VARIABLE FERTILIZER APPLICATION BASED ON YIELD GOAL, SOIL FERTILITY, AND SOIL MAP UNIT

Soil fertility and soil productivity are variable within farm fields. Over or under application of a nutrient in parts of every field can result from conventional fertilizer application. This study was conducted to determine whether yield and net returns could be increased by considering field variability when applying fertilizer. The influence of soil fertility variation was considered with soil sampling on a grid pattern and with detailed soil maps made of each field. The fields, located in eastern North Dakota, were mainly a mixture of Haploborols, Calciaquolls, and Argialbolls. Wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.) were grown. In 1989, variable rate fertilization based on grid soil sampling did not result in a greater yield of barley (32.1 bu/acre) than the conventional method (30.0 bu/acre). In 1990, the variable rate application yielded more barley (73.7 bu/acre) than the conventional method (67.7 bu/acre). The conventional method, however, resulted in the greatest net returns in both experiments. In 1991, the variable rate method based on a 50-ft soil sampling grid yielded more wheat (37.3 bu/acre) than the method based on soil mapping units (33.8 bu/acre). The greatest net return ($114/acre) was obtained by using the average soil test by map unit and a yield goal of 50 bu/acre. Yields of barley and wheat were increased by consideration of soil nutrient variability but the increased cost of grid soil sampling resulted in less net return than conventional fertilization and sampling methods.