Nitrate accumulation in spinach (Spinacia oleracea L.) tissues under different fertilization regimes

Accumulation of nitrate in spinach (Spinacia oleracea L. cv. Baladi) leaf and petiole as affected by different fertilizer types and application rates was examined at Al-Balqa' Applied University Research Station, during the 2008 and 2009 growing seasons. In addition, their impact on soil chemical properties was also evaluated. A randomized complete block design with seven treatments was followed. Organic fertilizer (composted manure) was applied at five application rates (10, 20, 30,40 and 50 t/ha). Chemical fertilizer was applied at a rate of 150 (N):150 (P):150 (K) kg/ha. Nitrate concentrations varied between spinach leaves and petioles, with general increment in the petioles over those of the leaves. The accumulated nitrate in the petioles was 110%, 108% and 107% of that accumulated in the leaves under organic, chemical and control fertilizer treatments, respectively. Tissue nitrate levels generally declined as the organic fertilizer rate increased. However, the highest rate of organic fertilizer produced spinach leaves with 59% and 83% of the nitrate accumulated in the leaves produced under chemical fertilizers and control treatment, respectively. The use of chemical fertilizer caused the highest nitrate concentration in both leaves and petioles. The highest spinach leaves yield (27.2 t/ha) was obtained from the chemical fertilizer treatment; which was similar to that of the highest organic fertilizer application rate which produced 27.1 t/ha. Largest leaf area was obtained by using chemical fertilizer or the highest organic fertilizer rates (40 and 50 t/ha). On the other hand, soil pH, EC and organic matter as well as N, P, K levels in the soil were increased by using organic fertilizers.