側条施肥栽培におけるペースト肥料の窒素組成の違いがホウレンソウの硝酸含量に及ぼす影響

To evaluate the effects of differences in nitrogen composition in paste-like fertilizers applied as side dressing, growth and nitrate concentration of spinach (Spinacia oleracea L. var. crispa) grown in containers were examined and compared to standard fertilization using a compound fertilizer (standard, N, P_2O_5 and K_2O applied to the soil at a rate of 120mg kg^<-1>). Three kinds of paste-like fertilizer, which differ in nitrogen source, i. e. urea, a 1:1 mixture of urea and residual liquid with fermented molasses (RLFM) and RLFM, were applied to the soil as side dressing at a rate that was 20% below the standard. Dry matter production and N uptake in spinach treated with paste-like fertilizers was comparable to that treated with standard fertilizer, while the nitrate concentration in spinach treated with paste-like fertilizers was lower than that treated with standard fertilizer. Among the paste-like fertilizers, the nitrate concentration in spinach decreased with the increasing rate of RLFM, in which the major N sources were composed of proteins and amino acids. To understand the possible explanation for better growth and low nitrate concentration in spinach treated with paste-like fertilizer despite the lower application rate, N concentrations of ammonium, nitrate and organic N were assessed temporally by leaching water from fallow plots. At the initial time of incubation, the nitrate concentration in the leaching water from the standard fertilizer was higher than that from paste-like fertilizer treatments. The paste-like fertilizer composed of urea, however, showed a higher concentration of organic N which was supposed to be urea. The fertilized-N of the standard and paste-like fertilizer composed of urea might be immediately eluviated by irrigation. However, N concentration in the leaching water after treatments with paste-like fertilizer composed of RLFM was lower than that of the former treatments, suggesting that fertilized-N might remain localized. The amount of N eluviated during incubation showed a negative correlation with the viscosity of the paste-like fertilizer. The highest viscosity was observed in the paste-like fertilizer composed of RLFM, followed by a 1:1 mixture of urea and RLFM, and the paste-like fertilizer composed of urea was the lowest in viscosity. Therefore, it was suggested that the paste-like fertilizer composed of RLFM or a 1:1 mixture of urea and RLFM showed limited N leaching due to the high viscosity. Therefore, spinach could take up N efficiently.