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2.Problem and its Countermeasure of Nitrate in Spinach (Spinacia oleracea) Cultivated with Organic Fertilizer in Greenhouse

Masahisa Abe, Koichi Shimizu(Institute for Agro-Microbiology)

In the agricultural field where spinach (Spinacia oleracea) was cultivated continuously over ten years by using the organic fertilizer in the greenhouse, there were some cases that the content of nitrate in spinach exceeded 900 mg / 100 g-fresh weight, and the spinach farmer requested us to improve the quality of spinach. In this study, we examined the relationship between the content of nitrate in soils and the quality of spinach, and verified some countermeasure for improving the quality of spinach. From the results of the investigation at the place of spinach production in 2001, it was observed that the content of nitrate in spinach, which was harvested from May to August, exceeded remarkably the standard value of nitrate in spinach. Since this phenomenon was related to the change of the accumulated atmospheric temperature, it was suggested that the mineralization of soil nitrogen was promoted by increasing the soil temperature in summer and then the content of nitrate in spinach increased consequently.
For the decrease of nitrate in soil, the new fertilizer-application method considering both the quality and use time of organic fertilizer or the removal of soil nitrogen by using cleaning crop such as sorghum (Sorghum vulgare) was considered to be effective. Moreover, since the intercropping with welsh onion (Allinm fistulosum) decreased in the content of nitrate in spinach, this cultivation technique was considered to contribute to the improvement of quality in spinach that was cultivated in summer period.

Keywords: spinach, organic fertilizer, nitrate nitrogen, cumulative temperature, rate of mineralization