The content of nitrates in vegetables varies greatly depending on the concentration of nitrates in the soil where they grow and therefore also on the use of fertilizer: for instance, nitrate concentration in spinach can vary from 24 to 390 mg/100g.
Therefore it is likely that "biologically grown" vegetables have a lower nitrate content than those grown with the use of fertilizer.
Nitrate content of >250mg/100g: beetroot, spinach, arugula, celery, lettuce, watercress.
Nitrate content of >100mg/100g: fennel, celeriac, endive, parsley.
Nitrate content <50mg/100g: carrot, cabbage, tomato, onion, garlic, potato, asparagus, artichoke, chicory, peas, mushrooms, pumpkin, zucchini.
When it is hard to reach the amount of nitrate needed to improve the
aerobic performance (500-600 mg/day) with your usual diet, you may want to add to it in the form of nitrate supplements.
I personally find the Beetroot Nitrate Performance Gels of ZIPVIP SPORT, UK, to be effective and well tolerated; containing 250 mg of nitrates from concentrate beetroot and 24g of CHO, they should be taken at a dose of 2-3 gels per day during the three days preceding the race, followed by 2 gels 1-2 hours before the event.