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.