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by  John Maber

It was once slow, expensive and tiring to apply fertiliser on hill country – contract workers would cart heavy bags of fertiliser on horseback and spread it by hand. Fast forward to the 1940s, and the return home of World War Two fighter pilots. Criss-crossing the skies over remote farmland in adapted aircraft, they dropped superphosphate by the hopper load. Farming expanded, and the economy boomed.

Fertilisers and spreading methods

The importance of fertiliser

Farmers, horticulturists and foresters all want their pastures, crops or trees to grow as fast as possible so that their business will be profitable. On most soils, fertilisers of various types are needed to improve plant growth rates.

Without an efficient fertiliser-spreading industry, New Zealand’s farming economy would not remain viable. This is especially so for hill-country farms, which rely on fertiliser being applied from the air (aerial topdressing).

Forms of fertiliser

Natural manure

Nutrients can be supplied by organic manure – the waste and residue of plant and animal life. The most common method is directly applying animal dung as a solid or slurry. It can also be applied in a liquid form. The nutrient content of natural manure varies widely.

Artificial fertiliser

Manufactured solid fertilisers usually have specific, relatively high, nutrient contents. These fertilisers are more efficient to transport and apply than other forms. In New Zealand the most widely used fertiliser is superphosphate. It is made from imported rock phosphate that is treated with sulfuric acid to produce a white, granulated powder.

Fertiliser spreading today

In New Zealand over 2 million tonnes of fertiliser is applied annually to pastures, crops and forests by truck or tractor, or by fixed-wing or helicopter aircraft. About 40% is spread by aircraft onto steep hill-country pastures and production forestry land – the highest proportion in the world. While most hill pastures receive fertiliser every one to three years, forests may be topdressed only at 10-year intervals.

Spreading fertiliser

Fertiliser is applied to maximise the production of pasture, crops or trees. It is always assumed that it will be spread evenly over the entire area, at the required rate.

A completely uniform spread is hard to achieve over a whole paddock. This is because the application rate (from both groundspreaders and aircraft) is usually highest directly behind the machine, and diminishes outwards on either side. This is called the swathe pattern, which is typically triangular. It is produced by the combined effects of the size of the fertiliser particles and the way they are discharged. It may also be affected by cross-winds.

When fertiliser is applied, the operator, farmer or pilot tries to partly overlap the previous swathe pattern with the next pass. This helps to compensate for the low rate at the edge of each swathe. Variation in the distance between successive passes can improve the uniformity of spread. However, as the distance reduces, the time taken to apply all the fertiliser will increase, and so will the cost of spreading.

When even spread is important

  • When a fast-acting fertiliser like nitrogen is used to get rapid grass growth over a short period. This is usually expensive and the farmer wants all the pasture to receive the right rate.
  • In horticulture, where there may be frequent applications – three or four times on some crops. Uneven application can cause crop variability, which leads directly to lower returns.
  • In large-scale forestry, particularly for high-value trees such as veneer or clear-wood logs. Fertiliser is typically applied at 10-year intervals, so there is no opportunity to counter or remedy uneven spread the next year.

When it is not so important

  • In routine annual application of phosphate fertiliser to pasture. This is because non-uniform application in one year tends to be partly compensated for in the following year.
  • When the nutrient being applied is only a small proportion of the level already in the soil.

Spreading standards

The New Zealand fertiliser industry has set standards for uniformity of application. They represent an acceptable compromise between the cost of application (which will rise to achieve a more even spread), and of lost production from uneven application.

Ground spreading

Early days

In the early years of pasture development in New Zealand, a horse-drawn, wheeled spreader box was used. Fertiliser was poured in from bags, and released from the bottom of the box as it moved along.

Later spreaders consisted of a hopper filled with fertiliser, mounted on the back of a tractor. A spinner underneath threw the fertiliser sideways and backwards as the tractor moved along. As the spread was much greater than the width of the tractor, this was a more popular method than the box spreader. It is the main type of equipment used today.

Spreading by contractors

Particularly on large farms, contract firms are increasingly being used. With large trucks and hoppers, they can do a farmer’s job in a fraction of the time.

Spreading by farmers

Some farmers own large tractors and spreading equipment, and spread fertiliser as needed.

On dairy farms, cows spend only one day or night grazing each paddock before being moved to the next. Nitrogen fertiliser is often applied to each paddock after each grazing. This does not require a contractor, so is often done using the four-wheel-drive farm bike with a spreader attachment behind.

Types of spreaders

Spinning disc

This is the most common type of ground spreader. Fertiliser is dropped onto a spinning disc that has several radial vanes, from which the particles are flung horizontally. Disc diameters are usually about 750 millimetres, and typical speeds range from 750 to 1,000 revolutions per minute. Most designs have two discs rotating in opposite directions, with the fertiliser fed between.

