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Abstract
A simple method was developed for estimating the nitrate nitrogen (NO3-N) concentration in standing sorghum (Sorghum bicolor Moench) crop at maturity. In the method, the concentration of NO3-N in whole plants on a dry matter basis can be accurately estimated by multiplying the concentration factor (CF, NO3-N concentration of stern on a fresh matter basis) by the dilution factor (DF, dry matter partitioning ratio of stern divided by the dry matter content of stern or fresh matter weight of stern divided by the dry matter weight of whole plant). CF was linearly correlated with the NO3-N concentration in stern juice squeezed from stern discs with a thickness of 1 cm obtained by cutting horizontally at a location corresponding to a ratio of 25% to the total stern length. CF can be estimated by multiplying the NO3-N concentration in stern juice by a factor of 0.68 or 0.53 for grain and dual purpose types or sorgo type, respectively. The values of the coefficients of determination (R 2) of the linear regression were above 0.7. When DF was plotted with the dry matter content of whole plants (DM%), regression curves were obtained for both sorghum types with R 2 > 0.8. The equations were as follows: DF= 0.00721X 2-0.503X + 10.72 for grain and dual purpose types, DF=0.00445X 2 -0.337X + 8.86 for sorgo type, where X is DM%. The NO3-N concentrations estimated by multiplying CF by DF enabled to predict the measured NO3-N concentrations with an R 2 of 0.873.