Soluble N compounds in sugar beet, summarised as harmful N, impair sugar recovery during manufacturing. Only amino N is included in the quality assessment. The objective of this study was to quantify the effect of genotype and environment on the composition of total soluble N in sugar beet and to evaluate whether amino N reliably represents the total soluble N. For this purpose, beet brei of 57 genotypes grown at 22 sites in 2000/01 was analysed for total soluble N, amino N, betaine, nitrate, and amino acid composition. The composition of total soluble N was more affected by environment than by genotype, whereby amino N was the only component which changed considerably. Its percentage increased from 25 to 35 % with increasing soluble N. In contrast, the concentration of betaine changed only slightly, so that its percentage decreased from 37 to 22 % with increasing total soluble N. Nitrate was the smallest and the calculated residual N was the most constant fraction of total soluble N. An increase of amino N was mainly due to the increase of glutamine, whereas the other amino acids hardly changed. The effect of genotype followed the same pattern as the impact of environment. Genotypes showed a high environmental stability for the percentage of amino N. For environments as well as for genotypes, low amino N was compensated by relatively higher betaine and vice versa. It is concluded that amino N is an acceptable estimate for total soluble N because of the close and consistent relation across environment and genotype. However, the quality assessment of sugar beet might even be improved by including betaine or total soluble N in addition to the standard parameters.