Previously reported studies on N fertilization of sugarbeets
(Beta vulgaris L.) in southern Idaho revealed considerable
variation among sites in amounts of residual
soil NO? 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 NO?--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 (NO?- + NH?+)-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 NO?--N. The relative
contributions of these two sources of soil derived N, respectively,
were approximately 66 and 75%, as estimated
from multiple regression analyses
