What is the secret to the luxuriant growth of nursery plants despite such small pot with minimal soil and root ball?
It seems in most cases that nitrogen is otherwise a limiting factor. But not just any nitrogen. Proteins and their decomposition products, peptides and amino acids. The fungi and bacteria that decompose have probiotic and prebiotic effects for plants the same was with human and animal diets.
Commercial nurseries often use fish emulsion but perhaps for reasons of convenience and availability.
>Plants can’t make use of large molecules such as oils and proteins; see Organic Fertilizer – What is it’s Real Value? for more details. When these molecules are added to soil, microbes digest them and turn them into small molecules like nitrate, and phosphate. It is only then that plants can make use of these molecules.
Since the large molecules need to be degraded before plants can use them, there is little difference – to the plant – between proteins and oils from fish, cows (manure), or even plants. I have found no support for the claim that fish fertilizer is better than any other organic fertilizer.
The main thing plants need from fertilizer is a source of nitrogen. Garden soils usually have enough P and K and the other minor nutrients. Nitrogen is the thing that is missing in soils. Given this fact, fish fertilizer is no better or worse than other types of fertilizer. (https://www.gardenmyths.com/fish-fertilizer-worth-buying/)
A factoid worth investigating and testing among the comments section of the article: that adding lactic acid bacteria fermentation to the fish or othe stinky emulsions will degrade the noxious smell.