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

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In the garden, effective micro-organisms are mainly used for two things: supporting the compost, and fermenting large quantities of green material, starting with grass clippings. These are two very different uses. The first takes place in the presence of air, the second without air, 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 practice: how to use EM if you choose to, at what doses, 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, then worms and small creatures break down the material in the presence of air. The heap heats up, settles, and within a few months becomes stable humus.

Fermentation, as in bokashi or silage, takes place 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 complete its transformation in the soil or in a compost bin.

EM are first and foremost ferments, rich in lactic acid bacteria. In a well-aerated compost, they do not change the nature of the process: they simply add to the micro-organisms already present. This is why independent composting trials show, at best, a modest gain. A 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 without a control receiving molasses alone: it is hard to say how much is down to the micro-organisms. Independent assessments consider the effect of EM in composting plausible, provided that moisture, air and temperature are well managed.

Key points

Good compost comes from the right mix, air and moisture; EM can support it. Good fermentation comes from excluding air, by compacting and covering airtight; here EM play a central role.

Spraying EM in your compost bin

The dose

The manufacturer of EM-A Microferm Active Solution recommends around 1 litre of product per m³ of material, diluted in twice its volume of non-chlorinated water and applied layer by layer. Other instructions go up to 2 litres per m³. For a 400-litre garden compost bin, that comes to about half a litre of product over the whole filling: with each addition, a few tablespoons (20 to 30 ml) diluted in half a litre of water are enough.

How to do it

1. Add your material, mixing greens and browns as usual: balance remains 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 do not replace these tasks.

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

If you empty a bucket of kitchen bokashi into your compost bin, spread it out and mix it with browns rather than dropping it in as a lump: the method is described in detail in the guide Bokashi and garden compost.

Fermenting your grass clippings under a tarpaulin

In spring, grass clippings arrive in such quantities that a compost bin cannot take them all. Piled in a thick layer, they compact and rot. Fermentation offers another route: a kind of "garden bokashi", similar to farmers' silage.

The method

1. Let the grass dry off for a few hours after mowing. If it is too wet, it ferments poorly, producing liquid and a rancid butter smell. If it stays very damp, mix in a little drier material: dead leaves, chopped straw.
2. Moisten it with EM: a 5% solution, i.e. 50 ml of EM-A per litre of non-chlorinated water, sprayed while mixing, so as to dampen the grass without soaking it. For a large volume, the manufacturer allows around 2 litres of Microferm diluted in 10 litres of water per m³ of material.
3. Optionally add crushed shell limestone (often sold as "shell lime"), in handfuls between the layers: grass acidifies quickly as it ferments, and this coarse, slow-acting limestone moderates the drop in pH without preventing it. Never use quicklime or hydrated lime, which are caustic to skin and micro-organisms alike.
4. Compact firmly, layer by layer, to drive out the air.
5. Cover airtight with a thick tarpaulin, weighted down all the way round. For small quantities, thick, tightly closed bags or a bin 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 heap or in cool weather. Do not lift the tarpaulin in the meantime.

Emy's tip

Build the heap on a tarpaulin laid on the ground, fold it over the top, then tread the whole thing down before weighting it with planks 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, often less well fermented, can go into your next heap or the compost bin. Like any bokashi, it is an acidic pre-compost, not finished compost: mix it into the surface of the soil or add it to the compost, and wait at least two weeks before sowing or planting in that spot. Do not spread it in a thick layer around the base of plants.

Two limits to be aware of. Do not use clippings from a lawn treated with weedkiller. And there is no evidence that fermentation destroys weed seeds: it is the heat of a well-managed compost that takes care of that. To reduce the volume of clippings at source, also consider mowing without collecting, 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: it is to avoid it that they let grass wilt before making silage.

Manure and farm waste: the lasagne method

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

• Build the heap in successive layers of manure, straw and green waste, spraying diluted EM between each layer.
• Add shell limestone and, if you wish, clay. As a guide, the manufacturer gives, per tonne of material, 2 litres of Microferm, 12 kg of shell limestone and 12 kg of clay minerals.
• Compact, cover airtight with a tarpaulin and leave to ferment for at least 8 weeks.

Be cautious about the claims: in a trial carried out in Norway on cattle slurry, 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 heap, or one opened "to have a look": air sets off mould and putrefaction again.
• Grass clippings alone piled in a thick layer in the compost bin: without dry material or proper fermentation, they turn into a slimy mass.
• Quicklime or hydrated lime: being caustic, it destroys the very micro-organisms you are 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 to choose?

These methods complement rather than compete with each other. Traditional compost remains the simplest solution for garden waste and produces stable humus. EM fermentation is useful for dealing with large volumes of green material in one go, and kitchen bokashi accepts leftovers that compost struggles to digest. Vermicompost, for its part, is very well suited to flats. 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 around 1 litre per m³ of material, diluted and applied layer by layer, never replacing the right mix, air and moisture; their effect on the speed of composting remains modest. Their real strength is fermentation: grass clippings left to dry off, moistened with a 5% solution, optionally with added shell limestone, firmly compacted 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 in lasagne layers, with no promise of a nitrogen gain.

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Other topics in the guide

Vermicomposting30 sheets →Composting25 sheets →Bokashi24 sheets →