Do We Now Know How to Recreate Terra Preta?
Betsy Kettle has been featured here a number of times. You can get to know her & her work from CCC S1E5. Betsy has provided the following report covering seven years of work that has covered community and Auckland council engagement but also encompasses composting systems, soil development, innovation and bananas. It is an interesting journey & thought-provoking read… (and plenty more photos & information from the website link at the end of the article, ed.)
For seven years the City to Farm Project has been applying bokashi-biochar composted food scraps to feed a banana farm north of Auckland. The goal was twofold – to profitably divert a rich, but difficult organic material from landfill and see if Terra Preta could be recreated. Proof of both would be the creation of a productive banana farm growing in transformed soil.
Rodney’s “Ultic” soils are non-reactive and contain a high proportion of kaolinite clay, similar to those found in the Amazon Basin. Local farmers, such as Phil, often describe them as “hydrophobic” because they have a very low CEC and fail to retain water.
Kaolinite is essentially “dead” clay, the end result of the breakdown of original clays. In both the Amazon and Northland, extensive mineral mining by native forest roots and over 50,000 years of rainfall have left these soils infertile and incredibly hard. Given these conditions, it is remarkable to consider how pre-Columbian Amazonians successfully transformed this kaolinic clay into highly productive, fertile agricultural land.
Before starting out, the Hibiscus Coast Zero Waste team studied what little was known about Terra Preta. It seemed to contain all manner of organic waste-faeces, branches, leaves, food scraps, bones, and shells, along with pottery shards, biochar and fungi. We knew that biochar and bokashi would help with odour control and that biochar needed to be “charged and inoculated” before land application. The solution was a specially sealed wheelie bin with woody mulch and biochar in the bottom. These were then delivered to commercial kitchens to layer food scraps with bokashi “zing” for a weekly or biweekly collection.
Phil the Farmer got out his digger to dig swales—shallow ditches parallel to the contour. My husband David broadforked biochar into the bottom with difficulty. Layers of food scraps from the wheelie bins and woody mulch were applied several times a year. Phil started reporting that the stream no longer dried up in the summer and was now running all year round. The extensive irrigation laid out for the bananas broke down but it turns out it wasn’t needed. The bananas were doing fine, despite the Auckland Drought of 2019-2020. Horses were grazing in the banana orchard as it was the only place that still had green kikuyu.
Somewhere between year two and three (2021-2022) we noticed water no longer ponded in the first swale but still ponded in the second and third swales. Oddly, the next year water was no longer ponding in any of the swales. What was happening?
We all know that natural topsoil formation in clay starts from the top down with a change in soil porosity. When we researched deeper it was found that biochar develops porosity in clay subsoils three different ways. The first is electro-chemically. Usually, gypsum or lime is used to “un-plate” clay soil structure, but it doesn’t work very well with non-reactive kaolinite. Biochar has better success, especially when it has a high negative electrical charge. This happens at the electron microscope scale 4000x.
In nature, the second stage starts when soil bacteria move in, to clump bits of organic matter, clay and themselves together with their slime, to create “micro-aggregates”. These micro-aggregates further increase air and water through the subsoil. In nature, this process is very slow. You may have heard the phrase “it takes a 1000 years to create an inch of topsoil”? Well, it doesn’t take quite that long, especially in sandy soil but topsoil doesn’t naturally form at all under NZ forests nor Amazon jungles.
Native trees use mycorrhizal fungi to quickly break down and take up what has fallen on the forest floor. In most Northland forest soils only a few centimetres of “brown stuff” are found below the duff. Unlike grasslands, NZ forest soils tend to be shallow with the bulk of the biomass in the bush and very little in the soil surface. The NZ bush feeds the fungi what it needs to find the water and minerals it wants. It is not as dependent on bacteria for fertility like grasslands. But fruit trees, bananas, nut trees and veggie gardens like a higher ratio of bacteria to fungi so man-made intervention can speed up the process.
Our man-made intervention of food scraps and bokashi increased the bacterial to fungal ratio in the soil. In fact, our theory is that when food scraps break down they generate a wide range of starches and sugars to feed the soil microbiome, imitating what a biodiverse plant community growing on the soil surface would, so a diverse soil microbiome is created rapidly. Also discovered was the acids plants exude to dissolve rock and clay minerals are the same as that found in bokashi digestate. So perhaps we inadvertently greatly speeded up the soil formation process. Is this what the Amazonians did, too?
When Miles Pope of Soil Pro was asked about the standing water disappearing in the swales and the change in soil structure observed he commented that one of the reasons so many Pukekohe growers were using biochar was to clump the fines together preventing erosion and slowing sediment loss.
But when holes were dug in 2025, we saw composted woody mulch with a dark layer of biochar in the bottom and pale, orange subsoil under that. The only major change was the color of the subsoil from whitish gray to pale orange. It didn’t look like the pictures of Terra Preta at all.
But this year, in 2026, we discovered the last and biggest change in the subsoil under the swales. When Phil and Paul dug a deep trench all the way across the swale and bund it was easy to see what was happening in the cross-section. Instead of rock-hard the subsoil was soft, rounded and aggregated into visible “peds”. It was full of soft, fleshy banana roots—another indication of the softness of the soil. In all my 45 years growing I had never seen any soil like this.
Our theory is that we were witnessing the last stage of topsoil formation which happens when the mycorrhizal soil fungi bind the micro-aggregates created by bacteria into macro-aggregates. This living ball is high in carbon both from the increased plant roots, their root exudates of starches and sugars, the increase in microbiome living in and around the biochar, the increase in soil fungi and the biochar itself. The subsoil was easily dug and water penetrated easily even though it was not black.
Did we discover the beginning of Terra Preta formation? We are hoping to find a soil scientist, familiar with both Terra Preta and the soil microbiome who will look at the project to inspect these discoveries and compare the subsoil under swales in other, newer areas of the banana plantation. In the meantime, the many new banana clumps are coming into full production, Phil is selling kilos of bananas to a local greengrocer and we are eating a LOT of beautiful, organic bananas. And the City to Farm project has diverted over 300 tonnes of food scraps from landfill to local farms.
We thank David, Phil and Jenny Grainger for their unflagging support and shared vision of this project, as well as Paul Candy, our resident soil expert who now advocates for biochar. We also appreciate Matthew Priestly, whose vision of the potential of biochar inspired his Masters Thesis and will highlight its role in soil carbon sequestration.
Betsy Kettle
021 0826 8196
Trustee, The Sustainable North Trust
hibiscuscoastzerowaste@gmail.com
http://www.Citytofarm.co.nz
