THE RELATION OF SOIL TEST VALUES TO FERTILIZER RESPONSE BY THE POTATO: II. NITRATE PRODUCTION AND NITROGENOUS FERTILIZER REQUIREMENTS

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Abstract
Bray's modified Mitscherlich equation was utilized to determine the relationship of potato yields (expressed as percentages of the maximum) to soil nitrate production and to rates of nitrogen fertilization at 18 locations over a 3-year period. A closer relationship was obtained using fresh soil samples than using those air-dried for 6 months.The influence of various factors on absolute tuber yields (bushel/acre) was also assessed by analysis of variance. Highly significant effects were obtained for "rates of nitrogen" and for "soil test values" but not for the interaction of these two factors. The polynomial response curves derived from the analysis of variance showed that maximum yields were approached at a rate of 200 pounds/acre of applied N, whether soil nitrate production values were low, medium or high. Tuber yields were also influenced by “soil series" and the amount of N fertilizer required for maximum yield was greater for some series than for others.