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

Abstract

AbstractPreviously 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, ΔN, over a 6‐month period were derived using the expression, ΔN/ Δt = kWN (k = fraction of N mineralized during each month, Δt, 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 α 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.