Aerial topdressing
The first known aerial application of agricultural materials was by John Chaytor, who spread seed over a swamped valley floor in Wairoa, New Zealand, in 1906 using a hot air balloon with mobile tethers.
The first known use of a heavier-than-air machine in aerial application was on 3 August 1921 when, as a result of advocacy by Dr Coad, a USAAC Curtiss JN4 Jenny piloted by John A. Macready was used to spread lead arsenate to kill catalpa sphinx caterpillars near Troy, Ohio, United States.
Crop dusting poisons enjoyed a boom in the US and Europe after World War II until the environmental impact of widespread use was recognised following the publication of Rachel Carson's Silent Spring in 1963.
Spreading superphosphate by agricultural aircraft was independently suggested in 1926 by two New Zealanders, John Lambert of Hunterville and Len Daniell[note 1] of Wairere.
There was some publicity when in 1936 Hawkes Bay farmer Harold McHardy used a de Havilland Gypsy Moth to sow clover seed on his own land.
This led the Soil Conservation and Rivers Control Council to decide to fund aerial sowing and topdressing trials in 1937 to prevent erosion, but little progress was made, despite strong advocacy by Doug Campbell.
Burying the hatchet, Harrison and Prichard spent that evening experimenting with methods of dispersal, before settling on sewing a sack onto a piece of downpipe.
The following morning, 8 March 1939, Prichard flew over Ninety Mile Beach while Harrison, on his signal, held the downpipe out a window and emptied the sack.
As a result of Prichard's experiments, in 1945 the Department of Agriculture estimated aerial topdressing would cost about £4 per ton of fertiliser (on a basis of 2 cwt per acre), which was economic (actually, this price turned out to be a significant overestimate).
Immediately after the war, he obtained permission to build a sheet metal hopper for ZK-AFH to test the spread of bluestone crystals.
In August 1947 trials with cobalt sulphate in liquid form were conducted on the farm of K. M. Hickson near Taumarunui, with a horseback-mounted radio used to convey results to the pilot.
It responded enthusiastically to Campbell's suggestion, initially proposing to use Tiger Moth and DC-3 aircraft, but concerns about corrosion lead them to use "expendable" war surplus Grumman Avengers.
The superphosphate was too powdery but a more granular form was found before final trials measuring distribution pattern of spread by air on 16 September 1948.
For 1949 a Research and Development flight was formed under Stan Quill, equipped with the three Avengers and a Douglas DC-3, while instructions were sent to England to modify 2 RNZAF Miles Aerovans then on the production line to carry one-ton hoppers.
The "Topdress III" trials culminated on 21 May 1949 with a demonstration drop on 11 different properties close to Masterton in front of large numbers of farmers and press.
Following these successful trials, in 1950, farmers' groups lobbied the government to have the RNZAF provide subsidised topdressing with the Bristol freighters and even advocated using large Handley Page Hastings.
But by this time government work was being overtaken by private enterprise as ex-airforce pilots bought New Zealand-built De Havilland Tiger Moth biplanes cheaply, placed a hopper in the front seat and went into business flying from the paddocks of any farmer willing to pay.
By the end of 1949 there were five firms; Airwork had five Tiger Moths, James Aviation three, Aircraft Services three, Gisborne Aerial topdressing (which was to become Field Air) had one, and Southern Scenic Airtrips had converted an Auster.
Acting on Worral's suggestion, Airwork arranged a public demonstration on Sir Heaton Rhodes's property at Tai Tapu, south of Christchurch, on 27 May 1949.
The arrangement was typical of the system adopted by all early firms; a steep sided hopper was installed in the Tiger Moth's front seat, which the pilot, by pulling a lever, opened the vent at the bottom to release its load.
When the club complained the hopper could not be removed from the passenger seat without causing structural damage, Field bought Barbara II and started the Gisborne Aerial Topdressing Company on 2 August 1949.
Fieldair developed the tractor-mounted hopper loader, adopted throughout the industry, and became the largest topdressing firm in the country by the time of Field's death in 1981.
Wally Harding, a pioneer Waiouru farmer, converted his Tiger Moth into a top dresser in 1949 to use on his own not particularly productive high country station.
In 2004 the family business was bought out by Ravensdown Fertiliser Cooperative, although two of Wally's grandsons remain in the company: Bruce, chief pilot, and Rick, operations Manager.
Ossie James was another pilot and farmer who started with a Tiger Moth salvaged from floodwaters in 1948 and progressed to owning the largest fleet of Fletchers in the country.
At the beginning of the 1950s there were no specialist designs for even crop dusters, due to the proliferation of World War II surplus trainers.
The DHC Beaver was purchased in numbers and there were abortive plans to build it under license in New Zealand, but its high wing and bulky cabin were unsuited to the role.
It had more than three times the load capacity of the Tiger Moth and the cockpit located well forward, ahead of the hopper, giving the pilot all round view.
These generally had poorer forward vision and lesser payload to weight ratios than the Fletcher, which continued to dominate the New Zealand market—however, in places where aircraft primarily were used to drop insecticide, these American designs were superior.
By enabling sheep to be run profitably on steep hillsides, the topdressing industry stopped reforestation of otherwise uneconomic land, contributing to the erosion it was originally designed to prevent.
