts of Microferm diluted in 2 gallons of water per cubic yard of material (2 liters in 10 liters of water per m³).
3. If you like, add crushed shell limestone (often sold as "shell lime"), in handfuls between the layers: grass turns acidic quickly as it ferments, and this coarse, slow-acting limestone softens the drop in pH without preventing it. Never use quicklime or hydrated lime, which are caustic to skin and microorganisms alike.
4. Pack it down firmly, layer by layer, to force out the air.
5. Cover it airtight with a heavy-duty tarp, weighted down all the way around. For small quantities, thick, tightly closed bags or a trash can with a lid will do the job.
6. Wait: at least 6 weeks according to the manufacturer, more like 8 to 10 weeks for a large pile or in cool weather. Don't lift the tarp in the meantime.

Emy's tip

Build the pile on a tarp laid on the ground, fold it over the top, then stomp the whole thing down before weighting it with boards or soil. The less air left inside, the cleaner the fermentation.

The result and how to use it

Well-fermented grass smells sour, like silage, and keeps its structure as it turns brown. The edges, which often ferment less well, can go into your next pile or the compost bin. Like any bokashi, it's an acidic pre-compost, not finished compost: mix it into the top layer of soil or add it to your compost, and wait at least two weeks before sowing or planting in that spot. Don't spread it in a thick layer around the base of plants.

There are two limitations to keep in mind. Don't use clippings from a lawn treated with weed killer. And there's no evidence that fermentation destroys weed seeds: it's the heat of a well-managed compost pile that takes care of that. To cut down the volume of clippings at the source, also consider mowing without bagging, covered in our guide A natural lawn with EM.

Did you know?

The rancid butter smell of overly wet grass comes from butyric acid, produced by bacteria other than lactic acid bacteria. Farmers know it well: that's exactly why they let grass wilt before making silage.

Manure and farm waste: the lasagna method

For horse, sheep, or poultry manure mixed with straw, and for large volumes of yard waste, the manufacturer suggests a layered method with no turning:

• Build the pile in alternating layers of manure, straw, and yard waste, spraying diluted EM between each layer.
• Add shell limestone and, if you like, clay. As a guideline, the manufacturer recommends, per metric ton of material, 2.1 quarts (2 liters) of Microferm, 26 lb (12 kg) of shell limestone, and 26 lb (12 kg) of clay minerals.
• Pack it down, cover it airtight with a tarp, and let it ferment for at least 8 weeks.

Be cautious about the claims: in a trial carried out in Norway on liquid cow manure, fermentation was more intense with EM, but the nitrogen and dry matter content of the manure was no different from that of the control.

Mistakes to avoid

• Waterlogged material: it rots instead of fermenting.
• A poorly sealed pile, or one opened "just to check": air lets mold and rot take hold again.
• Grass clippings alone dumped in a thick layer in the compost bin: without dry material or a controlled fermentation, they turn into a slimy mass.
• Quicklime or hydrated lime: it's caustic and destroys the very microorganisms you're trying to encourage.
• Chlorinated water for diluting EM.
• Using it too soon: freshly fermented material placed in contact with roots or seedlings can harm them.

EM, compost, or vermicompost: which should you choose?

These methods complement each other more than they compete. Traditional compost is still the simplest solution for yard waste and produces stable humus. EM fermentation is useful for handling large volumes of green material all at once, and kitchen bokashi accepts leftovers that compost struggles to break down. Vermicomposting, for its part, is a great fit for apartments. To compare these options for your situation, see our guide Compost, vermicompost, bokashi, or EM: which method to choose.

In summary

In a compost bin, EM are used at about 0.8 quart per cubic yard of material (1 liter per m³), diluted and applied layer by layer, and they never replace the right mix, air, and moisture; their effect on how fast compost breaks down remains modest. Where they really shine is fermentation: grass clippings left to dry off, moistened with a 5% solution, with shell limestone added if you like, firmly packed down, and covered airtight for 6 to 10 weeks, produce an acidic pre-compost to mix into the soil or the compost, with a two-week wait before sowing. The same approach applies to manure built up in lasagna layers, with no promise of a nitrogen gain.

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Other topics in the guide

Vermicomposting30 fact sheets →Composting25 fact sheets →Bokashi24 fact sheets →