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Hot compost: food a plant can use right away

By · First published

A young plant is hungry from the day it is planted. Its roots are small, and it needs food it can take up immediately, not food that will become available months later.

In a Nabhi bag, that early food comes from hot compost, which we make ourselves at our Gudibande farm. It is the second of the four pillars, and the one that carries a plant through its first weeks.

Why hot, not cold

Most home compost is cold compost: a pile left to rot slowly over many months. It works, but it rarely gets hot enough to do two important jobs.

In a well-built heap, heat-loving bacteria multiply so fast that the core gets very hot. That heat kills weed seeds and the organisms that cause plant disease. A study for the US Sustainable Agriculture Research and Education programme found that compost held at 55 to 77 °C for 15 days, and turned regularly, killed both the seeds of a common weed and the spores of early blight, a disease of tomatoes and potatoes (SARE).

That matters in a grow bag. Weed seeds in the soil compete with your plant, and disease carried in compost can follow it to your terrace. Hot compost brings neither.

How we make it

  1. Start with desi cow manure. Farmyard manure from indigenous cows is the base of every batch.
  2. Wake it up. We activate it with ghanajeevamrutha, made by Subhash Palekar's method, mixed into water with pure jaggery and left for two days. More jaggery water follows, and the manure is mixed through thoroughly.
  3. Wait for the first heat. We watch the core temperature. Once it climbs past 60 °C (140 °F), the heap is alive and ready to be fed.
  4. Feed it greens. Chopped weeds and fresh green plant material go in, generously.
  5. Shape it into cones. Each heap is about 3 feet wide at the base and 3 feet tall. That size holds the core between 65 and 71 °C (150 to 160 °F): hot enough to clean the compost, not so hot that the useful life inside it dies.
  6. Turn it when it cools. Whenever the core drops below 60 °C, usually every two or three days as the microbes use up their food, we turn the heap to give it fresh air. It heats up again.
  7. Stop when the heat stops. We keep turning until the heap no longer heats up after a turn.
  8. Let it cure. The compost then rests for about 15 days, until it is at room temperature all the way through.

What comes out is dark, crumbly and clean, with nutrients in a form plants can take up straight away.

Watch the heat

Here is one of our heaps on its second day, with the thermometer reading about 65 °C inside.

We check the temperature constantly. It is the only reliable way to know when to feed the heap, when to turn it, and when it is done.

Why compost alone is not enough

Compost on its own, in different preparations, was one of the things I tried in my grow bag trials. What those trials taught me is that no single input does the whole job.

Compost is the fast food of a Nabhi bag. It carries a plant through its first weeks, while the other pillars take over for the long run: forest microorganisms that help roots reach nutrients for months, and an aged-wood base that feeds the soil slowly over the life of the bag.

In the bag, the compost is blended with garden soil and the microorganism culture into our potting mix. Read how all the pillars work together in the Nabhi method, or choose a plant grown in it.

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