this price turned out to be a significant overestimate). Prichard now found an ally who could officially sanction further trials. Doug Campbell Doug Campbell was an agricultural academic concerned about soil erosion. He had been suggesting the spread of both seed and fertiliser for erosion control and aerial spreading of trace minerals since the 1930s, but had not conducted trials until he met Prichard. Campbell brought official backing and academic responsibility to Prichard's work. Immediately after the war, he obtained permission to build a sheet metal hopper for ZK-AFH to test the spread of bluestone crystals. In 1946 the first pure topdressing flight was conducted without seed. Mixtures of bluestone crystals, sulphate of ammonia, slaked lime and carbon black were used. The lack of a lid for the hopper initially resulted in irritating dust spreading through the aircraft in turbulence: in cold wet conditions it was necessary to heat the hopper to prevent the fertiliser coagulating, while in dry conditions the powder tended to disperse in the wind before reaching the ground. Nevertheless, in July Campbell arranged for ZK-AFH to topdress 1,100 acres (4.5 km2) of a copper-deficient farm. 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 was soon suggested that cobaltised superphosphate would be easier to spread, although it was felt a specialised aircraft would be needed to do this. Campbell published his research in the New Zealand Journal of Science and Technology, Volume X, 1948 as "Some observations on top dressing in New Zealand". Convinced by the trials, Campbell formed the co-ordinating and advisory committee on aerial topdressing with representatives from the Ministry of Public Works, Department of Agriculture, Department of Air, DSIR and the Soil Conservation Council. At the committee's first meeting on 27 November 1947 it resolved to ask the Royal New Zealand Air Force for assistance. RNZAF trials Between the second world war and the cold war, the RNZAF was a large and competent organisation without a lot of work to do. 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. Experiments were resumed on 5 September 1948 using a Miles Whitney Straight and three Grumman Avengers; the RNZAF put superphosphate