Previously, we reported that the content of total oxalic acid of spinach in the solution culture decreased along with increasing ratios of NH_4-N to NO_3-N in the solution. In the present field study, we attempted to decrease the content of oxalic acid of spinach by growing with slow-releasing fertilizers, since we could expect the absorption of NH_4-N by spinach just after being released from these fertilizers. Three cultivars of spinach were grown in an Ando soil with ammonium sulfate(AS), coated urea(CU)and coated ammonium phosphate(CAP), at 15 g N m^<-2>. The seeds were sown on the top of the row and the fertilizers were applied 6 cm below the seeds with 10 cm width. NH_4-N and NO_3-N in the soil block which contained the fertilizers were extracted with a 100 g L^<-1> KCl solution and measured colorimetrically. The NH_4-N contents 22 d after fertilizing were 1.9mg/100 g dry soil in the AS plot, but they were 10.0 and 16.3 mg/100 g dry soil in the CU and CAP plots, respectively, and a significant content of NH_4-N(16.1 mg/100 g dry soil)was detected only in the CAP at harvest. It was thought that a part of these NH_4-N were absorbed by spinach before nitrification. The contents of oxalic acid in the CU and CAP plots, compared with those in the AS plot, were remarkably decreased in spinach applied with CAP(47-72% of the value found in the AS plot). Nitrate was further decreased and sugar was increased in the both CU and CAP plots in two cultivars, and total ascorbic acid was increased in the CU plot in three cultivars. So application of slow-releasing nitrogen fertilizers induced a desirable effect on the quality of spinach.
