Abstract
In September 1973, an aerial spraying of 336 kg/ha of urea was made over an area of 34 ha covered by firs 20–70 years old. Within the 35 permanent sample plots each comprising 0.04 ha and established at random on equidistant lines every 100 m, open cans had been placed one week before the spraying. They were taken 2 weeks after the end of the spraying and after checking that the urea intercepted by the trees had fallen. An average quantity of urea equivalent to 347 kg/ha was collected. Approximately 29% of the area received 336 ± 56 kg/ha of urea, 37% received less, and 34% received more. Five cylindrical cans having a diameter of 23 cm and a height of 24 cm in each of the 10 plots were sufficient to determine the exact rate of spraying, but at least 35 plots were necessary to know the distribution of urea in such an area. However, 10 cans were necessary to determine with sufficient accuracy the quantity of urea sprayed on each plot.
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