Soil amendments used in organic farming may limit nitrogen availability and be cost prohibitive. Our objective was to determine if Brassicaceae seed meal, a byproduct of oil extraction that contains allelochemical-producing compounds called glucosinolates, can increase plant-available nitrogen and yields of lettuce (Lactuca sativa ) and beet (Beta vulgaris). Three seed meals with varying glucosinolate concentrations and profiles, Brassica juncea (BJ), Brassica napus (BN), and Sinapis alba (SA), were added to field soils at two different rates (3 and 10 Mg ha^-1) in each of two years. Crop yields and nitrogen uptake varied with precipitation, meal type, and amendment rate; however, the highest rate of BJ meal most often produced the largest increases in biomass and nitrogen uptake, except for lettuce in year two. Greater moisture in year two may have increased the release of allelochemicals from BJ meal, which inhibited germination of the small-seeded lettuce crop. The greatest amount of net nitrogen mineralization (extractable soil nitrogen + plant tissue nitrogen) also occurred in the high-rate BJ treatment, suggesting the allelochemical produced by this meal (2-propenyl isothiocyanate) stimulated mineralization of soil organic matter beyond that of the other seed meals. Laboratory incubations conducted with the same soils and meals indicated that 52 to 114% of the nitrogen added in the meals was mineralized in 21 days and that nitrification was inhibited by both BJ and SA meals. Combined field and laboratory studies demonstrate that allelochemicals released by mustard seed meals stimulate substantial mineralization of soil organic nitrogen, but only temporarily inhibit nitrification. Brassicaceae meals have the potential to improve crop yields by increasing plant-available nitrogen in organic vegetable production systems, but overall effects on crop growth must include consideration of water inputs, potential inhibition of seed germination, and glucosinolate type and concentration.