Abstract
Komatsuna ‘Natsurakuten’ was cultivated during three successive crop seasons (early summer, late summer, autumn) in the same soil consisted of andisols in a greenhouse. Four fertilization formulae were tested in order to reduce nitrate concentration in plants. The amount of nitrogen (N) fertilizer in the control fertilization (N7) was 7 kg·10 a−1. In fertilizations based on soil diagnosis of nitrate-N before fertilization, the summed amounts of N-fertilizer and soil nitrate-N were 7 or 5 kg·10 a−1. The other fertilization (N0) was formulated without N-fertilizer. Using the fertilizations based on soil diagnosis, nitrate concentrations in plants were hardly changed compared with those using N7 in the summer, but these concentrations were lower than those of N7 in the autumn. However, nitrate concentrations in N0 were much lower than those of other fertilizations. We gained sufficient yields with N0 when the length of cultivation was prolonged by 5 days in late summer. About 3 kg·10 a−1 organic N was mineralized during cultivation in summer, and this amount was higher than the amount of N assimilated in plants. Thus it is necessary to consider the amount of mineralized N and assimilated N in plants in order to reduce nitrate concentration in plants during high temperatures.
