It is customary to add nitrogen fertilizer in the spring, but the range being wide, you will have to choose the one or those that will be most appropriate for newly established plants or those that have been decorating your exterior for some time.
From April until September, you can bring dried blood , rich in nitrogen, guano , rich in nitrogen and also containing phosphorus good for rooting, as well as fertilizers based on algae and fish waste. Nitrogen is added at the time of growth, in early spring, and/or after summer. Nitrogen supply during flowering or fruiting would be harmful:risk of flower sag, fruit drop, etc.
In any case, do not force the doses. Excess nitrogen makes plants more susceptible to disease and aphids. Vegetables can also have a high nitrate content, which is bad for the health of consumers.
Plant manures are used as growth stimulators, especially nettle and comfrey manures
