Soil health
“I spent 11 years working in the land division of Māori Affairs, followed by 12 years supervising Māori Trusts in a private capacity,” Petersen said.
“Since 2008, I have focused on enhancing soil fertility and economic viability for farmers and growers.”
Most farms followed the traditional NPK fertiliser system.
Soil tests were routinely carried out, but there was little real expertise in interpreting individual results, so farmers tended to apply the same inputs year after year.
“There was no one to discuss soil health with in those days,” he said.
“During a course near Brisbane on soil fertility, lots of pennies dropped for me.”
During his time as a farm supervisor, Petersen explored products and methods from six different companies in order to identify the best options.
“Correctly balancing the soil should be the goal of any soil fertility programme, and we need to move away from the more harmful products, which are often the cheapest.”
Kinsey Agricultural Services produced manuals showing how to calculate nutrient requirements from soil samples using specific formulae.
These were based on the work of Dr William Albrecht, who authored more than 400 papers on soil fertility and developed the Albrecht System of Soil Fertility – an approach focused on feeding the soil and letting the soil feed the plants.
These manuals and papers form the foundation of Petersen’s soil management approach.
High-quality crop yields tend to share similar soil chemistry: calcium at 60-70%, magnesium at 10-15%, potassium at 5%, sodium at 1.5%, with defined levels of nitrogen, phosphate, sulphur and trace elements.
Ripening bananas and a banana flower. Photo / Catherine Fry
“The Lincoln University Dairy Farm [LUDF] achieved a best return on investment [ROI] of 4.5%,” he said.
“On one of the Māori Trust properties I supervised, our worst ROI was 4.6%, and our average was 8.5%, using only 10% of the nitrogen applied at LUDF.”
Tending land
In 1980, Petersen purchased 4ha on Ohakana Island in Ōhiwa Harbour, where he built a home and grew tamarillos and passionfruit, quickly gaining organic certification for the land.
He continued advising on soil health and sharing his knowledge through agricultural publications, including writing a column for Coast & Country News for several years.
Brett Petersen in his banana plantation. Photo / Catherine Fry
Now semi-retired, Petersen continues to advise several farms.
He has lived on a 5.5ha property near Awakeri since 1991, where he has established a banana plantation.
Bay bananas
“For the first two years, I grew canna lilies as a cash crop,” he said.
“I planted my first bananas 30 years ago. They were an inedible variety, but they didn’t seem to mind the climate.”
Today, Petersen has a 30m by 50m area planted with edible banana varieties introduced in 2023, including Misi Luki, Dwarf Cavendish, Mons Mari and Goldfinger.
He said they prefer free-draining soil.
“I don’t look at what the plant wants – I look at what the microbiome beneath the plant wants.
“Base saturation percentages are the key to getting things right.”
Petersen aimed for a soil structure that maximises drainage, consisting of 50% pore space (for air and water), 45% mineral content and 5% organic matter.
Banana plants have a pseudostem made of tightly bound leaves. Photo / Catherine Fry
He applies all of his soil expertise to the banana plantation.
While it is not a commercial venture and functions more like a test plot, he happily gives away the bananas he grows.
Most of the banana plants thrive in the sunny Edgecumbe location, but some struggle regardless of the row they are planted in, leading Petersen to suspect a climatic issue.
Banana plants have a pseudostem – a trunk-like structure composed of tightly wrapped leaf sheaths surrounding a soft core.
Globally, banana pseudostems are used for human food, animal feed, fibre production, organic fertiliser and compost, mulch, and traditional medicines.
Bees, moths and butterflies are attracted to the flowers, but the female flowers do not require pollination to produce fruit.
“When the flowers appear, it takes between 150 and 300 days – around 250 days on average – for the bananas to form and ripen.”
Petersen hopes to continue refining his banana-growing techniques and gather long-term data that could be useful to others in the future.