Spinning disc spreaders are robust, and can handle a wide range of fertilisers and application rates. Swathe widths of 25–30 metres can be obtained, depending on the size of fertiliser particles. A 1-millimetre-diameter particle can be flung 3 metres from the disc, whereas a 5-millimetre-diameter particle would travel 15 metres.

Oscillating spout

This machine uses a spout that moves rapidly from side to side. The speed and length of the spout determines the spread pattern. Although more complex than a disc, it can achieve a very accurate spread, with swathe widths up to 15 metres.


This type of spreader is very simple, and is often used to place fertiliser in areas difficult to access. However, a uniform spread is hard to achieve. A mixture of particle sizes seems best, with an optimum diameter of 5–6 millimetres.

With all these spreaders the fertilised pattern and rate will be affected by the spreader design and speed, the fertiliser type, and the accuracy of the driver in spacing successive passes.

Origins of aerial topdressing

The hard way

On hill country, the only way to get fertiliser and seed onto the land was to carry bags on horse back to the area, and then spread the material by hand. Contract labour was often used. It was extremely hard work, and stories abounded of farmers, having paid the contractors, finding bags of unspread fertiliser thrown into gullies.

In the 19th century and the first half of the 20th, there was no other way to do it. Any alternative was therefore eagerly anticipated.

Early proposals

Some pilots returning from the Second World War were keen to keep flying, and the aerial application of fertiliser seemed to offer possibilities.

The impetus for the use of aircraft came from advocates of aerial sowing onto bare areas to control erosion. The first aerial sowing on a New Zealand farm was in November 1947 at Ōmarama in North Otago. After initial reluctance, the Public Works Department, the Departments of Air, Scientific and Industrial Research, Agriculture, and the newly formed Soil Conservation and Rivers Control Council became involved in proposals for aerial topdressing of fertiliser. Doug Campbell, chief advisor on soil conservation to the Soil Conservation and Rivers Control Council, was one of the main advocates.

Taking off

John Lambert of Hunterville is often cited as the first to suggest using aircraft to spread fertiliser. He did this in a letter to his MP in 1926. But on 8 March 1941 Alan Prichard, at Ninety Mile Beach, was the first to actually sow seeds (of lupin) from the air, pouring them out the side window of a plane, a Miles-Whitney Straight. This gave the idea some credibility. All this was at a time when it was illegal to drop anything from an aircraft – even over your own farm.

Ōhakea trials

The first trials were at Ōhakea, near Palmerston North, in September 1948. The pilot flew a Grumman Avenger torpedo bomber aircraft from the New Zealand air force, testing fertiliser spread at aircraft altitudes ranging from 21 to 180 metres and a speed of about 200 kilometres per hour.

In fixed-wing aircraft the hopper is behind the pilot, its doors opening on the underside of the fuselage. They release the fertiliser, which falls into the slipstream of the aircraft. This creates turbulence, helping to spread the fertiliser laterally.

It was soon apparent that the type of superphosphate fertiliser had a large effect on the spreading result. The ideal form was granular, with as little fine material as possible.

Several flights at an altitude of 120 metres and a track spacing of 28 metres gave satisfactory results with superphosphate, at 2 hundredweight/acre (250 kilograms/hectare).

Hill-country trials

Trials followed on a typical hill-country farm near Raglan in the Waikato, also with the Grumman Avenger. The first results were poor because airframe vibration caused the superphosphate to compact in the hopper, and restricted the flow. Later trials with granular fertiliser showed a good spread at a rate of 250 kilograms per hectare.

Spreading lime

An attempt to spread lime was made in May 1949 using two Grumman Avenger aircraft. However, the planes had to return to their base because the lime would not flow at all. The problem was solved, at least temporarily, by putting a small amount of superphosphate into the hopper first, to start the flow.

By 1949 it was generally agreed that aerial topdressing had great possibilities, and that further development lay in the hands of commercial operators and not the air force.

Commercial developments of aerial topdressing


The first commercial aerial topdressing was in Canterbury. In May 1949, Tiger Moth ZK-ASO, piloted by John Brazier of Airwork (NZ) Ltd, applied superphosphate at a rate of 56 kilograms per hectare on Sir Heaton Rhodes’s property Otahuna, at Tai Tapu. The plane carried 181 kilograms of fertiliser, and each trip took 7.5 minutes. The cost was calculated to be less than half of that for manual spreading.

The Wairarapa

About the same date, L. T. Daniell, a Wairarapa farmer, arranged for 127 tonnes of fertiliser to be dropped on 11 different properties close to Masterton aerodrome. Within two months, 200 farmers had 101,000 hectares topdressed by air.

Adapted aircraft

Before the end of 1949, war-surplus de Havilland Tiger Moths were being converted, with one cockpit transformed into a hopper capable of carrying 272 kilograms of fertiliser. These could fly from hill-country airstrips (rather than aerodromes) or the nearest flat land, reducing the turnaround time for each trip.

