In case of excess spreading,some fertilizers are not absorbed by the plants.These fertilizers may reach into canals and rivers causing water pollution and encourage the growrh of algae.
Fertilizer is any substance used to add nutrients to the soil to promote soil fertility and increase plant growth. Notice how nothing in there mentions the actual soil health; that's because not all fertilizers are made the same and not all are healthy for the soil.
Soil health relies on a balance of macronutrients and micronutrients, as well as microbial health. It's vastly more complicated then simply adding nitrogen, phosphorus and potassium (NPK) to the ground and calling it a day.
Why doesn't this work? Because science doesn't always get it right. In the past scientists once thought food only consisted of macronutrients, but have since discovered micornutrients, antioxidants and so on. The same goes for soil health: scientists have focused almost exclusively on NPK since their "discovery" in the mid-1800s. It's this reductionist science that has us assuming we understand the chemistry of Nature and soil and thinking we can solve it with a quick application of chemical fertilizers.
Unfortunately, as unsustainable farming and gardening practices continue our soil is stripped of its health and we depend on amendments even greater. This leads to some pretty negative effects of chemical fertilizers.
Negative Effects of Chemical Fertilizers
The biggest issue facing the use of chemical fertilizers is groundwater contamination. Nitorgen fertilizers break down into nitrates and travel easily through the soil. Because it is water-soluble and can remain in groundwater for decades, the addition of more nitrogen over the years has an accumulative effect.
At the University of Wisconsin, Madison, they discovered the effects of chemical fertilizers are compounded when mixed with a single pesticide. They discovered altered immune, endocrine and nervous system functions in mice, as well as influence on childen's and fetus's developing neurological, endocrine and immune systems. These influences "portend change in ability to learn and in patterns of aggression."1
One popular fertilizer, urea, produces ammonia emanation, contributes to acid rain, groundwater contamination and ozone depletion due to release of nitrous oxide by denitrification process. With it's increased use and projections of future use, this problem may increase several fold in the coming decades.2
Groundwater contamination has been linked to gastric cancer, goitre, birth malformations, and hypertension3; testicular cancer4 and stomach cancer.5
Excessive air- and water-borne nitrogen from fertilizers may cause respiratory ailments, cardiac disease, and several cancers, as well as can "inhibit crop growth, increase allergenic pollen production, and potentially affect the dynamics of several vector-borne diseases, including West Nile virus, malaria, and cholera.