Mark Sweet – The ABC’s of Soil

“What’s so special about the Heretaunga Plains,” Tom asked me when we were talking about Hastings District Council’s Planning for a Sustainable Future document. The Council has identified the Plains as a priority for ‘protection of productive land capacity’.

“Because they’re incredibly fertile,” I replied. “Yeah but why?,” he asked, “What makes them so fertile? What’s the ABC’s here?”

I took that to mean he wanted to know how the Heretaunga Plains came into being. And the A, as it happens, stands for Australia.

As that clever advertisement on television portrays, Australians claim credit for pavlova, Far Lap, Split Enz and Dragon, when in fact they originated in New Zealand. But what Australians should never be told, because it cannot be refuted, is that our country was once part of theirs.

We parted company with the east coast of the Gondwana continent about 65 million years ago and drifted for a few million more before settling down on our present site. Like an ancient Noah’s Ark, the separated land mass carried passengers, including birds of the ratite family, who were the ancestors of the kiwi and moa.  Tuatara, earthworms, centipedes, millipedes, spiders, weevils, slugs, snails and weta were all onboard too, but no mammals, because they were still evolving.

The fossilised dinosaur bones Joan Wiffen discovered in the Maungahouanga Stream in 1975 carbon dated at 65 million years. It’s hard to know whether they experienced the departure from Gondwanaland or not, but they do indicate the extreme age of the compost of the Plains.

But I digress. Let’s skip to around 20,000 BC. The sea level was 100 metres lower than it is today, so the shoreline was about 30 kilometres further east, roughly in a line from the Mahia Peninsula to Cape Kidnappers. By then the Ruahine, Kaimanawa and Kaweka ranges were fully formed, and their catchments were served, as they are today, by the three great rivers, the Tuki Tuki, Ngaruroro, and Tutaekuri. For nearly 10,000 years the alluvial plain was layered with eroded material from the mountain ranges.

With the ending of the last ice age, sea levels rose and inundated most of the old alluvial plain creating a new shoreline along the base of Poraite and Taradale hills, covering Hastings, and the Te Awanga flats. Remnants of wave action eroding the hills can be seen today at Park Island, and hillsides behind Taradale and Korokipo. Sea borne silts and the remains of marine flora and fauna fell to the sea floor adding to the compost of what would eventually become the Heretaunga Plains.

As if Nature had a plan — to ensure that the potential fertility of the Plains would one day be realised — the rivers began reclaiming land from the sea. Over time river shingle deposits, and gravel eroded from the cliffs between Clifton and Cape Kidnappers, formed a spit linking to Bluff Hill, and later another spit formed northward enclosing the Ahuriri lagoon, joining up with the shingle deposits from the Esk river at Bay View.

Now that a sea barrier had been created, estuaries, lagoons and deltas developed, and the rivers deposited more and more material on the enclosed plain.

For Maori the Heretaunga flat lands provided an abundant source of eels and waterfowl but were mostly swamp and far too prone to flooding for the growing of crops or habitation.

With the arrival of European settlers, the opportunity for establishing pastoral farming and horticulture in the style of their homelands was recognised, but firstly the land needed to be drained and cleared, and the rivers controlled. This was a mind boggling undertaking, because as William Colenso described in 1844, the Heretaunga Plains were, ‘an interlaced jungle of cutting grass and flax, the only way through it being along the creeks by canoe’.

Up until the big flood of 1867, the Ngaruroro flowed from Roy’s Hill through where Heretaunga street is today to the foot of the Havelock hills then in a route to the sea where the Karamu creek flows today. It joined with the Tuki Tuki at Clive where together they flowed into the sea.  In the flood the Ngaruroro cut a new course from Fernhill to Pakowhai then onto the sea roughly in the same position it is today. The Tuki Tuki was separated and a new outlet to the sea formed at Haumoana. The Tutaekuri flowed into the Ahuriri lagoon and wasn’t settled into its present course, which meets the Ngaruroro at the sea near Clive, until 1933.

With the rivers contained by stop banks, and the land cleared and drained, the productive capacity of the Plains has been at the core of growth and prosperity in Hawke’s Bay for over a hundred years. There are indications, however, that our treatment of the soil has dangerously reduced fertility, and that urgent remedial action is needed if we are to continue to benefit  from Nature’s gift, which took millions of years to make.

If Hastings District Council is serious about ‘protection of productive land capacity’ of the Heretaunga Plains, it needs to liaise with the Regional Council and other interested parties to ensure that best practice for sustainable soil fertility is established and implimented.