Kōrero: Topdressing

Whārangi 6. Topdressing today

Ngā whakaahua me ngā rauemi katoa o tēnei kōrero

In 2007 about 750,000 tonnes of fertiliser was applied by air in New Zealand. There were 33 operators using fixed-wing aircraft and 88 using helicopters. Groundspreader contractors numbered about 200, of which about 150 were members of the Groundspreader Association.


Helicopters normally operate with a large bucket that hangs below the aircraft. This can carry 250–800 kilograms of fertiliser, and is fitted with a spinning disc. The effect is similar to that of a groundspreader. However, with a helicopter the forward speed is much greater, and so the mass flow of fertiliser onto the spinning disc must be much higher to achieve the same application rate. The helicopter bucket is usually filled from large bags of fertiliser lifted by a tractor.

The flying Harding brothers

In 2007 John Harding, from Rangitikei Air Services, completed 50 years as an agricultural pilot. Based in Taihape, he had logged over 32,000 hours of flying time – unlikely to have been equalled anywhere in the world. In 2006 his brother Richmond (Ditch), from Wanganui Aero Work, was awarded the Queens Service Medal for his services to the agricultural aviation industry.

Fixed-wing aircraft

The ground speed of fixed-wing aircraft is typically about 160 kilometres per hour. For application rates lower than about 150 kilograms per hectare, which is equivalent to a flow rate of 750 kilograms per minute, aerodynamic, or ‘ram air’ spreaders are fitted below the hopper outlet on the underside of aeroplanes. Fertiliser dropped from the hopper into these spreaders is moved laterally with the airflow as it leaves the wider rear outlet of the spreader. Swathe widths of 25 metres or more can be achieved, compared with about 12 metres from an unmodified aircraft, which is about the same as the wingspan.

A flow rate of more than 750 kilograms per minute is too high for a spreader to cope with. Fertiliser is simply dropped from the hopper outlet, with airflow providing some spread.

From on high

As well as spreading fertiliser, planes and helicopters are used to spray crops and forestry, and in urban areas insecticides are sprayed to control insect pests. Aircraft, including helicopters, also spread poisoned bait for rabbit control, and the poison 1080 to control possums in native forests.

Global positioning systems

With both aerial and groundspreaders, accurate and consistent track spacings are essential to achieve uniform application. GPS (Global Positioning Systems) technology is increasingly being used. This enables drivers or pilots to operate consistently, and to produce a map to show that they did so.

Me pēnei te tohu i te whārangi:

John Maber, 'Topdressing - Topdressing today', Te Ara - the Encyclopedia of New Zealand, http://www.TeAra.govt.nz/mi/topdressing/page-6 (accessed 6 July 2020)

He kōrero nā John Maber, i tāngia i te 24 Nov 2008