Plants are living organisms, as such they live transforming into energy what nature gives them; energy is obtained from plants thanks to chlorophyll photosynthesis, which uses sunlight; in order to develop their own cells, plants acquire a lot of nutrients from the soil, which are absorbed by the roots. These elements are mainly water, nitrogen, phosphorus and potassium; many other nutrients are acquired to a lesser extent, such as iron, manganese, magnesium and other mineral salts, which are in fact only present in traces in the soil.
In a well-balanced ecosystem, the plants take minerals from the soil which will then be released again into the substrate, in fact the leaves in decomposition, the fruits not consumed by the animals, branches and bark fallen to the ground, decomposing give back to the soil the mineral salts they had taken . So in an ecosystem like a forest in general the soil remains constantly fertile, even with the passage of time.
In a garden, on the other hand, the grass is commonly cut and the mowing is removed from the turf; the leaves fallen from the trees are removed from the ground and so are the fruits; in the flowerbeds we plant small plants which at the end of their life cycle will be uprooted to make way for other essences; in this way with the passage of time we tend to deplete the soil of the mineral elements that are commonly present there. For this reason, if we want to have a luxuriant and healthy garden, it is necessary to provide fertilization, to return the minerals that are removed from the plants we grow to the substrate.