Composting Done Right: The Mix, Moisture, Hot Phase and Fixing a Smelly Heap

Every farm produces waste: straw, stalks, leaves, weeds, manure, spoiled fruit and vegetable trimmings.

Burnt, it pollutes the air and is lost.

Left in a careless heap, it smells, breeds flies and leaks its nutrients away.

Composted well, the same waste becomes dark, crumbly organic matter that improves soil structure, holds water, feeds soil life and returns nutrients to the field.

Good compost is not luck. It is a controlled process, run by microbes that need the right food, water and air. In this video: how composting works, the right mix of materials, moisture and air, the hot phase that kills weed seeds and pathogens, how to fix a heap that smells or will not heat, how to tell when compost is ready, and how to use it safely.

These principles are the same on a small plot or a large farm, in any climate.

Composting is decomposition by bacteria, fungi and other organisms that need oxygen.

In a well-built heap it runs in phases.

First, as the microbes start on the easy sugars, the temperature rises within a day or two. Then comes the hot, or thermophilic, phase,

when the centre of the heap reaches about fifty-five to sixty-five degrees Celsius. This heat breaks down material fast, and kills most weed seeds and disease organisms.

As the easy food runs out, the heap cools, and slower fungi and small animals take over. Finally, the compost cures, or matures, for several more weeks. From start to finish, the process usually takes two to six months, depending on the materials, the climate and how well the heap is managed.

Microbes need carbon for energy and nitrogen to build their bodies,

in a ratio of roughly twenty-five to thirty parts carbon to one part nitrogen.

You do not need to measure it exactly.

Think of browns and greens.

Browns are dry, carbon-rich materials: straw, dry leaves, maize and cotton stalks, wood chips, sawdust and shredded cardboard.

Greens are moist, nitrogen-rich materials: fresh manure, green leaves and weeds, grass clippings, legume residues, and fruit and vegetable waste.

A practical starting point is about two to three parts browns to one part greens, by volume. Too many greens, and the heap turns wet, slimy and smells of ammonia, as nitrogen escapes as gas. Too many browns, and it stays cool and dry and barely decomposes.

Chop or shred coarse material into pieces a few centimetres long, so the microbes have more surface to work on, but keep some coarse material to leave air spaces.

Moisture comes next.

The heap should be about as moist as a wrung-out sponge.

Squeeze a handful hard: a few drops of water should appear between your fingers, but no stream.

If no water appears, it is too dry, and decomposition slows.

If water runs out, it is too wet, the air spaces fill with water, and the heap turns anaerobic and starts to smell.

Water each layer as you build, and in dry climates, cover the heap or build it in a shallow pit to save moisture. In very wet climates, build it on a slight mound and cover it against rain.

The microbes also need air.

Turning the heap brings fresh oxygen to the centre, and moves the cooler outer material into the hot core.

Many farmers turn when the temperature starts to fall after the first peak, often every one to two weeks during the hot phase. A heap that is too big and too tightly packed will go airless in the middle; one that is too small will lose its heat. For a simple heap, about one and a half to two metres wide at the base and about one to one and a half metres high works well.

The hot phase is what makes compost safe.

Push a long thermometer into the centre of the heap, or a metal rod: if the rod is too hot to hold after a few minutes, the heap is working.

For killing weed seeds and pathogens, the aim is for every part of the heap to spend several days at fifty-five degrees or more. That is why turning matters: the outside of a heap never gets hot. Some compost standards for large windrows require at least fifteen days at fifty-five degrees or more, with at least five turnings.

Do not let the heap get much hotter than about seventy degrees for long.

Very high temperatures kill the useful microbes too,

and dry heaps can even catch fire.

If it gets too hot, turn it and add some water.

Most compost problems have simple causes.

A smell of ammonia means too much nitrogen:

mix in more browns. A rotten-egg or sour smell means too wet and airless:

turn the heap and add dry browns. A heap that will not heat up is usually too dry, too small, or short of greens: add water, make it bigger, or add fresh manure or green material. If it is damp only in the middle and dry outside, it needs turning and watering.

Flies and rodents are drawn to exposed food waste:

bury it in the centre and cover it with browns.

Some things should stay out of a farm heap.

Meat, fish, bones, dairy and cooking oil attract pests.

Pet and human faeces need special treatment.

Plastics, glass and treated wood never break down.

And be careful with manure or straw from fields treated with persistent herbicides from the synthetic auxin group, HRAC group four, such as some pyridine products.

Their residues can survive composting and damage sensitive crops like tomato, beans and potato.

Ask the supplier which herbicides were used.

Mature compost is dark brown to black, crumbly, and smells like a forest floor, not of ammonia or rot. You can no longer recognise the original materials, apart from a few woody pieces, and the heap does not heat up again after you turn and water it.

For a simple check, fill two pots, one with the compost and one with ordinary potting mix or garden soil, and sow fast seeds such as cress, radish or wheat in both. If the seeds in the compost germinate and grow as well as the others, the compost is mature. If they are stunted or yellow, let it cure for a few more weeks. Immature compost can tie up nitrogen in the soil and harm young roots.

Compost is a soil conditioner first, and a fertiliser second.

It typically contains around one to two percent nitrogen on a dry-weight basis, released slowly over months and years, with useful phosphorus, potassium and micronutrients.

So count its nutrients in your fertiliser plan, but do not expect it to replace fertiliser on its own on a hungry crop.

Spread it before sowing or planting and mix it into the top layer of soil, or use it as a mulch around trees and vegetables.

Rates depend on the compost and the soil, so let your soil test guide you. For vegetables that are eaten raw, use only fully composted material,

and apply it well before harvest.

Where earthworms are available, vermicomposting is another option.

Worms turn food waste and manure into fine, rich vermicast. But a worm bin never gets hot, so it does not kill weed seeds or pathogens.

To recap.

Mix about two to three parts browns to one part greens.

Chop coarse material, and keep the heap as moist as a wrung-out sponge.

Build it big enough to heat, about one to one and a half metres high. Aim for fifty-five degrees or more in every part for several days,

and turn it to make that happen.

Fix smells with browns and air, and a cold heap with water, size or greens.

Keep out meat, plastics and herbicide-treated straw and manure. Test maturity with a simple seed test.

And count compost's nutrients in your fertiliser plan.

For more on soil health, visit laisha.ai. Thank you for watching.