Effect of partial urea application on nutrient absorption by hydroponically grown spinach (Spinacia oleracea L.)

Spinach (Spinacia oleracea cv. Okame) was grown in hydroponic pot culture with an Enshi nutrient solution amended with 0, 20, or 50% urea with or without nickel addition (Ni; 0.05 mg L-1), while the total concentration of N (17.33 mmol L-1) remained constant in all the cases to evaluate the effect of partial urea application, with or without the addition of Ni, on the absorption of NO3-N, urea-N, NH4-N, and minerals (e.g. Ca, K, Mg, and P) by plants. Fresh and dry weight of the shoots was highest when a 20% urea solution with Ni addition was used. The variation in spinach yield was related to the absorption of total-N by the plants. The absorption of total-N, attributed mainly to NO3-N and urea-N, differed between the treatments. In the case of short-term absorption, determination by using N-15-urea and N-15-KNO3 showed that, the urea-N absorption significantly increased with the increase in the urea concentration in the nutrient solution. When the urea solutions were used, regardless of Ni addition, the absorption of NO3-N was more than four times higher than that of urea-N. The addition of Ni in the urea solutions stimulated and increased both urea-N and NO3-N absorption. In the case of long-term absorption, the NO3-N absorption decreased with the decrease of the NO3-N concentration when NO3-N was partially replaced with urea in the nutrient solution. The addition of Ni in the urea solutions resulted in the increase of the absorption of both urea-N and NO3-N, but the NO3-N absorption remained lower in all the treatments compared to the control. In the urea solutions, the absorption of urea-N with or without the addition of Ni increased at a lower rate over time (sampling stages). Application of urea, with or without the addition of Ni in the nutrient solution, increased Ca absorption but decreased K and Mg absorption, whereas, P absorption was unaffected. It is suggested that spinach could grow adequately in an Enshi nutrient solution modified with 20% urea with the addition of 0.05 mg Ni L-1, when urea totally replaced NH4-N and partially replaced NO3-N.