Abstract
Absorbance at 280 nm in soil extracts with 0.4 M sulfate solution was highly correlated with the concentration of total N in the extracts of different soils. Therefore, we proposed an easy detection procedure for evaluating available N in soil. To estimate the total N uptake of spinach grown triannually under a rain shelter, the amount of 0.4 M sulfate-extractable organic nitrogen in the soil was determined before cultivation in the Andosol area in Iwate prefecture, Japan. Total nitrogen uptake of spinach grown without fertilizer in 8 experimental sites showed a tendency to increase when the amount of 0.4 M sulfate-extractable organic nitrogen in soil was increased, while N uptake efficiency tended to decrease when there was over 540 mg•kg−1 of the extractable organic N in soil. Application of manure compost was2 effective in increasing the yield, while reducing internal nitrate concentration of spinach. However, the internal nitrate concentration in spinach exceeded the criterion value (3,000 mg•kg−1) when there was over 540 mg•kg−1 of 0.4 M sulfate-extractable organic nitrogen in soil. These findings indicate that the determination of 0.4 M sulfate-extractable organic nitrogen in soil might contribute to the fertilization of rain shelter house-grown spinach.
