Biochar, black gold and bananas
What started as a simple experiment to improve soil in the direction of terra preta—a famously fertile, friable soil that seems to sequester carbon permanently—has become a national field trial for the use of biochar in New Zealand.
Betsy Kettle from Hibiscus Coast Zero Waste explains in words and pictures.
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One of the programmes of our environmental education trust is City to Farm, which involves schools, retirement villages and cafes collecting their food scraps in wheelie bins, along with bokashi to add microorganisms and biochar to control odour and build topsoil.
The bins are transported to a local farm for composting.
Jenny and Phil Grainger have been experimenting at their Waitoki farm, in Rodney district north of Auckland, creating compost swales, enriching the soil and growing bananas.
Seven years later, the results are in. The compost swales created at the Graingers’ farm—layered with mulch, food scraps, and biochar, then planted with bananas—have transformed from hard, lifeless clay into something resembling the black, soft soil of the Amazon. Soil tests from Hill Labs and the Bioeconomy Science Research Lab confirm it.
ABOVE: Phil and Jenny Grainger
WHAT IS BIOCHAR? Biochar is a form of stabilised carbon that can last for decades, centuries, or even millennia in the soil without decomposing – read more about it on the Biochar Network’s website here.
The black gold of terra preta
Archaeologists who have analysed terra preta tell us that ancient Amazonians placed organic waste of many kinds, along with biochar, into trenches to transform their infamous clay into their famous, long-lasting topsoil.
So we followed their lead. We imitated the process by collecting food scraps from town, using donated biochar from Soil Conditioner Products (SoilPro), and keeping both the composted area and the area around it planted.
The biggest mystery, though, is this: how long does it take to make terra preta? We know there are areas of this ‘black earth’ that are centuries, perhaps even millennia, old. But does it really take that long to create it? Landcare Research told us it would take at least five years before we would notice even the slightest change in soil carbon. We were a little concerned, but we carried on.
ABOVE: Phil Grainger checks out the soil in the compost swales
Compost swales
Compost swales are designed especially for clay soils to catch water from above and help it soak into the subsoil. Long, shallow excavations are dug across the contours of a hill, sloping gently downhill. The excavated soil is mounded on the downhill side and planted.
The hollowed area is then layered again and again with woody mulch and food scraps with biochar, or another nitrogen source, until the desired height is reached.
After that, it is simply left to compost. The compost is not turned and not moved. Instead, the combination we used of biochar and bokashi in the food scraps keeps the heap fresh and healthy, if it is not sitting in standing water. The bokashi ‘zing’ (microorganism mix) in the food scraps supports some anaerobic breakdown, and worms move in once the heap has cooled. Fungal hyphae grow undisturbed.
Climate action tool
Swales are useful climate action tools because in times of drought the biochar and deepened topsoil act as a reservoir that plants can draw from. They can also help ease flooding by encouraging runoff to soak into the subsoil. And when the swales are saturated, the bunds provide a drier, more aerated area for roots.
Compost swales are also climate action because they use the stabilised carbon that is biochar—making it a carbon sink in its own right. But this is not the biggest carbon sink. The growth of the living soil microbiome may be much larger.
Biochar acts as a long-lasting home for soil microorganisms, giving them more surface area to colonise. Fungal hyphae also seem to be stimulated by biochar and add to the growing carbon mass. But the fungi and bacteria must still rely on a steady source of food from the starches and sugars that plants pump into the soil.
ABOVE: Banana trees growing in compost swales
Holistic approach
So, the swale, biochar, compost, plants, and microorganisms all work together symbiotically and synergistically to transform the soil.
Will this work in other parts of New Zealand? We may have been fortunate: we were working with the same ‘dead’ kaolinite clay found in much of Rodney’s soils, that are also found in the Amazon.
We encourage growers throughout the country to experiment—not only with biochar, but with creating diverse living systems that work together to create living carbon sinks that are resilient to an ever-changing climate.
Betsy Kettle has been a zero-waste advocate and composter for at least three decades and is a trustee of the Sustainable North Trust. She thanks Auckland Council for their support for the City to Farm project over the past eight years.
