New Zealand has 43 million acres of farm land of which fully two-thirds are too steep for the use of tractor-drawn implements. These steep and hilly areas can be divided into three main types of vegetation. Sown pasture covers 11 million acres, mainly in the North Island, while 12 million acres are predominantly tussock, mainly in the South Island. Finally, scrub and weeds grow on about 5 million acres.
Until aerial topdressing gained momentum, the mineral fertility of the soils on these large areas had been slowly declining. On most of the 11 million acres of sown hill pasture this meant increasingly less productive swards, difficulties in maintaining the carrying capacity of sheep and cattle, a decline in weights of wool and in receipts from store stock, and less money for repairing and renewing fences and buildings. A thinning of pastures, which allowed the ingress of useless scrub, was often the final result of this deterioration of soil fertility.
A different spiral of decline has been emerging on the 12 million acres of tussock lands in the South Island where the native vegetation developed without grazing animals. When sheep (and rabbits) were introduced, the more palatable species were slowly eaten out. The less palatable species which remained had to be burned periodically so that the sheep could eat the young regrowth. Where the climate has been not too cold and soil moisture retentive, browntop (Agrostis tenuis) and other exotic grasses, as well as weeds, have slowly replaced the native tussock grasses. In the drier districts, however, colonisation with weeds and weed grasses has been slow or non-existent. In these dry areas the disappearance and weakening of tussock by grazing and fire has often been followed by soil erosion.
Finally, there are the 5 million acres of hilly scrub lands which for the most part represent a failure to maintain sown pasture. They cannot be reclaimed without the generous use of phosphatic fertilisers. To change the fate of our hill country from one of decline to one of improving productivity, a higher level of mineral fertility was sorely needed. Superphosphate was the fertiliser required and, at times, molybdenum and extra sulphur.
Labour, however, was not available to apply these fertilisers by hand during the period of full employment after the war. Moreover, it was slow and expensive to spread fertilisers by hand on steep and inaccessible hill country. Blowers operating from bulldozed tracks were used for a while by some farmers, but the gross unevenness of distribution of superphosphate by these means proved unsatisfactory and only small areas could be covered.
It seems that the idea of aerial topdressing goes back at least to 1926. In that year J. Lambert suggested that aerial topdressing of hill land might be worth trying, but his suggestion was rejected as impracticable. That this method of topdressing might be feasible was shown in 1936 by a farmer in Hawke's Bay. He used a light aircraft to oversow clover seed. In 1938 A. Pritchard, then a pilot in the Public Works Department, experimented with sowing lupin seed by air on coastal sand country. A year later fertiliser dropping was tested: the war halted further progress.
After the war the cooperation of the Royal New Zealand Air Force and the Department of Agriculture led to measurements of the distribution pattern of granulated superphosphate spread by air. The results of these tests were quite encouraging. In 1949 a private firm began experimenting with aerial topdressing. Since then expansion has been rapid. The reasons can be enumerated as follows:
Good prices for wool and meat during and after the war have given the farmer the necessary money.
The war left a surplus of small training aircraft which could be adapted cheaply to carry fertilisers.
The war had created the skilled pilots needed for this work.
The low costs and high speed of spreading fertilisers by air were quickly recognised.
The simultaneous introduction of clovers into deteriorated pastures by surface seeding proved successful. This made possible the full utilisation of applied fertilisers.
This expansion of aerial topdressing is illustrated in the following table:
| Year | Fertilisers and Lime Applied | Area Topdressed |
| tons (000) | acres (000) | |
| 1950 | 5 | 48 |
| 1952 | 90 | 802 |
| 1954 | 205 | 1,930 |
| 1956 | 405 | 3,850 |
| 1960 | 475 | 3,960 |
| 1961 | 590 | 5,240 |
| 1962 | 620 | 5,607 |
| 1963 | 612 | 5,169 |
| 1964 | 746 | 6,588 |
For the year ending 31 March 1964, aircraft distributed 746,795 tons of fertiliser and lime on New Zealand pastures, or 22 per cent more than in 1962–63. More than half this total (404,958 tons) was dropped in the Auckland area. Altogether, the North Island accounted for 650,358 tons and the South Island for 96,437.
Aircraft now help agriculture in many ways other than in spreading fertilisers. Each year poison baits for rabbits are spread on 80,000–100,000 acres, clover seed is sown, fencing materials are delivered to inaccessible places, and spraying is carried out with insecticides, fungicides, and weedkillers.
In 1945 a committee of the Department of Agriculture estimated that the probable cost of aerial topdressing alone would be about £4 per ton of fertiliser (on a basis of 2 cwt per acre). Prices charged in the early 1950s were slightly lower than this estimate. Even 10 years later, and in spite of increases in the cost of all essential components, topdressing charges have not increased. The following factors are responsible for this stability during a period when most other services were forced to increase their charges:
A greater number of farm airstrips has reduced the distance between airstrip and target. There are now about 10,000 privately owned airstrips.
A steady improvement in the quality of the airstrips themselves has led to increases in payloads, less wear and tear of aircraft, and greater safety.
New and more efficient aircraft have been introduced.
The methods of the mechanical handling of fertilisers have been improved. Fertilisers supplied in bags have been replaced in many instances by fertilisers supplied in bulk, with a speeding up of handling.
Fertiliser bulk stores at frequent intervals along railway lines have been constructed, allowing quicker delivery at short call. This development is mainly confined to the North Island.
More specialised training programmes have increased the efficiency of pilots. Many of these factors have contributed to a substantial increase in the fertiliser spread per flying hour. A mean of 2·8 tons per flying hour was spread in 1951; in 1958 it was 6·5 tons, and by 1962 it rose to 7·9 tons.
Overall efficiency of the aerial topdressing industry could be further increased by a more evenly distributed demand for aerial topdressing throughout the year. In the years 1960–62 nearly 50 per cent of all fertilisers supplied were spread during four months of the year, from February to May inclusive. In many cases, however, autumn topdressing may not possess marked advantages over topdressing at other times of the year.
Aerial topdressing has been the key to an increase in the productivity of our hill country. Without its development large areas of hill land would no doubt have gone out of production and many farms would have become uneconomic units. The full utilisation of greater soil fertility, however, demands changes in farm management such as closer subdivision, heavier and more carefully controlled stocking, and, in some cases, larger areas of supplementary crops and hay to feed the greater number of sheep and cattle during periods of feed shortage. Better provision of drinking water, more access tracks, closer shepherding, but more worries about diseases in stock, such as seasonal hypomagnesaemia in cattle (grass staggers) and pregnancy toxaemia in ewes, are also an inevitable adjunct of higher productivity.
Aerial topdressing has greatly stimulated investment on hill-country farms. It also demands greater managerial skill from the farmer. In all, it has become one of the most important and most beneficial features of our farming industry and one without which we could not progress at the rate needed.
by Cornelius During, B.AGR.SC., formerly Farm Advisory Service, Department of Agriculture, Wellington.