In order to be able to live, plants derive their nourishment from sunlight, through photosynthesis, as animals derive their nourishment from food; as is the case for animals, plants also need water to live at their best, moreover, they also need mineral salts, which they cannot obtain through photosynthesis, as is the case for a human being who eats oranges for example to have vitamin C, which not found in other foods. The mineral salts that plants need are naturally present in the soil, in general, and are manifold; the main ones are Nitrogen, Phosphorus, and Potassium, but we could add Calcium, Iron, Boron, Magnesium and some other important minerals.
Not always in the garden soil are all the mineral salts that our plant needs, and sometimes they are present but not in forms easily available to the roots; for this reason, in agriculture and gardening, mineral salts are added to the soil to make sure that the plants find everything they need.
Chemical fertilizers generally consist of salts containing the minerals the plant needs; those to be dissolved in the water remain in the ground for a short period of time, generally for a couple of waterings after the one with the fertilizer, the slow release fertilizers instead are washed out slowly, sometimes even for 4-5 months, before to dissolve completely. Watering and rains dissolve these salts, make them more easily available to plants, but at the same time they dilate them, transporting them to aquifers and rivers.
As with many things, also chemical fertilizers are useful in agriculture, but they are very harmful if used in excess, both because they can easily ruin the plant to which they are administered, and in the long term because the washout due to bad weather leads to adding large quantity of fertilizers in rivers or groundwater, polluting them.
An example that we all remember is that of the mucilages present near the mouth of the river Po, the uncontrolled proliferation of these algae is often also due to the massive presence of chemical fertilizers in the rivers.
So remember to use these products with caution, and avoid reckless excesses.