Aircraft including Piper Cub, Piper Pawnee, Auster and the de Havilland Beaver aircraft were later used. On the East Coast of the North Island, severe soil erosion threatened pastoral farming, and aerial topdressing provided a way to sow and maintain new pasture.

By any measure, the developments in 1949 proved a turning point for New Zealand agriculture. Fertiliser use expanded, and stock numbers doubled over the next 20 years.

Fill ’er up

Each fixed-wing topdressing plane has a truck driver who follows it around from farm to farm, using a loader to scoop fertiliser from a concrete bin. The driver fills a bucket fitted with a canvas or plastic chute, while the plane is flying. When it lands, he releases the fertiliser through the chute and into the aircraft hopper. This takes only a few seconds, and the aircraft is ready for take-off again.

Topdressing companies

By the end of 1949 five firms were spreading fertiliser: Airwork, James Aviation, Aircraft Services, Gisborne Aerial Topdressing (later Feildair), and Southern Scenic Airtrips. Wanganui Aero Work was founded in 1950. Over the next five years another 50 companies joined in, and by 1958 there were 73.

An ideal aircraft

As aerial topdressing expanded, so did the search for the ideal fixed-wing aircraft. In 1952 the Civil Aviation Department set out specifications that included, not surprisingly, the ability to operate from short airstrips in variable weather conditions while carrying at least 1,000 pounds (454 kilograms) of fertiliser. Included in the 23-point list were initial climb rates of 300 metres per minute with a full load, good ground-handling, and safety for the pilot in the event of a crash.

Building an aircraft specifically for aerial topdressing was a somewhat revolutionary request for manufacturers at the time.

The Fletcher is born

In 1953, Wendell Fletcher, of Fletcher Aviation Corporation in the US, presented a proposal to the annual conference of the Aviation Industry Association which resulted in the production of the Fletcher FU 24 aircraft. The first model was all metal, with a 225-HP engine.

The first prototype was flown in 1954, and since then over 300 Fletchers in various configurations have been built, including a modified Fletcher, called a Cresco, powered by a turbine engine. The Fletcher remains the most common fixed-wing aircraft used for aerial topdressing in New Zealand.

Topdressing today

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.

Legislation, quality assurance and safety

The Resource Management Act 1991

Fertiliser nutrients have an impact on the environment. This may be through direct application to waterways, or from nutrients leaching from enriched soil into waterways. Added nitrogen or phosphorus in lakes or rivers can cause a rapid increase in algal or weed growth.

The purpose of the Resource Management Act 1991 is to promote the sustainable management of natural and physical resources, and one of the basic principles is to ‘avoid, remedy or mitigate adverse effects on the environment’. This is done through regional council plans and consent processes. Normally topdressing will be permitted in a regional council plan, but there may be conditions, including:

  • preventing the direct application of fertiliser into specified areas such as water bodies and wetlands
  • asking for evidence or assurance that ‘good agricultural practice’ is being followed. This may relate to avoiding very uneven application, where some soils receive an unnecessarily high level of nutrients, and there is therefore a higher risk of their being washed into the waterways.

Quality assurance schemes

Quality assurance schemes provide information for regional councils. One is the Spreadmark scheme, operated by the Fertiliser Quality Council. This aims to ensure that fertilisers are applied where they will be of the most benefit, and cause the least environmental harm.

The New Zealand Fertiliser Quality Council consists of representatives from Federated Farmers, FertResearch, fertiliser companies (with registered products), the research community, the New Zealand Institute of Primary Industry Management, and the Ground and Aerial Fertiliser Spreading associations.


Aerial topdressing

Every year, preventable accidents occur in the aerial topdressing industry. Reasons include:

  • the poor condition and siting of airstrips
  • the lime or fertiliser is not ‘free flowing’. It may contain lumps of turf picked up at the fertiliser bin site, or be too moist, which can block the fertiliser hopper and upset the aircraft’s aerodynamics.
  • aircraft hitting fences, overhead power or telephone lines that the pilot was not aware of.

Increasing awareness of health and safety has reduced the aircraft accident rate. In 1999–2000 there were five fatal accidents – two involving fixed-wing aircraft and three involving helicopters; in 2005–6 there was only one fatal accident, involving a fixed-wing plane.

In 2006 a guideline was prepared for all those involved in the aerial application of lime or fertiliser. This sets out where problems can occur, and identifies who should be responsible for dealing with them: the farmer or airstrip owner, the transport company that carts the fertiliser, the fertiliser or lime suppliers, and the aerial topdressing companies.


The ground-spreading industry is not without casualties. Each year there are injuries and sometimes fatalities, often as a result of the groundspreader vehicle rolling over on steep hills.

Hononga, rauemi nō waho

More suggestions and sources

How to cite this page: John Maber, 'Topdressing', Te Ara - the Encyclopedia of New Zealand, (accessed 24 June 2024)

He kōrero nā John Maber, i tāngia i te 24 o Noema 2008