If you don't want to use a chemical fertilizer I recommend 20-20-20 NPK ratio, or as close as possible. But, if you don't want to do that then go fishing and throw the fish guts and head into the soil.

N.P.K. stands for the "N" Nitrogen, "P" Phosphores, "K" Potassium break down in fertilizer. The numbers should appear on all packages of fertilizer such as 20-20-20 or 18-24-12. (a common starter fertilizer)

Most fertilizers even though man made are made with simple elements and are totally safe. There are many fertilizers labled as "organic" and are actually the same thing as the other. It is a play on words because "organic" by the dictionary definition only means that it is carbon based.

Like the Bearswoman said using fish buried under each plant is a good idea, and there are other natural fertilizers on the market as well. Personally I like using the man made stuff. It usually give better results and it is basically all the same stuff anyway.

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Mike Maier
aka
The Plant Doc

There are many forms of fertilizer that you can purchase that is suitable for using on potted plants and vegetables. Here in the States, Miracle Grow is the most popular brand name. You can also get fish emulsion, which is a by product from fish. I am not really sure what brands and types of fertilizer are availible to you over there in Lithuania.
Warmer water will do nothing, and if you were to do it with too warm of water you could harm the plant.

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Mike Maier
aka
The Plant Doc