It’s all in the lack of dirt!
Soil is a key terroir factor mostly because of its influence on the amount of water that is available to the vine through the season, and how the vine gets it. In the unirrigated Old World mother nature is the entire determinant of that. In the New World irrigation often plays an important role dependent on what the vine can do by itself.
The soil, or lack of it, is a very significant factor in the terroir of the GIMBLETT GRAVELS WINE GROWING DISTRICT. The soils here are so stony that in 1988 an internationally owned concrete company purchased almost 25% of what is now the total planted area of the district and applied to mine it for gravel! If it wasn’t for a great legal result by winegrowers, now members of this society, a good part of this land may have been used to make the road that you travel on to get to Hawke’s Bay!
The GIMBLETT GRAVELS WINE GROWING DISTRICT was in 1867 the Omahu channel of the Ngaruroro River i.e. it was under water! At that time the river, as its velocity slowed, was dumping all the heavy bits (gravels and heavier sands and silts) around Roys Hill, with the lighter bits (silts and clays) dumped on the larger expanse of the Heretaunga Plains because they stayed in suspension of the river longer. That is essentially why the only significant area of these gravely soils is found around Roys Hill in what is known as the GIMBLETT GRAVELS WINE GROWING DISTRICT. The soils are appropriately called the Omahu Gravels.
Basically these soils are made up of gravel as deep as you want to go. The gravels are formed from compacted sands called greywacke who began their life under the sea some 200 million years ago. They were thrust out of the sea about 5 million years ago to form the mountains that are the backbone of both islands. The forces of nature split lumps of rocks off these mountains and the rivers transported these lumps to the plains, on the way breaking the big lumps into small lumps varying in size from a toenail to a football. These rocks are known as greywacke rocks. The process also created lots of smaller bits of sand, silt and clay.
The soils in the GIMBLETT GRAVELS WINE GROWING DISTRICT are essentially pure gravel beds with lenses of sand, silt and clay at various depths. These lenses contain up to 20% silt and 9% clay and along with the 10-40cm of topsoil present, is where all the feeder roots are located. Basically a vine, if given an unimpeded path, will send a tap root down until it finds one of these lenses and then send out the water and nutrient sucking feeder roots for sustenance. Having said that it has to work for it as these lenses are often few and far between.
The soils are naturally low in fertility and there is no particular mineral contribution from the soils or rocks. Unusually apart from regular applications of basic nutrients including lime, magnesium and phosphorous these soils seem to require minimal amelioration with fertilisers over the long term.
These coarse textured, weakly structured soils are very free draining and have very little water holding capacity, mostly between 9 and 20% soil moisture. In combination with the low summer rainfall, the soil is unable to provide the vines with enough water for them to survive, let alone produce grapes. Irrigation is necessary in nearly every vineyard, and is mostly applied as carefully controlled drip irrigation. Without the water there would be no terroir expression as there would be no living vines! While some commentators and producers endorse the attributes of dryland farming, we endorse the “use it don’t abuse it” philosophy on irrigation that significantly benefits wine quality and keeps the vines alive! With most modern research indicating that it is the effect of soil texture and structure on the hydrological properties of a soil that most determines the terroir influences of soil, the soils of the GIMBLETT GRAVELS WINE GROWING DISTRICT offer exceptional influence on vine performance, manipulated or not. More on that later.
Initial conclusions from a PhD thesis yet to be published by Mr Dejan Tesic, who researched viticultural environments of the Hawke’s Bay region, has shown that the “soil factor”, an interaction between soil temperature, soil moisture, soil texture and rooting depth, had a significant effect on vine performance. Those sites with high soil factors showed earlier flowering, veraison and harvest dates. Higher sugars and phenolics and lower acidity. Soils with the lowest soil moistures and highest soil factor scores produced the best wines. The sites with the highest soil factor score were the GIMBLETT GRAVELS WINE GROWING DISTRICT, and those with the shallow pan soils only in the driest years. These shallow pan soils occur around Havelock North and on the gentle slopes adjacent to the Tukituki River.
While some small parcels of these gravely soils are found in other areas in the Hawke’s Bay region, generally these localities don’t share the same climatic benefits that the GIMBLETT GRAVELS WINE GROWING DISTRICT does.