In the inter-war period, the practice of topdressing farms with artificial fertilisers had become widespread, and by 1939, nearly a quarter of the pastures were being topdressed annually. The high carrying capacity of much New Zealand pasture was made possible by this regular topdressing. Fertiliser usage was well over 600,000 tons a year in each of the two seasons preceding the war. It had doubled in less than twelve years.
Roughly three-quarters of this annual use of fertiliser was superphosphate, for which New Zealand was dependent on supplies of rock phosphate from Nauru and Ocean Islands in the Pacific, and from North Africa. The sulphur needed for the superphosphate came from the United States, Japan or Italy.
By the outbreak of war precautions had been taken against interruption of supplies, and 80,000 tons of rock phosphate was in store, equivalent to about one-third of a year's requirements. Fortunately, there was no immediate break in supply, although imports of basic slag and other phosphates declined from over 80,000 tons a year page 196 before the war, to under 35,000 tons a year in 1940 and in 1941. In the first two years of war, the use of fertilisers increased considerably, to reach nearly 700,000 tons in 1940–41, 14 per cent above 1938–39. However, in November 1940, five phosphate ships were sunk and the Nauru Island installations were bombarded by German raiders. In December 1941 Japan's entry into the war imperilled Pacific communications. Nauru Island installations received damaging attacks from Japanese bombers and, in August 1942, Japanese forces occupied the island.
Fertiliser supplies from alternative sources were scarce, and usage fell to 500,000 tons in 1941–42, to 360,000 in 1942–43 and to 285,000 tons in 1943–44. This was well under half of pre-war usage, a truly drastic fall. Rationing was necessary,1 and falls in output were expected to result. They did not eventuate. Ross suggests there was a residual in the soil from earlier applications which saved the situation.2
The limited supplies of fertilisers available from alternative, more distant, sources were dearer but were subsidised to hold the price reasonably close to its pre-war level.
Use of lime had also increased rapidly in the pre-war decade and, when wartime difficulties created shortages of phosphate fertilisers, even more lime was used. Lime being available in ample quantities in New Zealand, the main problem was cartage of a commodity with a very great weight for value. Rail and road cartage were subsidised.3 By 1943–44 the area topdressed with lime only was well over three times the pre-war area.
Imports of phosphatic fertiliser were at their lowest in 1942 and 1943, when arrivals were less than one-third of 1939. Thereafter they increased steadily, but did not reach the 1939 quantity again until 1949. To eke out supplies, serpentine superphosphate was made, including one part in four of ground serpentine rock. This produced a fertiliser with some advantages over superphosphate.4 The phosphate quarries at Clarendon, in Otago, were reopened after being inactive since 1924, and produced small quantities of rock for making superphosphate. In spite of all these endeavours, the area of grassland topdressed in 1944–45 was still 10 per cent below 1938–39.page 197
Chart 41 shows the areas topdressed in each year from 1934–35 to 1948–49, including topdressing with lime, artificial fertiliser, or both.
1 The Phosphatic Fertilizer Control Notice, June 1941.
3 Free carriage of lime by rail had been a long-standing concession to farmers.
4 Primary Production in New Zealand, 1946, p. 26.