In the mid-19th century the open country in the eastern North and South islands provided the pastures for sheep grazing and the new wool industry. These open areas had been largely created earlier by forest fires – both accidental (from lightning strikes and volcanic activity) and deliberately lit by Polynesian and European settlers to clear the bush.
After forest was burnt in the drier eastern regions, the absence of grazing animals allowed tussock grasslands to develop. South Island hills, downlands and plains were dominated by tall tussock (Chionochloa species), with larger snow tussock at higher altitudes.
The natives preferred by grazing animals were wheatgrass (Elymus species), plume grass (Dichelachne crinita), blue tussock (Poa colensoi) and holy grass (Hierochloe redolens). Farmers soon found native grasses inadequate due to their poor growth rate and low nutrient value. They believed that introduced species, especially clovers, were needed if farming was to succeed.
Introduced pasture species
Most of the plants that now grow in New Zealand pastures came from England. There grasses were used not only for feeding animals, but also for roof thatching, for which the common reed grass was preferred.
English grasses and clovers were introduced into New Zealand in the mid-1800s. Many weeds were also accidentally introduced in seed mixtures.
In 1861 there were 158,000 acres (63,200 hectares) of planted pasture, which increased to 3.5 million acres (1.4 million hectares) in 20 years. The success of these pastures encouraged more forest burning and land clearance to develop new grasslands. In a letter to the London Times in 1882, the New Zealand agent general in London asserted that New Zealand had more land laid down in English grasses than all of Victoria, New South Wales, Queensland and South Australia put together. By 1925 pasture covered 16.5 million acres (6.6 million hectares), almost a quarter of the country.
In the early years of pasture development, especially on hill country, bush-burn mix was often sown on land that had been cleared by burning. The seed mixture included grass, legumes and herbs, in the hope that if you put them all in, something would grow. Seed was applied generously – usually 20–40 kilograms per hectare.
One of the most popular introduced English grasses was cocksfoot (Dactylis glomerata), which thrived on the Banks Peninsula hills, where it was sown after the native tussock and remnant forest had been burnt. It was in demand for pastures in the North Island and for a few years Banks Peninsula farmers earned good money from selling cocksfoot seed – growing 83% of New Zealand’s supply in 1905.
Maintaining soil fertility
Once the initial supply of nutrients from ash and organic matter were depleted, introduced pasture plants needed fertiliser to continue to grow well. In areas where English grasses could not survive, native grasses were preferred.
In 1905 the first fertiliser trials showed that phosphate was needed by pasture. Superphosphate was a particularly useful fertiliser because it allowed other nutrients to be easily added into the mix where needed.
Nitrogen in soils
Apart from a lack of moisture, a shortage of nitrogen is usually the main thing that limits pasture growth. The cheapest form of nitrogen is that provided by legumes.
Legumes – plants like peas, clovers, soya, gorse and broom – all develop nodules or growths on their roots due to infection from bacteria in the soil. The bacterial activity converts nitrogen from the air in the soil into a form that can be taken up by other plants when the nodules die and disintegrate.
Legumes have the ability to fix atmospheric nitrogen into a form that other plants can use. This avoids the need to apply expensive nitrogen fertiliser, and has allowed New Zealand to develop relatively low-cost, year-round, pasture-based farming systems. No other country relies so heavily on legume-based pastures for its primary industries.