Very soluble salts of nitric acid (Potassium, Magnesium, Calcium, Ammonium nitrates); all plants assimilate them easily.
Nitrogen excess when wheather is cold and cloudy provoking nitrates accumulation in the plant (In France, for instance, authorized maximum contents are : 4000 ppm of NO3- for lettuce, 300 ppm for carrot and 50 ppm for babies food.
  • Plants' nitrates content increases with lightness (less on evening)
  • Young plants contain more nitrates than old plants. Nitrates are not held back by soil absorptive power.
Excess water irrigation provoke a nitrate leaching which can pollute ground water.