Environmental factors and cultural measures affecting the nitrate content in spinach

Environmental factors and cultural measures affecting the NO₃-content in spinach were studied indoors, in water-, sand- and soil-culture experiments. In the field, the influences of variations in N-fertilizing practices and in spinach varieties were also tested.High NO₃-contents in spinach were found with low light intensities, with low soil-moisture contents, and with high temperatures. NO₃-contents increased with increasing K-dressing (less so with KCl than with K₂SO₄), but decreased with increasing soil pH. In pot experiments, positive results were obtained with sulphur-coated urea, with farmyard manure and with pigmanure slurry.Application of Mo as a spray onto spinach leaves, and variations in P-dressings and in soil P-status were found not to affect the NO₃-content in spinach.In pot experiments, NO₃-contents decreased with progressing plant age (in autumn less so than in spring). Within spinach plants, NO₃-contents were highest in petioles and older leaves. Varietal differences in NO₃-contents were observed in a pot- and a field experiment.In pot- and field experiments, partial or complete replacement of NO₃-N by NH₄-N in general caused the NO₃-content in spinach to decrease. However, such a replacement was shown not always to result in lower NO₃-contents. Additional factors involved are e.g. the use and effectiveness of nitrification inhibitors, the soil type and the amount of available N.The amount of N added and, in the field, the amount of N available in the soil before sowing, strongly affected the NO₃-content in spinach. Under field conditions, nitrogen appeared to be taken up from the top 60 cm of the soil profile.The effects of variations in timing of nitrogen applications were absent in a pot experiment and not consistent in field experiments.List of abbreviations A = sum of inorganic anions (Cl + H₂PO₄ + SO₄ + NO₃) in meq per kg DM (H₂PO₄ stands for total P) - ADI = acceptable daily intake - C = sum of inorganic cations (Ca + Mg + Na + K) in meq per kg DM - C-A = excess of inorganic cations over inorganic anions in plants, meq per kg DM; NH₄-ions are not included - CEC = cation exchange capacity - DCD = dicyandiamide (C₂H₄N₄) - DM = dry matter - FW = fresh weight - FYM = farmyard manure - JECFA = Joint FAO/WHO Expert Committee on Food Additives - N-serve = nitrification inhibitor, containing 2-chloro-6-(trichloromethyl) pyridine (C₆H₃NCl₄), known as nitrapyrin, as active compound - org N = organic nitrogen (= total N — NO₃-N) - PMS = pig-manure slurry - SCU = sulphur-coated urea - WHC = water-holding capacity - WHO = World Health Organization