entry barriers, have ensured the Antipodean style of topdresser did not compete with the cockpit behind the hopper designs of American manufacturers. Mature industry The amount of fertiliser used in aerial topdressing rose from almost nothing in 1950 to over 250,000 tons in 1955, to over 450,000 tons in 1960 and to over 900,000 tons in 1965, approximately doubling every five years. By 1958 there were 73 aerial topdressing firms in New Zealand, flying 279 aircraft—although the amount of superphosphate dropped and the area it fell on would continue to increase—from then on the numbers of companies, aircraft and pilots dropped as the larger more expensive Fletchers came to dominate the market and the one-man companies that began in the 1940s were amalgamated. By 1965 the million tons of superphosphate dropped annually was being spread over 9 million acres (36,000 km2). The amount an aircraft dropped had increased from 2.5 tons to 8 tons and there were 10,000 privately owned airstrips for topdressing in New Zealand. Other work was also done by agricultural aircraft, as in foreign countries, particularly outside the February to May prime season. Clover seed is sown and spraying is carried out with insecticides, fungicides and weedkillers as well as general utility work. Aerial Topdressing has been attributed with vastly increasing agricultural production—in New Zealand alone, sheep numbers increased from 40 million to over 70 million, the majority of the increase being attributed to the increased feed that superphosphate made available. Environmental impact Ironically, given the industry was started by government research aimed at soil conservation, a number of negative impacts have emerged. The two major criticisms are the run off of fertiliser into streams and waterways which encourages marine plant growth, leading to choking of the waterways and altering the fresh water ecosystem, disadvantaging many fish, (and frustrating anglers). To minimise impact, topdressing is now prohibited within certain distances of water. The second impact is less direct. 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. The mining of superphosphate from guano deposits on the tiny South Pacific island of Nauru temporarily made the island one of the richest nations in the world on a per capita basis, but removed most of the soil from the island making 80% of it unusable. See also Aerial application