“Hi Jerry, I belong to a community garden in Brisbane. Where do we find great compost to buy? I’m looking for 3 cubic metres. Thanks”, Clare
Reply: Anyone can buy compost in bulk. But these products are often highly variable and sometimes even unsuitable. The traditional alternative is to sow green manures and to make your own compost. If you need bulk compost, consider the sandwich mulching technique. You’ll need a soil pH Test kit and, ideally, get your soil laboratory tested to check for nutrient deficiencies and contaminants.
Take a pH Test kit, and test before you buy. It’s a quick and simple way of ensuring it isn’t too alkaline before you buy. The importance of soil pH is that it determines if added nutrients, like manures and fertilisers, are chemically available for plant use and it influences the availability of micronutrients and contaminants that, while present in small amounts, can be powerful influencers of plant health. This is explained in the kit;
Problems can also arise when too much poultry manure has been added, making mixes too alkaline for plant growth. Other manures, like feed-lot manure, may also contain high enough levels of zinc to stunt or damage plants. A lot of nitrogen can be lost – outgassed as ammonia – as these manures rapidly decompose in sale yards;
Consider green manures and making compost
Originally, I had a 400 square metre back garden and I couldn’t afford to buy compost. Instead, I buried the lawn under a 15cm deep layer of soil excavated to make a pit to accommodate an in-ground rainwater tank.
I then used the traditional farming alternative to compost making: green manuring. I sowed three successive green manures, slashing each down and digging them in so they could rot.
To accelerate soil improvement, before I sowed the third green manure, I turned the entire back garden into a compost heap 0.5m deep.
I first broadcast a bucket of millet seed to bind the soil and prevent erosion by summer stormwater. I applied fertiliser and soil conditioner to correct the deficiencies and acidity level of my soil identified by laboratory soil testing*. The soil was so compacted it broke the tines on a walk behind rotary hoe (see video clip), so I hired a ride on rotary hoe.
Next, I sowed sunflowers. In Brisbane they can be sown all year round for this purpose. Their fibrous root systems broke up the heavy cracking clay – subsoil which had been excavated for the rainwater tank. I slashed them down and rotary hoed them in.
Sandwich mulching technique
For the compost, I used a sandwich mulch technique – another traditional method – to create a compost heap 400 sq metres in area and 0.5m high. Four months later when it was decomposed it had dropped to a depth of 15cm. I rotary hoed that in, set out the footpaths and raised garden beds, and started growing.
A sandwich technique involves adding materials in successive layers, ensuring a blend of soft, green, leafy, nitrogen-rich materials which are food for bacteria, and brown, woody, carbon rich materials, food for fungi. Bacteria start the composting process and fungi finish it.
The bottom layer must be porous to allow air and water to percolate through which prevents the heap from becoming anaerobic. I bought baled barley straw that had been ruined by rain. It was very cheap;
Next, a layer of fresh lawn mower clippings. I checked they were free of herbicides. They were free because mowing companies have to pay to tip them;
Then a layer of mulched powerline tree prunings. This contains a mixture of wood, twigs and leaves. This was free;
I topped this with a layer of mushroom compost. You can buy this fresh and unused at a premium price, but after it has been used in a mushroom farm it can be cheap;
Lastly, I sowed my third and final green manure, a mixture of peas, barley and sunflowers suited to the cool seasons;
This was in 2004, during the Millennium Drought, when my garden received 922mm of rain and watering restrictions prevented irrigation. Heat and drought slowed the composting process.
ABC television lent me an old camera they no longer used to record what my duplex soil profile looks like, the day the walk behind rotary hoe broke its tines on my hard soil, the process of creating the sandwich mulch. At the start, my compacted, impoverished soil had no earthworms in it. At the end, it was a nursery for worms, as it has been ever since.
*I had my soil laboratory tested to check for:
* Industrial/ traffic contaminants like heavy metals (lead, copper, zinc);
* Fungicidal contaminants, like copper chromium arsenate (until recently, a widely used timber treatment);
* Common nutrient deficiencies, like phosphorous, iron, calcium, potassium and magnesium that might compromise the health value of food grown in it and the vitality of crops;
‘Soil Wars’ video, see:
Director, Seed Saver’s Network Inc.
Patron, National Toxics Network Inc.
28th March 2020