Mulch: why bare soil isn't clean, and why it's the crown of our method
Look at most terrace gardens and you will see the same thing: neat pots of bare, dark soil, swept clean of every leaf. It looks tidy. Most people believe it is healthy.
It is not.
To be clean, you do not have to be shaved and naked. So why do we strip the clothes off Mother Earth, shave her bare, and then call what we see clean?
In nature, soil is almost never bare. Forest floors are covered in fallen leaves. Grasslands wear a mat of dry grass. Only where people interfere does soil lie naked to the sun and rain, and that is where it begins to die.
Mulch is that covering, given back. In the Nabhi method, four pillars feed the plant: the olla, hot compost, indigenous microorganisms and the hugel base. The mulch feeds none of them directly. It protects all of them. That is why we call it the crown, and why I think it is the most important part of any natural growing system.
Six weeks without rain
Our farm in Gudibande is dryland: there is no irrigation, and the crops live on whatever rain falls. On land like this, the difference mulch makes is not subtle. It is the difference between a harvest and nothing.
This monsoon brought some of the longest dry spells we have seen. For up to six weeks, it was as hot as summer, with not a single drop of rain.
Where the soil was mulched, the plants held up. Where it was not, everything dried.
Same land, same weeks, same sun. The only difference was a layer of dry grass.
What mulch does for your plant
It keeps the soil from cooking
On a sunny afternoon, put one hand on bare soil and the other under a layer of mulch. The bare soil feels like a hot pan. Under the mulch, the soil is cool, and you will want to keep your hand there. The University of Georgia's guide to mulching vegetables puts it simply: mulch keeps soil cooler in summer (UGA Extension). On a terrace, where heat rises off the floor as well as beating down from above, that matters even more.
It holds water where the roots are
Mulched soil soaks up water better and loses less of it to evaporation (UGA Extension). The surface does not bake into a crust, so what water there is stays below, near the roots. On cool mornings, the grass also catches mist and dew, adding a little moisture of its own.
It stops disease climbing the plant
When heavy rain hits bare soil, it throws mud up onto the lowest leaves and fruit. That splash carries soil-borne disease with it, which then spreads up through the plant. I have seen it damage not only tomatoes, chillies and brinjal, but cucumbers, gourds and many others. Mulch takes the force of the rain, and keeps leaves and fruit off the soil altogether.
It keeps weeds out, and food in
Weed seeds landing on mulch cannot reach the light, and young weeds die beneath it (UGA Extension). Every weed that never grows is one less plant competing for the nutrients you meant for your vegetables.
It brings back the earthworms
Lift the mulch after a few weeks and you will often find earthworms underneath. Earthworms are a sign that the whole soil food web is coming alive: the fungi, bacteria and small creatures that turn dead matter into food for plants. They only stay where the soil is cool, moist and covered.
Life returns, even on bare rock
Parts of our land are rocky slopes where the rain has been washing the topsoil away for a generation. In places, less than 3 inches of soil sits over the rock.
Even there, when we lay mulch and leave it, life comes back underneath. The soil darkens, it holds moisture, and the small creatures that make soil alive return.
If mulch can bring life back to a rocky slope that lost its soil a generation ago, imagine what it does in a grow bag that starts with good soil.
Our mulch: kashi grass
The grass we use most is called kashi. It grows wild in the crevices between rocks around Gudibande. When you cut it, it smells like meditation grass. It is tough and fibrous, and it breaks down very slowly, often staying good for a year or more. That is exactly what a mulch should do: one that rots in weeks has to be replaced, while one that lasts the life of the crop never needs touching.
We gather it from two places, and neither takes anything from the animals or the land.
The roofs of monsoon huts. Every monsoon, farmers around Gudibande build small huts in their fields so they can stay and guard their crops from wild animals coming out of the forest. The roofs are thatched with dry grass that cattle will not eat. When the season ends and the huts come down, we take the roof grass. The wood from the same huts becomes the hugel base of our bags.
The grasslands, before the fire season. Every year, the dry grass on the open land around us is lost to wildfires. Grass we cut there would have burnt anyway. And when enough of the dry brush is cleared, there is less fuel for a fire to spread.
Grass that would have been firewood or wildfire fuel becomes a year of shade and moisture for a vegetable plant.
Before any grass goes onto a bag, we cut off its flower heads by hand. Kashi is a hardy grass, and we do not want to take any chance of it seeding in your grow bag.
