Moindreffor wrote:
after what is an excess of nitrogen? too fertile ground?
For my part, this term (or the expression I often use: "overdose" - nitrogen is a bit of a dope of the gardener, well of the vegetable patch from the gardener's point of view), in fact covers two things:
a) during the period of vegetable growth, an excess of mineral nitrogen soluble in the soil solution, which results in exaggerated vegetative growth (to the eye: often dark green to bluish colors - of course to be put into perspective according to species and varieties). This exaggerated growth is linked to two things (at least): a growth / development imbalance (therefore less fruit set, in fruit vegetables) AND a greater sensitivity to diseases, due to too fragile tissues, 'a too strong attractiveness for certain suckers-stingers (in search of nitrogenous nutrients and which "know" how to detect plants having this richness; aphids "drip" the plants!
[It is also often linked to a lower quality: antioxidants, flavors, vitamins, minerals are in fact "diluted" in a larger volume of biomass!]
[I do not distinguish the "chemical" or "organic" origin of the overdose: in my opinion, although the "organic" more often use complex and organic fertilizers, which de facto relativizes the overdose, dilutes it over time, an excess of organic nitrates has nothing to envy to an excess of chemical nitrates. The "organic" too often has a good back! "Shut up, it's organic" should replace the famous "Eat, it's organic" of sore meals]
b) at the end of the season, these residues of mineral nitrogen, especially nitrates (or ammonium likely to be further nitrified): this will be linked to the risk of leaching; it is not a question of "production" of the vegetable garden; it is an ethical question: can we criticize farmers and do just as badly, or even worse? (the farmers have nitrogen fertilization plans which take into account the residues!).
The excess nitrogen is therefore not to be taken in absolute terms. The nitrogen contained in organic matter is not in question (it is about tons of N per ha !!!). We should always say "excess of directly assimilable mineral nitrogen". This is the one vegetables can gorge on. And it is that one, and that one only, that can be washed away.