A simple method for estimating the nitrate nitrogen concentration based ondry matter in standing sorghum (Sorghum bicolor Moench) using stem juice test and dry matter content

A simple method was developed for estimating the nitrate nitrogen (NO3-N) c oncentration 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 stem on a fresh matter basis) by the dilution fa ctor (DF, dry matter partitioning ratio of stem divided by the dry matter c ontent of stem or fresh matter weight of stem divided by the dry matter wei ght of whole plant). CF was linearly correlated with the NO3-N concentratio n in stem juice squeezed from stem discs with a thickness of 1 cm obtained by cutting horizontally at a location corresponding to a ratio of 25% to th e total stem length. CF can be estimated by multiplying the NO3-N concentra tion in stem juice by a factor of 0.68 or 0.53 for grain and dual purpose t ypes or sorgo type, respectively. The values of the coefficients of determi nation (R-2) of the linear regression were above 0.7. When DF was plotted w ith the dry matter content of whole plants (DM%), regression curves were ob tained for both sorghum types with R-2>0.8. The equations were as follows: DF= 0.00721 X-2-0.503X+10.72 for grain and dual purpose types, DF=0.00445 X -2 -0.337X+8.86 for sorgo type, where X is DM%. The NO3-N concentrations es timated by multiplying CF by DF enabled to predict the measured NO3-N conce ntrations with an R-2 of 0.873.