Why not other mulches?
Almost any natural covering is better than bare soil. But a good mulch also has to last, and be available locally without taking something that people or animals need. This is how the common choices compare for us in Gudibande:
| Mulch | Good points | Why it is not our main mulch |
|---|---|---|
| Kashi grass | Lasts a year or more; free; would otherwise burn | (Our choice) |
| Dry leaves | Good mulch where plentiful | Breaks down faster; availability varies by season and place |
| Coconut coir | Holds water well | Coconuts do not grow in plenty here |
| Paddy straw | Easy to spread | It is feed for cattle; we will not take it from them |
| Sugarcane trash | Coarse and long-lasting | Cane needs irrigation; nobody grows it on dry land here |
| Plastic sheet | Convenient; stops weeds | Harms the soil and leaches chemicals (see below) |
The problem with plastic mulch
Plastic sheets are common on farms because they are convenient. But under a plastic sheet, heat and humidity build up, and the life in the soil cooks. A farmer then has to add chemicals to make up for the soil life that was lost. We work the other way: we feed the soil food web instead of destroying it.
Plastic also leaves something behind. A 2023 study of a farm in Finland found that soil under polyethylene mulch film contained significantly more of certain plasticisers, including phthalates, than nearby soil that had never been mulched. The researchers concluded that plastic mulch films are a source of these chemicals in farm soil, and called for urgent study of the risks to ecosystems and health (Scopetani et al., Frontiers in Environmental Science).
So why is plastic so widely used? Convenience is part of it. I suspect the rest is the same reason the research on indigenous microorganisms is so thin: plastic mulch makes money for an industry, and dry grass from a hut roof makes money for nobody.
How the crown goes on a Nabhi bag
The mulch is the last layer of every bag we build. We chop the dry grass and twigs into short pieces and lay them at least 3 inches deep, until only the lid of the olla shows through.
The plant then grows on at our farm before it comes to you. In its first week or two, we pull any weeds that manage to appear. By the time it reaches your balcony, the crown has settled and the surface is clean.
After that, there is nothing for you to do with it:
- Do not remove it. It may look like dry rubbish. It is the most important layer in the bag.
- Do not water on top of it. Water goes only into the olla.
- Do not top it up or dig it in while the plant is growing.
The mulch needs refreshing only when the plant has finished its life and you want to replant the bag.
Questions people ask about mulch
Isn't bare soil cleaner?
It looks cleaner, but it is not healthier. Bare soil bakes in the sun, crusts over, loses water fast, splashes disease onto plants in the rain, and loses the life that feeds your plant. Covered soil stays cool, moist and alive.
What is the best mulch for a terrace garden in Bengaluru?
Any tough, dry, natural material that breaks down slowly: dry grass, dry leaves, or coconut coir if you can get it easily. Avoid fresh green clippings, which rot quickly and turn slimy, and avoid plastic sheet.
How thick should mulch be?
At least 3 inches. A thin sprinkling lets light and weeds through. Keep it loose rather than packed tight, so air still reaches the roots (UGA Extension).
Will mulch bring pests or weeds?
Not if it is dry and seed-free. That is why we remove the flower heads from our grass before it ever touches a bag. Under a good mulch you are far more likely to find earthworms than pests.
Should I move the mulch aside to water?
In a Nabhi bag, no. You never water the surface, only the olla, so the mulch can stay exactly where it is.
Can I use plastic mulch in a grow bag?
We would not. It traps heat and humidity that cook the life in the soil, and research has found it can leave plasticisers such as phthalates behind in the soil.
Give the soil back its clothes
Whether you grow in a Nabhi bag or anywhere else, the single most useful thing you can do for your plants is to stop leaving the soil bare. Cover it, leave it covered, and let the life underneath do its work.
Mulch is the crown because it protects everything beneath it. Take it away, and every other part of the system has to work twice as hard.
In a Nabhi bag, the crown rests over four pillars: the olla, hot compost, indigenous microorganisms and the hugel base. Read how they work together in the Nabhi method, or choose a plant that arrives already crowned.
Sources
- Mulching Vegetables, University of Georgia Cooperative Extension
- Chemical leaching from polyethylene mulching films to soil in strawberry farming, Scopetani, Selonen, Cincinelli and Pellinen, Frontiers in Environmental Science, 2023