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Residual Nitrate and Mineralizable Soil Nitrogen in Relation to Nitrogen Uptake by Irrigated Sugarbeets¹

Previously reported studies on N fertilization of sugarbeets (Beta vulgaris L.) in southern Idaho revealed considerable variation among sites in amounts of residual soil NO₃ and N mineralized during short-term laboratory incubations. Consequently, the amount of N fertilizer needed to achieve near-maximum yields of sucrose differed markedly. The purpose of this study was to determine the feasibility of estimating amounts of N mineralized in the root zone during the season, taking into account site variations in temperature and soil water regimes. Residual soil NO₃^--N and mineralizable N to approximate rooting depth were estimated for 21 field sites in 1971 and six sites in 1972. The relative contributions of these two N sources to total N uptake by the crop, in the absence of applied fertilizer N, were then assessed. Estimates of N mineralized in the upper 45-cm soil layer for each successive month, N, over a 6-month period were derived using the expression, N/ t = kWN (k = fraction of N mineralized during each month, t, adjusted for average air temperature; and W = the estimated soil water content expressed as a fraction of the available water storage capacity). Resulting estimates of the fraction of potentially mineralizable N converted to (NO₃^- + NH₄^-)-N between 1 April and 30 September ranged from 0.15 to 0.22 (mean ± S.D. = 0.18 0.02) in 1971 and 1972. On the average, mature sugarbeets recovered about 73% of the estimated N mineralized (6 months) plus residual NO₃^--N. The relative contributions of these two sources of soil derived N, respectively, were approximately 66 and 75%, as estimated from multiple regression analyses.

Key Words: Temperature • Soil water content • N use efficiency • N requirement

² Soil scientists, Agricultural Environmental Quality Institute, Beltsville, MD 20705.

³ Soil scientists, Snake River Conservation Research Centre, Kimberly, ID 83341.

Received for publication August 30, 1976.

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