In the last 50 years they have been widely used to meet the farmers' requirements for productivity. As a matter of fact, nitrogen, phosphorus and potassium fertilizers don't only improve plant growth but dramatically increase crop yields. If fertilizers, adequately used, are good for plant development, they're not so for soil life. Some potassium fertilizers are lethal to earthworms so, as time goes by, the soil loses its fertility. Moreover, too much potassium makes the plants walls thin and too much nitrogen causes a watery growth. Therefore, the plants become more susceptible to pests and diseases.
Soluble fertilizers move in three different directions: downwards, sidewards and out of the farm boundaries.
The downward movement is harmful, as fertilizers go down to the deepest layers and pollute the water tables.
The sideward movement is also dangerous, as fertilizers reach rivers and waterways, causing the growth of water weeds and algae: the consequence is that insects and fish die.
Fertilizers which go out of the farms can be found inside the vegetables in our diet, particularly in leafy vegetables.
However, they shouldn't be confused with chemical pesticides and fungicides, which are highly polluting and have devastating effects on human health and the environment.
(Thanks to Lorenzo S. for helping me writing this post)
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